Introduction
The human eye is one of mankind's most fascinating tools. It has greater precision and accuracy than many of the most sophisticated cameras. It has unique focusing capabilities and has the ability to work in conjunction with the human brain so that it can be trained to find specific details or characteristics in a part or test piece. It has the ability to differentiate and distinguish between colors and hues as well. The potential of this valuable tool, however, is most often overlooked. To examine parts, many companies employ or specify tests that are very expensive to perform and require expensive and elaborate equipment. In many cases, a simple visual examination would serve the purpose in a very satisfactory manner.

 

Common Usage of Visual Inspection
Almost any visual characteristic that can be enumerated can be assessed by the human eye or the human eye with a visual aid. The eye can perform accurate inspections to detect size, shape, color, depth, brightness, contrast, and texture. Some industries have already realized the value of this type of inspection with respect to visual characteristics and dimensional requirements and have mandated specific requirements within the applicable codes and standards. Many inspection factors have been standardized so that categorizing them as major and minor characteristics has become common. Surface finish verification of machined parts has even been developed, and classification can be performed by visual comparison to manufactured finish standards. In the fabrication industry weld size, contour, length, and inspection for surface discontinuities are routinely specified. In fact, American Welding Society (AWS) mandated inspection of welds under AWS D 1. 1 Structural Welding Code using a qualified, certified weld inspector (CWI) in 1977. Many industries balked at the idea that all inspectors would need certification and shortly thereafter the document was relaxed to express, "when required by contract." Since then many companies have mandated the need for qualified and certified visual weld inspection. This is the case particularly in the power industry, which requires documentation of training and qualification of the inspector. Forgings and castings are normally inspected for surface indications such as laps, seams, cold shuts, mismatch, and other various surface conditions. 

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Without adequate vision, even the best trained inspector may perform poorly.

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Nondestructive testing is another area where visual inspection is frequently used. Penetrant inspection using visible red or fluorescent dye relies on the inspector's ability to identify surface indications. Magnetic particle inspection falls into the same category of visible and fluorescent inspection techniques, and radiography relies on the interpreter's visual judgment of the radiographic image - either on film or on a video monitor. Recently, microscopic inspection has greatly expanded and many industries now use video cameras to image and magnify parts for inspection. The electronic IC and video board market utilizes microscopic and visual inspection to a large degree. Surface temper (nital etch) inspection is another form of nondestructive inspection that has been employed for a number of years. It is used extensively on aircraft parts and ground gear teeth. This involves acid etching of part surfaces to detect evidence of grinding burn or overheating. The indications are usually white on gray, black on gray, or gray on a lighter gray background.

The nuclear power industry has long realized the value of visual inspection and developed the four different code requirements VT-l through VT-4, later consolidated into three categories, for visual inspection during in service inspection. The sites for inspections include welds, hangers, valves, pumps, pipe restraints, supports, snubbers, spring sway braces, and integral components attached to critical systems. Many of these inspections are used to identify erosion, corrosion, loose or missing parts, adjustments, and settings. Leak testing, either hydrostatic or soap bubble type, is another discipline that requires visual observation to determine conformance to code requirements and is a routine inspection during in service inspection and plant start up. Other power and petroleum industry inspections involve visual inspections of tanks and storage vessels, since leakage is costly from production and environmental standpoints.

Because visual inspection is so frequently used, several companies now manufacture gages to assist visual inspection. Welding fabrication uses fillet gages, hi-lo gages for fit-up, undercut gages, angle gages, skew fillet weld gages, pit gages, contour gages, and a host of other specialty items to ensure product quality. At times remote viewing is necessary. Mirrors, borescopes, fiber optic scopes, telescopes, and remote cameras are necessary to perform inspections where direct observation is impossible.

 

Visual Inspection in the Coating Industry
Visual inspection is the predominant tool in the coating industry. The base metal may be in a variety of different conditions ranging from clean to scaled, rusted, or heavily pitted and rusted. The Steel Structures Painting Counsel (SSPC) has developed color visual standards regarding various base metal conditions. In addition to these color visual standards are sand or shot blasted standards such as commercial, near white, or white metal blast. These assist inspectors in verifying surface cleanliness before coating. After coating, visual inspection is performed to identify under thickness, sags, mud cracking, blistering, delamination, pores, and numerous other conditions that may be present in the top coating layer. In service visual inspection is also a valuable tool for coatings. Conditions that can be corrected can be identified before a total failure occurs in the coating. Erosion, corrosion, improper or lack of cathodic protection, mechanical damage, and other detrimental conditions are easily detected visually.

 

Visual Requirements In Nondestructive Testing
A number of quality control tools can be used in nondestructive testing to verify inspection conditions. Initially penetrants were examined by placing a drop of new and used penetrant next to each other on a paper towel for a visual comparison. Now fluorescent penetrant inspection employs photofluorometry to determine the brightness of penetrant materials. Most penetrant specifications require replacement of the penetrant when the brightness drops more than 10 percent. However, in these specific test situations, the human eye can only detect a change of about 15 percent or more in brightness. Because lighting conditions impact the effectiveness of a visual examination of a part, the intensity of visible white light and blacklight is verified with calibrated light meters to ensure that there is enough light at the inspection surface. Normal visual inspection is typically performed in the range of 50 to 100 foot candles (normal office lighting). Inspection of fine detail may require up to 2 x 10³ lx (200 ft. candles) (100 W mercury arc spot type bulb). Blacklight (UV) inspection uses high intensity bulbs with a coating or filter in front of the bulb. This produces blacklight with a wavelength of about 365 nanometers (1.4 x 10² µin.) The measurement of these intensities is of the order of 800-1200 µW (124-186 in.²) minimum, depending on what specification requirements are invoked. Signal to noise ratio is the factor which determines whether or not a discontinuity is detected. For this reason, testing materials are made so that they provide a contrast between the discontinuity and the balance of the part surface. Dry powder magnetic particles can be made red to provide a contrast with a gray shot blasted surface on a casting. Visible red dye penetrants are developed with a white chalky developer to provide a high contrast with the background. Fluorescent penetrant and magnetic particle materials stand out as a bright yellow green when excited by the blacklight, while the background remains bluish black or a more pale yellow green. Sensitivity panels and test pieces are used to confirm the acceptability of a test system and demonstrate that the defect can be seen against the part background.

Radiography is another common test method used on many different product forms. Radiography detects a change in thickness down to about one percent. The orientation and morphology of the void determine the degree of detectability. Light gray defects on a light gray background provide little contrast and can hamper detection. Standard sensitivity devices called penetrameters are placed on the part or shim blocks approximating part thickness next to the part and imaged along with the production part. Penetrameters are manufactured to represent either one or two percent of the part thickness. In addition, holes are drilled in the plaque that are 1, 2, or 4T in diameter (T represents the thickness of the penetrameter). The imaging of the outline of the penetrameter and the applicable T hole qualifies the radiograph and allows interpretation of indications detected on the radiograph. The human eye can detect or distinguish up to about 64 shades of gray. New computed radiography allows the computer to evaluate up to 4096 shades, and thus it provides a method to detect items missed by human inspection.

 

Vision Testing Standards
Vision testing of inspectors has been in use for about 40 years. Along with the development of the present day standards and specifications, many changes have taken place in the testing methods. New revisions have incorporated much of the new technology developed during the recent years. Interestingly, vision testing has changed little over time. Table 1 illustrates the different vision requirements that have been based on the present governing standards. Note that little has been done to standardize any of the vision tests used in the industrial sector.

Table 1 - Different vision requirements that have been on the present governing standards
Industry Study	Near	Far	Color Vision	Brightness Discrimination
SNT-TC-1A 1992	J2*	-	Differentiate	-
SNT-TC-1A 1975	J2	-	Differentiate	-
SNT-TC-1A 1968	J1	-	Differentiate	-
NAVSEA 250-1500	J1	-	Differentiate	RT contrast
MIL-STD-271	J1	-	Differentiate	RT contrast
MIL-STD-2132	J1	-	Differentiate	-
ASME SECTION III	J1	20/20	Differentiate	-
ASME SECTION XI	J1	20/20	Differentiate	0.79 mm (0.31 in.) Line on gray card
MIL-STD-410E	J1	-	Differentiate	-
AWS D1.1-96	20/40	20/40	Differentiate	-
*Ortho-rater 8 equivalent

Present vision tests measure the visual function at high contrast level, a level that does not occur in visual inspection tasks. For example, radiographic personnel should have the ability to differentiate radiographic contrast. NAVSEA 250-1500 and MIL-STD-271 address the need to perform a test to verify the inspector's capability, but there is no test method or acceptance criteria specified. The standards simply state that the film interpreter should be able to discern reject discontinuities of varying sizes and densities.

Many companies have radiographic penetrameters of various sizes and have asked the inspector to identify the smallest hole they can discern. One company imaged a blank penetrameter with no holes, and some inspectors actually identified holes in the blank. These false responses illustrate the need for better standardized tests to qualify inspection personnel.

Visual recognition capability at low contrast levels can be quickly and repeatedly measured using low contrast visual acuity charts. This visual function is called contrast sensitivity. Large objects are more easily seen at lower contrast levels than small objects and thus the boundary between objects seen by a person and those invisible is depicted by a curve (see Figure 1). In visual inspection the defects are often small and at low contrast. Therefore, our interest in evaluation of vision is in the right end of the curve which is nearly a straight line. Like any straight line, it is defined by two points, the one at high contrast is the visual high contrast acuity, the other is visual acuity at the one to two percent level. If both of these values are measured, even the slight bend of the normal curve is recorded.

Figure 1 - (a) Large objects are visible at lower contrast levels than small objects. In this picture contrast decreases toward the upper edge of the picture. Close to the X axis the numbers are at high contrast. The area where the low contrast numbers disappear is the boundary between objects seen and those invisible. (b) This boundary is called contrast sensitivity curve. The right end is the usual high contrast visual acuity value and the upper part can be assessed by measuring visual acuity at the 1 percent or 2 percent level.

Since there is variation in the declination of the right hand slope of the curve, a person with better high contrast visual acuity may have a lower low contrast acuity than another person with better high contrast acuity (see Figure 2). This means that visual acuity at high contrast does not always predict the quality of low contrast vision. This needs to be measured. This measurement is equal to the routine measurement of visual acuity.

Figure 2 - There is variation in the contrast sensitivity values of normally sighted persons. Also the declination of the right hand slope varies. Therefore, person A may have lower visual acuity at high contrast but better vision at low contrast than person B who has better visual acuity at high contrast level. This is an important finding, for example, when choosing people for tasks where small objects need to be detected at low contrast.

In a contrast sensitivity test, the person is asked to tell which number or letter (on an optotype chart) is the first or last one on each row until there is an incorrect answer or hesitation. The person is then asked to read all the numbers or letters on the previous line. If that line is read correctly, the person is asked again to try and read the next row with smaller characters. The threshold is specified as the line on which at least 3 out of 5 numbers or letters were correctly recognized. If the person recognized 4 characters on line 20/40 at 2.5 percent contrast (that is there was one wrong response), the result is recorded 20/40 (-I) at 2.5 percent.

Visual acuity at high contrast has been routinely measured in each eye separately. That detects small changes in refractive error and sometimes a change in the function of visual pathways. Since most workers use their eyes together, binocularly, the measurement of visual acuity at both high and low contrast levels that depicts a person's functional capability is the binocular measurement, measurement when both eyes are used. If a person does not have binocular vision, the value measured with both eyes open is equal to that of the preferred eye. In binocular persons the binocular value is usually one half to two lines better than that of the preferred eye.

Over the years a number of different tests have been used to address visual acuity at near distances. The Jaeger card is the one most often used for near vision testing. Other testing methods and cards have been substituted over the years and include the Snellen, Sloan, Wells, and Lebensohn cards, as well as the Ortho-Rater and the Titmus viewer. Depending on the manufacturer, Jaeger cards may vary in print size by half a point. To be comparable with the distance visual acuity, visual acuity at near distance can be measured with the same test symbols or optotypes as used for visual acuity at distance (see Figure 3). When testing a presbyopic person, the test card has to be kept at the distance where the person's glasses make the image clearest.

Figure 3 - Distance vision charts at high (-100 percent) and low (2.5 percent) contrast levels, near vision card, and the lightboxes for even, standard illumination. The large ETDRS lightbox is for research purposes, and the small lightbox is for routine use.

Visual acuity at distance is measured with high contrast number or letter charts. The threshold is specified similarly as when measuring at low contrast. There are numerous charts that can be used at different standard distances. The present international recommendation on distance visual acuity testing is based on a measurement at 4 m (13 ft), because a 6 m (20 ft) distance is not possible in many present day examining rooms. Typical visual requirements range from 20/20 to 20/40.

Luminance level of the test affects visual acuity values and, therefore, should always be the same. This is possible when the test charts are back lit in a lightbox which has even illumination. The illumination in the large EDTRS lightbox is 200 cd/m² (3 x 10⁵ cd/in.²) and 125 cd/m² (1.9 x 10⁵ cd/in.2) in the small lightbox (see Figure 3). When visual acuities at lower luminance  levels are required to be measured, luminance level can be reduced by using neutral density filters in front of the test.

The usual screening tests for color vision are the American Optical Association (AOA) or the Ishihara Pseudo Isochromatic Plates with numbers. At one time the colored yarn test was used as a form of color testing. This involved laying out several pieces of yarn in an array of colors. The person being tested simply identified the colors. This was a good test, but it was difficult to standardize. The color plates are very sensitive screening tests and therefore give false positive results. All false positive results need to be quantitatively assessed by using so called sorting tests like the Farnsworth Panel D-15 or Precision Vision PV-16 test. In these tests the person has to arrange the colored caps in order starting with the blue "pilot" cap so that the next cap is closest in color with the previous. The merit of these tests is that there is nothing that could be memorized by the person being tested. When the caps are mixed on the table the test situation is new. The tests reveal both the axis of the color deficiency, whether red-green or blue-yellow or irregular, and also the severity of the deficiency. All measurements of color vision need to be done under a day light lamp with a color temperature close to 6774K, the Commission Illumination de E' Clairage (CIE ) standard illuminant.

 

Conclusion
Vision testing is an important and vital aspect for humans responsible for quality control. Without adequate vision, even the best trained inspector may perform poorly. New contrast sensitivity and color vision tests are now available at very reasonable cost. They should be available in the quality control department or the company nurse's office so that they could be used any time when as a periodic health check or when a question arises as to an inspector's visual capabilities. This would reduce costs associated with medical screening, particularly when a large number of inspection personnel must be tested.

Practical application of vision testing can improve inspection and thus be economically profitable. With the advent of these new tools, the existing standards should be reviewed and meaningful tests should be incorporated to form a spectrum of tests that would accurately characterize an inspector's vision. Use of standardized tests would also provide better control for quality control. Uniform methods will ultimately enable a more accurate evaluation of visual capabilities for inspection personnel.

*Alloyweld Inspection Company. 796 Maple Lane, Besenville, IL 60106; (630) 595-2145; fax (630) 595-2128; e-mail ALLOYWELD@ MCS.NET.

⁺ Precision Vision, 944 1st Street, LaSalle, IL 61301; (815) 223-2022; fax (815)223-2224; e-mail precisionvision@mindspring.com.

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