Figure 6-43. Dark Adaptation Diagram.

Table 6-20. Visual Size versus Physical Size

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 6-94 The film reading illuminator or illuminators should transmit at least two (2) foot-candles of light through the film at the viewing surface of the film. This quantity of light is sufficient to view radiographs with a density of three (3) H and D units. There should also be a high intensity illuminator with a variable light intensity capable of transmitting the required light through densities in the order of four (4) to four and a half (4.5) H and D units for interpreting these high densities. All film viewers SHALL be of the type that provides a uniform level of illumination over the entire viewing surface. Figure 6-43. Dark Adaptation Diagram. 6.8.11.1 Limitations of Eye. The eye is the evaluation medium in radiography. Visual accuracy varies considerably from one individual to another. Oddly enough, a perfect eye does not necessarily mean a perfect visual system. Certain "defects" can be present. Vision must not only record shapes and sizes, but also the variation of light intensities. In this area the eye is especially unreliable. The relative brightness of two light sources, for example, can be gauged only approximately. And even such approximate evaluation is possible only when the light sources are close to the same order of brightness. For example, a bright object or area appears brighter when viewed against a dark field. Conversely, the object will appear darker than it really is when the surrounding area is comparatively brighter. 6.8.11.2 Visual Size. In any task requiring critical examination, we are usually more conscious of size than anything else. The minimum size of an object that can be seen under a given set of conditions is called the threshold size. This varies greatly depending on brightness-contrast between the immediate background and the detail being examined. It also varies with the level of brightness. The physical size of an object can easily be measured, but it is difficult for most individuals to interpret physical size into visual size. The type in which this is printed has a definite physical size measured in points, a point being about 1/72 of an inch. The visual size, however, depends on the distance from the page to the eye. The visual size of the letters at two feet is only one-half that obtained for a page-to-eye distance of one foot. The visual size is the angle subtended at the eye by an object at a distance. The threshold size of a critical detail (such as this black print on a white background) is about one minute (l/60 of one degree) for persons of normal vision. An individual with sub-normal vision will be able to pick up an object of just about twice the visual size required for normal vision. The relation of a visual size of one minute of a degree to physical size for different viewing distances is given in Table 6-20.

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