VISUAL TESTING
II. THEORY AND PRINCIPLES
Object Factors
In order to understand the physics of vision, it is necessary to first consider the character- istics of the eye. The eye can be compared to a radiation detector. Different wavelengths of light travel through the lens and reach the retina, which is located at the back of the FIGURE 3-3 Typical fiberoptic borescope. (Courtesy of Olympus Industrial, with permis- sion.)
eye. The rods and the cones of the retina in the human eye can sense wavelengths from about 400 nm up to approximately 760 nm. The eye performs the function of a spectrum analyzer that measures the wavelengths and intensity, as well as determining the origin of the light (from the sun or an artificial source). The light strikes the object to be viewed and is reflected towards the eye, through the lens and onto the retina as an image. The brain analyzes this image. The retina is similar to an array of tiny photosensitive cells.
Each of these elements (cells) is connected to the brain through individual optic nerves.
The optic nerves linking the eye to the brain can be compared to a bundle of electric ca- bles. The major parts of the eye are shown in Figure 3-5.
The iris opens and closes, thus varying the amount of light reaching the retina. The light then passes through the lens, which by changing shape, focuses the light and pro- duces the image on the retina at the rear of the eye. Here a layer of rods and cones are found. The neurological connection from the rods and the cones pass through the rear of the eye via the optic nerve, which transmits the neurological signals to the brain. The brain processes the signals as perceptions of colors and details that vary in light intensity and color. It is necessary for a certain minimum level of light to be present before the eye can produce an image. This level is known as the “intensity threshold.” Contrast is some-
FIGURE 3-4 How charge coupling works.
thing that shows differences between images placed side by side. Lighting requirements are frequently expressed in terms of ratios, due to the eye’s ability to perceive a percent- age of change rather than an absolute change in brightness. The retina will only retain an image for a certain amount of time. This varies according to the size of the object and speed at which it is moving. The main limitations of vision are intensity threshold, con- trast, visual angle, and time threshold.
Visual acuity is the ability to distinguish very small details. For example, as the distance from the eye to the object increases, two lines that are close together appear as one heavy, dark line. The normal eye can distinguish a sharp image when the object being viewed sub- tends an arc of one-twelfth of a degree (five minutes), irrespective of distance from the eye to the object. Practically speaking, a person with “normal” vision would have to be within eight feet of a 20-inch TV monitor to resolve the smallest detail displayed.
White light contains all colors. Newton proved that color is not a characteristic of an object but, rather, various wavelengths of light that are perceived as different colors by the eye. Color can be described as having three measurable properties: brightness, hue, and saturation. The color of an object, ranging from light to dark, emitting more or less light, is known as brightness. Different wavelengths give us different perspectives of col- ors; this is known as hue. How green something is as opposed to white, is how saturated it is with green.
In the United States NDT environment, visual acuity examinations are a requirement for certification. The visual inspector’s natural visual acquity must be examined. The Jaeger (J) test is used in the United States for near-distance visual acuity. It consists of an English language text printed on an off-white card. The parameters for near-distance visu- al acuity are described in personnel certification and qualifications programs. Visual acu- ity requirements will vary depending upon the needs of specific industries.
Light
For decades, light wavelengths have been measured in angstrom units (10–10meters) and currently are also measured using the nanometer (nm), which is 10–9meters. Brightness of light is an important factor in test environments. The apparent brightness of a test sur-
FIGURE 3-5 Key components of a human eye.
face is dependent on the intensity of the light and the reflectivity of the surface reflecting the light to the eye. Excessive brightness will interfere with the ability to see and will
“white out” the object. Inadequate light can cause excessive shadows and result in insuffi- cient light being reflected from the surface, preventing observation of the surface attrib- utes. Some codes require a minimum intensity of 15 foot candles (fc) for general visual testing and a minimal 50 fc for critical and fine detail viewing. The Illumination Engi- neering Society requires 100–300 fc for critical work. The inverse square law governs the intensity of light noted or measured. It states that illuminance (E) at a point on a surface varies directly with the luminous intensity of the source (I) and inversely as the square of the distance (d) between the surface and the source (see Equation 1 and Figure 3-6).
E = (3-1)
where:
E= luminance
I= intensity of the source
d= distance between the source and the surface
This equation is accurate within 0.5% when dis at least five times the maximum dimen- sion of the source, as viewed from the point on the surface.
Cleanliness
The amount of light that reaches the eye from an object is dependent on the cleanliness of the reflecting surface. In visual testing, the amount of light may be affected by distance, reflectance, brightness, contrast or the cleanliness, texture, size, and shape of the test ob- ject. Cleanliness is a basic requirement for a successful visual test. Opaque dirt can mask or hide attributes, and excessively bright surfaces cause glare and prevent observation of the visual attributes.
Brightness
Excessive brightness within the field of view can cause an unpleasant sensation called glare. Glare interferes with the ability to see clearly and make critical observations and judgments.
Surface Condition
Scale, rust, contaminants, and processes such as milling, grinding, and etching may affect the ability to examine a surface. This will be further discussed in the section relating to tools, equipment, and accessories.
ᎏI d2
FIGURE 3-6 Inverse square law.
Shape
The shape of an object can influence the amount of light reflected to the eye, due to vari- ous angles that can determine the amount of light that will be reflected back to the eye.
Size
The size of an object will determine the type of scan pattern that may be used to view 100% of the object or it may determine that some magnification is necessary to get a clos- er view of details otherwise unobservable.
Temperature
Excessive temperature may cause distortion in viewing due to the heat wave effect. Most are familiar with the heat waves coming off a desert, resulting in a mirage; this is known as “heat wave distortion.” In a nuclear reactor, underwater components are frequently dis- torted due to heat waves rising from the core that can interfere with the view from an un- derwater camera as it scans a particular component during visual examination.
Texture and Reflectance
One of the greatest variables in viewing an object is the amount of light that is reflected from it, and the angle at which the light strikes the eye. Excessive rust or roughness can cause diffusion of the light and limit the light returning to the eye. This can easily be cor- rected by increasing the amount of light or improving the surface condition of the object under examination.
Human Factors Environmental
The amount of light required for a visual test depends on several factors, such as speed or accuracy, reflection from background, and other inspection variables. These inspection variables include physiological processes, psychological states, and the inspector’s experi- ence, health, and fatigue. All of these factors contribute to the accuracy of a visual inspec- tion. One of the key factors in viewing a lighted object is the difference (contrast) between the light on the object and the background. A contrast ratio of 3:1 between the test object and the background is desirable. If the background is dark, a ratio of 1:3 is recommended between the test object and lighter surroundings, 3 being the most intense light in both cas- es. Psychological factors can also affect a visual inspector’s performance. Surrounding col- ors and patterns can have an effect on the inspector’s attitude. Dark walls can absorb up to 50% of the light. A high contrast on the pattern being inspected can cause eye fatigue. It is recommended that blue colors be utilized and brilliant colors avoided.
Physiological
The act of seeing something is not a passive activity. The observer must be active in keep- ing track of what is going on. Constant eye shifting back and forth from one location to another or scanning a large area at a rapid speed causes the muscles in the eye to fatigue.
If the eye isn’t focusing quickly when changing directions, the image can be lost altogeth- er. Any fatigue on the part of the observer can result in reduced efficiency and accuracy in interpreting the visual data.
Psychological
Individuals can be in various psychological states. They can be suffering from tensions, emotions, and other influences. These may influence the appraisal and ability to visualize
an object and may also influence performance of a visual task. The intention of a viewer may affect perception. A great deal of information is potentially available immediately af- ter viewing, but if one does not expect to find certain attributes, one may well overlook the physical evidence that has been viewed and not perceive them. One of the ways to overcome this is to know ahead of time what to expect, the attributes of what is to be seen, and what the greatest possibility is of these attributes existing.
Perception
The ability of the eye to sense a variety of views is not constant. If one is well rested in starting out an inspection activity, perception can be greater than when one is fatigued.
Grossly changing light levels may cause painful glare. After a long period of relative darkness, normal light may seem painful. (Normal light is that which is comfortable to the eyes.) Sudden exposure to full sunlight can cause discomfort. It may take up to 30 min- utes for the iris to adjust and regain normal vision. As the iris becomes tired and the mus- cles that adjust the lens become fatigued due to age, overuse, drugs, disease, or emotions, vision can be greatly affected. Vision examinations are usually administered annually to provide assurance that the inspector meets the requirements for performing VT. Daily in- fluences external to the inspector, such as emotions, drugs, excessive light, inadequate sleep, etc., can cause temporary loss of visual acuity.
Another influence on perception is appearance. The two lines represented in Figure 3- 7 appear to have different lengths due the perceptions created by the “Vee” extensions on each end. In fact, the two lines are identical in length; the brain perceives the lengths dif- ferently.
Visual Angle and Distance
Visualize two pencils representing two parallel lines. If these are perpendicular to the incident angle of view, two pencils can be clearly seen. As the two pencils rotate and become relatively parallel to the incident view of the eye, the two pencils appear to be- come one, since one is now behind the other. The quantitative ability of a person to re- solve objects is determined in a practical manner from the distance of the object to the eye and the angle of separation of two points that can be resolved by the eye. This is known as resolving power. For the average eye, the minimal resolvable angular separa- tion of two points on an object is about one minute of arc (1/60th of one degree). This means that at about 12 inches (300 mm) from the test surface, the best resolution to be expected is 0.0035 inches (0.09 mm); at 24 inches (600 mm), the best anticipated reso- lution is about 0.007 inches (0.18 mm). The best result from a visual test is obtained
FIGURE 3-7 Which line is longer?
when the object is brought close to the eye and a large visual angle is obtained. There is a limitation to distance; the eye cannot sharply focus if it is nearer to 10 inches (250 mm) to an object. Therefore, direct visual tests are best performed at a distance of 10–24 inches (250–600 mm). Also of importance is the viewing angle between the line of site that the eye makes with the test site and the plane of the test surface. As de- scribed above, two pencils can become one as the viewing angle of the eye changes. For practical consideration, this angle should not be less than 30° of view off the plane of the surface under inspection, as shown in Figure 3-1.