Infrared Thermal Testing Reading III- SGuide-IRT Part 1 of 2
My ASNT Level III Pre-Exam Preparatory Self Study Notes 29th May 2015
Charlie Chong/ Fion Zhang
Infrared Thermography
Charlie Chong/ Fion Zhang
Infrared Thermography
Charlie Chong/ Fion Zhang
Infrared Thermography
Charlie Chong/ Fion Zhang
DEADLY French Military Mistral Anti Aircraft Missile System
â– https://www.youtube.com/embed/_3c0NpYapM0 Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=_3c0NpYapM0
See Through & Fun Thermal Camera Experiments
â– https://www.youtube.com/embed/pXAzZoWLzSo Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=pXAzZoWLzSo
LEAKED Body Scan Images From The TSA!
■ https://www.youtube.com/embed/QRkWmRVs-nk Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=QRkWmRVs-nk
How to see through clothing 2
â– https://www.youtube.com/embed/0wQlyCNPw8M Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=0wQlyCNPw8M
Bf4 little bird ah-6j night vision infrared real combat footage helmet cam montage funker tactical. – 金头盔
■ https://www.youtube.com/embed/dRra63kOwWE Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B
Apache IR Thermal Weaponry
â– https://www.youtube.com/embed/XfXShaTzAhI?list=PL7D451B08CD9A119B Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B
Infrared Electrical Testing
â– https://www.youtube.com/embed/DgXsmvv7Q9o Charlie Chong/ Fion Zhang
https://www.youtube.com/watch?v=DgXsmvv7Q9o
Charlie Chong/ Fion Zhang
Fion Zhang at Shanghai 29th May 2015
http://meilishouxihu.blog.163.com/
Charlie Chong/ Fion Zhang
Charlie Chong/ Fion Zhang
Charlie Chong/ Fion Zhang
http://greekhouseoffonts.com/
Charlie Chong/ Fion Zhang
IVONA TTS Capable.
Charlie Chong/ Fion Zhang
http://www.naturalreaders.com/
SGuide-IRT Content Part 1 of 2 ■ Chapter 1 - Introduction to Principles & Theory ■ Chapter 2 - Materials and Their Properties ■ Chapter 3 – Thermal Instrumentation Part 2 of 2 ■ Chapter 4 – Operating Equipment and Understanding Results ■ Chapter 5 – Applications ■ Appendices A, B, C
Charlie Chong/ Fion Zhang
Chapter 1 Principles & Theory
Charlie Chong/ Fion Zhang
1.1
Introduction to Principles & Theory
Infrared/thermal testing involves the use of (1) temperature and (2) heat flow measurement as a means to predict or diagnose failure. This may involve the use of contacting or noncontacting devices, or a combination of both. A fundamental knowledge of heat flow and the thermal behavior of materials is necessary to understand the significance of temperature and temperature changes on a test sample. Contacting devices include thermometers of various types, thermocouples, thermopiles and thermochromic coatings. Noncontacting devices include convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment. Infrared thermography is the nondestructive, non-intrusive. noncontact mapping of thermal patterns on the surface of objects. It is usually used to diagnose thermal behavior and, thereby, to assess the performance of equipment and the integrity of materials, products and processes. Charlie Chong/ Fion Zhang
Keywords: Principles: temperature and heat flow measurement as a means to predict or diagnose failure. Techniques: contacting or noncontacting devices, or a combination of both. Contacting devices include: thermometers of various types, thermocouples, thermopiles and thermochromic coatings. Noncontacting devices include: convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment.
Charlie Chong/ Fion Zhang
The infrared thermal imaging equipment used in infrared thermography is available in numerous configurations and with varying degrees of complexity. The thermal maps produced by infrared thermal imaging instruments are called thermograms. To understand and interpret thermograms, the thermograpber must be familiar with the fundamentals of temperature and heat transfer, infrared radiative heat flow and the performance of infrared thermal imaging instruments and other thermal instruments. An understanding of the equipment, materials and processes being observed is also important to effectively assess the full significance of infrared/thermal measurements. A more detailed discussion of the performance parameters of infrared thermal imaging instruments is provided in Chapter 3. Keywords: â– infrared thermography - The thermal maps produced by infrared thermal imaging instruments are called thermograms.
Charlie Chong/ Fion Zhang
1.2
Fundamentals of Temperature and Heat Transfer
Heat is a transient form of energy in which thermal energy is transient. What is often referred to as a heat source (such as an oil furnace or an electric heater) is really one form or another of energy conversion – the energy stored in one object being converted to heat and nowing to another object. Heat flow is thermal energy in transit and heat always flows from warmer objects to cooler objects. (transient Temperature is a measure of the thermal energy contained in an object - the degree of hotness or coldness of an object that is measurable by any of a number of relative scales. Comments:
“HBNDEv C9 -Transfer of heat energy can be described as either steady-state or transient 暂态 . In the steady-state condition, heat transfer is constant and in the same direction over time.” – However, In this PPT, both steady state and transient are both transient form of energy. Charlie Chong/ Fion Zhang
The three modes of heat transfer are: ■ conductive, ■ convective and ■ radiative. All heat is transferred by one of these three modes. In most situations, beat is transferred by a combination of two or all three modes. Of these three modes of heat transfer, infrared thermography is most closely associated with the radiative process, but it is essential to study all three to understand the meaning of thermograms and to pursue a successful program of thermography. As a result of heat transfer, objects tend to increase or decrease their temperature until they come to thermal equilibrium with their surroundings. To maintain a steadystate heat flow condition, energy must be continuously supplied by some means of energy conversion so that the temperature differential, and hence the heat flow remains constant.
Charlie Chong/ Fion Zhang
The three modes of heat transfer are: ■ conductive, ■ convective and ■ radiative.
Charlie Chong/ Fion Zhang
http://www.chem.purdue.edu/gchelp/liquids/character.html
The three modes of heat transfer are: Water in 3 phases. http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.swf
Charlie Chong/ Fion Zhang
http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.html
Temperature and Temperature Scales Temperature is expressed in either absolute or relative terms. There are two absolute scales called Rankine (English system) and Kelvin (metric system). There are two corresponding relative scales called Fahrenheit (English system) and Celsius or centigrade (metric system). Absolute zero is the temperature at which no molecular action takes place. This is expressed as zero Kelvin or zero degrees Rankin (0 K or 0° R). Relative temperature is expressed as degrees Celsius or degrees Fahrenheit (°C or °F). The numerical relations among the four scales are as follows: converting ºC to ºF, (9/5 x ºC +32) ºF converting ºF to ºC, (5/9 x ºF -32) ºC T Rankine T Kelvin
= T Fahrenheit+ 459.7 = T Celsius + 273.16
Exercise: Temperature (not temperature interval) 0 ºC = 32 ºF thus -273.16 ºC = (-273.16 x 9/5 + 32) ºF = 459.7 ºF Charlie Chong/ Fion Zhang
Temperature and Temperature Scales
â– http://www.mathsisfun.com/temperature-conversion.html Charlie Chong/ Fion Zhang
Temperature and Temperature Scales REMEMBER
0ºC = 32ºF for my ASNT exam
converting ºC to ºF, (9/5 x ºC +32) ºF
Charlie Chong/ Fion Zhang
Boston Tea Party – New governances not the Old Fahrenheit & ⅝”.
Charlie Chong/ Fion Zhang
Boston Tea Party – New governances not the Old Fahrenheit & ⅝”.
Charlie Chong/ Fion Zhang
The Mighty Fahrenheit & ⅝”, English System.
Charlie Chong/ Fion Zhang
The Mighty Fahrenheit & ⅝”, English System.
Charlie Chong/ Fion Zhang
Absolute zero is equal to - 273.16 °C and also equal to approximately - 459.7 °F. To conveIt, a change in temperature or delta T (∆T) between the English and metric systems, the simple 9/5 (1.8 to 1) relationship is used: ∆T Fahrenheit (or º Rankine) = 9/5 x ∆T Celsius (or Kelvin) or simply; ∆T Fahrenheit (or º Rankine) = 1.8 x ∆T Celsius (or Kelvin) Table 1.1 (pages 12 to 14) is a conversion table that will assist in the rapid conversion of temperature between fabrenheit and celsius values. Instructions for the use of the table are shown at the top of the table. (not in this PPT)
Charlie Chong/ Fion Zhang
Conductive Heat Transfer Conductive beat transfer is probably the simplest form to understand. lt is the transfer of beat in stationary media. It is the only mode of heat flow in solids, but it can also take place in liquids and gases. Conductive heat transfer occurs as the result of atomic vibrations (in solids) and molecular collisions (in liquids) whereby energy is moved, one molecule at a time, from higher temperature sites to lower temperature sites. An example of conductive heat transfer is when one end of a section of metal pipe warms up after a flame is applied to the other end. There are physical laws that allow the amount of conductive heat flow to be calculated, and they are presented here to show the factors on which conductive heat flow depends. Keywords: â– atomic vibrations â– molecular collisions (atomic collisions in inert gas)
Charlie Chong/ Fion Zhang
The Fourier conduction Law expresses the conductive heat flow, Q per unit area A, through a slab of solid material of thickness L as illustrated in Figure 1.1. Thermal resistance Rt is defined as:
Thermal conductivity is defined as:
Heat flow per unit area is defined as:
Charlie Chong/ Fion Zhang
Where: • Q/A = the rate of heat transfer through the slab per unit area (BTU/h∙ft2) or (W/m2) perpendicular to the flow, • L = the thickness of the slab (ft or m), • T1 = (°F) or (ºC) is the higher temperature (at the left), • T2 = the lower temperature (at the right) • k = the thermal conductivity of the slab material (BTU/h∙ft∙ºF) or (W/m∙K) • Rt = the thermal resistance of the slab material (°F∙h∙ft2fBTU) or (m2∙K/W)
Charlie Chong/ Fion Zhang
The Fourier conduction Law ( One dimension heat flow) The mathematical relationship that describes heat transfer as a function of the material that heat is conducting through is known as Fourier's law and is given below. Fourier’s Law: q = k ∙ A ∙ （ TH-TC) ∙ L-1 Where: q A k L
= heat transfer per unit time (W) = heat transfer area (m2) = thermal conductivity of material (W/m∙K) = material thickness (m)
Charlie Chong/ Fion Zhang
Thermal conductivity is highest for metals such as aluminum and lower for porous materials such as brick. It is inversely proportional to thermal resistance.
K= 1/Rt Comment: k Îą 1/R, R= thermal resistivity and the thermal resistance Rt = L∙R Thermal conductivity is highest for metals such as aluminum and lower for porous materials such as brick. It is inversely proportional to thermal resistance. In real terms, the Fourier expression means that the rate of heat flow increases with increasing temperature difference. increases with increasing thermal conductivity and decreases with increasing slab thickness. Heat flow may be expressed in English units or metric units.
Charlie Chong/ Fion Zhang
Convective Heat Transfer Convective heat transfer takes place in a moving medium and is almost always associated with heat transfer between a solid and a moving fluid (such as air). Forced convection takes place when an external driving force, such as a wind or an air pump, moves the fluid. Free convection takes place when there is no external driving force - the temperature differences necessary for heat transfer produce density changes in the fluid. The warmer fluid rises as a result of increased buoyancy. In convective heat flow, heat transfer takes effect by direct conduction through the fluid and the mixing motion of the fluid itself. Figure 1.2 illustrates convective heat transfer between a flat plate and a moving fluid.
Charlie Chong/ Fion Zhang
Figure 1.2: Convective heat flow
Charlie Chong/ Fion Zhang
Figure 1.2: Convective heat flow
Distance from boundary layer
T∞
Thermal Boundary layer
Tsurface fluid velocity
Charlie Chong/ Fion Zhang
The presence of the plate causes the velocity of the fluid to decrease to zero at the surface and influences its velocity throughout the thickness of a boundary layer. The thickness of the boundary layer depends on the free fluid velocity V∞ - the higher the free fluid velocity, the thinner the boundary layer. It is greatest for free convection where V∞ = 0. The rate of heat flow depends, in turn, on the thickness of the boundary layer as well as the temperature difference between Ts and T∞ , Ts being the surface temperature and T∞ being the free field fluid temperature outside the boundary layer.
Charlie Chong/ Fion Zhang
Newton's cooling law defines the convective heat transfer coefficient as:
where: h = (BTU/b-ft2-째F) or (W/m2-K) This is rearranged to obtain an expression for convective heat flow per unit area:
If Rc= 1/h is the resistance to convective heat flow, then:
Charlie Chong/ Fion Zhang
Rc is easier to use than h when determining combined conductive and convective heat transfer because then they are additive terms. In real terms, this expression means that the rate of convective heat flow increases with increasing temperature difference, increases with higher convective heat flow coefficient and decreases with increasing convective thermal resistance. Conductive and convective heat transfer are very similar. In both, the heat transfer is directly proportional to the temperature difference and the speed at which th is energy is transferred (rate of heat flow) depends on the transfer coefficient of the media or material through which the heat energy flows. By comparison, radiative heat transfer takes place in accordance with a different set of rules.
Charlie Chong/ Fion Zhang
Radiative Heat Transfer Radiative heat transfer is unlike the other two modes because: 1. it occurs by electromagnetic emission and absorption in a manner similar to light; 2. it propagates at the speed of light; 3. like light, it requires a direct line of sight; 4. the heat energy transferred is proportional to the fourth power T4 of the temperature of the objects; and 5. it can take place across a vacuum – in fact, a vacuum is the most efficient medium for radiative heat transfer. The electromagnetic spectrum is illustrated in Figure 1.3 and shows that Xrays. radio waves. light waves (ultraviolet and visible) and infrared radiation are all related. Radioactive heat transfer takes place in the infrared portion of the spectrum, from 0.75μm to about 100μm, although most practical measurements can be calculated to about 20μm . The symbols μm (μm is preferred) stand for micrometers or microns. A micron is one-millionth of a meter and the measurement unit for radiant energy wavelength. Wavelength is inversely related to frequency (longer wavelengths have lower frequencies). Charlie Chong/ Fion Zhang
Figure 1.3: Infrared in the electromagnetic spectrum
Practical Infrared Thermography λ; 2μm to 6μm and 8μm to 14μm
Charlie Chong/ Fion Zhang
Figure 1.4: Infrared radiation leaving a target surface (ρετσ)
ρ Ɛ τ
Charlie Chong/ Fion Zhang
1.3
Fundamentals of Radiative Heat Flow
Radiation Exchange at the Target Surface The measurement of infrared/thermal radiation is the basis for non-contact temperature measurement and infrared thermography. The surface to be evaluated is called the target surface. Thermal infrared radiation leaving a surface is called exitance or radiosity. It can be emitted from the surface, reflected by the surface, or transmitted through the surface. This is illustrated in Figure 1.4. The total radiosity is equal to the sum of the emitted component (We), the reflected component (Wr) and the transmitted component (Wt ). It is important to note that the surface temperature Te is related to the emitted component We only. Keywords: â– Exitance â– Radiosity
Charlie Chong/ Fion Zhang
Thermal infrared radiation impinging on a surface can be absorbed, reflected, or transmitted as illustrated in Figure 1.5. Kirchhoff's law states that the sum of the three components is always equal to the total received radiation, E t The fractional sum of the three components equals unity or 100 percent: Et = Eα + Eρ + Eτ , (for blackbody EƐ = Eα ) where: Et = total energy Likewise, the sum of the three material properties, transmissivity, reflectivity and emissivity, also always equals unity: τ + ρ + Ɛ =1
Charlie Chong/ Fion Zhang
Figure 1.5: Infrared radiation impinging on a target surface
Kirchhoff's law Charlie Chong/ Fion Zhang
Reflections off Specular and Diffuse Surfaces A perfectly smooth surface will reflect incident energy at an angle complementary to the angle of incidence as shown in Figure 1.5. This is called a specular reflector. A completely rough or structured surface will scatter or disperse all of the incident radiation. This is called a diffuse reflector. No perfectly specular or perfectly diffuse surface can exist in nature, and all real surfaces have some diffusivity and some specularity. These surface characteristics will determine the type and direction of the reflected component of incident radiation. When making practical measurements, the specularity or diffusivity of a target surface are taken into account by compensating for the effective emissivity (Ɛ*) of the surface. The thermographer's use of effective emissivity is reviewed as part of the detailed discussion of equipment operation in Chapter 5. Keywords: ■ Specular reflector ■ Diffuse reflector
Charlie Chong/ Fion Zhang
Reflections off Specular and Diffuse Surfaces
Charlie Chong/ Fion Zhang
Reflections off Specular and Diffuse Surfaces
Charlie Chong/ Fion Zhang
Transient Heat Exchange The previous discussions of the three types of heat transfer deal with steady state heat exchange for reasons of simplicity and comprehension. Heat transfer is assumed to take place between two points, each of which is at a fixed temperature. However, in many applications, temperatures are in transition so that the values shown for energy radiated from a target surface are the instantaneous values at the moment measurements are made. In many instances, existing transient thermal conditions are exploited to use thermography to reveal material or structural characteristics in test articles. In infrared nondestructive testing of materials, thermal injection or active thermography techniques are used to generate controlled thermal transient flow based on the fact that uniform structural continuity results in predictable thermal continuity. These techniques will be discussed in greater detail in Chapter 5.
Charlie Chong/ Fion Zhang
Radiant Energy Related to Target Surface Temperature All target surfaces warmer than absolute zero radiate energy in the infrared spectrum. Figure 1.6 shows the spectral distribution of energy radiating from various idealized target surfaces as a function of surface temperature (T) and wavelength (A.). Very hot targets radiate in the visible as well, and our eyes can see this because they are sensitive to light. The sun, for example, is at a temperature of about 6000 K and appears to glow white bot. The heating element of an electric stove at 800 K glows a cherry red and, as it cools, it loses its visible glow but continues to radiate. This radiant energy can be felt with a hand placed near the surface even though the glow is invisible. The idealized curves shown in Figure 1.6 are for perfect radiators known as blackbodies. Blackbodies are defined and discussed in greater detail later in this chapter. Figure 1.6 also shows two key physical laws regarding infrared energy emitted from surfaces.
Charlie Chong/ Fion Zhang
Radiant Energy Related to Target Surface Temperature All target surfaces warmer than absolute zero radiate energy in the infrared spectrum.
Charlie Chong/ Fion Zhang
The Stefan-Boltzmann law: W=
σƐT4
Where: W = radiant flux emitted per unit area (W/m2) Ɛ = emissivity (unity for a blackbody target) σ = Stefan-Boltzmann constant= 5.673 x I0-8 W/m-2∙K-4 T = absolute temperature of target (K) (Comments: for blackbody Ɛ=1, α=Ɛ.) illustrates that W, the total radiant flux emitted per unit area of surface, (the area under the curve) is proportional to the fourth power of the absolute surface temperature T4. It is also proportional to a numerical constant σ, and the emissivity of the surface, Ɛ.
Charlie Chong/ Fion Zhang
Figure 1.6: Typical blackbody distribution curves and basic radiation laws Stefan-Boltzmann Law Radiant Flux per Unit Area In W/cm2
W= σƐT4 Ɛ = emissivity (unity for a blackbody target) σ = Stefan-Boltzmann constant = 5.673 x I0-8 W/m-2∙K-4 T = absolute temperature of target (K)
Wien's Displacement Law λmax = b/T where: λmax = peak wavelength (μm) b = Wien's displacement constant (2897 or 3000 approximately)
Charlie Chong/ Fion Zhang
Figure 1.6: Typical blackbody distribution curves and basic radiation laws
Charlie Chong/ Fion Zhang
Wien's displacement law:
λmax = b/T Where: λmax wavelength of maximum radiation (μm) b Wien's displacement constant or 2897 (μm∙K) illustrates that the peak wavelength, λmax (μm) at which a surface radiates, is easily determined by dividing a constant b (approximately 3000) by the absolute temperature T (Kelvin) of the surface.
Charlie Chong/ Fion Zhang
1.4
Practical Infrared Measurements
ln practical measurement applications, the radiant energy leaves a target surface, passes through some transmitting medium. usually an atmospheric path, and reaches a measuring instrument. Therefore, when making measurements or producing a thermogram, three sets of characteristics must be considered: 1. characteristics of the target surface, 2. characteristics of the transmitting medium and 3. characteristics of the measuring instrument. This is illustrated in Figure 1.7.
Charlie Chong/ Fion Zhang
Figure 1.7: Three sets of characteristics of the infrared measurement problem
Ɛobj ρamb τ assumed = 0
Charlie Chong/ Fion Zhang
Ɛatm τatm
Characteristics of the Target Surface Target surfaces are separated into three categories; blackbodies, graybodies and nongraybodies (also called real bodies, selective radiators or spectral bodies). The target surfaces shown in Figure 1.6 are all perfect radiators (or blackbodies). A blackbody radiator is defined as a theoretical surface having unity emissivity at all wavelengths and absorbing all of the radiant energy impinging upon it. Emissivity, in turn, is defined as the ratio of the radiant energy emitted from a surface to the energy emitted from a blackbody surface at the same temperature. Blackbody radiators are theoretical and do not exist in practice. The surface of most solids are graybodies, that is, surfaces with high emissivities that are fairly constant with wavelength. Figure 1.8 shows the comparative spectral distribution of energy emitted by a blackbody, a graybody and a nongraybody, all at the same temperature (300 K).
Charlie Chong/ Fion Zhang
Figure 1.8: Spectral distribution of a blackbody, graybody and nongraybody
Charlie Chong/ Fion Zhang
Referring back to Figure 1.5, the total exitance available to the measuring instrument has three components: • • •
emitted energy (We), reflected energy (Wr) from the environment and other reflecting sources, and for nonopaque targets, energy transmitted through the target (Wt) from sources behind the target.
Because a theoretical blackbody has an emissivity Ɛ of 1.00, it will reflect and transmit no energy ρ = 0, τ = 0. Real targets, however, are not blackbodies. and figure 1.9 shows the three components that comprise Wx, the total exitance that an instrument sees when aimed at a real Ufe target surface. Because only the emitted component, We, is related to the temperature of the target surface, it becomes apparent that a significant part of the measurement problem is eliminating or compensating for the other two components. This is discussed in greater detail in Chapter 4. Charlie Chong/ Fion Zhang
Figure 1.9: Components of energy reaching the measuring instrument
Charlie Chong/ Fion Zhang
Characteristics of the Transmitting Medium Because lhe infrared radiation from the target passes through some transmitting medium on its way to the target, the transmission and emission characteristics of the medium in the measurement path must be considered when making non contact thermal measurement. No loss of energy or self emission (Ć?atm) is encountered when measuring through a vacuum. However. most measurements are made through air. For short path length (a few meters, for example), most gases (including the atmosphere) absorb and emit very little energy and can be ignored. However. when highly accurate temperature measurements are required, the effects of atmospheric absorption must be taken into account. (Ď„atm, Ć?atm).
Charlie Chong/ Fion Zhang
As the path length increases to more than a few meters, or as the air becomes heavy with water vapor, atmospheric absorption may become a significant factor. Therefore, it is necessary to understand the infrared transmission characteristics of the atmosphere. Figure 1.10 illustrates the spectral transmission characteristics of a 10 m (33 ft) path of ground level atmosphere at a temperature of 25 °C (77 °F) and 50 percent humidity. It is immediately apparent that the atmosphere is not as transparent in the infrared ponion of the spectrum as it is in the visible ponion. Two spectral intervals have very high transmission. These are known as the 3 to 5 μm and the 8 to 14μm atmospheric windows, and almost all infrared sensing and imaging instruments are designed to operate in one of these two windows. The absorption segments shown in Figure 1.10 were formed by carbon dioxide and water vapor, which are two of the major constituents in air. For measurements through gaseous media other than atmosphere, it is necessary to investigate the transmission spectra of the medium before validating the measurements, which is explained in greater detail in Chapter 2. Charlie Chong/ Fion Zhang
ms nar T egat necr e P
Figure 1.10; Transmission of 10m (33ft) of ground level atmosphere at 50 percent humidity and 25 °C (77ºF)
Charlie Chong/ Fion Zhang
Wave Length μm
When there is a solid material, such as a glass or quartz viewing port, between the target and the instrument, the spectral characteristics of the solid media must be known and considered. Figure 1.11 shows transmission curves for various samples of glass. Most significant is the fact that glass does not transmit infrared energy at 10μm where ambient (30 °C, 86 °F) surfaces radiate their peak energy. In practice, infrared thermal measurements of ambient targets can never be made through glass. One practical approach to this problem is to eliminate the glass, or at least a portion through which the instrument can be aimed at the target. If a window must be present for personal safety, vacuum, or product safety, a material might be substituted that transmits in the longer wavelengths. Figure 1.12 shows the spectral transmission characteristics of several infrared transmitting materials, many of which transmit energy past 10μm. In addition to being used as transmitting windows, these materials are often used as lenses and optical elements in infrared sensors and imagers. Of course, as targets become hotter, and the emitted energy shifts to the shorter wavelengths, glass and quartz windows pose less of a problem and are even used as elements and lenses in high temperature sensing instruments. Characteristics of the measuring instrument are addressed in Chapter 4. Charlie Chong/ Fion Zhang
Figure 1.11: Transmission, absorption and reflectance characteristics of glass
Charlie Chong/ Fion Zhang
Figure 1.12: Transmission curves of various infrared transmitting material
Charlie Chong/ Fion Zhang
Figure 1.12: Transmission curves of various infrared transmitting material
Charlie Chong/ Fion Zhang
Convective Heat Transfer Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow). The term convection can sometimes refer to transfer of heat with any fluid movement, but advection is the more precise term for the transfer due only to bulk fluid flow. The process of transfer of heat from a solid to a fluid, or the reverse, is not only transfer of heat by bulk motion of the fluid, but diffusion and conduction of heat through the still boundary layer next to the solid. Thus, this process without a moving fluid requires both diffusion and advection of heat, a process that is usually referred to as convection. Convection that occurs in the earth's mantle causes tectonic plates to move. Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called "natural convection". An example is the draft in a chimney or around any fire. In natural convection, an increase in temperature produces a reduction in density, which in turn causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any g-force). For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid, which falls. After heating has stopped, mixing and conduction from this natural convection eventually result in a nearly homogeneous density, and even temperature. Without the presence of gravity (or conditions that cause a g-force of any type), natural convection does not occur, and only forced-convection modes operate. The convection heat transfer mode comprises one mechanism. In addition to energy transfer due to specific molecular motion (diffusion), energy is transferred by bulk, or macroscopic, motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer. Because the molecules in aggregate retain their random motion, the total heat transfer is then due to the superposition of energy transport by random motion of the molecules and by the bulk motion of the fluid. It is customary to use the term convection when referring to this cumulative transport and the term advection when referring to the transport due to bulk fluid motion. Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Convective_heat_transfer
Chapter 1 Review Questions
Q&A 1. b
13. d
2. d
14. e
3. c
I5. d
4. a
16. e
5. c
17. b
6. d
18. d
7. b
19. a
8. b
20. d
9. d
21. b
10. d
22. e
11. a 12. a
Charlie Chong/ Fion Zhang
Q1. At a temperature of absolute zero: a. hydrogen becomes a liquid. b. all molecular motion ceases. c. salt water is part solid and part liquid. d. fahrenheit and celsius readings are the same. Q2. Conductive heat transfer cannot take place: a. within organic materials such as wood. b. between two solid materials in contact. c. between dissimilar metals. d. across a vacuum. Q3. The only three modes of heat transfer are: a. resistive, capacitive and inductive. b. steady state, transient and reversible. c. conduction, convection and radiation. d. conduction. convection and absorption.
Charlie Chong/ Fion Zhang
Q4. Heat can only flow in the direction from: a. hotter objects to colder objects. b. colder objects to houer objects. c. more dense objects to less dense objects. d. larger object to smaller objects. Q5. Thermal resistance is: a. analogous to electrical current. b. proportional to the fourth power of emissivity. c. inversely proportional to the rate of heat flow by conduction. d. a measure of material stiffness. Q6. The radiation of thermal infrared energy from a target surface: a. occurs most efficiently in a vacuum. b. is proportional to the fourth power of the absolute surface temperature. c. is directly proportional to surface emissivity. d. is all of the above.
Charlie Chong/ Fion Zhang
Q7. The mode of heat transfer most closely associated with infrared thermography is: a. induction. b. radiation. c. convection. d. conduction. Q8. To convert a fahrenheit reading to celsius: a. divide by 1.8. b. subtract 32 and divide by 1.8. c. multiply by 1.8 and add 32. d. add 273. Q9. Thermal radiation reaching the surface of an object can be: a. absorbed only in the presence of atmosphere. b. reflection and absorbed only in a vacuum. c. transmitted only if the surface is organic. d. absorbed, reflected and transmitted.
Charlie Chong/ Fion Zhang
Q10. The follow ing spectral band is included in the infrared spectrum: a. 0.1 to 5.5 μm. b. 0.3 to 10.6 μm. c. 0.4 to 20.0 μm. d. 0.75 to 100 μm. Q11. Mosl instruments used in infrared thermography operate somewhere within the; a. 2 to 14 μm spectral region. b. 5 to 10 μm spectral region. c. 10 to 20 μm spectral region. d. 20 to 100 J μm spectral region. Q12. As a surface cools, the peak of its radiated infrared energy: a. shifts to longer wavelengths. b. shifts to shorter wavelengths. c. remains constant if emissivity remains constant. d. remains constant even if emissivity varies.
Charlie Chong/ Fion Zhang
Q13. The peak emitting wavelength of a 300 °C (572 ° F) blackbody is approximately: a. 1.5 μm. λmax = b/T( in K) = 2897/573 μm b. 3 μm. 0. 10 μm. d. 5 μm. Q14. An opaque surface with an emissivity of 0.04 would be: a. transparent to infrared radiation. b. a fairly good emitter. c. almost a perfect reflector. (τ=0, Ɛ=0.04, ρ = 0.96) d. almost a perfect emitter. Q15. If a surface has an emissivity of 0.35 and a reflectivity of 0.45. its transmissivity would be: a. impossible to detennine without additional information. b. 0.80. c. 0.10. d. 0.20. [1-(0.35+0.45)] Charlie Chong/ Fion Zhang
Q16. In forced convection, the boundary layer: a. increases as the fluid velocity increases. b. remains the same as the fluid velocity increases. c. decreases as the fluid velocity increases. d. increases in proportion to the fourth power of the fluid velocity. Q17. When heating one end of a car key to thaw a frozen automobile door lock, heat transfer from the key to the lock is an example of: a. forced convection. b. conductive heat transfer. c. free convection. d. radiative heat transfer. Q18. The infrared atmospheric window that transmits infrared radiation best is the: a. 2.0 to 3.0 μm region. b. 3.0 to 6.0 μm region. c. 6.0 to 9.0 μm region. d. 9.0 to 11.0 μm region. Charlie Chong/ Fion Zhang
Q19. The spectral band in which glass transmits infrared radiation best is the: a. 2.0 to 3.0 μm region. b. 3.0 to 6.0 μm region. c. 6.0 to 9.0 μm region. d. 9.0 to 11.0 μm region. Q20. Reflectance of infrared radiation by a glass surface is greatest in the: a. 2.0 to 3.0 μm region. h. 3.0 to 6.0 μm region. c. 6.0 to 9.0 μm region. d. 9.0 to 11.0 μm region. Q21. A diffuse reflecting surface is: a. a polished surface that reflects incoming energy at a complementary angle. b. a surface that scatters reflected energy in many directions. c. also called a specular reflecting surface. d. usually transparent to infrared radiation.
Charlie Chong/ Fion Zhang
Q22. In the 8 to 14 Îźm spectral region: a. the atmosphere absorbs infrared radiant energy almost completely. b. the atmosphere reflects infrared radiant energy almost completely. c. the atmosphere transmits infrared energy very efficiently. d. infrared instruments do not operate very accurately.
Charlie Chong/ Fion Zhang
Chapter 2 Materials and Their Properties
Charlie Chong/ Fion Zhang
2.1
Materials Characteristics
A knowledge of the characteristics of materials is important to the thermographer for numerous reasons, but the two most important arc the need to know how a particular target surface e mits. transmits and refl ects infrared radiant energy. and the need 10 know how heat flows within a particular material.
2.2
Surface Properties of Materials
The surface properties of materials include emissivity. reflectivity and transmissivity.
Charlie Chong/ Fion Zhang
Emissivity Ć? When using infrared thermography to measure surface temperature of a target. it is essential to know the effective emissivity (Ć?*) of the surface material. This is the value that must be set into the instrument's menu under the specific conditions of measurement for the instrument to display an accurate surface temperature value. When attempting to make temperature measurements on a target of unknown emissivity. an estimate of emissivity may be the only available alternative. There are numerous reference tables available that list generic values of emissivity for common materials and these can be used as guides. Table 2.2 is an example of a reference table. As previously noted. emissivity depends on the material and the surface texture. It may also vary with surface temperature and with the spectral interval over which the measurement is made. These variations, though usually small , cannot always be ignored.
Charlie Chong/ Fion Zhang
For an emissivity reference table to be useful. conditions of target temperature and spectral interval (wavelength) must also be presented. If the temperature and wavelength listed do not correspond to the actual measurement conditions. the emissivity listed must be considered to be a rough estimate. Even if there is an exact match to the measurement conditions, the lookup method is not the best approach for accurate temperature measurement. Ideally. the way to determine effective emissivity is to measure it with one of the several established protocols. using a sample of the actual target surface material and the actual instrument to be used for the measurement mission. The protocols for measuring effective emissivity of material samples are discussed in Chapter 4.
Charlie Chong/ Fion Zhang
Reflectivity Ď Reflectivity of a surface generally increases as emissivity decreases. For opaque graybody surfaces Ď„=0. the sum of emissivity and reflectivity is unity (1.0). Therefore. an opaque graybody surface with a low effective cmissivity will be highly reflective, which can result in erroneous temperature readings even if the correct emissivity is set into the instrument. These errors can be the result of either point source reflections, background reflections or both entering the instrument . There are two components of reflected energy the diffuse componenl and the specular component. If the surface is relatively specular (smooth). most of the reflected energy is specular, that is. it reflects off the surface at an angle complementary to the angle of incidenct. If the surface is relatively diffuse (textured) most of the renected energy is scattered uniformly (haphazardly?) in all directions regardless of the angle of incidence. Keywords: Therefore. an opaque graybody surface with a low effective cmissivity will be highly reflective
Charlie Chong/ Fion Zhang
Errors because of point source reflections are usually larger when the target surfaces are specular, and errors because of background reflections are not affected by the specularity or diffusivity of the target surface. Both types of reflective errors are more serious when the target surface is cool compared to the temperature of the point source or the background because the point source makes a greater contribution to the total radiant exitance than the target does. In practice, the thermographer can learn to recognize and avoid errors due to point source reflections. The thermographer also can learn to measure and compensate for errors due to background reflection. This is discussed in Chapter 4.
Charlie Chong/ Fion Zhang
Transmissivity τ When the target surface is a non-graybody, the target material may be partly transparent to infrared radiation. This means the target material has a transmissivity greater than 0. Due to this transparency. radiant thermal energy may be transmitted through the target from sources behind the target. This energy may enter the instrument and cause temperature measurement errors even if the correct emissivity is set into the instrument and reflective errors are eliminated. Although errors due to transmission are the least common in practice. errors due to energy transmiued through the target usually require the most sophisticated procedures to correct them. In most cases, spectral filtering is the best solution. Methods for correcting these errors are discussed in Chapters 4 and 5. Keywords: ■ spectral filtering ■ non-graybody (could be any others like black body, selective emitter, could be a body with τ > 0)
Charlie Chong/ Fion Zhang
View Angle The angle between the instrument's line of sight and the surface material will have a minimal effect on the material properties described above, providing this angle is kepi as close as possible to normal (perpendicul ar) and no greater than Âą30 degrees from normal (for many nonmetallic surfaces this may be increased 10 as large as Âą60 degrees from normal. if unavoidable). If it is not possible to view a target at an angle within this range, the effective emissivity may Change. particularly if it is low to begin with. This will most likely compromise the accuracy of temperature measurements. Note that the emissivities listed in Table 2.2 are normal emissivities and are not valid at acute viewing angles. On curved (nonflat) surfaces. view angle can be even more critical and measurements should be made cautiously. Note: An acute angle is an angle whose degree measure is greater than 0 but less than 90.
Charlie Chong/ Fion Zhang
2.3
Heat Conducting Properties of Materials
The use of infrared themlography for nondestructive material testing is generally based on the assumption that uniform structural continuity provides uniform thermal continuity. Both unstimulated and stimulated approaches to thermographic material testing depend on this assumption. as will be discussed in greater detail in Chapters 4 and 5. It is necessary. therefore, that the thermographer have a clear basic understanding of the manner in which heat flows within a material and the material properties that affect this flow. Keywords: The use of infrared themlography for nondestructive material testing is generally based on the assumption that uniform structural continuity provides uniform thermal continuity.
Charlie Chong/ Fion Zhang
Thermal Conductivity Thermal conductivity k is the relative one dimensional capability of a material to transfer heat. It affects the speed (thus time, t) that a given quantity of heat applied to one point in a slab of material will travel a given distance within that material to another point cooler than the first. Thermal conductivity is high for metals and low for porous materials. It is logical. therefore. that heat will be conducted more rapidly in metals than in more porous materials. Although thermal conductivity varies slightly with temperature in solids and liquids and with temperature and pressure in gases, for practical purposes it can be considered a constant for a particular material. Table 2.1 is a list of thermal properties for several conunon materials.
Charlie Chong/ Fion Zhang
Heat Capacity The heat capacity of a malerial or a structure describes its ability to store heat. It is the product of the specific thermal energy Cp and the density Ď of the material. When thermal energy is stored in a structure and then the structure is placed in a cooler environment, the sections of the structure that have low heat capacity will change temperature more rapidly because less thermal energy is stored in them. Consequently, these sections will reach thermal equilibrium with their surroundings sooner than those sections with higher heat capacity, The term thermal capacitance is used to describe heat capacity in terms of an electrical analog. where loss of heat is analogous to loss of charge on a capacitor. Structures with low thermal capacitance reach equilibrium sooner when placed in a cooler environmcnt than those with high thermal capacitance. This phenomenon is exploited when performing unstimulated nondestructive testing of structures, specifically when locating water saturated sections on flat roofs. This is discussed in greater detail in Chapter 5,
Charlie Chong/ Fion Zhang
Thermal Diffusivity As in emissivity Ɛ. the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors. Thermal diffusivity α is the 3D expansion of thermal conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp, and its density ρ. Its equation is
α = k/ρ Cp cm2s-1. Because thermal diffusivity of a sample can be measured directly using infrared thermography, it is used extensively by the materials flaw evaluation community as an assessment of a test sample's ability to carry heat away, in all directions, from a heat injection site. Table 2.1 lists thermal diffusivities for several common materials in increasing order of thermal diffusivity. Several protocols for measuring the thermal diffusivity of a test sample are described by Maldague.
Charlie Chong/ Fion Zhang
Thermal Diffusivity
Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow.
Charlie Chong/ Fion Zhang
Partial 2.1
Charlie Chong/ Fion Zhang
Partial Table 2.1
Charlie Chong/ Fion Zhang
Partial Table 2.2
Charlie Chong/ Fion Zhang
Thermal Diffusivity As in emissivity Ɛ. the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors. Thermal diffusivity α is the 3D expansion of thermal conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp, and its density ρ. Its equation is
α = k/ρ ∙ Cp cm2s-1. for my ASNT exam
Charlie Chong/ Fion Zhang
Chapter 2 Review Questions
Q&A 1. c 2. b 3. a 4. d 5. a 6. b 7. a 8. b 9. b 10. b
Charlie Chong/ Fion Zhang
1. The best way to determine the effective emissivity of a target surface is: a. to look it up in a table. b. to calcu late it. c. to measure the effective emissivity of the material itself or a similar sample. d. all of the above. 2. For an opaque graybody target surface, emissivity equals: a. 1/refleclivity. b. 1-reflectivity. c. 1.0. d. reflectivity to the fourth power. 3. The effective emissivity of a surface is always affected by: a. the material, its surface texture and the viewing angle. b. the material, its thermal conductivity and humidity. c. the material, its surface texture and its thermal diffusivity. d. the material, its visible color and its thermal conductivity.
Charlie Chong/ Fion Zhang
4. When measuring the temperature of a nongraybody target: a, the viewing angle is not critical. b. always assume an emissivity of 1.0. c. reflections off the near surface may be ignored. d. errors may be caused by hot sources behind the target. 5. The effective emissivity of a target surface: a, can vary at different wavelengths. b. is the same for all wavelengths if the viewing angle is kept constant. c. is always higher at longer wavelengths. d. is always lower at longer wavelengths. 6. Unfinished, unoxidized metal surfaces usually have: a. high and uniform emissivities. b. low and uniform emissivities. c. non-graybody characteristics. d. low specular reflectivity.
Charlie Chong/ Fion Zhang
7. Thermal diffusivity is: a. high for metals and low for porous materials. b. the same for all metals. c, low for metals and high for porous materials. d. the same for all porous materials. 8. Thermal diffusivity is: a, the same as diffuse reflectivity. b. related more to transient heat flow than to steady Slale heat flow. c. related more 10 steady stale heat flow than to transient heat flow. d. the same as spectral transmittance. 9. Thermal capacitance: a. describes the heating of a condenser. b. expresses the heat capacity of a material in a form analogous to electrical capacitance. c. is zero for a blackbody radiator. d. describes the maximum temperature rating of a capacitor.
Charlie Chong/ Fion Zhang
10. A highly textured surface is said to be diffuse. A smooth surface is said to be: a. opaque. b. specular. c. convex. d. transparent.
Charlie Chong/ Fion Zhang
Chapter 3 Thermal Instrumentation
Charlie Chong/ Fion Zhang
3.1
Thermal Instrumentation Overview
Equipment for temperature measurement and thermography includes contacting as well as noncontacting devices. Contacting devices for temperature measurement include thermopiles. thermocouples, liquid thermometers, gas expansion devices (bourdon gas thermometers), liquid crystals (cholesterol crystals ?), heat flux indicators and fiber optic sensors. Aside from some specialized instruments, the vast majority of noncontacting temperature measurement devices are infrared sensing instruments and systems. Infrared sensing instruments and systems are divided into (1) point sensors (radiation thermometers), (2) line scanners and (3) thermal imagers. This chapter begins with a review of contacting thermal measurement instruments and a discussion of the basic configurations of infrared sensing and imaging instruments. This is followed by a discussion of performance parameters and, finally, descriptions of commercial thermal sensing and imaging equipment, thermographic image processing software and image hard copy recording accessories.
Charlie Chong/ Fion Zhang
What is Thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient. Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometers widely used by medical professionals to measure body temperature. They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer) and gas burner safety controls. The output of a thermopile is usually in the range of tens or hundreds of millivolts. As well as increasing the signal level, the device may be used to provide spatial temperature averaging. Thermopiles are also used to generate electrical energy from, for instance, heat from electrical components, solar wind, radioactive materials, or combustion. The process is also an example of the Peltier Effect (electric current transferring heat energy) as the process transfers heat from the hot to the cold junctions.
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Thermopile
Thermopile- Thermoelectric Seebeck module
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Thermopile
The Working Principle: Thermopile, composed of multiple thermocouples in series. If both the right and left junctions are the same temperature, voltages cancel out to zero. However if one side is heated and other side cooled, resulting total output voltage is equal to the sum of junction voltage differentials.
Charlie Chong/ Fion Zhang
Leopoldo Nobili (1784 - 1835) first used the thermoelectric effect for IR radiation measurement using a “pile” of Bismuth and Antimony contacts. The measure of this effect is called the thermoelectric- or Seebeck- coefficient. For most conducting materials this coefficient is rather low, only few semiconductors possess rather high coefficients. Since the voltage of a single thermoelectric cell is very low, lots of such cells arranged in a series connection achieve a larger signal, making a “pile” of thermo-elements.
Charlie Chong/ Fion Zhang
What is a IR Thermopile? (non-contact) A thermopile is a serially-interconnected array of thermocouples, each of which consists of two dissimilar materials with a large thermoelectric power and opposite polarities. The thermocouples are placed across the hot and cold regions of a structure and the hot junctions are thermally isolated from the cold junctions. The cold junctions are typically placed on the silicon substrate to provide effective heat sink. In the hot regions, there is a black body for absorbing the infrared, which raises the temperature according to the intensity of the incident infrared. These thermopiles employ two different thermoelectric materials which are placed on a thin diaphragm having a low thermal conductance and capacitance.
Charlie Chong/ Fion Zhang
http://www.ge-mcs.com/download/temperature/930-164A-LR.PDF
IR Thermopiles Sensor (non-contact)
Charlie Chong/ Fion Zhang
IR Thermopile Quad Sensor (non-contact)
Charlie Chong/ Fion Zhang
Thermocouple General description: Thomas Seebeck discovered in 1821 that when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, grounded or ungrounded. The thermocouple is normally directly connected to a standard temperature controller. Thermocouples are among the easiest temperature sensors used in science and industry and very cost effective. (usually less than $50.00)
thermocouple embedded in Dalton cartridge heater Charlie Chong/ Fion Zhang
http://www.deltat.com/thermocouple.html
Thermocouple A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (째C) can be difficult to achieve. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements. Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered major appliances.
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Thermocouple
Liquid or Gas Expansion Devices Many physical properties change with temperature, such as the volume of a liquid, the length of a metal rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a gas kept at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of matter to measure temperature change. The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, a flexible capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a local indicating temperature dial. A typical filled-system thermometer is shown in Figure 7-1. In this system, the filling fluid, either liquid or gas, expands as temperature increases. This causes the Bourdon tube to uncoil and indicate the temperature on a calibrated dial.
Charlie Chong/ Fion Zhang
Bourdon Gas Thermometers
Charlie Chong/ Fion Zhang
Liquid Crystal Thermometer A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. Temperature changes can affect the color of a liquid crystal, which makes them useful for temperature measurement. The resolution of liquid crystal sensors is in the 0.1째C range. Disposable liquid crystal thermometers have been developed for home and medical use. For example if the thermometer is black and it is put onto someone's forehead it will change colour depending on the temperature of the person. There are two stages in the liquid crystals: 1. the hot nematic stage is the closest to the liquid phase where the molecules are freely moving around and only partly ordered. 2. the cold smectic stage is closest to a solid phase where the molecules align themselves into tightly wound chiral matrixes. Liquid crystal thermometers portray temperatures as colors and can be used to follow temperature changes caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation. In medical applications, liquid crystal thermometers may be used to read body temperature by placing against the forehead. These are safer than a mercury-in-glass thermometer, and may be advantageous in some patients, but do not always give an exact result, except the analytic liquid crystal thermometer which show the exact temperature between 35.5 to 40.5째 Celsius.
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Liquid_crystal_thermometer
Liquid Crystal Thermometer A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal.
Charlie Chong/ Fion Zhang
Thermocouple Thermocouple grade wires Stainless steel sheath
Flexible SS sheath
Adjustable nut
Wire junction
Charlie Chong/ Fion Zhang
http://www.omega.com/temperature/z/pdf/z021-032.pdf
Bimetallic Thermometers
Charlie Chong/ Fion Zhang
http://www.omega.com/temperature/z/pdf/z021-032.pdf
Resistance Thermometers - Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature. They are slowly replacing the use of thermocouples in many industrial applications below 600 째C, due to higher accuracy and repeatability.
http://www.npl.co.uk/content/ConMediaFile/113
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Resistance_thermometer
In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics. Platinum has the temperature range of 650°C, and then the Copper and Nickel have 120°C and 300°C respectively. The figure-1 shows the resistance-temperature characteristics curve of the three different metals. For Platinum, its resistance changes by approximately 0.4 ohms per degree Celsius of temperature. The purity of the platinum is checked by measuring R100 / R0. Because, whatever the materials actually we are using for making the RTD that should be pure. If it will not pure, it will deviate from the conventional resistancetemperature graph. So, α and β values will change depending upon the metals.
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Resistance_thermometer
Platinum Resistance Thermometer
http://www.aoip.com/product/670-standard-platinum-resistance-thermometers/
Charlie Chong/ Fion Zhang
Platinum Resistance Thermometer
Charlie Chong/ Fion Zhang
Resistance Temperature Detector (RTD) - Principle of Operation, Materials, Configuration and Benefits by Innovative Sensor Technology Overview Innovative Sensor Technology is a world-class manufacturer of thin-film RTD temperature sensors, capacitive humidity sensors, and mass flow sensors at the component level. With our state-of-the-art manufacturing technology, Innovative Sensor Technology offers both standard and custom sensors to satisfy unique applications. Additionally, our highly qualified staff is now offering R&D consulting services for industrial applications. Our sensors have applications in the automotive, HVAC, appliance, controls, and test & measurement industries.
Resistance Temperature Detector (RTD) - Principle of Operation An RTD (resistance temperature detector) is a temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature. The relationship between an RTD resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD resistance can be calculated, and the temperature can be determined.
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
RTD Materials Different materials used in the construction of RTD offer a different relationship between resistance and temperature. Temperature sensitive materials used in the construction of RTD include platinum, nickel, and copper; platinum being the most commonly used. Important characteristics of an RTD include the temperature coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the tolerance classes. The TCR determines the relationship between the resistance and the temperature. There are no limits to the TCR that is achievable, but the most common industry standard is the platinum 3850 ppm/K. This means that the resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in temperature. The nominal resistance of the sensor is the resistance that the sensor will have at 0 degrees Celsius. Although almost any value can be achieved for nominal resistance, the most common is the platinum 100 ohm (pt100). Finally, the tolerance class determines the accuracy of the sensor, usually specified at the nominal point of 0 degrees Celsius. There are different industry standards that have been set for accuracy including the ASTM and the European DIN. Using the values of TCR, nominal resistance, and tolerance, the functional characteristics of the sensor are defined.
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
RTD Configurations In addition to different materials, RTD are also offered in two major configurations: wire wound and thin film. Wire wound configurations feature either an inner coil RTD or an outer wound RTD. An inner coil construction consists of a resistive coil running through a hole in a ceramic insulator, whereas the outer wound construction involves the winding of the resistive material around a ceramic or glass cylinder, which is then insulated. The thin film RTD construction features a thin layer of resistive material deposited onto a ceramic substrate through a process called deposition. A resistive meander is then etched onto the sensor, and laser trimming is used to achieve the appropriate nominal values of the sensor. The resistive material is then protected with a thin layer of glass, and lead wires are welded to pads on the sensor and covered with a glass dollop. Thin film RTD have advantages over the wire wound configurations. The main advantages include that they are less expensive, are more rugged and vibration resistant, and have smaller dimensions that lead to better response times and packaging capabilities. For a long time wire wound sensors featured much better accuracy. Thanks to recent developments, however, there is now thin film technology capable of achieving the same level of accuracy.
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
Operations of RTD An RTD takes a measurement when a small DC current is supplied to the sensor. The current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. To reduce self-heating on the sensor the supply current should be kept low. In general, around 1mA or less of current is used. An RTD can be connected in a two, three, or four-wire configuration. The two-wire configuration is the simplest and also the most error prone. In this setup, the RTD is connected by two wires to a Wheatstone bridge circuit and the output voltage is measured. The disadvantage of this circuit is that the two connecting lead wire resistances add directly two the RTD resistance and an error is incurred.
2-Wire Configuration
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
The four-wire configuration consists of two current leads and two potential leads that measure the voltage drop across the RTD. The two potential leads are high resistance to negate the effect of the voltage drop due to current flowing during the measurement. This configuration is ideal for canceling the lead wire resistances in the circuit as well as eliminating the effects of different lead resistances, which was a possible problem with the three-wire configuration. The four-wire configuration is commonly used when a highly accurate measurement is required for the application.
4-Wire Configuration
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
Benefits of Thin Film RTD There are many options when considering contact temperature measurement, including thermocouples, thermistors, and RTD (wire wound and thin film). While thermocouples can handle very high temperatures and thermistors are inexpensive, there are many advantages of RTD. Some of these advantages include their accuracy, precision, long-term stability, and good hysteresis characteristics. Even beyond these, there are advantages of thin film RTD over wire wound, including smaller dimensions, better response times, vibration resistance, and relative inexpensiveness. New advancements has even made thin film technology just as accurate as wire wound at higher temperatures ranges.
Charlie Chong/ Fion Zhang
http://www.azom.com/article.aspx?ArticleID=5573
Thermistor A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiter, temperature sensors (NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements. Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Thermistor
Thermistor
Charlie Chong/ Fion Zhang
http://swordrock.wordpress.com/category/robotic-2/
Thermistor
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Thermistor
3.2
Contacting Thermal Measuring Devices
The most commonly used contacting devices include bimetallic thermometers, thermochromic liquid crystals, thermocouples, resistance thermometer, thermistors and heat flux indicators. These devices are discussed briefly here. For more detailed information, refer to ASNT Nondestructive Testing Handbook. third edition: Volume 3. Infrared and Thermal Testing. â– Bimetallic Thermometers Bimetallic thermometers are sensors constructed of dissimilar metallic strips bonded together. Typically. different iron nickel alloys are used. The strips differ in temperature coefficient of expansion such that temperature changes result in predictable bending of the assembly. Arranged in a spiral or helical configuration. one end of the bimetallic element is fixed and the other end is attached to a pointer. Properly calibrated, the angular position of the pointer can be made to indicate temperature on a scale.
Charlie Chong/ Fion Zhang
â– Thermochromic Liquid Crystals Thermochromic liquid crystals (also called cholesterol crystals) change color with temperature. Coatings made of liquid crystals are commonly used as temperature threshold indicators. Depending on the mixture. a coating applied to a surface will change color predictably when the surface exceeds a threshold temperature. The color change may be reversible or irreversible. and the sensing range for most mixtures is limited to a narrow temperature span. Typically. a set of liquid crystal markers provides a selection of transition temperatures. This allows the user to select the appropriate marker for the desired temperature. Keywords: Threshold temperature
Charlie Chong/ Fion Zhang
â– Thermocouple Thermocouples are contact temperature sensors based on the thermoelectric effect. or Seebeck effect. Thomas Seebeck discovered that, when two dissimilar metals arc joined at both ends and these ends are at different temperatures, a predictable direct current will flow through the circuit. The thermoelectric coefficient determines the relationship between this current and the temperature difference between the two junctions. This coefficient is known for each type of thermocouple. To configure a thermometer. the circuit is broken and the open-circuit voltage is measured by a volt meter. One of the two junctions is then held al a reference temperature. such as an ice bath, and the voltage is calibrated to indicate the temperature of the other junction. which then becomes the temperature sensing junction. Thermopiles arc banks of thermocouples connected in parallel or in series to increase output gradient. The reference temperature is important because of the thermocouples' non linear response. Keywords: thermoelectric coefficient
Charlie Chong/ Fion Zhang
â– Resistance Thermometers Resistance temperature detector (RTDs) arc contact sensors thaI measure tcmpcralUrc by a predictable change in resistance as a function of temperature. Platinum is the most popular resistance temperature detector material because of its excellent stability and its linear response to temperature change. Other materials used include nickel. copper. tungsten and iridium. In operation. the resistance temperature detector may be placed in a bridge circuit such that the bridge output voltage is a measure of the resistance and hence the temperature at the resistance temperature detector. A more accurate measurement may be achieved by using a constant current source and a digital volt meter (DVM). such that the digital volt meter reading is proportional to the resistance temperature detector resistance and hence the temperature at the resistance temperature detector.
Charlie Chong/ Fion Zhang
â– Thermistors Thermistors arc also sensors that measure temperature by a predictable change in resistance as a fun ction of temperature. Thermistors are made of semiconductor materials. Whereas resistance temperature detectors are low impedance devices. thennistors are high impedance devices. Thermistors typically are more sensitive to temperature changes than resistance temperature detectors but thermistors are not as stable. Keywords: Thermistors typically are more sensitive to temperature changes than resistance temperature detectors
Charlie Chong/ Fion Zhang
â– Heat Flux Indicators Heat flux indicators are heat flow meters and are used to measure rates in conduction, convection, radiation and phase change systems such as building walls, boiler tubes and air conditioning ducts. A typical heat flux indicator consists of a sensitive thermopile, composed of many fine gage thermocouples connected in series on opposite sides of a nat core wilh known and stable thermal resistance. The entire assembly is covered with protective material. The voltage generated across the thermopile is calibrated to be a measure of the steady state heat flux through the device. Transient heat flux can be related to the transient thermopile output and the geometry of the device.
Charlie Chong/ Fion Zhang
3.3
Optical Pyrometers
Optical pyrometers include brightness pyrometers and infrared pyrometers. Infrared pyrometers are also called infrared radiation themlometers. Various types are discussed in the next section. Brightness pyrometers are also called matching pyrometers. They incorporate a calibrated light source (lamp) powered by a calibrated current supply. Looking through a viewer. the operator matches the brightness of the target to be measured with the brightness of the calibrated lamp. The adjustment control is cal ibrated in temperature units. such that when the brightnesses arc matched, the control indicates the temperature of the target to be measured.
Charlie Chong/ Fion Zhang
Pyrometer A pyrometer is a device that is used for the temperature measurement of an object. The device actually tracks and measures the amount of heat that is radiated from an object. The thermal heat radiates from the object to the optical system present inside the pyrometer. The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation. The biggest advantage of this device is that, unlike a Resistance Temperature Detector (RTD) and Thermocouple, there is no direct contact between the pyrometer and the object whose temperature is to be found out.
Optical (brightness) Pyrometer In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated. The working of an optical pyrometer is shown in the figure below. Charlie Chong/ Fion Zhang
http://www.instrumentationtoday.com/optical-pyrometer/2011/08/
Pyrometer A pyrometer is a type of remote sensing thermometer used to measure temperature. Various forms of pyrometers have historically existed. In the modern usage, it is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry and sometimes radiometry. The thermal radiation can be used to determine the temperature of an object's surface. The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its incandescence, or the light that is emitted by the body as caused by its high temperature. Modern pyrometers are capable of interpreting temperatures of room temperature objects by measuring radiation flux in the infrared spectrum. A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object. J* = εσT4 This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs).
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Pyrometer
Brightness Pyrometers
Charlie Chong/ Fion Zhang
http://www.instrumentationtoday.com/optical-pyrometer/2011/08/
Brightness Pyrometers – Wien’s Law
Charlie Chong/ Fion Zhang
http://www.instrumentationtoday.com/optical-pyrometer/2011/08/
The scientist Wilhelm Wien (1864–1928) has described the relation between a solid body’s temperature and its emitting peak wave length by following equation: λmax =2898 / T T = Temperature in K (Kelvin) λ = Wavelength in μm Using this law we can calculate the specific peak emission wave length of any material or body: A human body, of a surface temperature of approx. 35°C or 308 K calculates into a peak wavelength of 9,4 μm; a cat of 38°C temperature into 9,3 μm. According to Max Planck (1858 – 1947) the intensity curve of all emitted wave lengths for a solid body is rather broad. For our example above this means we cannot distinguish human from the cat by their infrared spectrum.
Charlie Chong/ Fion Zhang
http://www.instrumentationtoday.com/optical-pyrometer/2011/08/
Charlie Chong/ Fion Zhang
3.4
Basic Configurations of Infrared Radiation Sensing and Imaging Instruments
In terms of configuration and operation. most thermal imagers are considered to be extensions of radiation thermometers or radiation thermometers plus scanning optics. The performance parameters of thermal imagers are extensions of the performance parameters of radiation thermometers. To aid comprehension. the basic measurement problem is discussed in this chapter in terms of the measurement of a single point. It is then expanded to cover thermal scanning and imaging. Figure 3.1 illustrates the basic configuration of an infrared sensing instrument (infrared radiation thermometer), showing the components necessary to make measurements. Collecting optics (an infrared lens, for example) arc necessary for gathering the energy emitted by the target spot and focusing this energy onto the sensitive surface of an infrared detector.
Charlie Chong/ Fion Zhang
The processing electronics unit amplifies and conditions the signal from the infrared detector and introduces corrections for such factors as detector ambient temperature drift and target effective surface emissivity. Generally. a readout. such as a meter. indicates the target temperature and an analog output is provided. The output signal is used to record, display. alarm, control, correct or any combination of these.
Charlie Chong/ Fion Zhang
Figure 3.1: Basic configuration of an infrared radiation thermometer
Charlie Chong/ Fion Zhang
Infrared Detector An infrared detector is at the heart of every infrared sensing and imaging instrument. whatever its configuration. Infrared detectors can sense infrared radiant energy and produce useful electrical signals proportional to the temperature of target surfaces. Instruments using infrared detectors and optics to gather and focus energy from the targets onto these detectors are capable of measuring target surface temperatures with sensitivities better than 0.10 °C (0.18 ºF). and with response limes in the microsecond (Οs) range. An instrument that measures the temperature of a spot on a target in this manner is called an infra red radiation thermometer. An instrument that combines this measurement capability with a means or mechanism for scanning the target surface is called an infrared thermal imager. It can produce thermal maps, or thermograms, where the brightness intensity or color hue of any spot on the map represents the apparent temperature of the surface at that point.
Charlie Chong/ Fion Zhang
Figure 3.2 illustrates the spectral responses of various infrared radiation detectors. Radiant energy impinging on their sensitive surfaces causes all infrared detectors to respond with some kind of electrical change. This may be an impedance change. a capacitance change, the generation of an electromotive force (emf) known as Voltage, or the release of photons, depending on the type of detector. Infrared detectors are divided into (1) thermal detectors and (2) photon detectors. Thermal detectors have broad uniform spectral responses, somewhat lower sensitivities and slower response times (measured in millisecond): photon detectors (also called photo detectors) have limited spectral responses. higher peak sensitivities and faster response times (measured in microsecond). Thermal detectors usually operate at or near room temperature. whereas photon detectors are usually cooled to optimize performance. Keywords: â– Thermal Detector- broad uniform spectral responses/ slower â– Photon Detector- limited spectral responses/ faster
Charlie Chong/ Fion Zhang
Figure 3.2: Response Curves of Various Infrared Detectors
Charlie Chong/ Fion Zhang
Discussion Subject: Why (or How) there are 2 MCT; MCT(215K), MCT(77K)?
Charlie Chong/ Fion Zhang
The mercury cadmium telluride (HgCdTe) detectors shown in Figure 3.2 are photon detectors cooled to 77 K (-321° F) for operation from 8 to 12 μm and to 195 K (-109 ° F) for operation from 3 to 5 μm. Because of their fast response, these detectors are used extensively in high speed scanning and imaging applications. In contrast to the mercury cadmium telluride detector, the radiation thermopile shown in Figure 3.2, is a broad band thermal detector operating uncooled. It is used extensively for spot measurements. Because it generates a direct current electromotive force proportional to the radiant energy reaching its surface. it is ideal for use in portable, battery powered instruments. The lead sulfide (PbS) detector is typical of those used in radiation thermometers that measure and control the temperature of very hot targets. Its peak sensitivity at 3μm matches the peak energy emitted by a 1000K (727 °C = 1340 ° F) graybody. Because of the atmospheric absorption considerations previously discussed. most infrared thermal imagers operate in either the 3 to 5 μm or the 8 to 12 μm spectral region. Note: 195K = [(-273+195) x 9/5] + 32 = -108 ° F
Charlie Chong/ Fion Zhang
Figure 3.2: Response Curves of Various Infrared Detectors Indium Antimony Photon Detectors
Charlie Chong/ Fion Zhang
Infrared Optics - Lenses, Mirrors and Filters There are two types of infrared optics; (1) refractive (lenses. filters, windows) and (3) reflective (mirrors). Refractive optics transmit infrared wavelengths of interest. When used for higher temperature applications. their throughput losses can usually be ignored. When used in low temperature measurement instruments and imagers, absorption is often substantial and must be considered when making accurate measurements. Reflective optics. which are more efficient are not spectrally selective and somewhat complicate the optical path. Reflective optics are used more often for low temperature applications. where the energy levels cannot warrant throughput energy losses. When an infrared radiation thermometer is aimed at a target, energy is collected by the optics in the shape of a solid angle determined by the configuration of the optics and the detector.
Charlie Chong/ Fion Zhang
The cross section of this collecting beam is called the field of view (FOV) of the instrument and it detennines the size of the area (spot size) on the target surface that is measured by the instrument at any given working distance. On scanning and imaging instruments this is called the instantaneous field of view (lFOV) and becomes one picture element on the thermogram. An infrared interference filter is often placed in front of the detector to limit the spectral range of the energy reaching the detector. The reasons for spectral selectivity will be discussed later in this chapter. Processing Electronics The processing electronics unit amplifies and conditions the signal from the infrared detector and introduces corrections for factors such as detector ambient temperature drift and effective target surface emissivity. In radiation thermometers, a meter is usually provided to indicate the target’s apparent temperature. An analog or digital output signal is provided to record, display, alarm, control, correct or any combination of these.
Charlie Chong/ Fion Zhang
Field of View (FOV) A field of view (FOV) is a specification that defines the size of what is seen in the thermal image. The lens has the greatest influence on what the FOV will be, regardless of the size of the array. Large arrays, however, provide greater detail, regardless of the lens used, compared to narrow arrays. For some applications, such as work in outdoor substations or inside a building, a large FOV is useful. While smaller arrays may provide sufficient detail in a building, more detail is important in substation work. See Figure 4-7.
Charlie Chong/ Fion Zhang
Figure 4-7. The field of view (FOV) is a specification that defines the area that is seen in the thermal image when using a specific lens.
Charlie Chong/ Fion Zhang
What is IFOV? A measure of the spatial resolution of a remote sensing imaging system. Defined as the angle subtended by a single detector element on the axis of the optical system. IFOV has the following attributes: â– Solid angle through which a detector is sensitive to radiation. â– The IFOV and the distance from the target determines the spatial resolution. A low altitude imaging instrument will have a higher spatial resolution than a higher altitude instrument with the same IFOV
Charlie Chong/ Fion Zhang
http://www.ssec.wisc.edu/sose/tutor/ifov/define.html
What is IFOV? IFOV (instantaneous field of view) – smallest object detectable The IFOV (instantaneous field of view), also known as IFOVgeo (geometric resolution), is the measure of the ability of the detector to resolve detail in conjunction with the objective. Geometric resolution is represented by mrad and defines the smallest object that can be represented in the image of the display, depending on the measuring distance. The thermography, the size of this object corresponds to a pixel. The value represented by mrad corresponds to the size of the visible point [mm] a pixel at a distance of 1 m.
Charlie Chong/ Fion Zhang
http://www.academiatesto.com.ar/cms/?q=ifov
Instantaneous Field of View (IFOV) An instantaneous field of view (IFOV) is a specification used to describe the capability of a thermal imager to resolve spatial detail (spatial resolution). The IFOV is typically specified as an angle in milliradians (mRad). When projected from the detector through the lens, the IFOV gives the size of an object that can be seen at a given distance. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. See Figure 4-8. It is specified as an angle (in mRad) but is typically larger by a factor of three than the IFOV. This is due to the fact that the imager requires more information about the radiation of a target to measure it than it does to detect it. It is vital to understand and work within the spatial and measurement resolution specific to each system. Failure to do so can lead to inaccurate data or overlooked findings.
IFOV, θ in milli-radian H in mm = D∙ θ
Charlie Chong/ Fion Zhang
D in meter
H
Figure 4-8. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. IFOV is similar to seeing a sign in the distance while IFOV measurement is similar to reading the sign, either because it is closer or larger.
Instantaneous field of view (spatial resolution)/ IFOV measurement (measurement of resolution) Charlie Chong/ Fion Zhang
3.5
Scanning and Imaging
When problems in temperature monitoring and control cannot be solved by the measurement of one or several discrete points on a target surface. it becomes necessary to spatially scan - that is to move the collecting beam or the instrument's field of view relative to the target. This is usually done by inserting a movable optical element into the collecting beam as illustrated in Figure 3.3.
Charlie Chong/ Fion Zhang
Figure 3.3: Adding the scanning element(s) for imaging
Charlie Chong/ Fion Zhang
Line Scanning When the measurement of a single spot on a target surface is not sufficient. infrared line scanners can be used to assemble infonnalion concerning the distribution of radiant energy along a single straight line. Quite often, this is all that is necessary to locate a critical thermal anomaly. The instantaneous position of the scanning element is usually controlled or sensed by an encoder or potentiometer so that the radiometric output signal can be accompanied by a position signal output and be displayed on a recording device and/or fed out to a computer based process control system. A typical high speed commercial line scanner develops a high resolution thermal map by scanning normal to the motion of a moving target such as a paper web or a strip steel process. The resulting output is a thermal strip map of the process as it moves normal to the scan line. The scanning configuration is illustrated in Figure 3.4. The output signal information is in a real time computer compatible format and can be used to monitor, control or predict the behavior of the target.
Charlie Chong/ Fion Zhang
Figure 3.4: Line scanner scanning configuration
Charlie Chong/ Fion Zhang
Two-dimensional Scanning - Thermal Imaging The three common imaging configurations that produce infrared thermograms are (1) optomechanical scanning, (2) electronic scanning and (3) focal plane array imaging. Of the three, optomechanical scanning was the most common until the midI990s. Focal plane array imagers have replaced scanning imagers in most applications.
Charlie Chong/ Fion Zhang
Optomechanical Scanning To scan optomechanically in two dimensions generally requires two scanning elements. Although an almost infinite variety of scanning patterns can be generated using two moving elements. the most common pattern is rectilinear. This scanning pattern is most often accomplished by two elements, each scanning a line normal to the other. A representative rectilinear scanner is illustrated in the schematic of Figure 3.5. Its scanning mechanism comprises two oscillating mirrors behind the primary lens, a high speed horizontal scanning mirror and a slower speed vertical scanning mirror. One performance limitation of single-detector optomechanical scanners is a trade off between speed of response and signal-to-noise ratio of the detector. These instruments require high speed cooled photodetectors that are pushed to their performance limits as the desired real time scanning rate is increased. Multidetector scanners reduce the constraints on detector performance by adding detector elements that share the temporal spatial burden, allowing for faster frame rales with no reduction in signal-to-noise ratio or improving the signal-to-noise ratio with no decrease in frame rate.
Charlie Chong/ Fion Zhang
Figure 3.5: Optomechanlcally scanned infrared imager
Charlie Chong/ Fion Zhang
Electronic Scanning – Pyroelectric Vidicon Thermal Imagers Electronically scanned thermal imaging systems based on pyrovidicons and operating primarily in the 8 to 14 μm atmospheric window are commonly used. They provide qualitative thermal images and are classified as thermal viewers. A pyroelectric vidicon or pyrovidicon is configured the same as a conventional video camera tube except that it operates in the infrared (2 to 20 μm) region instead of the visible spectrum. Image scanning is accomplished electronically in the same manner as in a video camera tube.
Charlie Chong/ Fion Zhang
Pyroelectric Vidicon Thermal Imagers
Charlie Chong/ Fion Zhang
Focal Plane Array Imaging First introduced to the commercial market in 1987. cooled infrared focal plane array (IRFPA) imagers have evolved into compact, qualitative and quantitative thermal imagers without scanning optics. These devices have been replacing optomechanically scanned imagers for many applications. The first uncooled infrared focal plane array imagers have been used by the military for several years and became available to thermographers in 1997. Figure 3.6 is a schematic of a typical. uncooled infrared focal plane array imager. Microbolometer arrays are also available.
Charlie Chong/ Fion Zhang
Figure 3.6: Typical uncooled infrared focal plane array imager
Charlie Chong/ Fion Zhang
IRFPA - Large IR mosaic prototype array with 35 H2RG arrays. The array has a total of nearly 147 million pixels. Each of the H2RG arrays has 2,048Ă—2,048 pixels.
Charlie Chong/ Fion Zhang
http://www.osa-opn.org/home/articles/volume_19/issue_6/features/high-performance_infrared_focal_plane_arrays_for_s/
IRFPA
Charlie Chong/ Fion Zhang
http://ececavusoglu.girlshopes.com/cmoslineararraysirsensor/
Infrared sensors with 3D ROIC for cooled dual-band IR arrays
Charlie Chong/ Fion Zhang
http://www.militaryaerospace.com/articles/2013/07/army-irfpa-roic.html
3.6
Performance Parameters of Infrared Sensing and Imaging Instruments
To select an appropriate instrument for an application, or to determine whether an available instrument will perform adequately. it is necessary for the thermographer to understand its performance parameters. The performance parameters for point sensing instruments (infrared radiation thermometers) are temperature range, absolute accuracy, repeatability, temperature sensitivity, speed of response, target spot size and working distance (field-of-view-spatial resolution), output requirements. sensor environment and spectral range. For scanners and imagers the performance parameters include temperature range. absolute accuracy, repeatability, temperature sensitivity, total field of view (TFOV), instantaneous field of view (lFOV), measurement spatial resolution (IFOVmeas), frame repetition rate, minimum resolvable temperature (MRT), temperature sensitivity, image processing software, sensor environment and spectral range.
Charlie Chong/ Fion Zhang
Qualitative Versus Quantitative Thermography For scanners and imagers. one distinction based on instrument performance limitations is that between qualitative and quantitative thermography. A qualitative thermogram displays the distribution of infrared radiance over the target surface, uncorrected for target, instrument and media characteristics. A quantitative thermogram displays the distribution of infrared radiosity over the surface of the target. corrected for target, instrument and media charactcristics so as to approach a graphic representation of true surface temperature distribution.
Charlie Chong/ Fion Zhang
Performance parameters of qualitative thermographic instruments. therefore, do not include temperature accuracy, temperature repeatability and measurement spatial resolution. Generally, instruments that include the capability to produce quantitative thermograms are more costly than qualitative instruments and require periodic recalibration. Many applications can be solved without the time and expense of quantitative thermography, but others require true temperature mapping. A discussion of the most appropriate applications for quantitative and qualitative thermal imagers is included in Chapter 5. Keywords: Performance parameters of qualitative thermographic instruments. therefore, do not include temperature accuracy, temperature repeatability and measurement spatial resolution.
Charlie Chong/ Fion Zhang
Performance Characteristics of Point Sensing Instruments (Radiation Thermometers) The American Society for Testing and Materials defines infrared point sensing instruments as infrared radiation thermometers even though they do not always read out in temperature units. Some read out directly in apparent radiant power units such as W·m-2· s-1 (or BTU· ft -2∙ h-1), some provide a closure or alarm signal at a selectable temperature and some others provide only difference indications on a light emitting diode display.
Charlie Chong/ Fion Zhang
Temperature Range Temperature range is a statement of the high and low limits over which the target temperature can be measured by the instrument. A typical specification would be. for example. "temperature range 0 to 1000 °C (32 to 1832 ºF).“ Absolute Accuracy Absolute accuracy, as defined by the National Lnstitute of Standards and Technology (NIST) standard, entails the maximum error. over the full range, that the measurement will have when compared to this standard blackbody reference. A typical specification would be, for example. "absolute accuracy ±0.5 °C (±0.9 ºF) ± 1 percent of full scale.“
Charlie Chong/ Fion Zhang
Repeatability Repeatability describes how faithfully a reading is repeated for the same target over the short and long term. A typical specification would be, for example, "repeatability (short and long term) of ±0.25 °C (±0.45ºF) “. Temperature range and absolute accuracy will always be interrelated; for example, the instrument might be expected to measure a range of temperatures from 0 to 200 °C (32 to 392 OF) with an absolute accuracy ±2 °C (±3.6ºF) over the entire range. This could alternately be specified as ±1 percent absolute accuracy over full scale. On the other hand, the best accuracy might be required at some specific temperature, say 100 °C (212 ° F). In this case, the manufacturer should be informed and the instrument could be calibrated to exactly match the manufacturer's laboratory calibration standard at that temperature. Because absolute accuracy is based on traceability to the NIST standard. it is difficult for a manufacturer to comply with a tight specification for absolute accuracy. An absolute accuracy of ±0.5 °C (±0.9 ° F) or ±1 percent of full scale is about as tight as can be reasonably specified. Repeatability, on the other hand, can be more easily ensured by the manufacturer and is usually more important to the user.
Charlie Chong/ Fion Zhang
Temperature Sensitivity Temperature sensitivity defines the smallest target temperature change the instrument will dctect. Temperature sensitivity is also called thermal resolution or noise equivalent temperature difference (NETD). It is the smallest temperature change at the target surface that can be clearly sensed at the output of the instrument. This is almost always closely associated with the cost of the instrument. so unnecessarily fine temperature sensitivity should not be specified. An important rule to remember is that. for any given instrument. target sensitivity will improve for hotter targets where there is more energy available for the instrument to measure. Temperature sensitivity should be specified, therefore, at a particular target temperature near the low end of the range of interest. A typical specification for temperature sensitivity would be, for example, “temperature sensitivity of 0.25 °C (0.45 ºF) at a target temperature of 25 °C (77 ºF)." In this case, the sensitivity of the instrument would improve for targets hotter than 2 °C (36 °F). Keywords: Temperature sensitivity is also called thermal resolution or noise equivalent temperature difference (NETD). Charlie Chong/ Fion Zhang
Temperature sensitivity is also called: thermal resolution or noise equivalent temperature difference (NETD).
for my ASNT exam
Charlie Chong/ Fion Zhang
Speed of Response Speed of response is how long it takes for an instrument to update a measurement. It is defined as the time it takes the instrument output to respond to a step change in temperature at the target surface. Figure 3.7 shows this graphically. The sensor time constant is defined by convention to be the time required for the output signal to reach 63 percent of a step change in temperature at the target surface. Instrument speed of response is usually specified in terms of a large percentage of the full reading, such as 95 percent. As illustrated in Figure 3.7, this takes about five time constants, and is generally limited by the detector used (on the order of microseconds for photodctcetors and milliseconds for thermal detectors).
Charlie Chong/ Fion Zhang
A typical speed of response specification would be, for example. "speed of response (to 95 percent) = 0.05 s.“ It should be understood that there is always a tradeoff between speed of response and temperature sensitivity. As in all instrumentation systems, as the speed of response for a particular device becomes faster (instrumentation engineers call this a wider information bandwidth) the sensitivity becomes poorer (lower signal- to-noise ratio). If the speed of response is specified to be faster than is necessary for the application, the instrument may not have as good a temperature sensitivity as might be possible otherwise.
Charlie Chong/ Fion Zhang
Figure 3.7: Instrument speed to response and time constant
Charlie Chong/ Fion Zhang
Target Spot Size and Working Distance Targct spot size D and working distance d define the spalial resolution of the instrument. In a radiation thermometer, spot size is the projcction of the sensitive area of the detector at the target plane. It may be specified directly, “1 cm at I m (0.4 in. at 3 ft)," for example, but it is usually expressed in more general terms such as a field of view solid angle ( 10 mrad, 1 degree, 2 degree) or a field-of-view ratio (ratio of spot size to working distance - for example, d/15, d/30, d/75. A milliradian (mrad) is an angle with a tangent of 0.001. A d/15 ratio means that the instrument measures the emitted energy of a spot one-fifteenth the size of the working distance: 3 cm at 45 cm (1.2 in. at 18 in .) f or example. Figure 3.8 illustrates these relationships and also shows how spot size can be approximated quickly based on working distance and fieldof-view information furnished by the manufacturer. A typical specification for spot size would be. for example. "target spot size = 2 degrees from 1 m (39 in.) to ∞.“
Charlie Chong/ Fion Zhang
Figure 3.8: Instrument field-of-view determination
Charlie Chong/ Fion Zhang
This would take into account the shortest working distance at which the instrument could be focused (1 m or 39 in.). For some instruments designed for very close workiing distances, the simple d∙D-1 ratio does not always apply. If closeup information is not clearly provided in the product literature, the instrument manufacturer should be consulted. For most applications and for middle and long working distance (greater than 1m or 3 ft), the following simple calculation (illustrated in Figure 3.8) will closely approximate target spot size: where: D ≡ αd D = spot size (approximate), α = field-or-view plane angle in radians, d = distance to the target. A 17.5 mrad (1 degree) field of view means a d∙D-1 ratio of 60 to1 and a 35 mrad (2 degree) field of view means a d∙D-1 ratio of 30 to 1. (?) Charlie Chong/ Fion Zhang
D ≡ αd D = spot size (approximate), α = field-or-view plane angle in radians, d = distance to the target. A 17.5 mrad (1 degree) field of view means a d∙D-1 ratio of 60 to1 and a 35 mrad (2 degree) field of view means a d∙D-1 ratio of 30 to 1. (?) for D ≡ α∙d given that α = 17.5mrad, D=17.5mm if d=1000mm, thus d/D = 1000/17.5 = 57.296 ≈ 60 This is to say the IFOV measurement ration = 1000 ∙ 1/α where α in mRad.
Charlie Chong/ Fion Zhang
EXAM score!
D=σ∙d IFOV ratio = d/D or 1/σ (care on unit used!)
for my ASNT exam
Charlie Chong/ Fion Zhang
Output Requirements Output requirements for radiation thermometers can vary widely - from a simple digital indicator and an analog signal to a broad selection of output functions, including digital output (binary coded decimal); high, low and proportional set points; signal peak or valley sensors; sample and hold circuits; and even closed loop controls for specific applications. On board microprocessors provide many of the above functions on even inexpensive standard portable models of radiation thermometers.
Charlie Chong/ Fion Zhang
Sensor Environment Sensor environment includes the ambient extremes under which the instrument will perform within specifications and the extremes under which it can be stored without damage when not in operation. For a portable radiation thermometer. a typical specifi cation for sensor environment would be as followas. 1. Operating temperature is 0 to 37째C (32 to 100 째F) 2. Humidity is at 20 to 80 percent relative (not condensing). 3. Atmospheric pressure is at -610 m to +2440 m (-2000 to +8000 ft) above sea level. 4. Storage temperature (nonoperating) ranges from -15 to +60 째C (5 to 140 째F). Frequently in process control applications, the sensor must be permanently installed in a somewhat more extreme environment involving smoke, soot. high temperature and even radioactivity. For these applications, manufacturers provide a wide range of enclosures that offer special protective featu res such as air cooling, water cooling, pressurization, purge gases and shielding. Charlie Chong/ Fion Zhang
Spectral Range Spectral range denotes the portion of the infrared spectrum over which the instrument will operate. The operating spectral range of the instrument is often critical to its performance and, in many applications. can be exploited to solve difficult measurement problems. The spectral range is determined by the detector and the instrument optics. as shown in Figure 3.9. Here, the fiat spectral response of a radiation thermopile detector is combined with that of a germanium lens and an 8 to 14 Îźm band pass filter. The instrument characterized is suitable for general purpose temperature measurement of cool targets through atmosphere. The transmission spectrum of a 0.3 km (0. 19 mil) atmospheric ground level is also shown. An infrared interference filter is often placed in front of the detector to limit the spectral range of the energy reaching the detector.
Charlie Chong/ Fion Zhang
the following three classes of filters are common: 1. High pass ti lters pass energy only at wavelengths longer than a designated wavelength. 2. Low pass filters pass energy only at wavelengths shorter than a designated wavelength. 3. Band pass filters similar to the one shown in Figure 3.9. pass radiation within a designated spectral band (8 to 14 Îźm. for example).
Charlie Chong/ Fion Zhang
Spectrall y selective instrumems use band pass filters to allow only a very specific broad or narrow band of wavelengths to reach the detector. (A combination of a spectrally selective detector and a filter can also be used.) This can make the instrument highl y selective to a specific material whose temperature is to be measured in the presence of an intervening medium or an interfering background. Solving measurement problems by means of spectrally selective instruments is discussed in greater detail in Chapter 4. For general purpose use and for measuring cooler targets cooler than about 500 °C (932 °F). most manufacturers of radiation thermometers offer instruments operating in the 8 to 14 μm atmospheric window. For dedicated use on hotter targets. shorter operating wavelengths are selected. usually shorter than 3 μm. One reason for choosing shorter wavelengths is that this enables manufacturers to use commonly available and less expensive quartz and glass optics, which have the added benefit of being visibly transparent for more convenient aiming and sighting. Another reason is that estimating emissivity incorrectly will result in smaller temperature errors when measurements are made at shorter wavelengths.
Charlie Chong/ Fion Zhang
Thermographers have learned that a good general rule to follow, particularly when dealing with targets of low or uncertain emissivities, is to work at the shortest wavelengths possible without compromising sensitivity or risking susceptibility to reflections from visible energy sources.
Charlie Chong/ Fion Zhang
Figure 3.9: Spectral response of an instrument determined by detector and optics spectra
Charlie Chong/ Fion Zhang
MWIR OR LWIR? For general purpose use and for measuring cooler targets cooler than about 500 °C (932 °F). most manufacturers of radiation thermometers offer instruments operating in the 8 to 14 μm atmospheric window. For dedicated use on hotter targets. shorter operating wavelengths are selected. usually shorter than 3 μm. One reason for choosing shorter wavelengths is that this enables manufacturers to use commonly available and less expensive quartz and glass optics, which have the added benefit of being visibly transparent for more convenient aiming and sighting. Another reason is that estimating emissivity incorrectly will result in smaller temperature errors when measurements are made at shorter wavelengths. Thermographers have learned that a good general rule to follow, particularly when dealing with targets of low or uncertain emissivities, is to work at the shortest wavelengths possible without compromising sensitivity or risking susceptibility to reflections from visible energy sources.
Charlie Chong/ Fion Zhang
3.7
Performance Characteristics of Scanners and Imagers
Because an infrared thermogram consists of a matrix of discrete point measurements, many of fhe performance parameters of infrared thermal imager are the same as those of radiation thermometers. The output of an infrared line scanner can be considered as one line of discrete point measurements. The parameters of temperature range, absolute accuracy. repeatability, sensor environment and spectral range are esscntially the same for radiation thermometers, line scanners and imagers. Others are derived from or are extensions of radiation thermometer performance parameters. Qualitative thermal imagers (also called thermal viewers) differ from quantitative thermal imagers (also called imaging radiometers) in that thermal viewers do not provide temperature or thermal energy measurements. For thermographers requiring qualitative rather than quantitative thermal images, therefore, some performance parameters are unimportant.
Charlie Chong/ Fion Zhang
Total Field of View (FOVtotal) For scanners and imagers. total field of view denotes the image size in terms of total scanning angles for any given lens. An example of a typical total field of view specifi cation would be "TFOV = 20 degrees vertical x 30 degrees horizontal" (with standard Ix lens) and would define the thermogram total target size by a simple trigonometric relationship: tan θ/2 = V/2∙d-1 V = 2 ∙ tan (y/2) ∙ d, for θ = y
d = working distance, H = total horizontal image size, V = total vertical image size, x = horizontal scanning angle, y = vertical scanning angle. This is illustrated in Figure 3. 10. Charlie Chong/ Fion Zhang
θ = y or x
The total field of view for a line scanner consists of one scan line as shown in Figure 3.4 and Figure 3.10. The horizontal image size H is equal to the scan sector. The vertical image size V is equal to the instantaneous field of view. All other parameters are the same as for an imager.
Figure 3.4: Line scanner scanning configuration Charlie Chong/ Fion Zhang
Figure 3.10: Total field of view (TFOV) determination for an infrared imager
Charlie Chong/ Fion Zhang
Instantaneous Field of View IFOV Instantaneous field of view in an imager is very similar to that for a point sensing instrument: it is the angular projection of the detector element at the target plane. (resolution?) In an imager, however, it is also called imaging spatial resolution and represents the size of the smallest picture element that ean be imaged. An example of a typical instantaneous field of view specification would be "IFOV = 1.7 mRad at 0.35 MTF." The 0.35 MTF refers to 35 percent of the modulation transfer function test used to check imaging spatial resolution. This is described in detail in Chapter 4. The simple expression. D = Îąd, can be used to estimate imaging spot size at the target plane from manufacturer's published data by substituting the published instantaneous field of view for Îą. Keywords: IFOV, image spatial resolution, MTF-modulated transfer function
Charlie Chong/ Fion Zhang
EXAM score!
IFOV is also called;
image spatial resolution
for my ASNT exam
Charlie Chong/ Fion Zhang
Recalling! Temperature sensitivity is also called: thermal resolution or noise equivalent temperature difference (NETD).
for my ASNT exam
Charlie Chong/ Fion Zhang
Measurement Spatial Resolution Measurement spatial resolution (IFOVmeas) is the spatial resolution of the minimum target spot size on which an accurate measurement can be made in lenns of its distance from the instrument. An example of a typical measurement spatial resolution specification would be "IFOVmeas = 3.5 mrad at 0.95 SRF.“ The 0.95 SRF refers to 95 percent slit response function test used to check measurement spatial resolution. This is described in detail in Chapter 4. The simple ex pression, D = αd, can again be used to estimate measurement spot size at the target plane from manufacturer's published data by substituting published measurement spatial resolution for α. Keywords: SRF refers to 95 percent slit response function test used to check measurement spatial resolution. Comments: IFOVmeas – IFOV measurement
Charlie Chong/ Fion Zhang
IFOV - MTF
The 0.35 MTF refers to: 0.35 percent of the modulation transfer function test used to check imaging spatial resolution.
for my ASNT exam
Charlie Chong/ Fion Zhang
IFOVmeas - SRF
95 SRF refers to: 95 percent slit response function test used to check measurement spatial resolution
for my ASNT exam
Charlie Chong/ Fion Zhang
Fig. 2a. Slit Response Function. Camera sees slit lips of radiometric temperature T0 (back side radiometric temperature) and The body behind the slit of radiometric temperature T1 (“slit “ temperature). Slit width is d and D is the distance slit-camera (Figure is issue from reference 4)
Charlie Chong/ Fion Zhang
http://qirt.gel.ulaval.ca/archives/qirt2006/papers/025.pdf
Frame Repetition Rate Frame repetition rate replaces speed of response and is defined as the number of times every point on the target is scanned in one second. This should not be confused with field rate. Some imagers are designed to interlace consecutive fields. each consisting of alternate image lines. This results in images less disconcerting 令人不安的 to the human eye. The frame rate in this case would be one half the field rate. An example of a typical frame repetition rate specification for an imager would be "frame repetition rate = 30 frames per second." For a line scanner. the term line scan rate is used and it is expressed in lines per second. Comments: For interlace field rate scanning; The frame rate in this case would be one half the field rate.
Charlie Chong/ Fion Zhang
Minimum Resolvable Temperature Difference Minimum resolvable temperature (MRT) or minimum resolvable temperature difference (MRTD) replaces temperature sensitivity and is defined as the smallest blackbody equivalent larget lemperature difference Ihat can be observed OUI of system noise on a thermogram. As in radiation thennometry. this difference improves (becomes smaller) with increasing target temperature and is expressed in those terms. An example of a typical minimum resolvable temperature diffe rence speci fi cation for a line scanner or an imager would be "MRTD = 0.05 °C at 25 °C target temperature (0.09 OF at 77 OF),“ Minimum resolvable temperature difference may also depend on the spatial frequency imposed by the test discipline. The test techniques for checking minimum resolvable temperature difference is described in Chapter 4, Comments: Temperature sensitivity is also called: thermal resolution or noise equivalent temperature difference (NETD).
Charlie Chong/ Fion Zhang
Thermal Imaging Display and Diagnostic Software Overview Thermography applications often req uire extensive thermal imaging display and diagnostic software. Thermal imagers feature image processing capabilities that may be divided into five categories. one or more of which may be used in the same application. These categories are quantitativc thermal measurements of targets; detailed processing and image diagnostics; image recording. storage and recovery; image comparison (differential or multispectralthermography); and database and documenlalion. Applications using software capabilities, singly and in combination. will also be described in Chapter 5.
Charlie Chong/ Fion Zhang
EXAM score!
D=σ∙d IFOV ratio = d/D or 1/σ (when calculation IFOV ratio care on unit used!) for my ASNT exam
Charlie Chong/ Fion Zhang
FOV - Animation
Charlie Chong/ Fion Zhang
http://www.imagerchina.com.cn/fov_calculator.html
Charlie Chong/ Fion Zhang
Questions & Answers Subject: Answer this web queries from: http://www.thesnellgroup.com/community/ir-talk/f/9/p/1402/5433.aspx wonder if anyone can help me here. I am studying for my employer's Level 2 certification exam and I am using the ASNT supplement booklet to help. They ask a few question about IFOV and spot size calculation and I do not quite understand how they get the answers. basically it is not the answer I want but how they got to the answers. Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or mRad to something else?) Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit conversions going on here from meters to cm. So how is it done?) Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply) Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in). NOW this one I kind of figured out using: 1/180 = 0.0055 & 3 m = 3000mm therefore 0.0055 x 3000 = 16.5 Let me know if you all know how to do these problems. I think all I need is the formula and an understanding when and which units to convert.
Charlie Chong/ Fion Zhang
Answer: D= σ∙d, IFOV ration= 1/σ = d/D Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or mRad to something else?) Calculation: D= 1.9 x 1 = 1.9mm or 0.19cm, (100cm = 1m) Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit conversions going on here from meters to cm. So how is it done?) Calculation: D= 1.2 x 25m = 30mm = 3cm Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply) Calculation: 25.4 = σ x 20, σ = 1.27mRad Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in). Calculation: 1/ σ = d/D = 180, σ = 1/180, D = σ∙d, D = 1/180 x 3 = 0.01667m = 16.7mm (when calculating IFOV ratio, good to use the same unit for all inputs)
Charlie Chong/ Fion Zhang
Charlie Chong/ Fion Zhang
Break Time
– Kenya Coffee Picker
Charlie Chong/ Fion Zhang
http://www.kickstartcafe.com/journal/kenyan-coffee#.VWuZY52S3IU
3.8
Descriptions of Thermal Sensing and Imaging Equipment
Point Sensors (Radiation Thermometers) Point sensors (radiation thermometers) can be further divided into temperature probes. portable hand held devices. online process control devices and specially configured devices. ■ Temperature Probes Temperature probes are low priced, pocket portable, battery powered devices that usually feature a pencil shaped sensor connected to a small basic readout unit. Generally, they are optically pre-adjusted for minimum spot size at a short working distance. A 0.5cm (0.2 in.) spot al a 2 cm (0.8 in.) working distance is typical. Temperature usually ranges from about - 20 °C to 300 °C (- 4 ° F to 570 ° F) and a sensitivity of ±1°C (1.8 ° F) is achieved easily. Probes are designed for close-up measurements such as circuit board analysis. troubleshooting of electrical connections. inspect ion of plumbing systems and biological and medical studies.
Charlie Chong/ Fion Zhang
Portable Handheld Devices
Charlie Chong/ Fion Zhang
■ Portable Handheld Devices Portable handheld radiation thermometers are designed for middle distance measurements and, with few exceptions, operate in the 8 to 14 μm spectral region and are configured like a pistol for one-handcd operation and aiming. They are usually optically preadjusted for infinity focus. A typical 2 degree field of view resolves a 7.5 cm (3 in.) spot at a 150 cm (60 in.) working distance and a 30 cm (1 ft) spot at a 9 m (30 ft) working distance. (9 x tan(2º) = 0.314m=31cm)
Most instruments in this group incorporate microcomputers with limited memory and some have data logging capabilities. An open or enclosed aiming sight is provided and in some models a projected laser beam is used to facilitate aiming of the instrument as shown in Figure 3. 11. Note that the laser beam docs not represent the field of view. A measurement readout is always provided and usually the temperature is shown on a digital liquid crystal display. These instruments are powered with disposable batteries and have low power drain.Temperature ranges are typically from 0 to 1000 °C (30 to 1800 ºF).
Charlie Chong/ Fion Zhang
Temperature sensitivity and readability are usually 1 percent of scale 1°C (2 ºF) although sensitivities on the order of 0.1 °C (0.2 ° F) arc achievable. Response times are on the order of fractions of a second, usually limited by the response of the readout. Hand held radiation thermometers are used extensively in applications where spot checking of target temperatures is sufficient and continuous monitoring is not required. Handheld radiation thermometers have become an important part of many plant energy conservation programs. Process applications include monitoring mixing temperatures of food products. cosmetics and industrial solvents. Microcomputers enable handheld instruments to incorporate special features such as the ability to store sixty readings for future retrievals and printout.
Charlie Chong/ Fion Zhang
Figure 3.11: Hand held infrared radiation thermometer with laser aiming
Charlie Chong/ Fion Zhang
Hand Held Infrared Module
Charlie Chong/ Fion Zhang
Note that the laser beam docs not represent the field of view. Figure 1. Use the Fluke 66 within 5 m (15 ft.) of the intended target. At greater distances, the measured area will be larger (approximately the distance divided by 30). Field of view θ= tan-1 (1/30) = 1.91º
Charlie Chong/ Fion Zhang
http://www.fluke.com/fluke/m3en/products/thermometers
Note that the laser beam docs not represent the field of view. Figure 2. Use the Fluke 68 within 8 m (25 ft.) of the intended target. At greater distances, the measured area will be larger (approximately the distance divided by 50). Filed of view θ= tan-1 (1/50) = 1.14º
Charlie Chong/ Fion Zhang
http://www.fluke.com/fluke/m3en/products/thermometers
â– Online Process Monitoring and Control Devices Online monitoring and control sensors are for dedicated use on a product or a process. Permanently installed where it can measure the temperature of one specific target. this type of instrument remains there for the life of the instrument or the process. With few exceptions. these instruments operate on line power. The measurement value can be observed on a meter. but it is more often used to trigger a switch or relay or to feed a simple or sophisticated process control loop. Most of the online monitoring and control sensors send signals to universal indicator control units that accept inputs from various types of industrial sensors. Because this instrument group is selected to perform a specific task, a shopping list format is provided to the customer by the manufacturer so that all required features can be purchased. including environmental features such as water cooled housings. air purge fittings and air curtain devices.
Charlie Chong/ Fion Zhang
Emissivity set controls, located in a prominent place on a general purpose instrument are more likely to be located behind a bezel ĺľŒć§˝ / ć&#x;œ on the sensor on these dedicated units. where they are set once and locked. The spectral interval over which the sensing head operates is selected to optimize the signal from the target, to reduce or eliminate the effect of an interfering energy source or to enable the instrument to measure the surface temperature of thin films of material that are largely transparent to infrared radiation. The capability for spectral selectivity has made these instruments important in the manufacture of glass and thin film plastics. Applications in these atres are discussed in Chapters 4 and 5.
Charlie Chong/ Fion Zhang
IR Sensor Module
Charlie Chong/ Fion Zhang
IR Sensor Module
Charlie Chong/ Fion Zhang
IR Sensor Module
Charlie Chong/ Fion Zhang
IR Sensor Module
Charlie Chong/ Fion Zhang
■ Devices with Special Configurations Special configurations of infrared radiation thermometers include ratio pyrometers (also called two color pyrometers), infrared radiometric microscopes, laser reflection pyrometers and fiber-optic coupled pyrometers. 1. Two-color pyrometers or ratio pyrometers, are a special case of the online instrument. Ratio pyrometers are particularly useful in high temperature applications above 300 °C (572 ° F) and in measuring small targets of unknown emissivity, provided the background is cool. constant and uniform. The emissivity of the target need not be known if it is constant and relections are controlled. The target does not need to fill the field of view. provided the background is cool, constant and uniform. The measurement is based on the ratio of energy in two spectral bands. so impurities in the optical path resulting in broad band absorption do not affect the measurement. Ratio pyrometers are usually, not applicable to measurements below 300 °C (572 °F).
Charlie Chong/ Fion Zhang
Two-color Pyrometers or Ratio Pyrometers
Charlie Chong/ Fion Zhang
Two-color Pyrometers or Ratio Pyrometers
Charlie Chong/ Fion Zhang
Two-color Pyrometers or Ratio Pyrometers
Charlie Chong/ Fion Zhang
https://www.eutech-scientific.de/products-services/power-generation/euflame.html
Two-color Pyrometers or Ratio Pyrometers
Charlie Chong/ Fion Zhang
https://www.eutech-scientific.de/products-services/power-generation/euflame.html
Two-color Pyrometers or Ratio Pyrometers Ratio Radiation - Also called two-color radiation thermometers, these devices measure the radiated energy of an object between two narrow wavelength bands, and calculates the ratio of the two energies, which is a function of the temperature of the object. Originally, these were called two color pyrometers, because the two wavelengths corresponded to different colors in the visible spectrum (for example, red and green). Many people still use the term two-color pyrometers today, broadening the term to include wavelengths in the infrared. The temperature measurement is dependent only on the ratio of the two energies measured, and not their absolute values as shown in Figure 3-4. Any parameter, such as target size, which affects the amount of energy in each band by an equal percentage, has no effect on the temperature indication. This makes a ratio thermometer inherently more accurate. (However, some accuracy is lost when you're measuring small differences in large signals). The ratio technique may eliminate, or reduce, errors in temperature measurement caused by changes in emissivity, surface finish, and energy absorbing materials, such as water vapor, between the thermometer and the target. These dynamic changes must be seen identically by the detector at the two wavelengths being used. Emissivity of all materials does not change equally at different wavelengths. Materials for which emissivity does change equally at different wavelengths are called gray bodies. Materials for which this is not true are called non-gray bodies. In addition, not all forms of sight path obstruction attenuate the ratio wavelengths equally. For example, if there are particles in the sight path that have the same size as one of the wavelengths, the ratio can become unbalanced. Charlie Chong/ Fion Zhang
http://www.omega.com/literature/transactions/volume1/thermometers2.html
Figure 3-4: The “Two-Color� IR Thermometer
E1
T1 E2
T2 E1 E2
Charlie Chong/ Fion Zhang
http://www.omega.com/literature/transactions/volume1/thermometers2.html
Phenomena which are non-dynamic in nature, such as the non-gray bodiness of materials, can be dealt with by biasing the ratio of the wavelengths accordingly. This adjustment is called slope. The appropriate slope setting must be determined experimentally. Figure 3-5 shows a schematic diagram of a simple ratio radiation thermometer. Figure 3-6 shows a ratio thermometer where the wavelengths are alternately selected by a rotating filter wheel. Figure 3-5: Beam Splitting in the Ratio IR Thermometer
Charlie Chong/ Fion Zhang
http://www.omega.com/literature/transactions/volume1/thermometers2.html
Figure 3-6: Radio Pyometry Via a Filter wheel
Figure 3-7: Schematic of a Multispectral IR Thermometer.
Charlie Chong/ Fion Zhang
http://www.omega.com/literature/transactions/volume1/thermometers2.html
Some ratio thermometers use more than two wavelengths. A multi-wavelength device is schematically represented in Figure 3-7. These devices employ a detailed analysis of the target's surface characteristics regarding emissivity with regard to wavelength, temperature, and surface chemistry. With such data, a computer can use complex algorithms to relate and compensate for emissivity changes at various conditions. The system described in Figure 3-7 makes parallel measurement possible in four spectral channels in the range from 1 to 25 microns. The detector in this device consists of an optical system with a beam splitter, and interference filters for the spectral dispersion of the incident radiation. This uncooled thermometer was developed for gas analysis. Another experimental system, using seven different wavelengths demonstrated a resolution of +/-1°C measuring a blackbody source in the range from 600 to 900°C. The same system demonstrated a resolution of +/- 4°C measuring an object with varying emittance over the temperature range from 500 to 950°C Two color or multi-wavelength thermometers should be seriously considered for applications where accuracy, and not just repeatability, is critical, or if the target object is undergoing a physical or chemical change. Ratio thermometers cover wide temperature ranges. Typical commercially available ranges are 1652 to 5432° F (900 to 3000°C) and 120 to 6692°F (50 to 3700°C). Typical accuracy is 0.5% of reading on narrow spans, to 2% of full scale. Charlie Chong/ Fion Zhang
http://www.omega.com/literature/transactions/volume1/thermometers2.html
2. Infrared radiometric microscopes are configured like a conventional microscope and by using reflective microscope objectives and beam splitters, the operator can simultaneously view and measure targets down to 10 Οm in diameter with accuracy and resolution of about 0,5 °C (1 °F). 3. Laser reflection pyrometers use the reflected energy of an active laser to measure target reflectance. A built-in microcomputer calculates target effective emissivity and uses this figure to provide a corrected true temperature reading. This instrument. though expensive, is useful for measurement of high temperature specular target surfaces in adverse environments. 4. Fiberoptic coupled pyrometers make possible the measurement of normally inaccessible targets by replacing the optic with a flexible or rigid fiberoptic bundle. This limits the spectral performance and hence the temperature range to the higher values, but has allowed temperature measurements to be made when previously none were possible.
Charlie Chong/ Fion Zhang
Infrared Radiometric Microscopes
Charlie Chong/ Fion Zhang
Fiberoptic Coupled Pyrometers
Charlie Chong/ Fion Zhang
http://www.omega.com/temperature/pdf/4121_ir.pdf
Line Scanners Line scanners are divided into online process control devices and special purpose scanners. â– Online Process Control Devices Online (monitoring and control) line scanners are high speed online commercial line scanners that develop high resolution thermal maps by scanning normal to the motion of a moving target such as paper web or a strip steel process. The vast majority of commercial infrared line scanners are in this configuration. The output signal information is in a real time computer compatible format and can be used to monitor, control or predict the behavior of the target. Like the online point sensor, these line scanners are usually permanently installed where they monitor the temperature profile at one site of the process, remaining there for the life of the instrument or the process. Likewise they are usually fitted with environmental housings and preset emissivity compensation sets. The best applications for this scanner are in online, real time process monitoring and control applications where they are integrated with the process host computer system.
Charlie Chong/ Fion Zhang
It is not unusual to find line scanners at multiple locations in a process with all of them linked to the host computer. In the 1990s, infrared line scanners based on a linear focal plane array came into use. This type of instrument frequently uses an un-cooled array of thermal detectors radiation thermopiles. This scanner has no moving parts. The linear array is oriented perpendicular to a process or a target moving at a uniform rate. The scanner output may be used to develop a thermograms or the data for each pixel can be fed directly to a host computer and used to monitor and control the process. Instruments of this type have been used to monitor moving railroad cars for overheated wheels and brake assemblies.
Charlie Chong/ Fion Zhang
Special Purpose Devices Special purpose configurations of line scanners include one type of portable line scanner and a number of aerial mappers that scan a line normal to the motion of the aircraft and develop a thermal strip map. Many of these mappers have been replaced by low cost forward looking infrared scanners (FLIRs) based on staring focal plane arrays.
Charlie Chong/ Fion Zhang
FLIR- Forward Looking Infrared
Charlie Chong/ Fion Zhang
FLIR- Forward Looking Infrared
Charlie Chong/ Fion Zhang
Imagers (Thermographic Instruments) Imagers (thermographic instruments) consist of both qualitative and quantitative imagers. ■ Qualitative Thermal Imagers Qualitative thermal imagers arc also called thermal viewers. They include mechanically scanned, electronically scanned (pyrovidicon) and staring focal plane array FPA imagers. ● Mechanically Scanned Thermal Viewers Mechanically scanned thermal viewers are moderately priced battery powered scanning instruments that produce a qualitative image of the radiosity over the surface of a targct. The battery packs are rechargeable and usually provide 2 to 3 h of continuous operation. These one-piece, lightweight instruments, designed to be simple to operate, feature thermoelectric detector, cooling provided by a battery powered cooler. Although not designed for absolute temperature measurements, they can demonstrably sense temperature differences of tenths of degrees and can be used for targets from below 0 °C up to 1500 °C (32 of up to 2372 °F). Charlie Chong/ Fion Zhang
Typically, the total field of view is from 6 to 8 degrees high and from 12 to 18 degrees wide, with spatial resolution of 2 mRad 10 mm at 2.0 m (0.4 in. at 7 ft). Images are video recorded by means of a conventional video tape recorder output jack and video recorder accessories. The broad applications for thermal viewers are generally limited only to those in which the temperature measurements are not critical and recording quality does not need to be optimum. The combination of a thermal viewer (to locate thermal anomalies) and a hand held thermometer (to quantify them) can be a powerful and cost effective ombination.
Charlie Chong/ Fion Zhang
� Electronically Scanned Viewers (Pyrovidcon Imagers) Pyrovidicon imagers arc electronically scanned video cameras. The camera tube is sensitive to target radiation in the infrared rather than the visible spectrum. Aside from the tube and germanium lens, which are expensive, these systems use television recording accessories, in comparison with other infrared imaging systems, the picture quality and resolution are good, approaching conventional television format. The thermal image can be viewed or videotaped with equal convenience and no cooling is required. Pyrovidicon systems do not intrinsically offer quantitative measurement capability, but some manufacturers offer models in which an integrated radiation thermometer is bore sighted with the scanner and its measurement is superimposed on the video display along with a defining reticle in the center of the display thermal resolution of flicker free pyrovidicon instruments is between 0.2 and 0.4 °C (0.4 and 0.7 °F).
Charlie Chong/ Fion Zhang
Pyroelectric devices have no direct current response, and a basic pyrovidicon imager 's display will fade when the device is aimed at an unchanging thermal scene. Early pyrovidicon imagers needed to be panned to retain image definition. To enable fixed monitoring, crude, flag type choppers were devised to interrupt the image at adjustable chop rates. However, this resulted in a blinking image that was disconcerting to the eye. These choppers have been replaced by synchronous choppers that chop the image in synchronism with the electronic scan rate and produce flicker free images on the display. Pyrovidicon viewers operate well in the 8 to 14 Îźm atmospheric transmission window. Operating costs are very low because no cooler or coolant is required.
Charlie Chong/ Fion Zhang
â—? Staring Infrared Focal Plane Array Thermal Viewers Staring infrared focal plane array (lRFPA) thermal viewers are direct adaplations of devices developed for military and aerospace night vision and missile tracking applications. For these applications, performance emphasis is on picture quality rather than measurement capability. Instruments using cooled platinum silicide (PtSi) staring arrays with as many as 512 x 512 elements are available. Instrument using cooled indium antimonide (LnSb) focal plane arrays are available in models designed to compete with top-ofthe-line commercial thermal imagers. Some instruments in this category have the size and weight of a commercial video camera that fits in the palm of the hand, as illustrated in Figure 3.12.
Charlie Chong/ Fion Zhang
Figure 3.12: Infrared focal plane array imager for qualitative thermography
Charlie Chong/ Fion Zhang
Infrared focal plane array imager
Charlie Chong/ Fion Zhang
Infrared focal plane array imager
Charlie Chong/ Fion Zhang
Qualitative IrFPA
Charlie Chong/ Fion Zhang
Infrared focal plane array imager
Charlie Chong/ Fion Zhang
■ Quantitative Thermal Imagers Quantitative thermal imagers include (1) mechanicatly scanned thermal imagers (imaging radiometers) and (2) focal plane array radiometers. ● Mechanically Scanned Thermal Imagers Mechanically scanned thermal imagers (imaging radiometers) provide a means for measuring apparent target surface temperature with high resolution image quality and sometimes with extensive on-board diagnostic software. Mosl commercially available imaging radiometers use a single detector. but some manufacturers offer dual detector or multidctcctor (linear array) instruments. Most require detector cooling. Imaging radiometers use refractive reflective or hybrid scanning systems and operate in either the 3 to 5 μm or the 8 to 14 μm atmospheric window. They generally offer instantaneous fields of view on the order of 1 to 2 mrad with standard optics and minimum resolvable temperature differences of 0.05 to 0.10 °C (0.09 to 0.18 °F).
Charlie Chong/ Fion Zhang
On-board capabilities include isotherm graphics features, spectral filtering. interchangeable optics for different total field of views. color or monochrome (black and white) displays, flexible video recording capabilities and computer compatibility. Most feature compact, field portable, battery operable sensing heads and control/display units. A complete system including battery and video recorder can be handled by one person by mounting the components on a cart or by assembling them on a harness.
Charlie Chong/ Fion Zhang
� Focal Plane Array Radiometers Focal plane array radiometers are adaptations of military and aerospace forward looking infrared scanners. but are designed to measure the apparent temperature at the target surface and to produce quantitative thermograms. The capabilities of early infrared focal plane array imagers were slow in developing. The quality of measurement capabilities has improved since 1990. Infrared focal plane array cameras offer minimum resolvable temperature differences comparable to imaging radiometers (0.1 to 0.2 °C; 0.18 to 0.36 °F) and instantaneous field of views considerably better than imaging radiometers (1 mRad or better with standard optics). Commercially available quantitative infrared focal plane array cameras use detector arrays made of platinum silicide or indium antimonide, either of which requires cooling. Quantitative thermal imagers based on uncooled focal plane arrays (using bolometrie and ferroelectric detectors) have also been developed. With inherently faster response, no moving parts and superior spatial resolution infrared focal plane array cameras have been replacing infrared imaging radiometers for most applications.
Charlie Chong/ Fion Zhang
ccc
Charlie Chong/ Fion Zhang
http://sevutune.tumblr.com/microbolometer
Infrared focal plane array imager
Charlie Chong/ Fion Zhang
Platinum Silicide IrFPA
Charlie Chong/ Fion Zhang
http://www.bealecorner.com/trv900/thermal/therm.html
Quantitative IR Image
Charlie Chong/ Fion Zhang
Quantitative IR Image
Charlie Chong/ Fion Zhang
3.9
Thermal Imaging Display and Diagnostic Software
When the personal computer was introduced as part of thermal imaging systems, the typical imager produced raw radiometric data. whereas all of the diagnostic software was contained in an ancillary. separately packaged computer that performed all of the diagnostics back on the bench. With improved packaging technology in both computers and thermal imaging equipment, there has been a gradual trend toward providing more and more on board software so that more diagnostics can be performed on site. Depending on manufacturer and model, some software is incorporated into instruments and some is available only on computer driven software packages. Although thermographic diagnostic software packages are usually proprietary to a particular manufacturer, there is a trend toward universality in image storage. Common formats for storing electronic images include tagged image file format (TIFF) and other bitmapped formats. Retrieving images from these format is fast and easy.
Charlie Chong/ Fion Zhang
Quantitative Thermal Measurements Some qualitative thermograms can be converted to quantitative thermograms. The raw image produced by a quantitative imager may be converted to a quantitative thermogram; the raw image produced by a viewer may not. Quantitative thermal measurements provide the user with the true radiance or apparent temperature value of any or all points on the target surface. To present the thermogram in true radiance measurements, the system throughput attenuation must be considered as well as losses through the measurement medium (atmosphere, in most cases). To present the thermogram in true temperature values. the target effective emissivity must also be considered. When this capability is provided, a menu instructs the user to enter system calibration constants on initial setup and a system of prompts assures the operator that changes in aperture settings, target distance, inter-changeable lenses. etc., will be fed into the keyboard each time a change in operating conditions occurs.
Charlie Chong/ Fion Zhang
Changes in the corrections setting for target effective emissivity are also monitored. In addition. digital cameras are available to save visible images in computer compatible format for archiving with corresponding thermograms. For most systems. the displayed temperature readings are based on the assumption that the entire target surface has the same effective emissivity. Some systems. however. allow the assignment of several different emissivities to different areas of the target selected by the operator with the resulting temperature correction. A color scale or gray scale is provided along one edge of the display with temperature shown corresponding to each color or gray level in the selected range. The operator can place one or more spots or crosshairs on the image and the apparent temperature value of that pixel will appear in an appropriate location on the display. The isotherm feature allows the operator to select a temperature band or interval and all areas on the target within that band then appear enhanced in a predetennined gray shade or color hue. Detailed processing and image diagnostics relies on software that allows manipulation and analysis of each pixel in the thermogram prescnting information in a wide variety of qualitative and quantitative forms for the convenience of the user. Some of these capabilities are described in this chapler. Charlie Chong/ Fion Zhang
In addition to the spot measurement capability discussed previously. line profiles may be selected. The analog trace. in X, Y. or both. of the lines on the image intersecting at the selected spot will then appear at the edge of the display. Some systems allow the operator to display as many as seven sets of profiles simultaneously. Profiles of skew lines can also be displayed on some systems. Selected areas on the thermogram in the form of circles, rectangles or point-to-point free forms, can be shifted, expanded. shrunk or rotated or used to blank out or analyze portions of the image. Detailed analysis of the entire image or the pixels within the area can include maximum, minimum and verage values. number of pixels or even a frequency histogram of the values within the area. Color scales can be created from 256 colors stored in the computer. Electronic zoom features allow the operator to expand a small area on the display for closer examination. or to expand the colors for a small measurement range. Autoscale features provide the optimum display settings for any image if selected. Three-dimensional features provide an isometric thermal contour map or thermal profile map of the target for enhanced recognition of thermal anomalies. Charlie Chong/ Fion Zhang
Image Recording, Storage and Recovery Images and data can be stored in and retrieved from memory, hard disk, floppy diskette, video tape, optical disks (writable compact disks and digitalvideo disks) and Personal Computer Memory/Computer Industry Association (PCMCIA) cards. Commercial thermal imaging systems incorporate some means, such as a floppy disk drive or a PCMCIA card to store images in the field. Usually. about forty images. with all accompanying data, can be stored on a 3.5 in diskette. Some analysis usually can be done with on-board software; more extensive diagnostics usually require a separate computer. Options include IEEE or RS232 ports for access to additional storage and a video recorder option so that an entire measurement program can be recorded on video tape. Video tapes can be played back into the system and images can be saved to disk. Images can be stored from a frozen frame thermogram of a live target on operator command. or the operator can set up an automatie sequence and a preset number of images will be stored at preset time intervals.
Charlie Chong/ Fion Zhang
Stored images can be retrieved, displayed and further analyzed. Image comparison (differential thermography) allows the automatic comparison of thermograms taken at different times. This includes time based comparison of images taken of the same target as well as the comparison of images taken of different but similar targets. A special software program allows the operator to display two images sideby- ide or in sequence; and to subtract one image from another or one area from another; and to display a pixel-by-pixel difference thermogram. Comparison (subtraction) of images can be accomplished between two images retrieved from disk, between a live image and an image retrieved from disk and between a live image and an image stored in a computers random access memory, in this way, standard thermal images of acceptable components, assemblies and mechanisms can be archived and used as models for comparison to subsequently inspected items. It is also possible to subtract a live image from a previous baseline image for subsequent time based thermal transient measurements.
Charlie Chong/ Fion Zhang
Database and Documentation Records, files, data and documents can be saved in an orderly fashion. This capability provides thc thermographers with a filing system so that records of all measurement missions can be maintained on magnetic media, including actual thermograms, time, date, location, equipment, equipment settings, measurement conditions and other related observations. Most manufacturers of thermal imaging equipment have developed comprehensive report preparation software to facilitate timely and comprehensive reporting of the findings of infrared surveys and other measurement missions. These packages provide templates that allow thc thermographer to prepare reports in standard word processor formats into which tagged image file format (TIFF) images. imported from various imaging radiometers. can be directly incorporated. Additional diagnostic software is customarily provided in these packages so that analysis and trending can be added to reports.
Charlie Chong/ Fion Zhang
Calibration Accessories Infrared radiation reference sources are used by manufacturers to calibrate infrared sensing and imaging instruments in the laboratory before they are shipped. These same reference sources are used later at periodic intervals thereafter to ensure calibration stability. A radiation reference source is designed to simulate a blackbody radiator: that is. a target surface with a stable, adjustable known temperature and a uniform emissivity approaching 1.0 at all appropriate wavelengths. In addition to laboratory reference sources. there are field portable models suitable for periodic calibration checks of fielded thermographic equipment and for other tasks. The setup and deployment of radiation reference sources is discussed in Chapter 4.
Charlie Chong/ Fion Zhang
3.10 Photorecording Accessories for Hard Copies Since the advent of the personal computer and its integration with thermal imagers, magnetic storage and archiving of data (labels. dates. conditions of measurement. instrument settings. etc.) as well as thermograms have become routine. Soft copies can be made of real time images, processed images enhanced images and combined images on floppy disks, analog and digital magnetic tape, recordable optical disks and Personal Computer Memory/Computer Industry Association (PCMCIA) cards. Report preparation software allows images to be inserted into word processing documents and printed by conventional laser or inkjet printers. Making a hard copy directly from a stored or displayed image is done in a variety of ways. A number of devices were introduced before magnetic media were available for directly photographing the display between with conventional or instant film. Using them generally required considerable skill because the ambient lighting and the screen curvature had to be considered. For this reason. it was difficult to achieve repeatable results. online printers and plotters provide reliable, good quality copies when speed is not a consideration. Charlie Chong/ Fion Zhang
Online printers and plotters are relatively slow and may tie up the computer and related software during operation. For real time or high speed photorecording, portable video printers are usually selected. The video printer connects to the system's video output. It presents the current image on a remote display where it is frame grabbed and reproduced in real time under optimized conditions. Most video printers produce output on integral recorder paper. Available accessories allow a choice of direct instant hardcopies, negatives or slide transparencies. Although video printers are costly. they provide consistent quality in a reasonable time and do not require the use of the thermal imager or the computer during production time.
Charlie Chong/ Fion Zhang
Chapter 3 Review Questions
Q&A 1. b
14. b
2. d
15. a
3. a
16. b
4. b
17. d
5. d
18. b
6. a
19. e
7. c
20. a
8. c
21. d
9. d
22. a
10. d
23. a
11. b
24. d
12. a
25. b
13. b
Charlie Chong/ Fion Zhang
Q1. The thermal resolution of an instrument is the same as: a. the temperature accuracy. b. minimum resolvable temperature difference. c. temperature repeatability. d. the minimum spot size. Q2. The speed of response of an instrument is: a. the time constant of the detector. b. one half the time constant of the detector. c. the same as the field repetition rate. d. the time it takes to respond to a step change at the target surface. Q3. The instantaneous spot size of an instrument is related to the: a. instantaneous field of view and the working distance. b. thermal resolution. c. spectral bandwidth and the working distance. d. speed of response and the working distance.
Charlie Chong/ Fion Zhang
Q4. The performance parameters that are important for qualitative thermography are: a. absolute accuracy, repeatability and resolution. b. spatial resolution and thermal resolution. c. spatial resolution and absolute accuracy. d. measurement spatial resolution and thermal resolution. Q5. Thermal viewers do not provide: a. high resolution thermograms. b. recording capabilities. c. real time scan rates. d. quantitative thermograms. Q6. The thermal resolution of an instrument tends to: a. improve as target temperature increases. b. degrade as target temperature increases. c. remain constant regardless of target temperature. d. improve with increasing working distance.
Charlie Chong/ Fion Zhang
Q7. The 3 to 5 Οm spectral region is ideally suited for operation of instruments: a. measuring subzero temperature targets. b. measuring targets at extremely long working distances. c. measuring targets warmer than 200 °C (392 ° F). d. operating at elevated ambient temperature. Q8. The total field of view of an imaging instrument determines the: a. imaging spatial resolution (lFOV) of the instrument. b. measurement spatial resolution (IFOVmeas) of the instrument. c. image size at the target plane for any given working distance. d. operating spectral range of the instrument. Q9. The frame repetition rate of an imager is defined as the: a. number of imaging pixels in a thermogram. b. number of frames selected for image averaging. c. electronic image rate of the display screen. d. number of times every point on the target is scanned in one second.
Charlie Chong/ Fion Zhang
Q10. The purpose of adding an infrared spectral filter to an instrument may be to limit the spectral band: a. to only wavelengths longer than a specified wavelength. b. to only wavelengths shorter than a specified wavelength. c. to only wavelengths between two specified wavelengths. d. any of the above. Q11. To quickly calculate target spot size, a useful approximation is: a. π =3.1416. b. an instantaneous field of view of 1 degree represents a 60: 1 ratio between working distance and spot size. c. there are 2π radians in 360 degrees. d. a 1°F temperature change is equivalent to a 1.8 °C temperature change. Q12. For online process control instruments, important features are: a. environmental housings and long term stability. b. ready access to emissivity compensation setting. c. portability and battery life. d. precision sighting. Charlie Chong/ Fion Zhang
Q13. A line scanner can be used to produce a thermogram of a sheet process only when: a. emissivity is known. b. the sheet process is moving at a uniform rate. c. the process material is a non graybody. d. the sheet process is hotter than 200 째C (392 째F). Q14. Most quantitative infrared thermal imagers: a. are heavier than quantitative imagers and usually require line power. b. can store thermograms on floppy disks in the field. c. require frequent infusions of detector coolant in the field. d. use detectors that operate at room temperature. Q15. Infrared focal plane array imagers: a. have no scanning optics. b. cannot be used for quantitative thermography. c. cannot be used for very cool targets. d. cannot operate on rechargeable batteries.
Charlie Chong/ Fion Zhang
Q16. Most infrared focal plane array imagers: a. use more costly optics than scanning radiometers. b. offer better spatial resolution than scanning radiometers. c. offer better thermal resolution than scanning radiometers. d. offer more diagnostics features than scanning radiometers. Q17. The number of detector elements in an infrared focal plane array imager: a. affects the measurement accuracy of the imager. b. affects the thermal resolution of the imager. c. affects the spectral band of the imager. d. affects the spatial resolution of the imager. Q18. The fact that all elements in a focal plane array imager are always looking at the target make this kind of imager better suited than scanning imagers for observing: a. distant low temperature targets. b. targets with rapidly changing temperatures. c. targets with low emissivities. d. targets with high emissivities.
Charlie Chong/ Fion Zhang
Q19. For which of the following applications are quantitative thermograms most critical? a. Search and rescue. b. Nondestructive material testing. c. Process monitoring and control. d. Security and surveillance. Q20. Infrared thermal detectors: a. have a broad. flat spectral response. b. usually require cooling to operate properly. c. have much faster response times than photon detectors. d. have much greater sensitivity than photon detectors. Q21. The characteristics of infrared photodetectors (photon detectors) include: a. faster response times than thermal detectors. b. a requirement for cooling to operate properly. c. selective spectral response based on operating temperature. d. all of the above. Charlie Chong/ Fion Zhang
Q22. Filters, lenses and transmitting windows: a. are all examples of refractive optical elements. b. have negligible transmission loss in the infrared. c. are all examples of reflective optical elements. d. are not spectrally selective. Q23. Resistance temperature detectors and thermistors operate on the same principle. that is: a. a predictable change in resistance as a function of temperature. b. the inverse square law. c. the known expansion of dissimilar materials. d. the comparison of target brightness with a calibrated reference. Q24. Infrared radiation thermometers are used to measure temperature: a. without contacting the target. b. very rapidly. c. without causing a temperature change at the target. d. all of the above.
Charlie Chong/ Fion Zhang
Q25. Two-color (ratio) pyrometers measure the temperature of a target by: a. taking into account the size and distance to the target. b. comparing the radiant energy from the target in two narrow spectral bands. c. incorporating tables of known emissivity. d. calibrating and correcting for the infrared absorption in the measurement path.
Charlie Chong/ Fion Zhang
End Of Reading
Charlie Chong/ Fion Zhang
■ωσμ∙Ωπ∆º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λ
Charlie Chong/ Fion Zhang
NASA EDDY CURRENT TESTING RQA/M 1-5330 .17
■ωσμ∙Ωπ∆º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λ α ρτ
Charlie Chong/ Fion Zhang
Good Luck
Charlie Chong/ Fion Zhang
Good Luck
Charlie Chong/ Fion Zhang
https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang