Page 181 - Modern Optical Engineering The Design of Optical Systems

164 Chapter Eight

from cone vision to rod vision and also by an electrochemical mecha-
nism involving rhodopsin, the visual purple pigment). This process is
called dark adaptation. Figure 8.5 illustrates the adaptation process
as a function of the length of time that the eye is in darkness. The
“fovea only” curve indicates that after 5 or 10 minutes, the level of
brightness detectable by the portion of the retina used for distinct
vision is as low as it will ever get. At lower levels of illumination, only
the outer portions of the retina are useful; the fovea becomes a blind
spot. Figure 8.5 is for a target which subtends about 2°; the threshold
brightness is lower for larger targets and higher for smaller targets. As
indicated by the dashed lines, the conditions of the test have a great
bearing on the threshold of vision, and the data of Fig. 8.5 should be
regarded as indicating only an order of magnitude for the threshold.
The eye is a poor photometer; it is very inaccurate at judging the
absolute level of brightness. However, it is an excellent instrument for
comparison purposes, and can be used to match the brightness or color
of two adjacent areas with a high degree of precision. Figure 8.6 indicates
the brightness difference that the eye can detect as a function of the
absolute brightness of the test areas. At ordinary brightness levels, a
brightness difference of about 1 or 2 percent is detectable. (Note that
in comparison photometry, in which the eye is called upon to match two
areas, the precision of setting is increased by making a series of readings.
In half the readings, the brightness of the variable area is raised until
an apparent match is obtained; in the other half of the readings, the
brightness is lowered to obtain the apparent match. The average is
then much more accurate than either set.) Contrast sensitivity is best
when there is no visible dividing line between the two areas under
comparison. When the areas are separated, or if the demarcation
between areas is not distinct, contrast sensitivity drops markedly.

Figure 8.5 The threshold of
vision. The minimum brightness
perceptible drops sharply with
time as the eye adapts itself to
darkness. The upper and lower
dashed curves show the effect of
high and low illumination levels
(respectively) before adaptation
begins. For areas subtending
more than 5° the threshold is
almost constant, but rises rapidly
as target size is reduced. Curves
shown are for a target subtend-
ing about 2°.