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
Figure 6-43. Dark Adaptation Diagram.
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.
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.