Table 6-20. Visual Size versus Physical Size

Figure 6-44. Typical High Intensity Viewer

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 6-95 Table 6-20. Visual Size versus Physical Size. Viewing Distance (inches) Physical Size (inches) 10 12 15 20 24 0.0029 0.0035 0.0044 0.0058 0.0070 6.8.11.2.1 For a given viewing distance the visual size is maximum when the line of sight is perpendicular to the plane in which the object lies. Referring again to this printed page, this means a line from the eye perpendicular to the page. As the page is inclined (to decrease the angle between the line of sight and the page), the visual size of the print is decreased until at 45 degrees the type size is only 70 percent of what it was at 90 degrees. For a 45-degree angle, assuming an object of fixed physical size and fixed viewing distance, visibility equal to that at 90 degrees can be had only by increasing the illumination level by 2-1/2times. An aid in reading radiographic film is the pocket comparator with graduated reticules having linear and circular scales. They are able to measure the size of discontinuities and/or defects depicted on this film: 6.8.11.3 Visual Contrast. A certain level of contrast is desirable between small detail and its immediate surroundings. However, a high degree of contrast between those immediate surroundings and any large area outside the field in which the detail lies is unfavorable. The contrast between this print and the page is favorable. But a high contrast between the page and the desk on which it lies is detrimental to good vision. For each contrast there is a threshold size, and, conversely, for each threshold size there is a minimum contrast if the object is to be just visible. As the brightness level decreases, the difference in brightness levels must be greater and greater if the eye is to detect a difference. It is evident from this figure that the eye must have considerable time to adjust to low levels of light intensity. 6.8.11.4 Speed Of Sight. Sight is not instantaneous. It takes time to see. We do not see when the eyes are in motion. In reading this line the eyes focus on a point called the point of fixation. This point of fixation is then moved along the line in a series of jumps. The eyes come to a dead stop several times, about three times usually in reading a line of this print. What we do is read a portion of the line during each fixation period. The time of one of the fixation periods varies between 0.07 and 0.3 second. Hence, these times become the limiting periods in seeing. As a visual task increases in difficulty, these fixation periods become longer. Involved, too, is the problem of reaction time, that is, the time that elapses between seeing and acting. Any task involving sight becomes a series of complex time intervals. The time for seeing is naturally greatly influenced by experience, mental reaction time, brightness level, contrast-brightness, and visual size. The rapidity with which any visual examination can be carried out is a relation between these factors and the necessary accuracy or exactness of the examination.

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