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Characteristics of the Human Eye 161

Figure 8.2 The variation of visual acuity (relative to the fovea)
with the retinal position of the image. Note that because of the
logarithmic scales of the figure, the falloff in visual acuity is far
more rapid than the shape of the curve might indicate.

that at the fovea, which is arbitrarily set at unity) versus the angular
position of the test target in the field of view. Also note that the vertical
VA is 5 to 10 percent higher than horizontal and that the horizontal and
vertical VA are about 30 percent higher than oblique (45°) VA.
As the brightness of a scene is diminished, the iris opens wider and
the rods take over from the cones. At low illuminations, the eye is color
blind and the fovea becomes a blind spot, since the cones lack the nec-
essary sensitivity to respond to low levels of illumination. One result
of this process is that the visual acuity drops as the illumination drops.
This relationship is plotted in Fig. 8.3, which also indicates the normal
pupil size. Note that the brightness of the area surrounding the test
target affects the acuity. A uniform illumination seems to maximize
the acuity. Figure 8.4 shows that, as might be expected, reducing the
contrast of the target will also reduce the acuity.
Because the eye has about 0.75 D of chromatic aberration (C-light to
F-light; it is about 3 D from 380 nm to 780 nm), VA is affected by the
wavelength of light illuminating the target. Normally, VA is given for
white light. In monochromatic light, the acuity is very slightly higher
for the yellow and yellow-green wavelengths and slightly lower for red
wavelengths. In blue (or far red) light, VA may be 10 to 20 percent lower,
and in violet light the reduction in VA is 20 to 30 percent. The chromatic
of the eye can be corrected or doubled (by external lenses) without detec-
tion; a quadrupling is noticeable. The effect of the chromatic aberration
on the acuity of the eye is less than one might expect because the slightly
yellow lens blocks out the ultraviolet, and the macula lutea (which is