ASNT Level III- Visual & Optical Testing My Pre-exam Preparatory Self Study Notes Reading 4 Section 1 2014-August
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Reading 4 ASNT Nondestructive Handbook Volume 8 Visual & Optical testing- Section 1 For my coming ASNT Level III VT Examination 2014-August
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Fion Zhang 2014/August/15
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SECTION 1 FUNDAMENTALS OF VISUAL AND OPTICAL TESTING
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SECTION 1: FUNDAMENTALS OF VISUAL AND OPTICAL TESTING PART 1: Description of visual and optical tests 1.1 Luminous Energy Tests 1.2 Geometrical Optics PART 2: History of the borescope 2.1 Development of the Borescope 2.2 Certification of Visual Inspectors PART 3: Vision and light 3.1 The Physiology of Sight 3.2 Weber's Law 3.3 Vision Acuity 3.4 Vision Acuity Examinations 3.5 Visual Angle 3.6 Color Vision 3.7 Fluorescent Materials
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PART 4: Safety for visual and optical tests 4.1 Need for Safety 4.2 Laser Hazards 4.3 Infrared Hazards 4.4 Ultraviolet Hazards 4.5 Photosensitizers 4.6 Damage to the Retina 4.7 Thermal Factor 4.8 Blue Hazard 4.9 Visual Safety Recommendations 4.10 Eye Protection Filters
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Part 1: DESCRIPTION OF VISUAL AND OPTICAL TESTS 1.1.0
General:
Nondestructive tests typically are done by applying a probing medium (such as acoustic or electromagnetic energy) to a material. After contact with the test material, certain properties of the probing medium are changed and can be used to determine changes in the characteristics of the test material. Density differences in a radiograph or location and peak of an oscilloscope trace are examples of means used to indicate probing media changes. In a practical sense, most nondestructive tests ultimately involve visual tests- a properly exposed radiograph is useful only when the radiographic interpreter has the vision acuity required to interpret the image. Likewise, the accumulation of magnetic particles over a crack indicates to the inspector an otherwise invisible discontinuity. The interface of visual testing with other nondestructive testing methods is discussed in more detail in a later section of this volume.
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For the purposes of this book, visual and optical tests are those that use probing energy from the visible portion of the electromagnetic spectrum. Changes in the light's properties after contact with the test object may be detected by human or machine vision. Detection may be enhanced or made possible by mirrors, magnifiers, borescopes or other vision enhancing accessories. Keywords; â– Visible Spectrum (380nm ~ 770nm), â– Human or machine vision, â– Vision enhancing tools- Borescope, mirror and other enhancing accessories.
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1.2.0
Luminous Energy Tests
Visual testing was probably the first method of nondestructive testing. It has developed from its ancient origins into many complex and elaborate optical investigation techniques. Some visual tests are based on the simple laws of geometrical optics. Others depend on properties of light, such as its wave nature. A unique advantage of many visual tests is that they can yield quantitative data more readily than other nondestructive tests. Luminous energy tests are used primarily for two purposes: 1. testing of exposed or accessible surfaces of opaque test objects (including a majority of partially assembled or finished products) and 2. testing of the interior of transparent test objects (such as glass, quartz, some plastics, liquids and gases). For many types of objects, visual testing can be used to determine quantity, size, shape, surface finish, reflectivity, color characteristics, fit, functional characteristics and the presence of surface discontinuities.
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Keywords: Objects: Testing of opaque objects Testing of transparent objects VT is used to determined: quantity, size, shape, surface finish, reflectivity, color characteristics, fit, functional characteristics and the presence of surface discontinuities. Question: Does VT covers Translucent object?
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1.3.0
Geometrical Optics
1.3.1
Image Formation
Most optical instruments are designed primarily to form images. In many cases, the manner of image formation and the proportion of the image can be determined by geometry and trigonometry without detailed consideration of the physics of light rays. This practical technique is called geometrical optics and it includes the formation of images by lenses and mirrors. The operation of microscopes, telescopes and borescopes also can be partially explained with geometrical optics. In addition, the most common limitations of optical instruments can be similarly evaluated with this technique. Keyword: Geometrical optics
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1.3.2
Light Sources
The light source for visual tests typically emits radiation of a continuous or noncontinuous (line) spectrum. Monochromatic light is produced by use of a device known as a monochromator, which separates or disperses the wavelengths of the spectrum by means of prisms or gratings. Less costly and almost equally effective for routine tests are light sources emitting distinct spectral lines, These include mercury, sodium and other vapor discharge lamps. Such light sources may he used in combination with glass, liquid or gaseous filters or with highly efficient interference filters, for transmitting only radiation of a specific wavelength. Keywords: Continuous spectrum Non-continuous spectrum-Monochromatic light Monochromator used prisms or grating
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Keywords: Monochromatic light produces by vapor discharged lamp (Mercury/sodium etc.) with glass/ liquid & gaseous filter to produces only radition with specific wavelength
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Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, a plasma. The character of the gas discharge depends on the pressure of the gas as well as the frequency of the current. Typically, such lamps use a noble gas (argon, neon, krypton and xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury, sodium, and metal halides. In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons in the atomic orbitals of these atoms are excited by these collisions to a higher energy state. When the excited atom falls back to a lower energy state, it emits a photon of a characteristic energy, resulting in infrared, visible light, or ultraviolet radiation. Some lamps convert the ultraviolet radiation to visible light with a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp. http://en.wikipedia.org/wiki/Gas-discharge_lamp
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Compared to incandescent lamps, gas-discharge lamps offer higher efficiency, but are more complicated to manufacture, and require auxiliary electronic equipment such as ballasts to control current flow through the gas. Some gas-discharge lamps also have a perceivable start-up time to achieve their full light output. Still, due to their greater efficiency, gas-discharge lamps are replacing incandescent lights in many lighting applications.
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Vapor Discharged Lamp
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Vapor Discharged Lamp
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Vapor Discharged Lamp
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A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, single, and chroma, colour, and the Latin suffix -ator, denoting an agent. http://en.wikipedia.org/wiki/Monochromator
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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Monochromator used prisms or grating
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1.3.3
Stroboscopic Sources
The stroboscope is a device that uses synchronized pulses of high intensity light to permit viewing of objects moving with a rapid, periodic motion. A stroboscope can be used for direct viewing of the apparently stilled test object or for exposure of photographs. The timing of the stroboscope also can be adjusted so that the moving test object is seen to move but at a much slower apparent motion. The stroboscopic effect requires an accurately controlled, intermittent source of light or may be achieved with periodically interrupted vision.
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1 Stroboscopic Movement
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Stroboscopic Movement
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Stroboscopic Movement
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Stroboscopic Sources
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Stroboscopic Sources
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Stroboscopic Glasses
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千手观音
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千手观音
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千手观音
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1.3.4
Light Detection and Recording
Once light has interacted with a test object (been absorbed, reflected or refracted), the resulting light waves are considered test signals that may be recorded visually or photoelectrically. Such signals may be detected by means of photoelectric cells, bolometers or thermopiles, photomultipliers or closed circuit television systems. Electronic image conversion devices often are used for the invisible ranges of the electromagnetic spectrum (infrared, ultraviolet or X-rays) but they also may he used to transmit visual data from hazardous locations or around obstructions. Occasionally, intermediary photographic recordings are made. The processed photographic plate can subsequently be evaluated either visually or photoelectrically. Some applications take advantage of the ability of photographic film to integrate low energy signals over long periods of time. Photographic film emulsions can be selected to meet specific test conditions, sensitivities and speeds.
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Keywords： Photoelectricity detection
photoelectric cells, bolometers or thermopiles, photomultipliers or closed circuit television systems.
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Bolometer consists of an absorptive element, such as a thin layer of metal, connected to a thermal reservoir (a body of constant temperature) through a thermal link. The result is that any radiation impinging on the absorptive element raises its temperature above that of the reservoir — the greater the absorbed power, the higher the temperature. The intrinsic thermal time constant, which sets the speed of the detector, is equal to the ratio of the heat capacity of the absorptive element to the thermal conductance between the absorptive element and the reservoir. The temperature change can be measured directly with an attached resistive thermometer, or the resistance of the absorptive element itself can be used as a thermometer. Metal bolometers usually work without cooling. They are produced from thin foils or metal films. Today, most bolometers use semiconductor or superconductor absorptive elements rather than metals. These devices can be operated at cryogenic temperatures, enabling significantly greater sensitivity.
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Bolometer
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Bolometer
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Bolometer
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Thermopiles
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Thermopiles
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Thermopiles
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Multi-Junction Thermopiles The thermopile is a heat sensitive device that measures radiated heat. The sensor is usually sealed in a vacuum to prevent heat transfer except by radiation. A thermopile consists of a number of thermocouple junctions in series which convert energy into a voltage using the Peltier effect. Thermopiles are convenient sensor for measuring the infrared, because they offer adequate sensitivity and a flat spectral response in a small package. More sophisticated bolometers and pyroelectric detectors need to be chopped and are generally used only in calibration labs.
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Photo Detector Comparisons
http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/RYER/ch10.html
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Photomultiplier Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically vacuum phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and nearinfrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is very low. Unlike most vacuum tubes, they are not obsolete. The combination of high gain, low noise, high frequency response or, equivalently, ultra-fast response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), radar jamming, and high-end image scanners known as drum scanners. Elements of photomultiplier technology, when integrated differently, are the basis of night vision devices.
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Semiconductor devices, particularly avalanche photodiodes, are alternatives to photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated.
http://en.wikipedia.org/wiki/Photomultiplier
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Photomultiplier Secondary emission Photoelectric effect
Photon
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Electrons multiplying
Photomultiplier
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Photomultiplier
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Photoelectric Cell Photovoltaic Cell Photo emissivity Photovoltaic's (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Solar photovoltaics power generation has long been seen as a clean sustainable[1] energy technology which draws upon the planet’s most plentiful and widely distributed renewable energy source – the sun. The direct conversion of sunlight to electricity occurs without any moving parts or environmental emissions during operation. It is well proven, as photovoltaic systems have now been used for fifty years in specialized applications, and grid-connected systems have been in use for over twenty years
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Photovoltaic Cell
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Photovoltaic Cell
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Photoemissive Cell- Photoemissive cell (electronics) A device which detects or measures radiant energy by measurement of the resulting emission of electrons from the surface of a photocathode.
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Photoemissive Cell
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Photoemissive Cell: Analysis of sodium levels in junk food by flame photometer
http://www.pharmatutor.org/articles/analysis-sodium-levels-junk-food-flame-photometer?page=0,2
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1.3.5
Fluorescence Detection
A material is said to fluoresce when exposure to radiation causes the material to produce a secondary emission of longer wavelength than the primary, exciting light. Visual tests based on fluorescence play a part in qualitative and quantitative inorganic and organic chemistry, as a means of quality control of chemical compounds, for identifying counterfeit currency, tracing hidden water flow and for detecting discontinuities in metals and pavement.
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Fluorescence Detection
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More Reading on Light Light Measurement Handbook http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/RYER/index.html http://homepages.inf.ed.ac.uk/rbf/CVonline/
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Part 2: History of Borescope 2.1.0
Introduction
Development of the Borescope The development of self illuminated telescopic devices can be traced back to early interest in exploring the interior human anatomy without operative procedures. Devices for viewing the interior of objects are called endoscopes, from the Creek words for "inside view." Today the term endoscope in the United States is applied primarily to medical instruments. Nearly all of the medical endoscopes have an integral light source; some incorporate surgical tweezers or other devices. Industrial endoscopes are called horescopes because they were 'originally used in machined apertures and holes such as gun bores. There are both flexible and rigid, fiber optic and geometric light borescopes. Keywords: â– Endoscopes â– Horescopes
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2.1.1
Cystoscopes and Borescopes
In 1806 Philipp Bozzini of Frankfurt announced the invention of his Lichtleiter (German for "light guide"). Having served as a surgeon in the Napoleonic wars, Bozzini envisioned using his device for medical research. It is considered the first endoscope. In 1876, Dr. Max Nitze, a urologist, developed the first practical cystoscope to view the human bladder.' A platinum loop in its tip furnished a bright light when heated with galvanic current. Two years later, Thomas Edison introduced an incandescent light in the United States. Within a short time, scientists in Austria made and used a minute electric bulb in Nitze's cystoscope, even before the electric light was in use in America. The early cystoscopes contained simple lenses but these were soon replaced by achromatic combinations. In 1900, Reinhold Wappler revolutionized the optical system of the cystoscope and produced the first American models. The forward oblique viewing system was later introduced and has proved very useful in both medical and industrial applications. Direct vision and retrospective systems were also first developed for cystoscopic use.
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Borescopes and related instruments for nondestructive testing have followed the same basic design used in cystoscopic devices. The range of borescope sizes has increased, sectionalized instruments have been introduced and other special devices have been developed for industrial applications.
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2.1.2
Gastroscopes and Flexible Borescopes
A flexible gastroscope, originally intended for observing the interior of the stomach wall, was first developed by Rudolph Schindler' and produced by Georg Wolf in 1932. The instrument consisted of a rigid section and a flexible section. Many lenses of small focal distance were used to allow bending of the instrument to an angle of 34 degrees in several planes. The tip of the device contained the objective and the prism causing the necessary axial deviation of the bundle of rays coming from the illuminated gastric wall. The size of the image depended on the distance of the objective from the observed surface. It could be magnified, reduced or normal size but the image was sharp and erect with correct sides. Flexible gastroscopes are now available, with rubber tubes over the flexible portion, in diameters of approximately 14 mm (0.55 in.) and 8 mm (0.31 in.).
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Flexible borescopes for industrial use are more ruggedly constructed than gastroscopes, having flexible steel tubes instead of rubber for the outer tube of the flexible portion. A typical flexible borescope is 13 mm (0,5 in.) in diameter and has a 1 m (3 ft) working length, with flexibility in about 500 mm (20 in.) of the length. Extension sections are available in 1, 2 or 3 m (3, 6 or 9 ft) lengths, permitting assembly of borescopes up to 10 m (30 ft) in length. In such flexible instruments the image remains round and sharp when the tube is bent to an angle of about 34 degrees. Beyond that limit, the image becomes elliptical but remains clear until obliterated at about 45 degrees of total bending.
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Keywords: Conventional Borescope Bend angles & Images 34 Degree- Round and Clear 34 ~ 45 Degree- Elliptical but Clear > 45 Degree- Obliterated
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Digitized Borescope
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2.1.3
American Development of Borescopes
After the early medical developments, certain segments of American industry needed visual testing equipment for special inspection applications. One of the first individuals to help fill this need was George Sumner Crampton. George Crampton (Fig. 1) was born in Rock Island, Illinois in 1874. He was said to have set up a small machine shop by the age of 10 and his first ambition was to become an electrical engineer. He chose instead to study medicine and received his M.D. from the University of Pennsylvania in 1898. While he was interning at Pennsylvania Hospital, Crampton's mechanical and engineering abilities were recognized and he was advised to become an oculist.' He returned to the university, took a degree in ophthalmology and later practiced in Philadelphia, Pennsylvania and Princeton, New Jersey‘ In 1921, the Westinghouse Company asked Crampton to make a device that could be used to check for discontinuities inside the rotor of a steam turbine (Fig. 2). Crampton developed the instrument in his Philadelphia shop and delivered the prototype within a week- it was the first borescope produced by his company.
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Crampton continued to supply custom borescopes for testing inaccessible and often dark areas on power turbines, oil refinery piping, gas mains, soft drink tanks and other components (Fig. 3). Crampton soon was recognized for his ability to design and manufacture borescopes, periscopes and other optical equipment for specific testing applications. After retiring as emeritus professor of ophthalmology at the university Crampton continued private practice in downtown Philadelphia. At the same time, he worked on borescopes and other instruments in a small shop he had established in a remodeled nineteenth century coach house (Fig. 4).
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FIGURE 1. George Crampton, developer of the borescope
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FIGURE 2. Tests of forgings for a steam turbine generator shaft manufactured in the 1920s FIGURE 3. Inspectors use early borescopes to visually inspect piping at an Ohio oil refinery
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FIGURE 4. Periscope built in the 1940s is checked before shipment to a Texas chemical plant
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2.1.4
Wartime Borescope Developments
After World War II began, Crampton devoted much of his energy to the war effort, filling defense orders for borescopes (Fig. 5). Crampton practiced medicine until noon, then went to the nearby workshop where he visually tested the bores of 37 mm antiaircraft guns and other weapons. During the war, borescopes were widely used for testing warship steam turbines (particularly their rotating shafts). The United States Army also used borescopes for inspecting the barrels of tank and antiaircraft weapons produced in Philadelphia. An even more challenging assignment lay ahead. The scientists working to develop a successful nuclear chain reaction in the top secret Manhattan Project asked Crampton to provide a borescope for inspecting tubes near the radioactive pile at its guarded location beneath the stadium seats at the University of Chicago's Stagg Field. Crampton devised an aluminum borescope tube 35 mm (1.4 in.) in diameter and 10 m (33 ft) long. The device consisted of 2 m (6 ft) sections of dual tubing joined by bronze couplings which also carried an 8 V lighting circuit.
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FIGURE 5. Using a borescope, an inspector at an automobile plant during World War H checks the interiors of gun tubes for 90 mm antiaircraft guns
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The inspector standing directly in front of the bore was subject to radioactive emissions from the pile, so Crampton mounted the borescope outside of a heavy concrete barrier. The operator stood at a right angle to the borescope, looking through an eyepiece and revolving the instrument manually. The borescope contained a prism viewing head and had to be rotated constantly. It was supported in a steel V trough resting on supports whose height could be varied. Crampton also mounted a special photographic camera on the eyepiece. The original Manhattan Project borescope was later improved with nondarkening optics and a swivel-joint eyepiece that permitted the operator to work from any angle (this newer instrument did not require the V trough). It also was capable of considerable bending to snake through the tubes in the reactor. A total of three borescopes were supplied fbr this epochal project and they are believed to be the first optical instruments to use glass resistant to radioactivity.
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Manhattan Project
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Manhattan Project
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Manhattan Project
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2.1.5
Borescopes and Aircraft Tests
Aircraft inspection soon became one of the most important uses of borescope technology. In 1946, an ultraviolet light borescope was developed for fluorescent testing of the interior of hollow steel propeller blades. The 100 W viewing instniment revealed interior surface discontinuities as glowing green lines. Later, in 1958, the entire United States' B-47 bomber fleet was grounded because of metal fatigue cracks resulting from low level simulated bombing missions. Visual testing with borescopes proved to be the first step toward resolving the problem. The program became known as Project Milkbottle, a reference to the bottle shaped pin that was a primary connection between the fuselage and wing (Fig. 6). In the late 1950s, a system was developed for automatic testing of helicopter blades. The borescope, supported by a long bench, could test the blades while the operator viewed results on a television screen (Fig. 7). The system was used extensively during the Vietnam conflict and helicopter manufacturers continue to use borescopes for such critical tests.
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FIGURE 6. Inspector using a borescope to check for metal fatigue cracks in a B-47 bomber during grounding of the bomber fleet in 1958 FIGURE 7. Visual testing of the frame of a 10 m (32 ft) long helicopter blade using a 10 m (32 ftj borescope; the inspector could view magnified results on the television screen at bottom left
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After a half century of pioneering work, George Crampton sold his borescope business to John Lang of Cheltenham, Pennsylvania, in 1962.6•7 Lang had developed the radiation resistant optics used in the Manhattan Project borescope, as well as a system for keeping it functional in high temperature environments. He also helped pioneer the use of closed circuit television with borescopes for testing the inner surfaces of jet engines and wings, hollow helicopter blades and nuclear reactors. In 1965, the company received a patent on a borescope whose mirror could he very precisely controlled. This borescope could zoom to high magnification and could intensely illuminate the walls of a chamber by means of a quartz incandescent lamp containing iodine vapor. The basic design of the borescope has been in use for many decades and it continues to develop, accommodating advances in video, illumination, robotic and computer technologies.
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2.2.0
Certification of Visual Inspectors
2.2.1
Introduction
The recognition of the visual testing technique and the development of formal procedures for educating and qualifying visual inspectors were important milestones in the history of visual inspection. Because visual testing can be performed without any intervening apparatus, it was certainly one of the first forms of nondestructive testing. In its early industrial applications, visual tests were used simply to verify compliance to a drawing or specification. This was basically a dimensional check. The soundness of the object was determined by liquid penetrant, magnetic particle, radiography or ultrasonic testing. Following World War II, few inspection standards included visual testing. By the early 1960s, visual tests were an accepted addition to the American Welding Society's code hooks. In NAV SHIPS 250-1500-1, the US Navy included visual tests with its specifications for other nondestructive testing techniques for welds.
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By 1965, there were standards for testing, and criteria for certifying the inspector had been established in five test methods: liquid penetrant, magnetic particle, eddy current, radiographic and ultrasonic testing. These five were cited in ASNT Recommended Practice No. SNT-TC-1A, introduced in the late 1960s. The broad use of visual testing hindered its addition to this group as a specific method- there were too many different applications on too many test objects to permit the use of specific acceptance criteria. It also was reasoned that visual testing would occur as a natural result of applying any other nondestructive test method.
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2.2.2
Expanded Need for Visual Certification
In the early 1970s, the need for certified visual inspectors began to increase. Nuclear power construction was at a peak, visual certification was becoming mandatory and nondestructive testing was being required. In 1976, the American Society for Nondestructive Testing began considering the need for certified visual inspectors. ASNT had become a leading force in nondestructive testing and American industry had accepted its ASNT Recommended Practice No. SNT-TC-IA as a guide for certifying other NDT inspectors. In the spring of 1976, ASNT began surveying industry about their inspection needs and their position on visual testing. Because of the many and varied responses to the survey, a society task force was established to analyze the survey data. In 1977, the task force recommended that visual inspectors be certified and that visual testing be made a supplement to ASNT Recommended Practice No. SNT-TC-IA (1975). At this time, the American Welding Society implemented a program that, following the US Navy, was the first to certify inspectors whose sole function was visual weld testing.
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During 1978, ASNT subcommittees were formed for the eastern and western halves of the United States. These groups verified the need for both visual standards and trained, qualified and certified inspectors. In 1980, a Visual Methods Committee was formed in ASNT's Technical Council and the early meetings defined the scope and purpose of visual testing (dimensional testing was excluded). In 1984, the Visual Personnel Qualification Committee was formed in ASNT's Education and Qualification Council. In 1986, a training outline and a recommended reference list was finalized and the Board of Directors approved incorporation of visual testing into ASNT Recommended Practice No. SN T-TC -1 A.
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Part 3: VISION AND LIGHT 3.1
The Physiology of Sight
3.1.1
Visual Data Collection
Human visual processing occurs in two steps. First the entire field of vision is processed. This is typically an automatic function of the brain, sometimes called pre-attentive processing. Secondly, focus is localized to a specific object in the processed field. Studies at the University of Pennsylvania indicate that segregating specific items from the general field is the foundation of the identification process. Based on this concept, it is now theorized that various light patterns reaching the eyes are simplified and encoded, as lines, spots, edges, shadows, colors, orientations and referenced locations within the entire field of view. The first step in the subsequent identification process is the comparison of visual data with the long-term memory of previously collected data. Some researchers have suggested that this comparison procedure is a physiological cause of deja vu, the uncanny feeling of having seen something before.
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The accumulated data are then processed through a series of specific systems. Certain of our light sensors receive and respond only to certain stimuli and transmit their data to particular areas of the brain for translation. One kind of sensor accepts data on lines and edges; other sensors process only directions of movement or color. Processing of these data discriminates between different complex views by analyzing their various components. By experiment it has been shown that these areas of sensitivity have a kind of persistence. This can be illustrated by staring at a lit candle, then diverting the eyes toward a blank wall. For a short time, the image of the candle is retained. The same persistence occurs with motion detection and can he illustrated by staring at a moving object, such as a waterfall, then at a stationary object like the river bank. The bank will seem to flow because the visual memory of motion is still present.
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3.1.2
Differentiation in the Field of View
Boundary and edge detection can be illustrated by the pattern changes in Fig. 8. When scanning the figure from left to right, the block of reversed Ls is difficult to separate from the upright Ts in the center but the boundary between the normal Ts and the tilted Ts is easily apparent. The difficulty in differentiation occurs because horizontal and vertical lines comprise the L and upright T groups, creating a similarity that the brain momentarily retains as the eye moves from one group to the other. On the other hand, the tilted Ts share no edge orientations with the upright Ts, making them stand out in the figure. Differentiation of colors is more difficult when the different colors are in similarly shaped objects in a pattern. The recognition of geometric similarities tends to overpower the difference in colors, even when colors are the object of interest. Additionally, in a grouping of different shapes of unlike colors, where no one form is dominant, a particular form may hide within the varied field of view. However, if the particular form contains a major color variance, it is very apparent. Experiments have shown that such an object may be detected with as much ease from a field of thirty as it is from a field of three. Charlie Chong/ Fion Zhang
FIGURE 8. Pattern changes illustrating boundary and edge detection
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3.1.3
Searching the Field of View
The obstacles to differentiation discussed above indicate that similar objects are difficult to identify individually. During pre-attentive processing, particular objects that share common properties such as length, width, thickness or orientation are not different enough to stand out. If the differences between a target object and the general field is dramatic, then a visual inspector requires little knowledge of what is to be identified. When the target object is similar to the general field, the inspector needs more specific detail about the target. In addition, the time required to detect a target increases linearly with the number of similar objects in its general field. When an unspecified target is being sought, the entire field must be scrutinized. If the target is known, it has been shown statistically that only about half of the field must be searched.
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The differences between a search for simple features and a search for conjunctions or combinations of features can also have implications in nondestructive testing environments. For example, visual inspectors may be required to take more time to check a manufactured component when the possible errors in manufacturing are characterized by combinations of undesired properties. Less time could be taken for a visual test if the manufacturing errors always produced a change in a single property. Another aspect of searching the field of view addresses the absence of features. The presence of a feature is easier to locate than its absence. For example, if a single letter 0 is introduced to a field of many Qs, it is more difficult to detect than a single Q in a field of Os. The same difficulty is apparent when searching for an open 0 in a field of closed Os. In this case statistics show that the apparent similarity in the target objects is greater and even more search time is necessary
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Experimentation in the area of visual search tasks encompasses several tests of many 'individuals. Such experiments start with studies of those features that should stand out readily, displaying the basic elements of early vision recognition. The experiments cover several categories, including quantitative properties such as length or number. Also included are search tasks concentrating on single lines, orientation, curves, simple forms and ratios of sizes. All these tests verify that visual systems respond more favorably to targets that have something added (Q versus 0) rather than something missing. In addition, it has been determined that the ability to distinguish differences in intensity becomes more acute with a decreasing field intensity. This is the basis of Weber's law. The features it addresses are those involved in the early visual processes: color, size, contrast, orientation, curvature, lines, borders, movement and stereoscopic depth.
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3.2
Weber's Law
3.2.1
General
Weber's law is widely used by psychophysicists and entails the following tenets: (1) individual elements such as points or lines are more important singly than their relation to each other and (2) closed forms appear to stand out more readily than open forms. To view a complete picture, the visual system begins by encoding the basic properties that are processed within the brain, including their spatial relationships. Each item in a field of view is stored in a specific zone and is withdrawn when required to form a complete picture. Occasionally, these items are withdrawn and positioned in error. This malfunction in the reassembly process allows the creation of optical illusions, allowing a picture to be misinterpreted.
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The diagram in Fig. 9 represents a model of the early stages of visual perception. The encoded properties are maintained in their respective spatial relationships and compared to the general area of vision. The focused attention selects and integrates these properties, forming a specific area of observation. In some cases, as the area changes, the various elements comprising the observance are modified or updated to represent present conditions. During this step, new data are compared to the stored information.
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FIGURE 9. Stages of visual perception
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Charlie Chong/ Fion Zhang
http://art.nmu.edu/cognates/ad175/background.html
3.3.
Vision Acuity
3.3.1
General
Vision acuity encompasses the ability to see and identify, what is seen. Two forms of vision acuity are recognized and must be considered when attempting to qualify visual ability. These are known as near vision and far vision (acuity).
Charlie Chong/ Fion Zhang
3.3.2
Components of the Human Eye
The components of the human eye (Fig. 10) are often compared to those of a camera. The lens is used to focus light rays reflected by an object in the field of view. This results in the convergence of the rays on the retina (film), located at the rear of the eyeball. The cornea covers the eye and protects the lens. The quantity of light admitted to the lens is controlled by the contraction of the iris (aperture). The lens has the ability to become thicker or thinner, which alters the magnification and the point of impingement of the light rays, changing the focus. Eye muscles aid in the altering of the lens shape as well as controlling the point of aim. This configuration achieves the best and sharpest image for the entire system. The retina consists of rod and cone nerve endings that lie beneath the surface. They are in groups that represent specific color sensitivities and pattern recognition sections. These areas may be further subdivided into areas that collect data from lines, edges, spots, positions or orientations.
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The light energy is received and converted to electrical signals that are moved by way of the optic nerve system to the brain where the data are processed. Because the light is being reflected from an object in a particular color or combination of colors, the individual wavelengths representing each hue also vary. Each wavelength is focused at different depths within the retina, stimulating specific groups of rods and cones (see Figs. 10 and 11). The color sensors are grouped in specific recognition patterns as discussed above.
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FIGURE 10. Components of the human eye in cross section
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FIGURE 11. Magnified cross section showing the blind spot of the human eye
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To ensure reliable observation, the eye must have all the rays of light in focus on the retina. When the point of focus is short or primarily near the inner surface of the retina closest to the lens, a condition known as nearsightedness exists. If the focal spot is deeper into the retina, farsightedness occurs. These conditions are primarily the result of the eyeball changing from nearly orb shaped to an elliptical or egg shape. In the case of the nearsighted person, the long elliptical diameter is horizontal, If the long diameter is in a vertical direction, farsightedness occurs. These clinical conditions result from a very small shift of the focal spot, on the order of micrometers (ten-thousandths of an inch).
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3.3.3
Determining Vision Acuity
The method normally used to determine what the eye can see is based on the average of many measurements. The average eye views a sharp image when the object subtends an arc of five minutes, regardless of the distance the object is from the eye. The variables in this feature are the diameter of the eye lens at the time of observation and the distance from the lens to the retina. When vision cannot he normally varied to create sharp clear images, then corrective lenses are required to make the adjustment. While the eye lens is about 17 mm (0.7 in.) from the retina, the ideal eyeglass plane is about 21 mm (0.8 in.) from the retina. Differences in facial features must therefore be considered when fitting for eyeglasses. Under various working conditions, the glass lenses may not stay at their ideal location. This can cause slight variations when evaluating minute details and such situations must be individually corrected. For the majority of visual testing applications, near vision acuity is required. Most visual inspections are performed within arm's length and the inspector's vision should be examined at 400 mm (15.5 in.) distance. Examinations for far vision are done at distances of 6 m (20 ft). Charlie Chong/ Fion Zhang
Keywords: The average eye views a sharp image when the object subtends an arc of five minutes, regardless of the distance the object is from the eye. For the majority of visual testing applications, near vision acuity is required. Near vision should be examined at 400 mm (15.5 in.) distance. Far vision are done at distances of 6 m (20 ft).
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3.4
Vision Acuity Examinations
3.4.1
General
Visual testing may occur once or more during the fabrication or manufacturing cycle to ensure product reliability. For critical products, visual testing may require qualified and certified personnel. Certification of the visual test itself may also be required to document the condition of the material at the time of testing. In such cases, testing personnel are required to successfully complete vision acuity examinations covering specific areas necessary to ensure product acceptability. For certain critical inspections, it may be required for the eyes of the inspector to be examined as often as twice per year.
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3.4.2
Near Vision Examinations
The examination distance should be 400 mm (16 in.) from the eyeglasses or from the eye plane, for tests without glasses. When reading charts are used, they should he in the vertical plane at a height where the eye is on the horizontal plane of the center of the chart. Each eye should be tested independently while the unexamined eye is shielded from reading the chart but not shut off from ambient light. The Jaeger" eye chart is widely used in the United States for near vision acuity examinations. The chart is a 125 X 200 mm (5 x 8 in.) off-white or grayish card with an English language text arranged into groups of gradually increasing size. Each group is a few lines long and the lettering is black. In a vision examination using this chart, visual testing personnel may be required to read, for example, the smallest letters at a distance of 300 mm (12 in.). Near vision acuity examinations that are more clinically precise are described below.
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3.4.3
Far Vision Examinations
Conditions are the same as those for near vision examinations, except that the chart is placed 6 m (20 ft) from the eye plane. Again, each eye is tested independently.
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3.4.4
Grading Vision Acuity
The criterion for grading vision acuity is the ability to see and correctly identify 7 of 10 optotypes of a specific size at a specific distance. The average individual should be able to read six words in four to five seconds, regardless of the letter size being viewed. The administration of a vision acuity examination does not necessarily require medical personnel, provided the administrator has been trained and qualified to standard and approved methods. In some instances specifications may require the use of medically approved personnel. In these cases, the administrator of the examination may be trained by medically approved personnel for this application. In no instance should any of these administrators try to evaluate the examinations.
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If an applicant does not pass the examination (fails to give the minimum number of correct answers required by specification), the administrator should advise the applicant to seek a professional examination. If the professional responds with corrective lenses or a written evaluation stating the applicant can and does meet the minimum standards, the applicant may be considered acceptable for performance of the job.
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An eye chart is a chart used to measure visual acuity. Types of eye charts include the logMAR chart, Snellen chart, Landolt C, Lea test and the Jaeger chart. Procedure Charts usually display several rows of optotypes (test symbols), each row in a different size. An optotype is a standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols. The person is asked to identify the optotype on the chart, usually starting with large rows and continuing to smaller rows until the optotypes cannot be reliably identified anymore. Technically speaking, testing visual acuity with an eye chart is a psychophysical measurement that attempts to determine a sensory threshold (see also psychometric function).
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Ototype
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Snellen Chart- Far Vision Acuity
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Golovin-Sivtsev Table
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Jaeger chart
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3.4.5
Vision Acuity Examination Requirements
There are some basic requirements to be followed when setting up a vision acuity examination system. The distances mentioned above are examples but there are also detailed requirements for the vision chart. The chart should consist of a white matte finish with black characters or letters. The background should extend at least the width of one character beyond any line of characters. Sloan letters as shown in Fig. 12 were designed to be used where letters must be easily recognizable. Each character occupies a five stroke by five stroke space.
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FIGURE 12. Letters used for acuity examination charts (measurements in stroke units)
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The background luminance of the chart should be 85 ± 5 cd•m- 2. The luminance is a reading of the light reflected from the white matte finish toward the reader. When projected images are used, the parameters for the size of the characters, the background luminance and the contrast ratio are the same as those specified for charts. In no case should the contrast or illumination of the projected image be changed. A projection lamp of appropriate wattage should be used. When projecting the image, room lighting is subdued. This should not cause any change in the luminance of the projected background contrast ratio to that of the characters. The room lighting for examinations using charts should be 800 lx (75 ftc). Incandescent lighting of the chart is recommended to bring the background luminance up to 85 ±5 cd•m- 2. Fluorescent lighting should not be used for vision acuity examinations. Incandescent lamps emit more light in the yellow portion of the visible spectrum. This makes reading more comfortable for the examinee. Fluorescent lamps, especially those listed as full spectrum, are good for color vision examinations.
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Many of the lighting conditions for vision acuity examinations can be met by using professional examination units. With one such piece of equipment, the examinee views slides under controlled, ideal light conditions. Another common design is used both in industrial and medical examinations. With this unit, the individual looks into an ocular system and attempts to identify numbers, letters or geometric differences noted in illuminated slides. The examinee is isolated from ambient light. The slides and their respective data were developed by the Occupational Research Center at Purdue University, based on many individuals tested in many different occupations. Categories were developed for different vocations and are provided as guides for examinations required by various industries. Such equipment is expensive and accordingly eye charts are still very popular. Table 1 compares the results of these three vision acuity examination systems.
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TABLE 1. Eye examination system conversion chart
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There are slight differences between the reading charts and the slides. The reading chart distance for one popular letter card is 400 mm (16 in.). The simple slide viewer is set for near vision testing at 330 min (13 in.). There also are some differences between individual examination charts. Most of the differences are the result of variances in typeface, ink and the paper's ink absorption rate. Regardless of the examination system that is used, the requirements for the lighting and contrast remain the same.
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3.5
Visual Angle
3.5.1
Posture
Posture affects the manner in which an object is observed—appropriate posture and viewing angle are needed to minimize fatigue, eyestrain and distraction. The viewer should maintain a posture that makes it easy to maintain the optimum view on the axis of the lens. 3.5.2
Peripheral Vision
Eye muscles may manipulate the eye to align the image on the lens axis. The image is not the same unless it impinges on the same set of sensors in the retina (see Fig. 13). As noted above, different banks of sensors basically require different stimuli to perform their functions with optimum results. Also, light rays entering the lens at angles not parallel to the lens axis are refracted to a greater degree. This changes the quality and quantity of the light energy reaching the retina. Even the color and contrast ratios vary and depth perception is altered
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FIGURE 13. Vision acuity of peripheral vision
Charlie Chong/ Fion Zhang
The commonly quoted optimum, included angle of five (5) minutes of arc is the average in which an individual encloses a sharp image. There are other angles to be considered when discussing visual testing. The angle of peripheral vision is not a primary consideration when performing detailed visual tests. It is of value under certain inspection conditions: (1) (2) (3)
when surveying large areas for a discontinuity indication that has a high contrast ratio with the background and is observed to one side of the normal lens axis.
The inspector's attention is drawn to this area and it can then he scrutinized by focusing the eyes on the normal plane of the lens axis.
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3.5.3
Visual Testing Viewing Angle
The angle of view is very important during visual testing. The viewer should in all cases attempt to observe the target on the center axis of the eye. The angle of view should not vary more than 45 degrees from normal. Figure 14 shows how the eye perceives an object from several angles and how the object appears to change or move with a change in viewing angle.
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FIGURE 14. Shifting eye positions change apparent object size and location
Charlie Chong/ Fion Zhang
The same principle applies to objects being viewed through accessories such as mirrors or borescopes. The field of view should be maintained much in the same way that it is when viewed directly. On reflective backgrounds, the viewing angle should be off normal but not beyond 45 degrees. This is done so that the light reflected off the surface is not directed toward the eyes, reducing the contrast image of the surface itself. It also allows the evaluation of discontinuities without distorting their size, color or location. This is very important when using optical devices to view areas not available to direct line of sight.
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3.6
Color Vision
3.6.1
General
There are specific industries where accuracy of color vision is important: paint, fabrics and photographic film are examples. Surface inspections such as those made during metal finishing and in rolling mills are to determine manufacturing discontinuities. Color changes are not indicative of such discontinuities and therefore, for practical purposes, color is not as significant in these applications. However, heat tints are sometimes important and colors may be crucial in metallography and failure analysis. When white light testing is performed, it must be remembered that white light is composed of all the colors (wavelengths) in the spectrum. If the inspector has color vision deficiencies, then the test object is being viewed differently than when viewed by an inspector with normal color vision. Color deficiency may be as critical as the test itself. During visual testing of a white or near white object, slight deficiencies in color vision may be unimportant. During visual testing of black or near black objects, color vision deficiencies make the test object appear darker
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3.6.2
Color Vision Examinations
Ten percent of the male population have some form of color vision deficiency. The so-called color blind condition affects even fewer people truly color blind individuals are unable to distinguish red and green. But, there are many variations and levels of sensitivity between individuals with normal vision and those with color deficiencies. There are two causes of color deficiency: inherited and acquired. And each of these may be subdivided into specific medical problems. Most such subdivisions are typically discovered during the first vision examination. The most common color deficiencies are hereditary and occur in the redgreen range. About 0.5 percent of the affected individuals are female, in the red-green range. Women constitute about 50 percent of those affected in the blue-yellow range. Most such deficiencies occur in both eyes and in rare instances in only one eye. About 0.001 percent of the affected groups in the hereditary portion have their deficiency in the blue-green range. Individuals in the red green group may make misinterpretations of discontinuities in shades of red, browli, olive and gold.
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Color Vision Examinations-Ishihara Plates
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Color Vision Examinations-Ishihara Plates
http://www.nature.com/nmeth/journal/v8/n6/full/nmeth.1618.html
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Color Vision Examinations-Ishihara Plates
http://www.today.com/health/surprise-side-effect-new-specs-may-fix-color-blindness-1C8487550
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Acquired color deficiency is a greater problem to good color vision testing. The acquired deficiencies may affect only one eye and a change from acceptable color vision to a recognizable problem may he very gradual. Various medical conditions can cause such a change to occur (Table 2 lists conditions that produce color vision deficiencies in particular color ranges). Most acquired color vision problems vary in severity and may be associated with ocular pathology. If the disease continues for an extended period of time without treatment, the deficiencies may become erratic in intensity and may vary from the red-green or blue-yellow ranges. Aging can also affect color vision.
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TABLE 2. Causes of acquired color vision deficiencies Color Vision Deficiency Cause of Deficiency
Blue-yellow deficiency
Glaucoma Myopic retinal degeneration Retinal detachment Pigmentary degeneration of the retina (including retinitis pigmentosa) Senile macular degeneration Chorioretinitis Retinal vascular occlusion Diabetic retinopathy Hypertensive retinopathy Papilledema Methyl alcohol poisoning Central serous retinopathy (accompanied by luminosity loss in red)
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TABLE 2. Causes of acquired color vision deficiencies Color Vision Deficiency Cause of Deficiency
Red-green deficiency
Optic neuritis (including retrobulbar neuritis) Tobacco or toxic amblyopia Leber's optic atrophy Lesions of the optic nerve and pathway Papillitis Hereditary juvenile macular degeneration {Stargardt's and Best's disease) Blue-yellow deficiency Dominant hereditary optic atrophy Red-green or blue-yellow deficiency Juvenile macular degeneration
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3.6.3
Color Vision Classifications
Two functions that determine an individual's sensation range are their color perception and color discrimination. When a primary color is mistaken for another primary color, this is an error in perception. An error in discrimination is an error of lesser magnitude involving a mistake in hue selection. During a vision examination, these two functions are tested independently. A color vision examination performed with an anomaloscope allows the mixing of red and green lights to match a yellow light standard. Yellow and blue lights may be mixed to match a white light. An individual with normal vision requires red, blue and green light to mix and match colors of the entire color spectrum. A color deficient person may require fewer than the three lights to satisfy the color sensation. Table 3 indicates the type of deficiencies and the percent of the male population known to be affected
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Anomaloscope
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Anomaloscope Test
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TABLE 3. Classification of color vision deficiencies and percent of affected males
Color Vision Hereditary deficiencies trichromatism three colors: red, green. blue) normal vision anomalous (defective) dichromatism (two colors)* protanopia (red lacking) deureranopia (green lacking) trianopia {blue lacking) tetratanopia (yellow lacking) Acquired deficiencies tritan (blue yellow) protan-deutan (red-yellow)
Percent Males Affected
92 6 or 7 1 1 rare very rare data not available data not available
*Deficiency most often referenced when discussing color blindness
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TABLE 4. Naval Submarine Medical Research Laboratory color vision classification system
Class
Description
0 I ll
Normal Mild anomalous trichromat Unclassified anomalous trichromat (includes mild and moderate classes) Moderate anomalous trichromat Severely color deficient {includes severe anomalous trichromats, dichromats andmonochromats)
III IV
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For the practical purpose of classifying personnel affected by hereditary color deficiencies, the Naval Submarine Medical Research Laboratory has developed the classifications shown in Table 4. about 50 percent of color deficient people can be categorized in accordance with this table. Class I covers 30 percent of the color deficient population and Class III accounts for 20 percent. Individuals in Class I can judge colors used as standards for signaling, communication and identification as fast and as accurately as zero class persons can. The limitation of Class I people is when good color discrimination is necessary. Persons in Class III may be used in other areas such as radio repair, chemistry, medicine and surgery, electrical manufacturing or general painting. Class II encompasses staff members, managers or clerical help, whose need for color resolution is not critical. Individuals in Class IV must be restricted from occupations where color differentiation of any magnitude is required.
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As with vision acuity examinations, there are many different examinations for color vision. Color vision is often tested with pseudoisochromatic plates or cards on which the detection of certain figures depends on red-green discrimination. Unfortunately, most common vision acuity examinations were designed to identify hereditary red-green deficiencies and ignore blue-yellow deficiencies. A good, discriminating examination technique is illustrated in color Plates 1 to 7. The diagrams show the sequence in which the colors are arranged in each photograph for each deficiency, differing from the sequence according to normal vision illustrated in Plate 1. 21 (Caution: These plates are provided for educational purposes only. Photography, print reproduction and chemical changes all cause colors to vary from the original and fade with time. Under no circumstances should illustrations in this book be used for vision examinations.)
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Pseudoisochromatic plates
http://www.healthytimesblog.com/2011/04/facts-about-color-blindness/
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http://www.healthytimesblog.com/2011/04/facts-about-color-blindness/
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The exam consists of the examinee's arranging fifteen colored caps into a circle according to changes in hue progressing from a reference cap. To help evaluate the outcome, each cap is numbered on the back. A perfect score has the caps in numerical sequence. This test is used for those known to have a color vision deficiency. The test allows for the evaluation of the individual's ability and determines the specific area of the deficiency. The arrangement of colors allows confusion to exist across the quadrants of the circle. For instance, reds can be confused with blue-greens. One authority has stated that anyone who can pass this test should have no problem in any work requiring color vision acuity. Two types of red-green deficient patterns can be noted.
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Individuals in these categories confuse green (4) with redpurple (13) and blue-green (3) with red (12). The sequence then appears as 4, 13, 3 and 12. Persons with the blue-yellow deficiency confuse yellow-green (7) with purple (15), creating a sequence of 7, 15, 8, 14 and 9. As in the normal vision acuity examinations, lighting requirements and time must be controlled for color vision examinations. The illumination intensity of full spectrum fluorescent lighting should be no less than 200 lx (20 ftc). The rating of the light source is known as the color temperature. A low color temperature lamp such as an incandescent lamp makes it easier for persons with borderline color deficiencies to guess the colors correctly. A color temperature of 6,700 K is preferred. Too high a color temperature increases the number of reading errors. To eliminate glare, the light source should be 45 degrees to the surface while the patient is perpendicular to it. The reading distance should be about 400 to 600 mm (15 to 24 in.) or arm's length.
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To perform such an examination, two minutes should be allotted to arrange all fifteen caps in their appropriate positions. In summary, color deficiency can be acquired or inherited. Some color deficiencies may be treated, alleviated or minimized. Pseudoisochromatic plates in conjunction with the progressive hue color caps provide an adequate test for most industrial visual inspectors. Full spectrum lighting (6,700 K) is necessary for accurate test results. It should be added that, because the visible spectrum is made up of colors of varying wavelengths and the black and white colors consist of various combinations of colors, deficiencies in any part of the color spectrum has an impact on certain black and white inspection methods, including X-ray film review It is recommended that all nondestructive testing personnel have their color vision tested annually, while taking their vision acuity examination.
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Caps for Color Vision Examinations The exam consists of the examinee's arranging fifteen colored caps into a circle by a change in hue progressing from a reference cap. To help evaluate the outcome, each cap is numbered on the back. A perfect score has the caps in numerical sequence. The diagrams show the sequence in which the colors are arranged in each photograph for each deficiency, differing from the sequence according to normal vision illustrated in Plate 1. (Caution: These plates are provided for instructional purposes only. Photography, print reproduction and chemical changes all cause colors to vary from the Original and fade with time. Under no circumstances should illustrations in this book be used for vision examinations.)
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PLATE 1. Colored caps for normal color vision examination
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PLATE 2. Colored caps for normal color vision with minor errors
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PLATE 3. Colored caps for normal color vision with one error
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PLATE 4. Colored caps for red blindness
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PLATE 5. Colored caps for green blindness
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PLATE 6. Colored caps for blue blindness
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PLATE 7. Colored caps for anomalous trichromatic vision
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3.7
Fluorescent Materials
3.7-1
General
Fluorescence is a complex phenomenon that occurs in gases, liquids and solids. It has also proved to be the greatest and most efficient source of the so-called cold light. For the purpose of visual nondestructive testing, fluorescence is used in conjunction with long wave ultraviolet radiation as an excitation source (see Fig. 15). Visible light rays are made up of billions of photons, packets of particle-like energy. Photons are so small they have no mass. They do however carry energy and this is what we see when a light bulb is energized—the photons have carried energy from the bulb to the eye. Photons have different energies or wavelengths which we distinguish as different colors. Red light photons are less energetic than blue light photons. Invisible ultraviolet photons are more energetic than the most energetic violet light that our eyes can see. Studies show that the intensity of fluorescence in most situations is directly proportional to the intensity of the ultraviolet radiation that excites it.
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Fluorescence is the absorption of light at one wavelength and reemission of this light at another wavelength. The whole absorption and emission process occurs in about a nanosecond and because it keeps happening as long as there are ultraviolet radiation photons to absorb, a glow is observed to begin and end with the turning on and off of the ultraviolet radiation. Care must be taken when using short wave or wide bandwidth ultraviolet sources. A safe, general operating principle is to always hold the lamp so the light is directed away from you. Long wave ultraviolet is generally considered safe. However, individuals should use adequate protection if they are photosensitive or subjected to long exposure times. Commercially available fluorescent dyes span the visible spectrum. Because the human eye is still the most commonly used sensing device, most nondestructive testing applications are designed to fluoresce as close as possible to the eye's peak response. Figure 16 shows the spectral response of the human eye, with the colors at the ends of the spectrum (red, blue and violet) appearing much dimmer than those in the center (orange, yellow and green).
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While the fundamental aspects of fluorescence are still incompletely understood, there is enough known to ensure that nondestructive testing methods using fluorescence will continue to improve with the development of new dyes or new solvents to increase brightness or eye response matching.
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Keywords: Studies show that the intensity of fluorescence in most situations is directly proportional to the intensity of the ultraviolet radiation that excites it.
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FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region
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FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region
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FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region
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FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region
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FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region
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FIGURE 16. Human eye response at 1070 lx 1100 ftc)
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FIGURE 16. Human eye response to light
Digital ambient light sensor
http://www.michaelhleonard.com/sensorcape-reference/
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FIGURE 16. Human eye response to light
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FIGURE 16. Human eye response at 1070 lx 1100 ftc)
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Eye Spectra Responds: http://www.telescope-optics.net/eye_spectral_response.htm
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Fluorescence Testing
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Fluorescence Testing
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Fluorescence Testing
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Fluorescence Testing
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Fluorescence Testing
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Fluorescence Testing
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Fluorescence Testing
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Part 4: SAFETY FOR VISUAL AND OPTICAL TESTS 4.0
General:
This information is presented solely for educational purposes and should not be consulted in place of current safety regulations. Note that units of measure have been converted to this book's format and are not those commonly used in all industries. Human vision can be disrupted or destroyed by improper use of any light source. Consult the most recent safety documents and the manufacturer's literature before working near any artificial light or radiation source.
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4.1
Need for Safety
Developments in optical testing technology have created a need for better understanding of the potential health hazards caused by high intensity 'light sources or by artificial light sources of any intensity in the work area. The human eye operates optimally in an environment illuminated directly or indirectly by sunlight, with characteristic spectral distribution and range of intensities that are very different from those of most artificial sources. The eye can handle only a limited range of night vision tasks. Over time, there has accumulated evidence that photochemical changes occur in eyes under the influence of normal daylight illumination- short term and long term visual impairment and exacerbation of retinal disease have been observed and it is important to understand why this occurs. Periodic fluctuations of visible and ultraviolet radiation occur with the regular diurnal light-dark cycles and with the lengthening and shortening of the cycle as a result of seasonal changes. These fluctuations are known to affect all biological systems critically.
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The majority of such light-dark effects is based on circadian cycles and controlled by the pineal system, which can be affected directly by the transmission of light to the pineal gland or indirectly by effects on the optic nerve pathway. Also of concern are the results of work that has been done demonstrating that light affects immunological reactions in vitro and in vivo by influencing the antigenicity of molecules, antibody function and the reactivity of lymphocytes. Given the variety of visual tasks and illumination that confronts the visual inspector, it is important to consider whether failures in performance might be a result of excessive exposure to light or other radiation or even a result of insufficient light sources. A myth exists that 20/20 fovea vision, in the absence of color blindness, is all that is necessary for optimal vision. In fact, this is not so, there may be visual field loss in and beyond the fovea centralis for many reasons; the inspector may have poor stereoscopic vision; visual ability may be impaired by glare or reflection; or actual vision may be affected by medical or psychological conditions.
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4.2
Laser Hazards
4.2.1
General
Loss of vision resulting from retinal burns following observation of the sun has been described throughout history. Now there is a common technological equivalent to this problem with laser light sources. In addition to the development of lasers, further improvement in other high radiance light sources (a result of smaller, more efficient reflectors and more compact, brighter sources) has presented the potential for chorioretinal injury. It is thought that chorioretinal burns from artificial sources in industrial situations have been very much less frequent than similar burns from the sun.
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Because of the publicity of the health hazard caused by exposure to laser radiation, awareness of such hazards is probably much greater than the general awareness of the hazard from high intensity extended visible sources which may be as great or greater. Generally, lasers are used in specialized environments by technicians familiar with the hazards and trained to avoid exposure by the use of protective eyewear and clothing. Laser standards of manufacture and use have been well developed and probably have contributed more than anything else to a heightened awareness of safe laser operation.
Charlie Chong/ Fion Zhang
Laser standards of manufacture and use have been well developed and probably have contributed more than anything else to a heightened awareness of safe laser operation. Laser hazard controls are common sense procedures designed to (1) restrict personnel from entering the beam path and (2) limit the primary and reflected beams from occupied areas. Should an individual be exposed to excessive laser light, the probability of damage to the retina is high because of the high energy pulse capabilities of some lasers.
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However, the probability of visual impairment is relatively low because of the small area of damage on the retina. Once the initial flash blindness and pain have subsided, the resulting scotomas (damaged unresponsive areas) can sometimes be ignored by the accident victim. The tissue surrounding the absorption site can much more readily conduct away heat for small image sizes than it can for large image sizes. In fact, retinal injury thresholds (see Fig. 17) for less than 0.1 to 10 s exposure show a high dependence on the image size (0.01 to 0.1 W •mm - 2 for a 1,000 p.m image up to about 0.01 KV•mm- 2 for a 20 μm image. To put the scale into perspective, the sun produces a 160 μm diameter image on the retina.
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4.2.2
High Luminance Visible Light Sources
The normal reaction to a high luminance light source is to blink and to direct the eyes away from the source. The probability of overexposure to noncoherent light sources is higher than the probability of exposure to lasers, yet extended (high luminance) sources are used in a more casual and possibly more hazardous way. In the nondestructive testing industry, extended sources are used as general illumination and in many specialized applications. Unfortunately, there are comparatively few guidelines for the safe use of extended sources of visible light.
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FIGURE 1 7. Typical retinal burn thresholds
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4.3
Infrared Hazards
Infrared radiation comprises that invisible radiation beyond the red end of the visible spectrum up to about 1 mm wavelength. Infrared is absorbed by many substances and its principal biological effect is known as hyperthermia, heating that can be lethal to cells. Usually, the response to intense infrared radiation is pain and the natural reaction is to move away from the source so that burns do not develop. Keywords: Hyperthermia
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4.4
Ultraviolet Hazards
Before development of the laser, the principal hazard in the use of intense light sources was the potential eye and skin injury from ultraviolet radiation. Ultraviolet radiation is invisible radiation beyond the violet end of the visible spectrum with wavelengths down to about 185 nm. It is strongly absorbed by the cornea and the lens of the eye. Ultraviolet radiation at wavelengths shorter than 185 nm is absorbed by air, is often called vacuum ultraviolet and is rarely of concern to the visual inspector. Many useful high intensity arc sources and some lasers may emit associated, potentially hazardous, levels of ultraviolet radiation. With appropriate precautions, such sources can serve very useful visual testing functions. Keywords: 380nm- 185nm Ultraviolet radiation damaging to eye < 185nm- Vacuum UV is absorbed by air and not a concern to the inspector
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Studies have clarified the spectral radiant exposure doses and relative spectral effectiveness of ultraviolet radiation required to elicit an adverse biological response. These responses include kerato-conjunctivitis (known as welder's flash), possible generation of cataracts and erythema or reddening of the skin. Longer wavelength ultraviolet radiation can lead to fluorescence of the eye's lens and ocular media, eyestrain and headache. These conditions lead, in turn, to low task performance resulting from the fatigue associated with increased effort. Chronic exposure to ultraviolet radiation accelerates skin aging and possibly increases the risk of developing certain forms of skin cancer.
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It should also be mentioned that some individuals are hypersensitive to ultraviolet radiation and may develop a reaction following, what would be for the average healthy human, suberythemal exposures. However, it is extremely unusual for these symptoms of exceptional photosensitivity to be elicited solely by the limited emission spectrum of an industrial light source. An inspector is typically aware of such sensitivity because of earlier exposures to sunlight. In industry, the visual inspector may encounter many sources of visible and invisible radiation: incandescent lamps, compact arc sources (solar simulators), quartz halogen lamps, metal vapor (sodium and mercury) and metal halide discharge lamps, fluorescent lamps and flash lamps among others. Because of the high ultraviolet attenuation afforded by many visually transparent materials, an empirical approach is sometimes taken for the problem of light sources associated with ultraviolet: the source is enclosed and provided with ultraviolet absorbing glass or plastic lenses.
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If injurious effects continue to develop, the thickness of the protective lens is increased. The photochemical effects of ultraviolet radiation on the skin and eye are still not completely understood. Records of ultraviolet radiation's relative spectral effectiveness for eliciting a particular biological effect (referred to by photohiologists as action spectra) are generally available. Ultraviolet irradiance may be measured at a point of interest with a portable radiometer and compared with the ultraviolet radiation hazard criteria (Table 5). For the near ultraviolet region (from 320 nm to the edge of the visible spectrum), the total irradiance incident on the unprotected skin or eye should not exceed 1 mW.cm- 2 for periods greater than 1,000 s. For exposure times less than 1,000 s, incident irradiance on unprotected skin or eye should not exceed 1 J.cm-2 within an eight hour period. These values do not apply to exposures of photosensitive people or those simultaneously exposed to photosensitizing Agents.
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For purposes of determining exposure levels, it is important to note that most inexpensive, portable radiometers are not equally responsive at all wavelengths throughout the ultraviolet spectrum and are usually only calibrated at one wavelength with no guarantees at any other wavelength. Such radiometers have been designed for a particular application using a particular lamp. A common example in the nondestructive testing industry is the so-called blacklight radiometer used in fluorescent liquid penetrant and magnetic particle applications. These meters are usually calibrated at 365 nm, the predominant ultraviolet output of the filtered 100 W medium pressure mercury vapor lamp commonly used in the industry. Use of the meter at any other wavelength in the ultraviolet spectrum may lead to significant errors. To minimize problems in assessing the hazard presented by industrial lighting, it is important to use a radiometer that has been calibrated with an ultraviolet spectral distribution as close as possible to the lamp of interest.
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If the inspector is concerned about the safety of a given situation, ultraviolet absorbing eye protection and face wear is readily available from several sources. An additional benefit of such protection is that it prevents the annoyance of lens fluorescence and provides the wearer considerable protection from all ultraviolet radiation. In certain applications, tinted lenses can also provide enhanced visibility of the test object.
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TABLE 5. Threshold limit values for ultraviolet radiation' within an eight hour period* Wavelength (nanometers) 200 210 220 230 240 250 254 260 270 280 290 300 305 310 315 Charlie Chong/ Fion Zhang
Threshold Limit Values 100 40 25 16 10 7 6 4.6 3 3.4 4.7 10 50 200 1,000
*These values are presented for instructional purposes and not as guidelines. They do not apply to exposure of photosensitive people or those simultaneously exposed to photosensitizing agents. Consult current safety regulations, manufacturers data and inspection codes before any exposure to ultraviolet radiation.
4.5
Photosensitizers
While ultraviolet radiation from most of the high intensity visible light sources may be the principal concern, the potential for chorioretinal injury from visible radiation should not be overlooked. Over the past few decades, a large number of commonly used drugs, food additives, soaps and cosmetics have been identified as phototoxic or photoallergenic agents even at the longer wavelengths of the visible spectrum. Colored drugs and food additives are possible photosensitizers for organs below the skin because longer wavelength visible radiations penetrate deeply into the body.
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4.6
Damage to the Retina
It is possible to multiply the spectral absorption data of the human retina by the spectral transmission data of the eye's optical media at all wavelengths to arrive at an estimate of the relative absorbed spectral dose in the retina and the underlying choroid for a given spectral radiant exposure of the cornea. The computation should provide a relative spectral effectiveness curve for chorioretinal burns. In practice, the evaluation of potential chorioretinal burn hazards may be complicated or straightforward, depending on the maximum luminance and spectral distribution of the source; possible retinal image sizes; the image quality; pupil size; spectral scattering and absorption by the cornea, aqueous humor, the lens and the vitreous humor; and absorption and scattering in the various retinal layers. For convenience, the product of total transmittance of the ocular media Tx and the total absorptance of the retinal pigment epithelium and choroid 伪位, over all wavelengths may be defined as the relative retinal hazard factor R:
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where L位, could be any other spectral quantity. Qualitatively, the ocular media transmission rises steeply from somewhat less than 400 nm and does not fall off again until about 900 nm in the near infrared after which a peak at about 1,100 nm is exhibited. These values finally fall off to virtually zero at about 1,400 nm thus defining the potential hazardous wavelength range. For most extended visible sources, the retinal image size can be calculated by geometrical optics. As shown in Fig. 18, the angle subtended by an extended source defines the image size. Knowing the effective focal length f of the relaxed normal eye (17 mm), the approximate retinal image size d位can be calculated if the viewing distance r and the dimensions of the light source DL, are known.
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This analysis strictly holds only for small anglesâ&#x20AC;&#x201D;corrections must be made at angles exceeding about 20 degrees. Because the solid angles â&#x201E;Ś subtended by the source and retinal image are clearly identical, the retinal illuminance area AL and source luminance area Ar are likewise proportional. The source luminance L is related to the illuminance at the cornea Er as follows:
FIGURE 18. The extended source of length DL imaged on the retina with length dr
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Calculation of the permissible luminance from a permissible retinal illuminance for a source breaks down for very small retinal image sizes or for very small hot spots in an extended image caused by diffraction of light at the pupil, aberrations introduced by the cornea and lens and scattering from the cornea and the rest of the ocular media. Because the effects of aberration increase with increasing pupil size, greater blur and reduced peak retinal illuminance are noticed for larger pupil sizes and for a given corneal illumination.
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4.7
Thermal Factor
Visible and near infrared radiation up to about 1,400 nm (associated with most optical sources) is transmitted through the eye's ocular media and absorbed in significant doses principally in the retina. These radiations pass through the neural layers of the retina. A small amount is absorbed by the visual pigments in the rods and cones, to initiate the visual response, and the remaining energy is absorbed in the retinal pigment epithelium and choroid. The retinal pigment epithelium is optically the most dense absorbent layer (because of high concentrations of melanin granules) and the greatest temperature changes arise in this layer. For short (0.1 to 100 s) accidental exposures to the sun or artificial radiation sources, the mechanism of injury is generally thought to be hyperthermia resulting in protein denaturation and enzyme inactivation. Because the large, complex organic molecules absorbing the radiant energy have broad spectral absorption bands, the hazard potential for chorioretinal injury is not erected to depend on the coherence or monochromaticity of the source. Injury from a laser or a nonlaser radiation source should not differ if image size, exposure time and wavelength are the same.
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Because different regions of the retina play different roles in vision, the functional loss of all or part of one of these regions varies in significance. The greatest vision acuity exists only for central (foveal) vision, so that the loss of this retinal area dramatically reduces visual capabilities. In comparison, the loss of an area of similar size located in the peripheral retina could be subjectively unnoticed. The human retina is normally subjected to irradiances below 1 1.1.Wâ&#x20AC;˘mm- 2, except for occasional momentary exposures to the sun, arc lamps, quartz halogen lamps, normal incandescent lamps, flash lamps and similar radiant sources. The natural aversion or pain response to bright lights normally limits exposure to no more than 0.15 to 0.2 s. In some instances, individuals can suppress this response with little difficulty and stare at bright sources, as commonly occurs during solar eclipses.
Charlie Chong/ Fion Zhang
Fortunately, few arc sources are sufficiently large and sufficiently bright enough to be a retinal burn hazard under normal viewing conditions. Only when an arc or hot filament is greatly magnified (in an optical projection system, for examlarge can hazardous irradiance be imaged on a sufficiently large area of the retina to cause a burn. Visual inspectors do not normally step into a projected beam at close range or view a welding arc with binoculars or a telescope. Nearly all conceivable accident situations require a hazardous exposure to be delivered within the period of a blink reflex. If an arc is stnick while an inspector is located at a very close viewing range, it is possible that a retinal burn could occur. At lower exposures, an inspector experiences a short term depression in photopic (daylight) sensitivity and a marked, longer term loss of scotopic (dark adapted) vision. That is why it is so important for visual inspectors in critical fluorescent penetrant and magnetic particle test environments to undergo dark adaptation before actually attempting to find discontinuities. Not only does the pupil have to adapt to the reduced visible level in a booth but the actual retinal receptors must attain maximum sensitivity. This effect may take half an hour or more, depending on the preceding state of the eye's adaptation.
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4.8
Blue Hazard
The so-called blue hazard function has been used in conjunction with the thermal factor to calculate exposure durations that do not damage the retina. The blue hazard is based on the demonstration that the retina can be damaged by blue light at intensities that do not elevate retinal temperatures sufficiently to cause a thermal hazard. It has been found that blue light can produce 10 to 100 times more retinal damage (permanent decrease in spectral sensitivity in this spectral range) than longer visible wavelengths. Note that there are some common situations in which both thermal and blue hazards may be present.
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4.9
Visual Safety Recommendations
The American Conference of Governmental Industrial Hygienists (ACGIH) has proposed two threshold limit values (TLVs) for noncoherent visible light, one covering damage to the retina by a thermal mechanism and one covering retinal damage by a photochemical mechanism. Threshold limit values for visible light, established by the American Conference of Governmental Industrial Hygienists, are intended only to prevent excessive occupational exposure and are limited to exposure durations of 8 h or less. They are not intended to cover photosensitive individual.
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4.10
Eye Protection Filters
Because continuous visible light sources elicit a normal aversion or pain response that can protect the eye and skin from injury, visual comfort has often been used as an approximate hazard index and eye protection and other hazard controls have been provided on this basis. Eye protection filters for various workers were developed empirically but now are standardized as shades and specified for particular applications. Other protective techniques include use of high ambient light levels and specialized filters to further attenuate intense spectral lines. Laser eye protection is designed to have an adequate optical density at the laser wavelengths along with the greatest visual transmission at all other wavelengths. Always bear in mind that hazard criteria must not be considered to represent fine lines between safe and hazardous exposure conditions. To be properly applied, interpretation of hazard criteria must he based on practical knowledge of potential exposure conditions and the user, whether a professional inspector or a general consumer. Accuracy of hazard criteria is limited by biological uncertainties including diet, genetic photosensitivity and the large safety factors required to be built into the recommendations. Charlie Chong/ Fion Zhang
Charlie Chong/ Fion Zhang