Table 6-20. Visual Size versus Physical Size.
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:
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.
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.