Page 179 - Modern Optical Engineering The Design of Optical Systems

162 Chapter Eight

Figure 8.3 Visual acuity as a function of object brightness.
Visual acuity in reciprocal minutes. The dashed and dotted
lines show the effect of increased and decreased (respec-
tively) surround brightness (1 millilambert is approxi-
mately the brightness of a perfect diffuser illuminated by
1 footcandle). The open circle curve indicates the diameter
of the pupil; pupil diameters are larger in the young and
smaller in the old, especially at lower brightnesses.

Latin for yellow spot) filters out the blue and violet light; the spectral
response function of the eye is as shown in Figs. 8.8, 8.9 and 8.10.

Other types of acuity
Vernier acuity is the ability of the eye to align two objects, such as two
straight lines, a line and a cross hair, or a line between two parallel
lines. In making settings of this type, the eye is extremely capable. In
instrument design, it can be safely assumed that the average person

Figure 8.4 The object contrast
(
B/B max ) necessary for the eye
to resolve a pattern of alternat-
ing bright and dark bars of
equal width. Note that this
curve shifts upward in reduced
light levels and drops as the
light level is increased. For this
plot the bright bars had a
brightness of B max 23 foot-
lamberts.