Vi self study exam preparatory note part 4 section 2 by Charlie Chong

ASNT Level III- Visual & Optical Testing My Pre-exam Preparatory Self Study Notes Reading 4 Section 2 2014-August

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For my coming ASNT Level III VT Examination 2014-August

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Reading 4 ASNT Nondestructive Handbook Volume 8 Visual & Optical testing- Section 2 For my coming ASNT Level III VT Examination 2014-August

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Fion Zhang 2014/August/15

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SECTION 2 THE PHYSICS OF LIGHT

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SECTION 2: The Physics of Light PART 1: THE PHYSICS OF LIGHT 1.1 1.2 1.3 1.4 1.5 1.6

Radiant Energy Theories Light and the Energy Spectrum Blackbody Radiation Atomic Structure and Radiation Luminous Efficiency of Radiant Energy Luminous Efficiency of Light Sources

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SECTION 2: The Physics of Light PART 2: Measurement of the properties of light 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14

Photovoltaic Cells Photoconductor Cells Photoelectric Tubes Photodiodes and Phototransistors Photometry Principles of Photometry Photometers Photovoltaic Cell Meters Meters Using Photomultiplier Tubes Equivalent Sphere Illumination Photometers Reflectometers Radiometers Spectrophotometers Types of Photometers

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PART 1: THE PHYSICS OF LIGHT 1.0

General:

Light can be defined as radiant energy capable of exciting the human retina and creating a visual sensation. From the viewpoint of physics, light is defined as that portion of the electromagnetic spectrum with wavelengths between 380 and 770 nm. Visually, there is some variation in these limits among individuals. Radiant energy at the proper wavelength makes visible anything from which it is emitted or reflected in sufficient quantity to activate the receptors in the eye. The quantity of such radiant energy may be evaluated in many ways, including: radiant flux (measured in joules per second or in watts) and luminous flux (measured in lumens).

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1.1

Radiant Energy Theories

Several theories describing radiant energy have been proposed and the text below briefly discusses the primary theories. Corpuscular Theory The corpuscular theory was advocated by Sir Isaac Newton and is based on the following premises. 1. Luminous bodies emit radiant energy in particles. 2. These particles are intermittently ejected in straight lines. 3. The particles act on the retina of the eye, stimulating the optic nerves to produce the sensation of light.

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Wave Theory The wave theory of radiant energy was advocated by Christian Huygens and is based on these premises. 1. Light results from the molecular vibration in luminous material. 2. The vibrations are transmitted through the ether in wavelike movements (comparable to ripples in water). 3. The vibrations act on the retina of the eye, stimulating the optic nerves to produce visual sensation. The velocity of a wave is the product of its wavelength and its frequency.

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Electromagnetic Theory The electromagnetic theory was advanced by James Clerk Maxwell and is based on these premises. 1. Luminous bodies emit light in the form of radiant energy. 2. This radiant energy is propagated in the form of electromagnetic waves. 3. The electromagnetic waves act on the retina of the eye, stimulating the optic nerves to produce the sensation of light.

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Quantum Theory The quantum theory is an updated version of the corpuscular theory. It was advanced by Planck and is based on these premises. 1. Energy is emitted and absorbed in discrete quanta (photons). 2. The energy in each quantum is hv. The term h is known as Planck's constant and is equal to 6.626 x 10-34 jouleâ&#x20AC;˘second. The term v is the frequency in hertz.

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Unified Theory The unified theory of radiant energy was proposed by De Broglie and Heisenberg and is based on the premise that every moving element of mass is associated with a wave whose length is given by: 位 = h / mv (Eq. 1) Where: 位 h m v

= wavelength of the wave motion (meters); = Planck's constant or 6.626 x 10-34 Joule.second; = Mass in Kg = velocity of the particle (meters per second).

It is impossible to determine all of the properties that are distinctive of a wave or a particle simultaneously, since the energy to do so changes one of the properties being determined.

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The quantum theory and the electromagnetic wave theory provide an explanation of radiant energy that is appropriate for the purposes of nondestructive testing. Whether it behaves like a wave or like a particle, light is radiation produced by atomic or molecular processes. That is, in an incandescent body, a gas discharge or a solid state device, light is produced when excited electrons have just reverted to more stable positions in their respective atoms, thereby releasing energy.

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1.2

Light and the Energy Spectrum

The wave theory permits a convenient representation of radiant energy in an arrangement based on the energy's wavelength or frequency. This arrangement is called a spectrum and is useful for indicating the relationship between various radiant energy wavelength regions. Such a representation should not be taken to mean that each region of the spectrum is physically divided from the others- actually there is a small but discrete transition from one region to the next. The general limits of the radiant energy spectrum extend over a range of wavelengths varying from 10-16 to over 105 m. Radiant energy in the visible spectrum has wavelengths between 380 x 10-9 and 770 x 10-9 m. In the SI system, the nanometer nm (10-9 m) and the micrometer Îźm (10-6 m) are the commonly used units of wavelength in the visible region. In the cgs system, the angstrom A (10-10 m) was used to denote wavelength.

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All forms of radiant energy are transmitted at the same speed in a vacuum: 299,793 km•s -1 (186,282 mil•s-1). However, each form of energy differs in wavelength and therefore in frequency. The wavelength and velocity may be altered by the medium through which it passes but the frequency is fixed independently of the medium. Equation 2 shows the relationship between radiation velocity, frequency and wavelength.

v = λ v/ n

(Eq.2)

Where: v = velocity of waves in the medium (meters per second); n = the medium's index of refraction; λ = wavelength in a vacuum (meters); and v = frequency (hertz).

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Keywords:  The velocity of light change in different medium with different refractive index  The wavelength and velocity may be altered by the medium through which it passes but the frequency is fixed independently of the medium. When light travelling in different medium  Velocity change  Wavelength change  Frequency always remain the same in all mediums

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Planck and the Quanta In 1900, Max Planck was working on the problem of how the radiation an object emits is related to its temperature. He came up with a formula that agreed very closely with experimental data, but the formula only made sense if he assumed that the energy of a vibrating molecule was quantized--that is, it could only take on certain values. The energy would have to be proportional to the frequency of vibration, and it seemed to come in little "chunks" of the frequency multiplied by a certain constant. This constant came to be known as Planck's constant, or h, and it has the value 6.626x10-34 J x s http://web2.uwindsor.ca/courses/physics/high_schools/2005/Photoelectric_effect/planck.html

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Table 1 gives the speed of light in different media for a frequency corresponding to a wavelength of 589 nm in air. TABLE 1. Speed of light for a wavelength of 589 nanometers (sodium D-lines1

Medium

Speed 106 meters per second

Vacuum Air 100 kilopascals [760 mm Hg] at 0째 C Crown glass Water

299.793 299.724

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198.223 224.915

1.3

Blackbody Radiation

The light from common sources, particularly light from incandescent lamps, is often compared with light from a theoretical source known as a blackbody. For equal area, a blackbody radiates more total power and more power at any wavelength than any other source operating at the same temperature. For experimental purposes, laboratory radiation sources have been built to approximate closely a blackbody. Designs of these sources are based on the fact that a hole in the wall of a closed chamber, small in size compared with the size of the enclosure, exhibits blackbody characteristics. This can be understood with the help of Fig. 1. At each reflection, some energy is absorbed. In time, all incoming energy is absorbed by the walls. Conversely, a blackbody can be a perfect radiator. If the interior walls of the blackbody are uniformly heated, the radiation which leaves the small opening will he that of a perfect radiator for a specific temperature and its emission energy and wavelength spectrum will be independent of the nature of the enclosure. From 1948 to 1979, the international reference standard for the unit of luminous intensity was taken to be the luminance of a blackbody operating at the temperature of freezing platinum. Charlie Chong/ Fion Zhang

FIGURE 1. Small aperture in an enclosure exhibits blackbody characteristics

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Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body. The thermal radiation spontaneously emitted by many ordinary objects can be approximated as blackbody radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium. A black-body at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. Because of the specific colour responsiveness of the human eye, a black body, viewed in the dark at the lowest just faintly visible temperature, subjectively appears grey, even though its objective physical spectrum peaks in the red range. When it becomes a little hotter, it appears dull red. As its temperature increases further it eventually becomes blindingly brilliant blue-white.

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Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, in the sense that they absorb all the radiation that falls on them. It has been proposed that they emit black-body radiation (called Hawking radiation), with a temperature that depends on the mass of the black hole. The term black body was introduced by Gustav Kirchhoff in 1860. When used as a compound adjective, the term is typically written as hyphenated, for example, black-body radiation, but sometimes also as one word, as in blackbody radiation. Black-body radiation is also called complete radiation or temperature radiation or thermal radiation. http://en.wikipedia.org/wiki/Black-body_radiation

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1.3.1

Planck Radiation Law

Data describing blackbody radiation curves have been obtained using a specially constructed and uniformly heated tube as the source. Planck, introducing the concept of discrete quanta of energy, developed an equation depicting these curves. It gives the spectral radiance of a blackbody as a function of the wavelength and temperature. Figure 2 shows the spectral radiance of a blackbody as a function of wavelength for several values of absolute temperature, plotted on a logarithmic scale.

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FIGURE 2. Blackbody radiation curves showing Wien displacement of peaks for operating temperatures between 500 and 20,000 K

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FIGURE 2. Blackbody radiation curves

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FIGURE 2. Blackbody radiation curves

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FIGURE 2. Blackbody radiation curves- Peak Shifts

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FIGURE 2. Blackbody radiation curves- Intensity & Peak Shifts

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The color (chromaticity) of black-body radiation depends on the temperature of the black body; the locus of such colors, shown here in CIE 1931 x,y space, is known as the Planckian locus. http://en.wikipedia.org/wiki/Black-body_radiation

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1.3.2

Wien Radiation Law

In the temperature range of incandescent filament lamps (2,000 to 3,400 K) and in the visible wavelength region (380 to 770 nm), a simplification of the Planck equation, known as the Wien radiation law, gives a good representation of the blackbody distribution of spectral radiance. The Wien displacement law gives the relationship between the wavelength at which a blackbody at temperature T in degrees Kelvin emits maximum power per unit wavelength and the temperature T. In fact the product of absolute temperature T and the peak wavelength is a constant. It gives the relationship between blackbody distributions at various temperatures only with this important limitation.

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Wien Radiation Law

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1.3.3

Stefan-Boltzmann Law

The Stefan-Boltzmann law is obtained by integrating Planck's expression for the spectral radiant excitance from zero to infinite wavelength. The law states that the total radiant power per unit area of a blackbody varies as the fourth power of the absolute temperature. The Stefan-Boltzmann law is explained in introductory physics texts. Note that this law applies to the total power (the whole spectrum) and cannot be used to estimate the power in the visible portion of the spectrum alone.

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1.3.4

Spectral Emissivity

No known radiator has the same emissive power as a blackbody. The ratio of a radiator's output at any wavelength to that of a blackbody at the same temperature and the same wavelength is known as the radiator's spectral emissivity 蔚(位). 1.3.5

Graybody Radiation

When the spectral emissivity is uniform for all wavelengths, the radiator is known as a graybody. No known radiator has a uniform spectral emissivity for all visible, infrared and ultraviolet wavelengths. In the visible region, a carbon filament exhibits very nearly uniform emissivity and is nearly a graybody.

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1.3.6

Selective Radiators

The emissivity of all known materials varies with wavelength. In Fig. 3, the radiation curves for a blackbody, a graybody and a selective radiator (tungsten), all operating at 3,000 K, are plotted on the same logarithmic scale to show the characteristic differences in output.

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FIGURE 3. Radiation curves for blackbody, graybody and selective radiators operating at 3,000 K

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FIGURE 3. Radiation curves for blackbody, graybody and selective radiators operating at 3,000 K ~ 6,000 K

http://www.webexhibits.org/causesofcolor/3.html

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1.3.7

Color Temperature

The radiation characteristics of a blackbody of unknown area may be specified with the aid of the radiation equations by modifying two quantities: the magnitude of the radiation at any given wavelength and the absolute temperature. The same type of specification may be used with reasonable accuracy in the visible region of the spectrum for tungsten filaments and other incandescent sources. However, the temperature used in the case of selective radiators is not that of the filament but a value called the color temperature. The color temperature of a selective radiator is that temperature at which a blackbody must be operated so that its output is the closest approximation to a perfect color match with the output of the selective radiator. While the match is never actually perfect, the small deviations that occur in the case of incandescent filaments are not of practical importance.

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The apertures between coils of the filaments used in many tungsten lamps act something like a blackbody because of the interreflections that occur at the inner surfaces of the helix formed by the coil. For this reason, the distribution from coiled filaments exhibits a combination of the characteristics of the straight filament and of a blackbody operating at the same temperature. The use of the color temperature method to deduce the spectral distribution from other than incandescent sources, even in the visible region, usually results in appreciable error. Color temperature values associated with light sources other than incandescent are known as correlated color temperatures and are not true color temperatures.

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1.4

Atomic Structure and Radiation

The atomic theories first proposed in 1913 have been expanded and confirmed by much experimental evidence. The atom consists of a central positively charged nucleus about which revolve negatively charged electrons. In the normal state, these electrons remain in specific orbits or energy levels and radiation is not emitted by the atom. The orbit described by a specific electron revolving about the nucleus is determined by the energy of the electron (there is a particular energy associated with each orbit). An atom's system of orbits or energy levels is characteristic of each element and remains stable until disturbed by external forces. The electrons of an atom can he divided into two classes. The first includes the inner shell electrons which are removed or excited only by high energy radiation. The second class includes the outer shell or valence electrons which cause chemical bonding into molecules. Valence electrons are readily excited by ultraviolet or visible radiation or by electron impact and can be removed completely with relative ease. The valence electrons of an atom in a solid when removed from their associated nucleus enter the so-called conduction band and give the material the property of electrical conductivity. Charlie Chong/ Fion Zhang

After absorption of sufficient energy by an atom, the valence electron is pushed to a higher energy level further from the nucleus. Eventually, the electron returns to the normal orbit or to an intermediate orbit. In so doing, the energy that the atom loses is emitted as a quantum of radiation and this is the source of light. The wavelength (or frequency) of the radiation is determined by Planck's equation: E1 â&#x20AC;&#x201C; E2 = hv Where: E1 E2 h v

(Eq. 3)

= energy associated with the excited orbit; = energy associated with the normal orbit; = Planck's constant; and = frequency of the emitted radiation.

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Plankâ&#x20AC;&#x2122;s Equation

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This equation can he converted to a more practical form: 位 = 1239.76/ Vd Where: 位 = wavelength (nanometers); = the potential difference between two energy levels through which Vd the displaced electron has fallen in one transition (electron volts).

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1.5

Luminous Efficiency of Radiant Energy

Many apparent differences in intensity between radiant energy of various wavelengths are in fact differences in the ability of various sensing devices to detect them. The reception characteristics of the human eye have been subject to extensive investigations and the results may be summarized as follows. 1. The spectral response characteristic of the human eye varies between individuals, with time and with the age and health of an individual, to the extent that the selection of any individual to act as a standard observer is not scientifically feasible. 2. However, from the available data, a luminous efficiency curve has been selected to represent a typical human observer. This curve may be applied mathematically to the solution of photometric problems.

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The standard spectral luminous efficiency curve for photopic (light adapted) vision represents a typical characteristic, adopted arbitrarily to give unique solutions to photometric problems, from which the characteristics of any individual may be expected to vary. Some data indicate that most human observers are capable of experiencing a visual sensation on exposure to radiant energy of wavelengths longer than 770 nm, called infrared under most circumstances, provided the radiant energy reaches the eye at a sufficiently high rate. It also is known that ultraviolet radiation (wavelengths less than 380 nm) under most circumstances can be seen if it reaches the retina even at a moderate rate.

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Most observers yield only a slight response to ultraviolet radiation at the nearly visible wavelengths because the lens of the eye absorbs nearly all of it. Typically, human observers have a response that under normal lighting conditions extends from 380 to 770 manometers but some individuals have greater sensitivity at the blue and/or red ends of this range. Of course, at lower lighting levels even the average observer experiences a shift of the visible spectrum to shorter wavelengths and vice versa at higher lighting levels. The spectral range of visible response is therefore not static but greatly dependent on the lighting conditions.

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1.6

Luminous Efficiency of Light Sources

The luminous efficiency of a light source is defined as the ratio of the total luminous flux (lumens) to the total power input (watts or equivalent). The maximum luminous efficiency of an ideal white source (defined as a radiator with constant output over the visible spectrum and no radiation in other parts of the spectrum) is about 220 lmâ&#x20AC;˘W-1

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PART 2: MEASUREMENT OF THE PROPERTIES OF LIGHT 2.0

General:

The most widely used detector of light is the human eye. Other common, mechanical detectors are photovoltaic cells, photoconductive cells, photoelectric tubes, photodiodes, phototransistors and photographic film.

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2.1

Photovoltaic Cells

Photovoltaic cells typically include selenium barrier layer cells and silicon or gallium arsenide, photodiodes operated in the photovoltaic or unbiased mode. These devices depend on the generation of a current resulting from the absorption of a photon. The cell is comprised of 1. a p-type material, typically a metal plate coated with a semiconductor, such as selenium on iron; and 2. a semitransparent n-type material such as cadmium oxide. Unless there is an external circuit, electrons liberated from the semiconductor are trapped at the p-n junction after exposure to light. The device thereby converts radiant energy to electric energy, which can be used directly or amplified to drive a micro-ammeter (see Fig. 4). Photovoltaic cells can be filtered to correct their spectral response so that the micro-ammeter can be calibrated in units of illuminance. Factors such as response time, fatigue, temperature effects, linearity stability, noise and magnitude of current influence the choice of cell and circuit for a given application.

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FIGURE 4. Cross section of a barrier layer photovoltaic cell showing motion of photoelectrons through a micro-ammeter circuit

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FIGURE 4. Cross section of a barrier layer photovoltaic cell showing motion of photoelectrons through a micro-ammeter circuit

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FIGURE 4. Cross section of a barrier layer photovoltaic cell showing motion of photoelectrons through a micro-ammeter circuit

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FIGURE 4. Cross section of a barrier layer photovoltaic cell showing motion of photoelectrons through a micro-ammeter circuit

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2.2

Photoconductor Cells

Photoconductor cells depend on the resistance of the cell changing directly as a result of photon absorption. These detectors use materials such as cadmium sulfide, cadmium selenide and selenium. Cadmium sulfide and cadmium selenide are available in transparent resin or glass envelopes and are suitable for low illuminance levels less than 10-4 lx (10-5 ftc).

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Photoconductor Cells

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2.3

Photoelectric Tubes

The photoelectric effect is the emission of electrons from a surface bombarded by sufficiently energetic photons. If the surface is connected as a cathode in an electric field (see Fig. 5), the liberated electrons flow to the anode, creating a photoelectric current. An arrangement of this sort may be used as an illuminance meter and can be calibrated in lux or footcandles. The photoelectric current in vacuum varies directly with the illuminance level over a wide range (spectral distribution, polarization and cathode potential remain the same). In gas filled tubes, the response is linear only over a limited range. If the radiant energy is polarized, the photoelectric current varies as the orientation of the polarization is changed (except at normal incidence).

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FIGURE 5. By the photoelectric effect, electrons may be liberated from an Illuminated metal surface, flowing to an anode and creating an electric current that may be detected by a galvanometer (see Eq. 1 and 2)

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Photoelectric Effects

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Photoelectric Effects & Secondary radiation

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Photoelectric Effects

http://whs.wsd.wednet.edu/faculty/busse/mathhomepage/busseclasses/radiationphysics/lecturenotes/chapter12/chapter12.html

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2.4

Photodiodes and Phototransistors

Photodiodes or junction photocells are based on solid state p-n junctions that react to external stimuli such as light. Conversely, if properly constructed, they can emit radiant energy (light emitting diodes). In a photosensitive diode, the reverse saturation current of the junction increases in proportion to the illuminance. Such a diode can therefore be used as a sensitive detector of light and is particularly suitable for indicating extremely short pulses of radiation because of its very fast response time. Phototransistors operate in a manner similar to photodiodes but, because they provide an additional amplifier effect, they are many times more sensitive than simple photodiodes.

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Photodiode

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Schematic cross section of an integrated CMOS single-photon-counting avalanche diode (SPAD) device.2HV: High-voltage. p, n: Semiconductor materials.

http://spie.org/x93517.xml

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Photodiode

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Phototransistors

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Phototransistors

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2.5

Photometry

Progress in the sciences is often dependent on our ability to measure the physical quantities associated with the technology- each advance in measurement ability or accuracy provides a broadening of the science's knowledge base. The measurement of light and its properties is called photometry. The basic measuring instrument is known as a photometer. The earliest photometers depended on visual appraisal by the operator as the means of measurement and such meters are rarely used now. They have been replaced by nonvisual meters that are sensitive to light's physical properties, measuring radiant energy incident on a receiver, producing measurable electrical quantities. Physical photometers are more accurate and simpler to operate than their earlier counterparts.

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2.5.1

Observer Response Curves

Light measurements by physical photometers are useful only if they indicate reliably how the eye reacts to a certain stimulus. In other words, the photometer should be sensitive to the spectral power distribution of light in the same way that the eye is. Because of the substantial differences between individual eyes, standard observer response curves or eye sensitivity curves have been established. The sensitivity characteristics of a physical photometer should be equivalent to the standard observer. The required match is typically achieved by adding filters between the sensitive elements of the meter and the light source.

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2.5.2

Photopic and Scotopic Vision

The human eye contains two basic types of retinal receptors known as rods and cones. They differ not only in relative spectral response and other properties but by orders of magnitude in responsivity. The rods are the most sensitive and spectrally respond more to the blue and less to the red end of the spectrum. However, they do not actually give the sensation of color as the cones do. Luminance is measured in candelas (cd). When the eye has been subjected to a field luminance of more than 3.0 cdâ&#x20AC;˘m-2 (0.27 cdâ&#x20AC;˘ft -2) for more than a few minutes, the eye is said to he in a light adapted state in which only the cones are responsible for vision; the state is also known as photopic vision or fovea/ vision. At light levels five or more orders of magnitude below this, at or below 3.0 x 10-5 cd.m-2 (2.7 x 10-6 cdâ&#x20AC;˘ft -2), the cones no longer function and the responsivity is that of the rods. This is known as dark adapted, or scotopic vision or parafoveal vision. After being light adapted, the eye usually requires a considerable time to become dark adapted when the light level is lowered. The time needed depends on the initial luminance of the starting condition but is usually achieved in 30 to 45 minutes.

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Photopic and Scotopic Vision

http://www.solarlightaustralia.com.au/2013/05/30/photopic-scotopic-and-mesotopic-lumens/

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Photopic and Scotopic Vision

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Between the levels at which the eye exhibits photopic and scotopic responses the spectral and other responses of the eye are continuously variable; this is known as the mesopic state, in which properties of both cone and rod receptors contribute. Many visual tests are made under photopic conditions but most measurements of fluorescent and phosphorescent materials are made under scotopic and mesopic conditions. Because of the changes in the eye's spectral response at these levels it is necessary to take luminance into account when evaluating the results of such measurements.

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Photopic and Scotopic Vision

http://lumenistics.com/consider-photopic-scotopic-mesopic-vision-before-specifying-lumen-requirements/

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2.5.3

Measurable Quantities

As indicated in Table 2, many characteristics of light, light sources, lighting materials and lighting installations may be measured, including 1. 2. 3. 4. 5. 6. 7. 8. 9.

illuminance, luminance, luminous intensity, luminous flux, contrast, color (appearance and rendering), spectral distribution, electrical characteristics and radiant energy.

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TABLE 2. Measurable characteristics of light, light sources and lighting materials

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2.6

Principles of Photometry

2.6.0

General

Photometric measurements frequently involve a consideration of the cosine law and the inverse square law (strictly applicable only for point sources). 2.6.1

Inverse Square Law

The inverse square law (see Fig. 6a) states that the illumination E (in lux) at a point on a surface varies directly with the luminous intensity I of the source and inversely as the square of the distance d between the source and the point. If the surface at the point is normal to the direction of the incident light, the law may be expressed as: E= I/d2

(Eq. 5)

This equation is accurate within 0.5 percent when d is at least five times the maximum dimension of the source, as viewed from the point on the surface.

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Inverse Square Law

http://pondscienceinstitute.on-rev.com/svpwiki/tiki-index.php?page=Square%20Law

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2.6.2

Lambert Cosine Law

The Lambert cosine law (see Fig. 6b) states that the illuminance of any surface varies as the cosine of the angle of incidence. The angle of incidence 0 is the angle between the normal to the surface and the direction of the incident light. The inverse square law and the cosine law can be combined to yield the following relationship (in lux): E = I/d2 Cos ϴ

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(Eq. 6)

Lambert Cosine Law

http://webx.ubi.pt/~hgil/FotoMetria/HandBook/ch06.html

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2.6.3

Photometric Reference Standards

Reference standards for candlepower, luminous flux and color are established by national standard laboratories. A primary reference standard is reproducible from specifications and is typically found only in a national laboratory. Secondary reference standards are usually derived directly from primary standards and must be prepared using precise electrical and photometric equipment. Preservation of the reference standard's rating is very important. Accordingly, a standard is used as seldom as possible and is handled and stored with care. Photometric reference lamps are used when accuracy warrants the highest attainable precision. Because of the cost of reference standards, so-called working standards are prepared for frequent use A laboratory can prepare its own working standards for use in calibrating photometers. The working standard is not used to conduct a test, except where a direct comparison is necessary.

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2.6.4

Photometric Applications

Photometric measurements make use of the basic laws of photometry, applied to readings from visual photometric comparison or photoelectric instruments. Various procedures are discussed below Direct photometry is the simultaneous comparison of a standard lamp and an unknown light source. Substitution photometry is the sequential evaluation of the desired photometric characteristics of a standard lamp and an unknown light source in terms of an arbitrary reference. To avoid the use of standard lamps, relative photometry is often used. Relative photometry is the evaluation of a desired photometric characteristic based on an assumed lumen output of the test lamp. Alternately, relative photometry refers to the measurement of one uncalibrated light source to another uncalibrated light source. It is sometimes necessary to measure the output of sources that are nonsteady or cyclic and, in such cases, extreme care should be taken.

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2.6.5

Means of Achieving Attenuation

During photometric measurement, it often becomes necessary to reduce the luminous intensity of a source in a known ratio to bring it within the range of the measuring instrument. A rotating sector disk with one or more angular apertures is one means of doing this. If such a disk is placed between a source and a surface and is rotated at such speed that the eye perceives no flicker, the effective luminance of the surface is reduced in the ratio of the time of exposure to the total time (Talbot's law). The reduction is by the factor ϴ/360 degrees. The sector disk has advantages over many filters: (1) it is not affected by a change of characteristics over time and (2) it reduces luminous flux without changing its spectral composition. Sector disks should not be used with light sources having cyclical variation in output.

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Various types of neutral filters of known transmittance are also used for attenuation. Wire mesh or perforated metal filters are perfectly neutral but have a limited range. Partially silvered mirrors have high reflectance but the reflected light must be controlled to avoid errors in the photometer. When a mirror filter is perpendicular to the light source photometer axis, serious errors may be caused by multiple reflections between the filter and receiver surface. This can be avoided by mounting the filter at a small angle (not over 3 degrees) from perpendicular at a sufficient distance from the receiver surface to throw reflections away from the photometric axis. In this canted position, care must be taken not to reflect light from adjacent surfaces on to the receiver. Also, it is difficult to secure completely uniform transmission over all parts of the surface. So-called neutral glass filters are seldom neutral and transmission characteristics should be checked before use. In general, they have a characteristic high transmittance in the red and low transmittance in the blue, so that spectral correction filters may be required. However, this type of filter varies in transmittance with ambient temperature, as do many other optical filters.

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Neutral gelatin filters are satisfactory, although not entirely neutral. Some have a small seasoning effect (losing neutrality over a period of time). They must be protected by mounting between two pieces of glass and must be watched carefully for loss of contact between the glass and gelatin. Filters should not be stacked unless cemented, because of errors that may be created by interference between surfaces. With modem metering techniques, electronic alterations can be accomplished to keep the output of a receiver and amplifier combination in range of linearity and readability.

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2.7

Photometers

A photometer is a device for measuring radiant energy in the visible spectrum. Various types of physical instruments consist of an element sensitive to radiant energy and appropriate measuring equipment and are used to measure ultraviolet and infrared energy. When used with a filter to correct their response to the standard observer, such devices can measure visible light. In general, photometers may be divided into two types: 1. laboratory photometers are usually fixed in position and yield results of highest accuracy, 2. portable photometers are of lower accuracy for making measurements in the field.

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Both types of meters may be grouped according to function, such as the photometers used to measure luminous intensity (candlepower), luminous flux, illuminance, luminance, light distribution, light reflectance and transmittance, color, spectral distribution and visibility Illuminance Photometers. Visual photometric methods have largely been supplanted by physical methods. Because of their simplicity, vision based photometry methods are still used in educational laboratories for demonstrating photometric principles and for less routine types of photometry. Photoelectric photometers' may be divided into two classes: 1. those employing solid state devices such as photovoltaic and photoconductive cells and 2. those employing photoemissive tubes.

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Photometry

http://safety.fhwa.dot.gov/roadway_dept/night_visib/lighting_handbook/

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2.8

Photovoltaic Cell Meters

2.8.0

General

A photovoltaic cell is one that directly converts radiant energy into electrical energy. It provides a small current that is about proportional to the incident illumination and also produces a small electromotive force capable of forcing this current throtigh a low resistance circuit. Photovoltaic cells provide much larger currents than photoemissive cells and can directly operate a sensitive instrument such as a microammeter or galvanometer. However, as the resistance of their circuit increases, photovoltaic cells depart from linearity of response at higher levels of incident illumination. Therefore, for precise results, the external circuitry and metering should apply nearly zero impedance across the photocell.

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Some portable illuminance meters consist of a photovoltaic cell or cells, connected to a meter calibrated directly in lux or footcandles. However, with solid state electronic devices, operational amplifiers have been used to amplify the output of photovoltaic cells. The condition that produces the most favorable linearity between cell output and incident light is automatically achieved by reducing the potential difference across the cell to zero. The amplifier power requirements are small and easily supplied by small batteries. In addition, digital readouts may be conveniently used to eliminate the ambiguities inherent in deflection instruments.

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2.8.1

Spectral Response

The spectral response of photovoltaic cells is quite different from that of the human eye and color correcting filters are usually needed.."-" As an example, Fig. 7 illustrates the degree to which a typical commercially corrected selenium photovoltaic cell commonly used in illuminance meters approximates the standard spectral luminous efficiency curve. Cells vary considerably in this respect and for precise laboratory photometry each cell should be individually color corrected. The importance of color correction can be illustrated by comparing the human eye match under illumination generated by a monochromatic source. For example, if a predominant blue light source is used, the majority of the visible energy is concentrated near 465 nm. It can be seen in Fig. 7 that the relative eye response and the filtered receptor response are about 10 and 15 percent. This represents a 50 percent differential and indicates that the photoreceptor could read as much as 50 percent high under the blue light source. Care should be taken to correct for this difference.

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FIGURE 7. Average spectral sensitivity characteristics of selenium photovoltaic cells, compared with relative eye response (luminous efficiency curve)

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2.8.2

Transient Effects

When exposed to constant illumination, the output of photovoltaic cells requires a short finite rise time to reach a stable output. The output may decrease slightly over time because of fatigue. 4U-42 Rise times for silicon cells often are considerably shorter than for selenium cells.

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2.8.3

Effect of Incidence Angle

At high incidence angles, part of the light reaching a photovoltaic cell is reflected by the cell surface and the cover glass and some may be obstructed by the rim of the case. The resulting error increases with angle of incidence. When an appreciable portion of the flux comes at wide angles, an uncorrected meter may read illuminance as much as 25 percent below the true value. The cells used in most illuminance meters are provided with diffusing covers or some other means of correcting the light sensitive surface to approximate the true cosine response. The component of illuminance contributed by single sources at wide angles of incidence may be determined by positioning the cell perpendicular to the direction of the light and multiplying the reading by the cosine of the incidence angle.

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The possibility of cosine error must be taken into consideration for some laboratory applications of photovoltaic cells. One satisfactory solution to the problem consists of placing a nonfluorescent opal diffusing acrylic plastic disk with a matte surface over the cell. At high angles of incidence, the disk reflects the light specularly so that the readings are too low. This can be compensated by allowing light to reach the cell through the edges of the plastic. The readings at very high angles are then too high but can be corrected using a screening ring. In general it is important that the opal diffusing plastic disk with a matte surface should be nonfluorescent or erroneous values of illuminance may be obtained in the presence of blueviolet and ultraviolet radiations; such a situation is common in fluorescent penetrant and magnetic particle testing applications in which measurements of the ambient visible light, in he presence of the blacklight are required by certain industrial and military specifications. Certain photometers are actually provided with fluorescent diffusers and should be avoided in such situations.

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2.8.4

Effect of Temperature.

Wide temperature variations affect the performance of photovoltaic cells, particularly when the external resistance of the circuit is high. Prolonged exposure to temperatures above 50 째C (120 째F) permanently damages selenium cells. Silicon cells are considerably less affected by temperature. Measurements at high temperatures and at high illuminance levels should therefore be made rapidly to avoid overheating the cell. Hermetically sealed cells provide greater protection from the effects of temperature and humidity. When using photovoltaic cells at other than their calibrated temperature, conversion factors may be used or means may be provided to maintain cell temperatures near 25 째C (77 째F).

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2.8.5

Effect of Cyclical Variation of Light

When electric discharge sources are operated on alternating current power supplies, precautions should be taken with regard to the effect of frequency on photocell response. In some cases, these light sources may be modulated at several kilohertz. Consideration should then be given to whether the response of the cell is exactly equivalent to the Talbot's law response of the eye for cyclic varying light. Because of the internal capacitance of the cell, it cannot always be assumed that its dynamic response exactly corresponds to the mean value of the illuminance. It has been found that a low or zero resistance circuit is the most satisfactory for determining the average intensity of modulated or steady state light sources with which photovoltaic cell instruments are generally calibrated. Although a microammeter or galvanometer appears to register a steady photocell current, it may not be receiving such a current. The meter actually may be receiving a pulsating current which it integrates because its natural period of oscillation is long compared to the pulses. Meters are available that can average over a period of time, eliminating the effect of cyclic variation.

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2.8.6

Photometer Zeroing

It is important to check photometer zeroing before taking measurements. If an analog meter is used, this requires manual positioning of the indicator to zero. For any type of equipment using an amplifier, it may be necessary to zero both the amplifier and the dark current (current flowing through the device while it is in absolute darkness). When possible, it should be verified that the meter remains correctly zeroed when the photometer scale is changed. Alternately, any deviation from zero under dark current conditions may be measured and subtracted from the light readings.

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2.8.7

Electrical interference

With electronic meters, care should be taken to eliminate interference induced in the leads between the cell and the instrumentation. This can be achieved by filter networks, shielding, grounding or combinations of the above.

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2.9

Meters Using Photomultiplier Tubes

2.9.0

General

Photoelectric tubes produce current when radiant energy is received on a photoemissive surface and then amplified by a phenomenon known as secondary emission. These tubes require a high voltage to operate (2 to 5 kV) and an amplifier to provide a measurable signal. The resulting current may be measured by a deflection meter, oscillograph or a digital output device. Dark current (current flowing through the device while it is in absolute darkness) must be compensated for in the circuitry or subtracted from the lighted tube output. Meters using this device are often extremely sensitive. Photomultiplier tubes can he damaged by shock and the calibration of the meter can be altered by strong magnetic fields. In addition, the device is temperature sensitive and should be operated at or below room temperature. As with other photoelectric devices, the photomultiplier spectral response curve does not match the human eye and color correcting filters are required.

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Because of the large number of photomultiplier types available, manufacturers commonly supply the proper optical filter for their design. When a photomultiplier tube is used in conjunction with an optical lens system, the resulting luminance meter can be of high sensitivity and broad range. Keywords: Secondary emmision

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2.9.1

Luminance Photometers

The basic principles for the measurement of illuminance apply equally well for the measurement of luminance. Luminance meters are essentially a photoreceptor in front of a focusing mechanism. By suitable optics, the luminance of a certain size spot, when cast onto the receptor, generates an electrical signal that is dependent on the object luminance. This signal can be measured and, assuming that the necessary calibration has been performed, a reading is produced that directly measures luminance. Usually an eyepiece is provided so that the user is able to see the general field of view through the instrument. By changing the lens system in front of the photoreceptor, different fields of view and therefore different sizes of measurement area may be achieved. This can vary from areas subtending a few minutes of arc up to several degrees. Photoreceptors may be selenium but are usually silicon cells or photomultipliers. The meter reading may be analog or digital and either built into the meter or remote. Amplifier controls for zeroing and scale selection are usually provided. Other options include optical filters for color work or scale selection by means of neutral density filters.

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Luminance Photometers

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2.9.1

Brightness Spot Meter

The brightness spot meter is a photoelectric device for measuring the luminance of small areas, typically 0.25, 0.5 or 1 degree field of view. A beam splitter allows a portion of the light from the objective lens to reach a reticule viewed by the eyepiece. The remainder of the light is reflected in front of the photomultiplier tube. The output of the tube after amplification is read on a microammeter with a scale calibrated in candelas per square meter or footlamberts. One of the filters provided for such instruments corrects the response of the photomultiplier to the standard spectral luminous efficiency curve. Full scale deflection is produced by 10-1 to 10-7 cdâ&#x20AC;˘m-2

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2.9.3

High Sensitivity Photometer

One version of the photomultiplier photometer has interchangeable field apertures covering fields from arc minutes to 3 degrees in diameter. Full scale sensitivity ranges are from 10-4 to 108 cdâ&#x20AC;˘m-2. In this meter, readings of the measured light are free from the effects of polarization because there are no internal reflections of the beam. The spectral response of each photometer is individually measured. The filters to match it best to the standard spectral luminous efficiency curve are then inserted. Filters are also included to permit evaluation of polarization and color factors.

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High Sensitivity Photometer

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High Sensitivity Photometer

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2.10

Equivalent Sphere Illumination

2.10.1 Photometers Equivalent sphere illumination (ESI) may be used as a tool as part of the evaluation of lighting systems. The equivalent sphere illumination of a visual task at a specific location in a room illuminated with a specific lighting system is defined as that level of perfectly diffuse (spherical) illuminance that makes the visual task as visible in the sphere as it is in the real lighting environment. Measurements may be made visually and/or physically. In the visual method the measurement is made by comparison between a task viewed in the measured (actual) environment and the task viewed in a luminous sphere by using a visibility meter. The physical method, however, is based on certain algorithms and requires only measurements in situ of background illuminance Lb and task illuminance Lt. All physical equivalent sphere illumination devices measure Lb and Lt in one way or another. Measuring devices are discussed below in chronological order of development.

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2.10.2 Visual Task Photometer The visual task photometer is a basic, reference instrument against which others are often compared. Equivalent sphere illumination is not measured directly: Lb and Lt are measured and equivalent sphere illumination is subsequently calculated. The task to be evaluated is mounted on a target shifter and a telephotometer is aimed at it from the desired viewing angle. The task and telephotometer (usually mounted on a cart) are then positioned so that: â&#x2013; â&#x2013;

the task is in the location where the measurement is to he made and the telephotometer is facing the proper direction of view.

The standard body shadow (attached to the telephotometer) shades the task in a manner similar to an actual observer. The Lb value is measured, then the shifter is activated to bring the target into view of the telephotometer. The Lt value is then measured.

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2.10.3 Visual Equivalent Sphere Illumination Meter The visual equivalent sphere illumination meter' consists of an optical system, variable luminous veil, target carrier, luminous sphere, illuminance meter (inside the sphere) and a body shadow. A task is placed on the target carrier and viewed through the optical system. The contrast of the task is then reduced to threshold by adjusting a variable luminous veil. Field luminance is automatically kept constant so as not to alter the adaptation luminance of the observer's eye. The task is then carried inside the sphere and the optical system is adjusted until the target is again at threshold (task visibility is the same inside the sphere as it was outside). The illuminance in the sphere is measured directly to determine equivalent sphere illumination.

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