ASNT Level III- Visual & Optical Testing My Pre-exam Preparatory Self Study Notes Reading 4 Section 3 2014-August
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For my coming ASNT Level III VT Examination 2014-August
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At works
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Reading 4 ASNT Nondestructive Handbook Volume 8 Visual & Optical testing- Section 3 For my coming ASNT Level III VT Examination 2014-August
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Fion Zhang 2014/August/15
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SECTION 3 THE VISUAL AND OPTICAL TESTING ENVIRONMENT
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SECTION 3: THE VISUAL AND OPTICAL TESTING ENVIRONMENT PART 1: EFFECT OF DESIGN CRITERIA ON VISUAL AND OPTICAL TESTS 1.1 1.2
Visual Testing in Product Design Designing for Quality Assurance
PART 2: ENVIRONMENTAL FACTORS 2.1 2.2 2.3 2.4 2.5
Cleanliness Texture and Reflectance Lighting for Visual Tests Light Intensities Vision in the Testing Environment
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PART 3: PHYSIOLOGICAL FACTORS 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9
The Lens The Fovea Rods and Cones Receptors Perception Physiology of Vision Mechanism of Vision Color and Color Vision Observer Differences
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PART 4: VISUAL WELD TESTING PERFORMANCE STANDARDS 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16
Near Vision Acuity Color Perception Target Detection Acuity Variables Reserve Vision Acuity and Visual Efficiency Performance Standards for Visual Weld Testing Use of Visual Reference Standards Knowledge of Crack Pattern Recognition Scanning Techniques Lighting Practical Qualification Requirements Remote Visual Tests Vision Hardware Other Factors Affecting Perception Recommendations for Visual Weld Testing Conclusion
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PART 1: EFFECT OF DESIGN CRITERIA ON VISUAL AND OPTICAL TESTS 1.0
General
To use visual testing effectively for quality control of a manufactured component, the test method's capabilities must be considered early in the product's design phase (see Table 1). Realistic accept and reject criteria must be established as a first step in designing for process control but these realistic criteria are not always obvious. For example, what is the distribution of voids in nonstructural composite honeycomb that can be tolerated for satisfactory service life? What quality of surface finish must be achieved to make a product acceptable? Or to make a product marketable? What level and type of material anomalies can be reliably detected by visual testing? How must the product design be changed to accommodate visual testing procedures? If correct controls are to be established, these and similar questions must be considered and answered as early as possible. One of the most complex problems is determining when, during a fabrication or assembly process, visual testing is most effective and least expensive.
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TABLE 1. Summary of the visual and optical testing method Method Key process and basic result
Principles Probe medium or energy source Nature of signal or signature Detection or sensing method Indication or recording method Interpretation basis
Objectives Discontinuities and separations Structure Dimensions and metrology Physical and mechanical properties Composition and chemical analyses Stress and dynamic responses Signature analysis
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Direct visual and optically aided testing is applied to object surfaces for indications of unacceptable conditions Visible natural or artificial light Reflected or transmitted photons Eyes, optical aids, magnifiers, borescopes, video and film cameras Visual image, video and film Direct, used with other methods for direct interpretation (liquid penetrant, magnetic particle) Cracks, voids, pores and inclusions Roughness, grain and film Mechanically aided measurements None None Visible responses to stress None
TABLE 1. Summary of the visual and optical testing method Applications Applicable materials Applicable features and forms Process control applications in situ or diagnostic applications Typical structures and components
Limitations Access, contact or preparation Probe and object limits Sensitivity or resolution Interpretation limits Related techniques
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All Surfaces, layers, films, coatings, entire objects On-line and off-line monitoring or control All forms of nondestructive testing Machined parts, internal surfaces, indefinite range of test objects, materials, components assemblies.
Visual access Specialized optical aids often required Various degrees of magnification May require supplementation with other nondestructive test methods for discontinuity detection and measurement Borescopy, refractometry, diffractometry, interferometry, microscopy. telescopy, light radiometry, phase-contrast and Schlieren techniques
1.1
Visual Testing in Product Design
Product design typically comprises four steps: conceptual, preliminary, layout and detail. During the concept phase, compatibility with visual and other nondestructive testing procedures must be ensured. In the preliminary design phase, performance criteria and material selection should be made compatible with nondestructive testing. During layout, inspectability of the product must be determined. It is important that the efforts of qualified engineering, manufacturing and nondestructive testing personnel he closely coordinated during this determination. Producibility and quality should receive the greatest attention in the detail design phase but all disciplines must be considered.
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Complex structures may not be inspectable because of geometric constraints or accessibility. It is necessary either for ■ such components must be redesigned or ■ for the approval of the design to take un-inspectability into account. Nondestructive testing is an added cost but, when properly applied, it can substantially reduce total life-cycle costs. The visual testing specialist participates in the design process by providing knowledge of the visual testing function. This can best be accomplished by: ■ providing qualified NDT support during design, ■ revising design handbook data to cover nondestructive testing and ■ establishing nondestructive testing guidelines to govern testing as part of overall quality procedures.
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1.2
Designing for Quality Assurance
Quality assurance is the establishment of a program to guarantee the desired quality level of a product from raw materials through fabrication, assembly and delivery. Quality control is the physical and administrative actions required to ensure compliance with the quality assurance program. Quality control includes nondestructive testing at appropriate points in the manufacturing cycle. A quality assurance program consists of five basic elements.
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1. Prevention: a formalized plan for designing, for inspectability and costeffectiveness. 2. Control: documented workmanship standards and compatible procedures for training of and use by production and quality control personnel. 3. Assurance: establishing quality control check points and a rapid information feedback system. 4. Corrective action: implementation of the feedback system and necessary corrective action. 5. Audit: unbiased third party review of all aspects of the program, including vendor materials.
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Management must decide what quality level it will produce and support. Once this is established, production and testing personnel aim to maintain this level and not to depart from it either toward lower or higher quality. For example, when drawing a component, the designer sets tolerances on dimensions and finish. If a drawing specifies a certain dimension as 32 mm (1.25 in.) but fails to specify the tolerance, the machine shop supervisor could: â– reject the drawing as incomplete or â– assume a standard tolerance. In nondestructive testing, a quality tolerance (the acceptable limits on the characteristic of interest) must also be specified. For example, no defects is an unworkable quality acceptance criteria. The lack of this single requirement has caused much misunderstanding of nondestructive testing in general and visual tests in particular.
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PART 2: ENVIRONMENTAL FACTORS 2.0
General
An important environmental factor affecting visual tests is lighting. Often, emphasis is placed on equipment variables such as borescope view angle or degree of magnification. But if the lighting is incorrect, no magnification is going to improve the image. Other working conditions are also important including factors causing operator discomfort and fatigue.
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2.1
Cleanliness
The act of seeing depends on the amount of light reaching the eye. In visual tests, the amount of light may be affected by distance, reflectance, brightness, contrast or the cleanliness, texture, size and shape of the test object. Cleanliness is a basic requirement for a good visual test- it is impossible to gather visual data through layers of opaque dirt unless cleanliness itself is being examined. In addition to obstructing vision, dirt on the test surface can mask actual discontinuities with false indications. Cleaning typically may be done by mechanical or chemical means or both. Cleaning avoids the hazards of undetected discontinuities and improves customer product satisfaction.
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2.2
Texture and Reflectance
Vision is dependent on reflected light entering the eye. The easiest way to ensure adequate lighting is by placing the light source and eye as close to the test surface as the focal distance allows. Similarly, a magnifier should be held as close to the eye as possible, ensuring that the maximum amount of light from the target area reaches the eye. Reflectance and surface texture are related characteristics. It is important for lighting to enhance a target area, but glare should not be allowed to mask the test surface. A highly reflective surface or a roughly textured surface may require special lighting to illuminate without masking. Supplementary lighting must be shielded to prevent glare from interfering with the inspector's view. Reflected or direct glare can be a major problem that is not easily corrected. Glare can be minimized by decreasing the amount of light reaching the eye. This is done by increasing the angle between the glare source and line of vision by increasing the background light in the area surrounding the
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This is done by increasing the angle between the glare source and line of vision by increasing the background light in the area surrounding the glare source or by dimming the light source. Such solutions are simple to implement for direct glare from a supplemental light or the reflected glare from a small test object. Glare from permanent lighting fixtures is more difficult to control. Ceiling fixtures should be mounted as far above the line of sight as possible and must be shielded to eliminate light at an angle greater than 45 degrees to the field of vision. Task lighting should be shielded to at least 25 degrees from horizontal. Such shielding must allow a sufficient amount of light to reach the test area.
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Glare
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2.3
Lighting for Visual Tests
The amount of light required for a visual test is dependent on several factors, including the type of test, the importance of speed or accuracy, reflections from backgrounds and inspector variables. Physiological processes, psychological state, experience, health and fatigue all contribute to the accuracy of a visual inspection. The reflections and shadows from walls, ceiling, furniture and equipment must also be considered. Some reflectance from the environment must occur or the room will be too dark to be practical. Recommended reflectance values are: ceiling, 80 to 90 percent; walls, 40 to 60 percent; floors, not less than 20 percent; desks, benches and equipment, 25 to 45 percent. For visual and other nondestructive testing applications, a ratio of 3:1 between the test object and darker background is recommended. A 1:3 ratio is recommended for a test object and lighter surroundings. Keywords: 3: 1 ratio
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Certain psychological factors can also affect a visual inspector's performance. Wall colors and patterns have been shown to have a measurable effect on attitude and this is especially important when visually inspecting critical or small components. In general, a visual inspector's optimum attitude is relaxed but not bored, alert but not restless. To complement the illumination needed for visual testing, all colors in a room should be light tones. Otherwise, up to 50 percent of the available light can be absorbed by dark walls and flooring. A strong contrast of pattern or color can cause restlessness and eventually fatigue. Cool (blue) colors are recommended for work areas with high noise levels and heavy physical exertion.
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2.4
Light Intensities
The nanometer (tip), equal to 10-9 meters, has replaced the angstrom unit (A) as the preferred unit for measuring radiation wavelengths. There are ten angstrom units in a nanometer. To perform a visual test, there must be a source of natural or artificial light adequate in both intensity and spectral distribution. Even under optimum conditions the human eye can be stimulated by only a small part of the electromagnetic spectrum. The limits of this visible portion are ill defined, depending on the amount of energy available, its wavelength and the health of the eye. For most practical purposes, the visible spectrum may be considered to be between about 380 nm at the beginning of the violet and 770 nm at the end of the red, However, with especially intense sources and with a completely dark adapted eye, the shorter wavelength boundary may be extended down to 350 nm or shorter, with a corresponding reduction in the longest wavelength perceived. Similarly, with an especially intense longer wavelength source and an eye adapted to a higher level of light, the longer wavelength boundary may extend up to 900 nm. These ranges together are only a small part of the electromagnetic spectrum. Charlie Chong/ Fion Zhang
Brightness is an important factor in visual test environments. The brightness of a test surface depends on its reflectivity and the intensity of the incident light. Excessive or insufficient brightness interferes with the ability to see clearly and so obstructs critical observation and judgment. For this reason, light intensity must be tightly controlled. A minimum intensity of 160 lx (15 ftc) of illumination should be used for general visual testing. A minimum of 500 lx (50 ftc) should be used for critical or finely detailed tests. According to the Illuminating Engineering Society, visual testing requires light at 1,100 to 3,200 lx (100 to 300 ftc) for critical work.' A commercially available light meter can be used to determine if the working environment meets this standard. To ensure compliance with the minimum intensity requirement, a known light source held within a specified maximum distance must be used. Alternatively, a light measuring device such as a photocell or phototube must be used. Examples of known light sources are shown in Table 2.
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Keywords: A minimum intensity of 160 lx (15 ftc) of illumination should be used for general visual testing. A minimum of 500 lx (50 ftc) should be used for critical or finely detailed tests. According to the Illuminating Engineering Society, visual testing requires light at 1,100 to 3,200 lx (100 to 300 ftc) for critical work
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TABLE 2. Distances for minimum 500 Ix (50 ftc) illumination
Light 2 D cell flashlight 60 W incandescent bulb 75 W incandescent bulb 100 W incandescent bulb
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Maximum Source-to-Object Distance Source millimeters (inches) 250 (10) 250 (10) 380 (15) 460 (18)
Light measuring device- Photometer
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2.5
Vision in the Testing Environment
2.5.0
General
The eye is a critical variable in visual tests because of variations in the eye itself as well as variations in the brain and nervous system. For this reason, visual inspectors must be examined to ensure natural or corrected vision acuity. The frequency of such examinations is determined by code, standard specification, recommended practice or company policy and yearly examinations are common. 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 30 cm (12 in.).
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More clinically precise ways of measuring vision acuity involve recognition of Roman capital letters of various sizes from controlled distances. More on the determination of vision acuity may be found in the discussion of the physiology of sight. The exact requirements for near vision acuity examination are specified by the employer. If prescription lenses are required to pass a vision examination, then the subject must wear them during subsequent visual testing. Photograying lenses can be a problem where ultraviolet light is high, e.g., under some fluorescent lights.
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2.5.1
Visual Angle and Distance
The angle of vision and the distance of the eye from the test surface determine the minimum angular separation of two points resolvable by the eye. This is known as the eye's resolving power. For the average eye, the minimum resolvable angular separation of two points on an object is about one minute of arc (or 0.0167 degrees). This means that at 300 mm (12 in.) from a test surface, the best resolution to be expected is about 0.09 mm (3.5 mil). At 600 mm (24 in.), the best anticipated resolution is about 0.18 mm (0.007 in.). To complete a visual test, the eye is brought close to the test object to obtain a large visual angle. However, the eye cannot sharply focus on an object if it is nearer than 250 mm (10 in.). Therefore, a direct visual test should be performed at a distance of 250 to 600 mm (10 to 24 in.). Also of importance is the angle the eye makes with the test surface. For most indications, this should not be less than 30 degrees (see Fig. 1).
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FIGURE 1. Minimum angle for typical visual testing
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PART 3: PHYSIOLOGICAL FACTORS 3.1
The Lens
The human eye is a roughly spherical organ, set in a socket where it is free to rotate in all forward directions. (Refer to figure showing eye elsewhere in this book.) At the front, a compound lens (including the cornea) is set into an opening through which light enters the eye. This lens is of variable focal length and changes without conscious effort to focus objects at varying distances, forming images at the back of the eye. With aging, focusing becomes sluggish. Immediately in front of the lens is the iris, a circular pigmented membrane, perforated by an aperture known as the pupil. The iris, analogous to the diaphragm of a camera, adjusts spontaneously the area of the pupil to change the amount of light entering the eye by a maximum factor of about 16: 1. The pupil tends to be wider at low light intensities and smaller at higher intensities. It plays little part in color perception.
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The lens does not pass light of the shortest wavelengths and is largely responsible for the termination of response at the low end of the spectrum. As age increases, the lens yellows, increasing the absorption in the blue region and tending to increase the shortest wavelength that can be seen. This can be a factor in color differences reported between observers of different age, especially for tasks involving shorter wavelength perceptions.
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3.2
The Fovea
The photographic plate used in the camera is represented in the eye by the retina, which contains the end plates of the optic nerve. These receptors are extremely complicated structures called rods and cones. Figures showing the approximate location of these microscopic receptors are in the introductory discussion on the physiology of sight. Nerve impulses stimulated by light arise in these structures and are conducted along the visual pathways to the occipital region of the brain. When the eye looks directly at a small area in the field of view, the images impinge on a region called the foven centrails (see Fig. 2). This is the region of sharpest vision and the retina component most important for visual testing. It is convenient to consider the cone and rod distributions and their dependence on increasing distance from the fovea centralis. The central part of the fovea consists almost entirely
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of color sensitive cones, nearly all of which are connected individually to optic nerve fibers. The foveal cones are packed more tightly together and the structure above them is much thinner, forming a depression in the retina in this region. There is a sound physiological basis for the superiority of detail perception in the fovea centralis. This rod-free area extends outward to around 2 or 3 degrees as measured by the area's angular subtense in the external field; it is notably insensitive to shorter visible wavelengths. This may aid detail vision by offsetting chromatic aberrations of the eye's lens. Broadly speaking, no other part of the eye is used to perceive a momentary object of interest. At a distance of 500 mm (20 in.), the 2 degrees of rod-free surface corresponds to an object size of about 19 mm (0.75 in.). Visual testing of a component larger than 19 mm (0.75 in.) then becomes a series of quick and successive fixations along the object with occasional returns for verification.
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FIGURE 2. Cross section of the eye, showing fovea centralis (see also figures in discussion of the physiology of eyesight)
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FIGURE 2. Cross section of the eye, showing fovea centralis (see also figures in discussion of the physiology of eyesight)
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3.3
Rods and Cones
Proceeding outward from the fovea centralis, rods are found mingled with cones. With distance from the fovea, the percentage of cones decreases exponentially and the percentage of rods increases exponentially. At the same time, both rods and cones show a tendency to connect in groups to single nerve fibers. This tendency is much stronger for rods and these groups become larger with increased distance from the fovea. Vision for detail therefore decreases steadily but color perception persists, at normal light intensity levels. Partly overlapping the fovea and surrounding it out to around 10 degrees in the visual field is an irregular, diffuse ring of yellow pigment known as the macular lutae. Its importance in perception comes from its absorption of blue light, thus changing the spectral energy distribution of the light reaching receptors that are under it.
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Rods and cones differ in the minimum intensity of light to which they can respond. This difference is caused in rods by the presence of a photosensitive pigment called rhodopsin. This material is very easily bleached by light at low levels and is assumed to produce an electrochemical response in the rods. This visual response is essentially without color sensation and the sensitivity of the eye as a function of wavelength at these intensity levels corresponds to the wavelength absorption curve of rhodopsin. It is distinctly different from the wavelength response curve of the whole eye at higher intensity levels, which is representative of the sensitivity of the cones. Keywords: Rod- Rhodopsin
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Three classes of human cones have been identified, with a sensitivity peaking at 445 nm, 535 nm and 570 nm. It is known that the blue absorbing cones are relatively sparse in the fovea centralis, thereby explaining its insensitivity to shorter wavelengths. Because of the absence of rods in the fovea, there is DO response for low level light, even if the chromatic sensitivity is at its highest level and the iris is fully dilated. It is the level of the stimulus that is inadequate to elicit a chromatic response. To obtain any response at all, it is necessary to look off to one side of the stimulus so that at least some rods participate in the perception. However, any sufficient stimulus in the central field of view (a distant light source, for example, or a discontinuity filled with fluorescent penetrant irradiated with ultraviolet radiation) produces a chromatic response while everything else remains colorless.
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3.4
Receptors
Because receptors are grouped and the size of the groups increases rapidly with increasing distance from the fovea, peripheral vision is very indistinct and largely serves purposes of orientation and the detection of motion. The mechanism appears to play little if any part in perception of stationary objects at normal room and daylight intensities. The fibers from the various receptors cross the inner (vitreous humor) side of the retina and pass through it together in the optic nerve bundle. This transitional area is called the optic disk and is completely blind. Its surface area is comparable to that of the fovea. The optic disk lies about 16 degrees toward the nose from the fovea (outward in the visual field) so that corresponding parts of the visual field cannot fall on both disks simultaneously. An observer is not aware of this blind spot except when consciously arranging for an image to fall wholly within the optic disk. As mentioned above, the retina does not detect light uniformly over its area. The importance of this for perception is not so much the details overlooked because of the nonuniformity as the fact that even a rather keen observer is not normally aware of the nonuniformity unless an instance is pointed out.
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As we look about a scene (rather than at a fixed point), the image in the eye moves across the region of sharpest vision as well as all the other regions. This voluntary, though not usually conscious, movement corresponds to shifting focus of attention on details. During each pause, there is also a fairly rapid tremor of the eyes called saccadic movement. Both movements encourage contours in the image to cross the receptor elements of the retina. It is believed that this effect plays a role in contour perception and even appears to be a necessary condition for vision. If the center of such a field is rigidly fixated and viewed without blinking (both difficult), there is a gradual loss of both brightness and saturation over the whole area and this can eventually make the stimulus disappear. The progressive change can be interrupted at any point by either blinking or moving the eye quickly (changing the fixation point) from side to side. Together with other data, it is apparent that there has been a loss in sensitivity in the area of the retina covered by the image.
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3.5
Perception
3.5.0
General
In terms of its visible response, the sensitivity of the eye to light is not constant. The eye tends to respond more to differences in the field of view than to absolute values. It appears to do this automatically by adjusting sensitivities to something approaching the average of the stimuli. Sensitivity is also affected laterally by stimuli lying near the primary object. These are time-dependent factors, with the time scale being determined largely by the magnitude of change from the previous stimulation. Adaptation is essentially independent in the two eyes so that they may have quite different sensitivity levels at the same time. For gross light level changes, adaptation occurs as (1) the familiar and painful glare of a bright light after a long period in relative darkness, with adjustment sometimes taking as long as a minute and (2) the blindness after entering a normally lighted room from full sunlight, with adjustment taking as long as thirty minutes. Adaptation time increases with age.
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3.5.1
Influences on Perception
The science of perception is the study of (1) how ideas and other mental events become organized to yield impressions of objects and (2) the influence of the observer's mental and physical states. It is known that the perceived qualities of a viewed object may change with the state of the observer, based on knowledge of or assumptions about the causes of a stimulus. For example, under certain conditions, two lines of the same length can be perceived as different lengths, as in the well known Muller-Lyer illusion with double ended arrows (see Fig. 3).
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FIGURE 3. In the MUller-Lyer illusion, the shafts of two arrows are the same length- contrary to appearances
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FIGURE 3. In the MUller-Lyer illusion, the shafts of two arrows are the same length- contrary to appearances
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FIGURE 3. In the MUller-Lyer illusion, the shafts of two arrows are the same length- contrary to appearances
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For the purposes of visual and optical testing, it is important to know why physical reality may differ from perception and what are the effects of the observer's knowledge, fatigue, health and attitude. Perception is an active process in which the observer uses vision in combination with experience to maximize the wanted details and minimize the unwanted details. Most visual inspectors recognize that test objects exist, that they emit or reflect light, that this light causes neural activity and that the brain then synthesizes some representation of the original object. Other assumptions are specific to the application and may even be erroneous for example, what sort of defects are likely to occur or are of concern to the employer. The following discussion emphasizes somatic conditions that can impair the inspector's judgment. Sluggishness of the iris or of the muscles adjusting the lens can be caused by age, fatigue, drugs, disease or emotions. Such sluggishness in turn can affect what the observer sees and does not see.
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3.5.2
Effects of Fatigue
Seeing is not the passive formation of an image. It is an active process in which the observer keeps track of personal actions through a kind of feedback loop in which the perceived thing may be altered by the observer's actions. As one of the first steps in this complex feedback system, the image is formed by the lens of the eye on 100 million or so rods and cones in the retina. There are only about 1 million fibers that can carry the responses of these elements out of the eye through the optic nerve. Clearly, there must be grouping of these sensitive elements into single channels. Both this grouping and the distribution of rods and cones change systematically over the retina. In common with other psychological subjects, but unlike the physical sciences, the end result of seeing cannot be measured. It can only be described or compared to the effect of a previous, similar experience. In common with all other processes that require active participation, fatigue reduces the observer's efficiency for accurately interpreting visual data.
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3.5.3
Effect of Observer's Health
There are many somatic conditions that can directly or indirectly affect an individual's ability to see. Glaucoma is one such disease, characterized by increased intraocular tension which can cause vision impairments ranging from slight abnormalities to absolute blindness. In many cases, the cause of visual impairment is not known and not easily discovered. Some problems of perception are secondary effects supplemented by predispositions of heredity, emotional state or circulatory factors. In other cases, impairment can result directly from disease of the ocular structures, including intraocular tumors, enlarged cataracts or intraocular hemorrhage. Presbyopia is a condition in which the lens stiffens with age and so loses its ability to focus.
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Diabetic retinopathy is another condition that impairs normal vision. It can occur eight years after the onset of diabetes, with effects ranging from minor to severe. Diabetes can also lead to degenerative changes in a normally developed lens, characterized by gradual loss of transparency. Well developed, diffuse cataracts sometimes result from diabetes as well as other causes. The condition can reduce vision until only light perception remains. Sometimes myopia develops in the early stages of nuclear cataracts so that someone whose vision is presbyopic may be able to read without corrective lenses. Gradual loss of vision in middle age is characteristic of both cataracts and glaucoma. Prolonged use of the eyes with defective illumination and a strained position should always he avoided. It is also important to avoid fatigue of the eye muscles particularly when caused by errors of refraction. Inability to concentrate on the subject and a rhythmic oscillation of the eye and eyelids may occur as a result of eye muscle fatigue, leading to ineffective visual tests.
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Corrective lenses and rest often relieve simple forms of eye strain. Because of physiological changes in the lens with age, the lens is rendered less responsive to the process of accommodation and the resulting presbyopic individual is unable to focus well for near vision. Blurring, increasing awareness of photophobia, too-watery eyes, throbbing pains in the eyeballs, burning, eyeball tenderness, a feeling of discomfort in the eyes and sluggish reaction of the iris are some of the signs that a thorough eye examination is needed. Color blindness is discussed in the part of this book on the physiology of eyesight. It is the initial stage of impairment that commonly causes the most problems for the unknowing visual inspector. Because vision impairment typically progresses slowly, individuals may not be aware of a problem until it impairs performance. Any individual who needs frequent changes of corrective lenses, who notes diminished vision acuity, has mild headaches, sees halos around light sources or has impaired dark adaptation should have an eye examination as soon as the condition is discerned. This is especially true for individuals over the age of 40.
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3.5.4
Effect of Observer's Attitude
A complete representation of the visual field probably is not present in the brain at any one time. The brain must contain electrochemical activity representing some major aspects of a scene but such a picture typically does not correspond to how the observer describes the scene. This occurs because the observer adds experience and prejudices that are not themselves part of the visual field. Such sensory experience may reflect physical reality or may not. Sensory data entering through the eye are irretrievably transformed by their contexts—an image on the retina is perceived differently if its background or context changes. Perceptually, the image might be a dark patch in a bright background that can, in turn, appear to be a white patch if displayed against a dark background. No single sensation corresponds uniquely to the original retinal area of excitation.
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The context of a viewed object can affect perception and, in addition, the intention of the viewer may also affect perception. The number of visible objects in a scene far exceeds the typical description of the scene. And a great deal of information is potentially available to the observer immediately after viewing. If an observer has the intention of looking for certain aspects of a scene, only certain visual information enters the awareness, yet the total picture is certainly imaged on the retina. if a scene or an object is viewed a second time, many new characteristics can be discerned. This new nformation directly influences perception of the object, yet such information might not be available to the viewer without a second viewing. The selective nature of vision is apparent in many common situations. An individual can walk into a room full of people and effectively see only the face of an expected friend. The same individual can walk right by another friend without recognition because of the unexpected nature of the encounter. Vision is strongly selective and guided almost entirely by what the observer wants and does not want to see. Any additional details beyond the very broadest have been built up by successive viewing. Both the details and the broad image are retained for as long as they are needed and then they are quickly erased.
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The optical image on the retina is constantly changing and moving as the eye moves rapidly from one point to another- the sensing rods and cones are stimulated in ways that vary widely from one moment to the next. The mental image is stationary for stationary objects regardless of the motion of the optical image or, for that matter, the motion of the observer's head. It is very difficult to determine how a unique configuration of brain activity can be the result of a particular set of sensory experiences. A unique visual configuration must be a many-to-one relationship requiring complex interpretation. If an observer does not apply experience and the intellect, it is likely that a nondestructive visual test will be inadequate.
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3.6
Physiology of Vision
3.6.1
Visual Functions
Vision comprises a number of factors, including perception of light, form, color, depth and distance. Form perception occurs when light from an object is focused in the eye. This visual image is affected by the lens system in almost the same way that any inorganic lens brings rays of light to a focus and forms an image. The focus of the lens system in the eye can be changed like that of a camera. A diaphragm (the iris) regulates the quantity of light admitted. The retina is a light sensitive plate on which the image is formed. Adjustments of focus are made by changing the thickness and curvature (the focusing power) of the lens. Increasing the lens thickness is called accommodation. This is done by the action of tiny muscles attached to the lens.
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3.6.2
Refractivity and Binocular Vision
In the normal eye, the length of the eyeball and the refractive power of the cornea and lens are such that images of objects at a distance of 6 m (20 ft) or more are sharply focused on the retina when the muscles of accommodation are relaxed. Errors in these relationships require correction with specially prepared lenses. In a farsighted individual, the situation can be corrected with convex lenses. These bring light from distant objects to a focus without contracting the accommodation muscles which make the lens more convex. In the nearsighted person, light rays from distant objects come to a focus in front of the retina. This causes blurring of all objects located beyond a critical distance from the eye. By use of concave lenses, distant objects can be seen clearly by the nearsighted individual. Keywords: In the normal eye, the length of the eyeball and the refractive power of the cornea and lens are such that images of objects at a distance of 6 m (20 ft) or more are sharply focused on the retina when the muscles of accommodation are relaxed
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Myopia
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Myopia
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http://www.tedmontgomery.com/the_eye/eyephotos/Farsightedness-grphc.html
Hyperopia
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Hyperopia
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Hyperopia
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3.6.3
Distance Judgment
Binocular vision is an important aid in accurate judgment of distance. Distance judgment is the basis for depth perception or stereoscopic vision. Stereoscopic vision depends, at least in part, on the fact that each eye gets a slightly different view of close objects. The right eye sees a little more of the right hand surface of the object. The left eye sees a little less of this surface but more of the left hand surface. When the images on the two retinas differ in this way, the object is perceived as three-dimensional.
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3.7
Mechanism of Vision
3.7.0
General
The photographic plate used in the camera is represented in the eye by the retina, which contains the end plates of the optic nerve. These receptors are the rods and cones. Nerve impulses stimulated by light arise in these structures and are conducted along the visual pathways to the occipital region of the brain.
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3.7.1
Photochemical Processes
The mechanism of converting light energy into nerve impulses is a photochemical process in the retina. The so called visual purple, a chromoprotein called rhodopsin, is the photosensitive pigment of rod vision. It is transformed by the action of radiant energy into a succession of products, finally yielding the protein called opsin plus the carotenoid known as retinene. This process occurs by the action on the visual purple of a small number of quanta of radiant energy in the visible range of wavelengths. It has been shown that the peak and slope of the curve of scotopic (night vision) luminosity sensation are almost identical with the absorption curve of rhodopsin.
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Chromoprotein
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3.7.2
Light Receptors
The two kinds of light receptors in the retina, the rods and the cones, differ in shape as well as function. At the point where the optic nerve enters the retina, there are no rods and cones. This portion of the retina, called the blind spot, is insensitive to light. At the other extreme, the maximum vision acuity at high brightness levels exists only for that small portion of the image formed on the center of the retina. This is the fovea centralis discussed in detail earlier. Here, the layer of blood vessels, nerve fibers and cells above the rods and cones is far thinner than in peripheral regions of the retina.
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Light Receptors
http://www.sciencephoto.com/media/308752/enlarge
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Light Receptors
http://ncifrederick.cancer.gov/atp/imaging-and-nanotechnology/electron-microscopylaboratory/eml-protocols-and-resources/eml-image-gallery/retina3/
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3.7.3
Daylight Vision
Daylight vision, which gives color and detail, is performed by the cones, mainly in the fovea centralis. There are at least three different kinds of cones, each of which is in some way activated by one of the three fundamental colors, as discussed earlier in this section.
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Cones
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html
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3.8
Color and Color Vision
3.8.0 General Color vision is one of the most interesting aspects of the function of the human eye. Color vision occurs only in the light-adapted eye and is dependent on the acuity of the cones. Light is the specific stimulus for the eye but the eye is sensitive only to rays of certain wavelengths. Within those wavelengths, the stimulus must have a certain minimum intensity. The sensation of color varies according to the intensity of the light, the wavelength of the different radiations and the combinations of different wavelengths. In daylight vision, yellow is the brightest color.
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3.8.1
Color Characteristics
Every color has three characteristics: (1) tone or hue, (2) saturation or purity and (3) brightness or luminosity. Hue is associated with a range of wavelengths in the spectrum and is usually what an observer means when describing a color (red or blue, for instance). An estimated seven million or more colors can be discriminated but, because the transition from one hue to the next is gradual, the demarcations are ill defined and to some extent a matter of opinion. For practical purposes only a few main colors are commonly distinguished, with the following approximate wavelengths: violet, 380 to 450 nm; blue, 450 to 480 nm; blue-green, 480 to 510 nm; green, 510 to 550 nm; yellow-green, 550 to 570 nm; yellow, 570 to 590 nm; orange, 590 to 630 nm; and red, 630 to 730 nm. Light from a limited part of the spectrum is called monochromatic.
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A hue may also vary in brightness, according to the intensity of its predominant radiation. Indigo, with wavelengths approximately from 425 to 455 nm, is sometimes included between violet and blue, perhaps because of the name Roy G. Biv, a mnemonic comprising initials of the colors of the rainbow. Another color characteristic is saturation. This is a relative or comparative characteristic and may be described as a hue's dilution with white light.
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3.8.2
Color Changes
The critical evaluation of color and color change is one of the basic principles of most visual tests. Corrosion or oxidation of metals or deterioration of organic materials is often accompanied by a change in color imperceptible to the eye itself. For example, minute color changes on the surface of meat may not be detectible by the human eye but can be detected with photoelectric devices designed for the automatic inspection of meat before canning.
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3.8.3
Brightness Characteristics
Brightness contrast is generally considered the most important factor in seeing. The brightness of a diffusely reflecting colored surface depends on its reflection factor and the quantity of incident light (lux or footcandles of illumination). Excessive brightness (or brightness within the field of view varying by more than 10:1) causes an unpleasant sensation called glare. Glare interferes with the ability of clear vision, critical observation and judgment. Glare can be avoided by using polarized light or other polarizing devices. Keywords: â– â–
Glare 10:1 brightness differences Polarizing devices as a mean to reduce glare.
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3.9
Observer Differences
The visibility of an object is never independent of the human observer. Human beings differ inherently in the speed, accuracy and certainty of seeing, even though they may possess average or normal vision. Individuals vary particularly in threshold measurements and in their interpretations of visual sensations. Their psychological state, tensions and emotions influence their appraisals of the visibility of objects and influence their performance of visual tasks under many conditions. The importance of an inspector's attitude cannot be overemphasized. Because many visual testing decisions may involve marginal material, all interpretations must be impartial and consistent. A defined policy of test procedure and standards should be adopted and followed faithfully.
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PART 4: VISUAL WELD TESTING PERFORMANCE STANDARDS 4.0
General
The text below focuses on direct and remote visual weld testing with emphasis on crack detection. Visual weld testing for cracks is one of the most prevalent nondestructive tests. However, of all the nondestructive methods, visual testing has the least defined performance procedures for qualifying or quantifying minimum test performance. Three procedures are used to verify a visual inspector's performance or sensitivity: (1) near vision acuity, (2) color recognition and (3) target detection. The validity of the procedure for verifying field reliability can be improved by understanding the use and limitations of performance oriented tests.
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4.1
Near Vision Acuity
The majority of recommended practices or standards require 20/30 uncorrected or corrected vision in one eye. Section V of AS ME's Boiler and Pressure Vessel Code requires 20/30 near vision acuity and Section XI requires 20/20. 5 While this provides a baseline for vision performance, it does not measure stereo vision or other vision problems such as astigmatism that can significantly affect detection reliability. Measurement of physiological vision capability involves several tests that can have complex interactive variables. As a screening standard, 20/20 or 20/30 near vision acuity in both eyes is a reasonable beginning for the vision requirements of weld testing. However, near vision acuity measurement alone is not sufficient for predicting probability of detection for fine discontinuities.
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4.2
Color Perception
Although color recognition is not part of typical visual testing specifications, it is a part of the requirements for inspector qualification. Color recognition screening is usually a requirement for nondestructive tests that are distinctly color based. For example, magnetic particle testing in many cases requires the ability to see red or green (fluorescent) indications. Individuals with good vision acuity and red/green color deficiency can often pass practical tests based only on contrast recognition. Color can be a significant factor for pattern recognition of color based information during weld testing. However, it is difficult to qualify or quantify visual weld testing performance criteria. The fundamental question of what degree of color deficiency disqualifies a visual weld inspector is not well defined. The American Welding Society's Certified Welding Inspector Programs states that color vision acuity is desirable in some specific applications but is not considered essential for all inspections.
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4.3
Target Detection
A critical performance standard for some visual tests is the detection of a line to verify a system's sensitivity. This procedure is often called a resolution test. Detection may be defined as the task of perceiving the absence or presence of an object. In vision physiology and psychology,' resolution is the ability of a vision system to discriminate between the critical elements of a stimulus pattern. Detection of a single line does not fulfill the standard definition of resolution. Single line detection for a direct visual examination is usually performed using a 750 Îźm (30 mil) width black line on an 18 percent neutral gray flat uniform background. Some performance criteria" require detection of a 25 Îźm (1 mil) black line for remote visual tests in critical applications. Because simple line detection is a relatively gross task, it can be a poor performance standard, allowing detection of a highly blurred image. This does not emulate sharpness quality recognition for evaluation of weld discontinuities.
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A 750 μm (30 mil) black line can be reliably detected by individuals classified as legally blind (20/200 corrected both eyes). The 750 μm (30 mil) and even the smaller 25 μm (1 mil) widths should not be used as performance standards because they do not determine image sharpness. Image sharpness is critical to discontinuity recognition and is a key feature for pattern recognition of welding discontinuities. Figures 4 to 6 show a 750 μm (30 mil) line detection in which detection occurs with both 20/20 and 20/200 near vision acuity One means of simulating the effect of 20/200 near vision acuity is to observe objects underwater with the naked eye (assuming 20/20 near vision acuity as a baseline).
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FIGURE 4. General view of 20/20 near visual acuity card and line card of 18 percent neutral gray background
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FIGURE 5. Photograph of 0.75 mm (0.033 in.) line on 18 percent neutral gray card taken with an equivalent 20/20 near vision acuity
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FIGURE 6. Photograph of 0.75 mm (0.033 in.) line on 18 percent neutral gray card taken with an equivalent 20/200 near vision acuity (line is detectable but blurred)
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4.4
Acuity Variables
Several variables affect vision acuity including target movement, lighting, target angle, target knowledge and psychophysics. Information about these variables is helpful for quantifying visual performance standards for measuring test system sensitivity. 4.4.1 Kinetic Vision Acuity Near vision acuity examinations are performed with the eye chart (target) in a stable position. Performing visual testing tasks that require the object to be scanned results in a dynamic observer- or video camera- to target movement. The term kinetic vision acuity' is used for acuity measurements with a moving target. Studies indicate that 10 to 20 percent of visual efficiency can be lost by target movement.
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4.4.2
Lighting and Target Angle
Near vision acuity tests are performed under uniform lighting and on targets that do not cast shadows. Because the target characters have a uniform luminance contrast for both the figures and the background, near vision acuity tests are not designed to measure detection of detail in rough surface topographies such as welds. While a visual testing specification may specify a viewing angle, near vision acuity charts are made with the eye or video camera perpendicular to the target, resulting in optimum vision acuity
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4.4.3
Target Knowledge
Target knowledge is the key feature for detection and recognition. Targets such as letters, numbers and straight lines are simple for human recognition, especially on a uniform background. Such targets have little transferability for the discontinuities of interest during a visual weld test. In fact, there are several different near vision acuity tests based on varying targets. One of the problems with using well known patterns such as letters is that the individual may be responding to visual clues and filling in a partially visible pattern deduced from letter shape. This is known as closure. Optometrists score and measure vision acuity based on both the number of errors and response time. Additionally, vision acuity is a function of the observer's acuity for a given time. For example, acuity may be diminished if the observer has been performing strenuous detailed vision tasks. Because of these variables, near vision acuity is not a precise quantified measurement but one having the accuracy required to fit a high probability of eyesight correction to the 20/20 standard. Variability in eyesight measurement is such that a 10 percent difference in measurement is possible based on the type of acuity test and the individual's performance for that time.
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4.4.4
Psychophysics
Psychophysics is the interaction between vision performance and physical or psychological factors. One example is the so-called vigilance decrement, the degradation of reliability based on performing visual tasks over a period of time. If not identified as a significant variable and controlled, vigilance decrement can result in diminishing visual performance Keywords: vigilance decrement, the degradation of reliability based on performing visual tasks over a period of time
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4.5
Reserve Vision Acuity and Visual Efficiency
The 20/20 standard for near vision acuity is a baseline designed in the late 1800s as a means for standardizing eyesight relative to the ability to read fine print and to provide a means for prescribing corrective lenses. The standard was not intended to identify vision acuity relative to the detectability of fine lines such as cracks but measurements of near vision acuity are transferable to the ability to detect cracks. Visual systems with a near vision acuity of 20/20 can detect cracks with widths of 10 Îźm (0.4 mil) on polished surfaces. Such systems can detect hairline weld fractures with widths near 25 Îźm (1 mil) in the toe of a weld. The term reserve vision acuity refers to the ability of an individual to maintain acuity under poor viewing conditions. u An individual with 20/20 near vision acuity observing under degraded viewing conditions has considerable reserve vision acuity compared to an individual with 20/70 near vision acuity. The term visual efficiency uses 20/20 near vision acuity as a baseline for 100 percent visual efficiency. The concept is useful for defining the reliability of a visual system based on detection relative to visual efficiency.
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4.6
Performance Standards for Visual Weld Testing
In addition to specific calibration or verification standards, the majority of nondestructive testing specifications include use of test objects with known discontinuities. These reference standards have intentionally fabricated discontinuities or discontinuities from production cutouts. Reference standards with known discontinuities have three disadvantages: (1) procurement of the test object, (2) validity of the test object and (3) standardization of discontinuity sizes. Fabrication of tight visual cracks is controllable and such standards can be manufactured or purchased for a reasonable cost. The simplest method for creating a tight visual crack is to butt two highly machined plates together with a surface weld head minimally joining the faying surface. The weld is then broken and the plates reassembled mechanically or by tack welding (see Figs. 7 to 9). Another method for creating toe cracks is to fabricate a highly restrained welded object with an invisible crack that becomes visible as the plate cools. These two methods produce toe weld cracks that are representative of the most prevalent inservice welding discontinuity. Charlie Chong/ Fion Zhang
FIGURE 7. Two plates machined to a 4 (100 nm) rms finish and bolted together to appear as one plate
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FIGURE 8. Same plate as in Figure 8 with 0.025 mm (0.001 in.) width separation achieved by spacing with a feeler gage
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FIGURE 9. Same plate as in Figure 8 with a purposely cracked surface weld bead at the faying surface; can be used to show the effects of line of sight and width opening; separation here is 0.5 mm (0.02 in.) to show fabrication method
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Transverse cracks can be fabricated by grinding a notch transverse to the weld and then filling the notch with copper. When a stringer bead is run over the copper, a tight visual transverse crack is produced. Transverse cracks can also be produced by restraining high tensile low alloy steel such as A514 or A517 and welding with 10018, 11018 or 12018 weld electrode. The amount of restraint and other variables, such as the moisture content of the weld electrode flux and inadequate preheat or post heat, will determine the size of the cracks created. The use of the weld discontinuity reference standards has two significant advantages: (1) they represent actual conditions that cannot be accurately simulated by vision acuity eye tests or line detection and (2) reference standards are critical for training inspectors in pattern recognition as well as proper detection and evaluation methodology Plastic reference standards replicated from actual cracks are sometimes used in training programs. These plastic standards are convenient and transportable but they often lack the realism essential for effective training.
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4.7
Use of Visual Reference Standards
Visual testing inspectors with 20/20 or 20/30 near vision acuity in one eye should reliably pass the 8 Îźm (0.32 mil) detection test (based on transferring predicted acuity to detection). Therefore, for sensitivity verification, the line test does not provide a means for determining which visual inspectors cannot detect actual cracks. In other nondestructive testing techniques, verification and practical tests are designed to determine sensitivity and can result in some personnel failures (the pass/fail rate is dependent on the testing technique and the application). For example, there is typically a greater pass rate for ultrasonic discontinuity detection. Likewise, the pass rate for an intergranular stress corrosion crack detection program can produce a lower pass rate than typical ultrasonic detection methods. A greater pass rate can be expected for generic magnetic particle testing than for ultrasonic tests.
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Therefore, a reliable method must be established for qualifying visual inspectors for crack detection- one solution is to use valid visual reference standards containing the types of cracks predicted and required to be detected. Since 1985, major oil and gas companies have used performance demonstration programs to test underwater inspectors for nondestructive testing qualification. These programs are strongly based on practical demonstration for proficiency. The reference standards typically contain non visual magnetic particle testing indications. In 1990, oil and gas company operators placed additional emphasis on visual testing and instituted a program using visual reference standards for performance demonstration. Magnetic particle and visual testing trials were carried out concurrently because the two methods are complementary for underwater weld tests.
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Before visual and magnetic particle testing, personnel were given basic near vision acuity and color recognition screening tests. Near vision measurements were recorded for each eye and for both eyes. The majority of diving personnel fell into the 20/20 to 20/30 range with a small percentage in the 20/40 to 20/50 category. In some cases, diving personnel with minor near vision acuity deficiencies do not wear corrective lenses (wearing bifocals in the diving helmet is somewhat tedious and uncomfortable). Contact lenses are not recommended for diving. The initial qualification program indicated that personnel with near vision efficiency less than 20/20 did not perform detection of cracks as well as those near 20/20. The initial observation was based on a small sample population but was noted as a potential problem. Subsequent testing of a larger population indicates that individuals with less than near 20/20 have significantly poorer detection ability.
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There are several integrated visual testing variables beyond equating near vision acuity to performance, including 1. 2. 3. 4. 5. 6. 7.
knowledge of crack pattern recognition, knowledge of scanning techniques, lighting, orientation of the test objects, test instructions, feedback from the test administrator and psychophysical factors.
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4.8
Knowledge of Crack Pattern Recognition
The tour main weld crack categories are weld toe, transverse, face and heataffected zone cracks. Most nondestructive tests require the observer to focus attention on reasonably well defined targets and patterns. In visual tests for weld discontinuities, detection is constrained by a lack of knowledge about the patterns to be detected. Tight hairline weld cracks are not well defined targets and can be discrete, based on their position in the weld. If reference standards or photographic examples are not used, the inspectors' reliability of detection is determined almost solely by experience. For inservice weld tests, the frequency of fine cracks is small and does not provide a high degree of pattern recognition based on experience. Detection of relatively gross cracks with tight ends may supply the inspector with some knowledge of hairline crack recognition. Training programs should include discontinuities that have crack like appearances but cannot be fully evaluated without supplemental nondestructive testing. This results in lowering the false positive alarm rate (there are some weld conditions that have suspect crack like appearances). Although visual testing is often considered a stand alone test, knowledge of penetrant testing or magnetic particle testing is essential.
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4.9
Scanning Techniques
Detection is a function of both scanning coverage and speed. Other nondestructive testing techniques define these parameters by physically positioning a probe or the measurement material. Visual testing is primarily noncontact and controls for scanning and coverage are difficult to quantify. Coverage and scanning rate are determined by the type of discontinuity to be detected. Fine crack detection requires considerably more control than detection of gross cracks, to ensure 100 percent area coverage at a reasonable speed. While the human eye and machine vision have required sensitivity to detect extremely fine cracks, the vision system must be positioned in the proper orientation to detect the discontinuity. If a hairline crack is present in the weld toe opposite from the primary viewing angle, there is a high probability that the crack will be missed.
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Because most visual tasks do not require a high degree of angular probing, there is a tendency for inspectors to view the weld from a single position, resulting in a measurable loss of test sensitivity. This problem can be minimized by specifying that the inspector view the weld from several different angles. Visual testing is analogous to ultrasonic crack detection, in which the probability of detection is increased by using different angle probes and defining maximum scanning speeds.
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Scanning coverage requires a visual mapping plan to compensate for the fact that the human memory is not optimally equipped for scanning tasks. While the brain is processing the area under test, little data can he retained about past coverage and no hard copy documentation is produced to show the coverage area. In some qualification tests, crack reference standards are usually placed with the cracks opposite the observer's initial viewing angle. If the inspector does not visually scan the test object from more than one angle, the crack is usually missed. When the reference standard is placed so that the crack is in direct line of sight, crack detection significantly increases.
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4.10
Lighting
Lighting for visual testing has two functions: (1) providing luminance contrast for discontinuity detection and (2) illuminating the object to assist in scanning guidance. There are subtle lighting thresholds at which cracks become detectable and it may accurately be said that optimal lighting conditions increase visual sensitivity. Some visual testing specifications require 500 Ix (50 ftc) at the test site but light characteristics (hue, for example) are not given. Many specifications refer only to light levels adequate for test inspection. Other nondestructive testing specifications require visual detection of physical indications, as in magnetic particle and liquid penetrant testing." Such techniques typically require 1,000 to 2,000 lx (100 to 200 ftc) on the viewing area. In addition, these surface techniques often make use of high contrast backgrounds to maximize indication detection. The 500 lx (50 ftc) minimum does not give optimum visual detection.
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The other key feature of artificial lighting is that the lighted area provides a guidance system for the visual testing and aids the mapping required for coverage. With video systems, use of side lighting can further optimize visual testing when component geometry creates shadows that degrade visual performance. Simple empirical tests on the effects of lighting can be performed using reference standards. In qualification tests, lighting is an important variable that has a measurable influence on performance.
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Florescence MPI
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Dye Penetrant Testing
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4.11
Practical Qualification Requirements
Performance verification can be achieved using three distinct methodologies. 1. Use a completely quantified test regime in which performance is measured with no prior cues regarding test object information (sometimes referred to as a blind test). 2. Use a written review to provide guidelines on means to optimize visual testing techniques and basic test object characteristics without giving knowledge of the discontinuities. 3. Provide the test candidate with some degree of real-time performance feedback.
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The blind test regime represents the most severe environment for visual testing. For quality assurance reasons, the blind test may be required but there are drawbacks to exclusive use of blind testing programs. Many visual testing qualification programs benefit by providing test candidates with some knowledge of required detection criteria. The real time feedback process allows the test administrator to determine if lack of detection is an eyesight acuity problem or a matter of poor technique. In some cases, when the candidate is given the exact location of a hairline crack, the individual still cannot trace the crack. This strongly indicates that lack of detection is a vision acuity problem that can he remedied with corrective lenses and retesting. If the missed crack can be traced once the candidate has knowledge of the location, it can be possible that improper technique was a factor.
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Detection may be such that the candidate's recognition is at a threshold level and a range of crack sizes should be used. The larger crack widths are first used to separate technique problems from vision acuity deficiencies. One unique feature of underwater testing is the presence of an oral nasal mask in the diving helmet. This oral nasal mask is in the field of view when performing close visual tasks and can create potential problems with visual disturbance and binocular vision. This is especially true when visual tasks must he performed for long periods of time.
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Expert Diver at works
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Expert Diver at works
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Expert Diver at works
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4.12
Remote Visual Tests
Remote visual testing is used in hostile environments unsafe for human intervention or in areas of inaccessibility. All of the variables that apply to direct visual testing can be applied to remote visual testing. The main differences are: (1) some loss of depth cues caused by the two-dimensional medium, (2) more difficulty in scanning the test site with full coverage line of sight and (3) inability to easily implement supplemental nondestructive tests. Of these, the inability to use supplemental nondestructive testing is the most severe constraint. This is critical because a percentage of visual targets that appear as crack-like discontinuities cannot be separated into non relevant or relevant indications without additional nondestructive testing. Although the number of suspect crack targets may be low, the inability to provide evaluation with a high confidence level is a significant limitation of the remote visual testing method. The potential for false positive alarms must be critically evaluated before effecting a remote visual testing plan over a direct visual testing plan. Charlie Chong/ Fion Zhang
Keywords: ď Ž some loss of depth cues caused by the two-dimensional medium, ď Ž more difficulty in scanning the test site with full coverage line of sight and ď Ž inability to easily implement supplemental nondestructive tests. Of these, the inability to use supplemental nondestructive testing is the most severe constraint. This is critical because a percentage of visual targets that appear as crack-like discontinuities cannot be separated into non relevant or relevant indications without additional nondestructive testing.
Charlie Chong/ Fion Zhang
The line detection procedure is often used to qualify video systems. This is a poor test because the line can be detected at acuity levels much lower than the optimal 20/20. Image size should be consistent with magnification allowed only to aid in acuity. Magnification levels should be set so that features necessary for recognition are still identifiable. Other tests cited in the use of video quantification are a variety of resolution tests. Video cameras are often stated to be high resolution. The term high resolution can be misleading if thought to be based on actual image quality using a specific system. Resolution is a function of the complete system's ability to resolve minimum line pairs. As a visual standard, line pair resolution can be quantified with greater precision than simple vision acuity tests. However, line pair resolution is a relatively cumbersome concept. It must be determined if 400 horizontal television lines are adequate for detection and with what ease the visual inspector can identify resolution with crack detection.
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The few visual standards that reference remote visual testing state that remote visual tests must be equivalent to direct visual requirements. This implies that the video system should have a near vision acuity equivalent to that of direct visual testing. In 1990, the term equivalent 20/20 near vision acuity was introduced to resolve this inconsistency for remote video testing systems. 16 Most near vision acuity cards are designed to be read at a defined distance. Using the equivalent 20/20 near vision acuity criterion, the observer reads the near vision acuity 20/20 characters on a video monitor. Camera distance from the object is not critical but field of view and depth of field are set according to the needs of the test. For example, a specification may require equivalent 20/20 near vision acuity for a 100 cm 2 (16 in.2 ) viewing area with a depth of field of 25 mm (1 in.). If the depth of field is extremely shallow, constant focusing is required and this produces operator fatigue. Medium wide angle lenses with high f-stops (achievable with high intensity lights) usually produce equivalent 20/20 near vision acuity. The 20/20 standard is better for remote than 20/30 because of the inherent loss of visual sensitivity caused by some lost depth cues. In fact, 20/10 is a preferable acuity but magnification should not be so great as to remove key pattern recognition features. Charlie Chong/ Fion Zhang
As stated, 20/20 near vision acuity is a good guideline for hairline crack detectability. However, use of actual cracked test objects is preferred for performance testing and training. In remote visual testing, both 20/20 near vision acuity characters and reference standards with specified discontinuity sizes can often be mounted on the robotic system for performance checks. On steel structures, where remote video is performed using manipulators, 20/20 near vision acuity characters can be magnetically positioned at test sites to verify visual performance.
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Remote Visual Tests
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Remote Visual Tests
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Remote Visual Tests
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Remote Visual Tests
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4.13
Other Factors Affecting Perception
Most nondestructive testing techniques require specific learning regimes while visual testing is wrongly assumed to be an innate human process. There are several factors that make individuals good observers and these factors are sometimes counterintuitive. In one case, for example, the best observer had poor near vision acuity but was able to find objects more rapidly than observers with good eyesight. A plausible explanation lies in the fact that the test objects were more discernible from the background when slightly blurred. This is not the case for detection of hairline cracks but it does confirm the complexity of using the human vision system.
Charlie Chong/ Fion Zhang
Visual inspectors are highly encouraged to discuss vision considerations and problems with an optometrist. However, it should be recognized that optometrists are trained to examine eyes and not welded test objects. Interesting results are sometimes obtained when optometrists are given near vision acuity tests using unfamiliar crack reference standards. In one experiment, an optometrist with 20/20 near vision acuity was unable to detect a hairline crack and unable to trace the crack after given its location. The crack reference standard was validated as detectable when the optometrist's six year old daughter was able to detect the crack without prior knowledge of its location.
Charlie Chong/ Fion Zhang
4.14
Recommendations for Visual Weld Testing
There is a need to improve training and standards for visual weld testing. Personnel qualification and certification in visual testing were first formalized by ASNT in 1988 with completion of a training outline but there is no ASNT certification as such for visual inspectors. The American Welding Society's Certified Welding Inspector Certification is a broad program of which visual testing for cracks is a small part. The majority of visual testing specifications are typically short (one or two pages) and provide minimum performance criteria that can be applied to practical conditions. The following recommendations have proved to be effective. 1. Adopt a policy of using valid crack reference standards for training and verification of a system's performance sensitivity. 2. Use a 1,070 lx (100 ftc) minimum for lighting on the test site.
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3. Require scanning based on predicted line of sight for the discontinuity 4. Require yearly eye tests with a minimum of 20/30 near vision acuity in both eyes. The eye tests should be performed by an optometrist. 5. Remove the detection of a 8 Îźm (0.32 mil) wide line as a visual testing performance standard. 6. Develop courses and specifications especially designed for weld testing.
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4.15
Conclusion
Improved training and specifications for visual weld testing which address vision acuity, lighting and line of sight requirements will measurably increase visual testing sensitivity. More attention must be placed on detection methodology- present training tends to focus on categorizing discontinuity types. Because the training issues are academically fundamental, visual weld testing methods will be easy to implement. Enhancement and miniaturization of video hardware will allow equivalent 20/20 near vision acuity testing using remote techniques. Career testing personnel should have yearly check-ups by optometrists to ensure there are no vision problems which can affect visual testing. Without yearly testing, vision degradation may go unnoticed and may be more difficult to correct.
Charlie Chong/ Fion Zhang
Charlie Chong/ Fion Zhang