Page 182 - Modern Optical Engineering The Design of Optical Systems

Characteristics of the Human Eye 165

Figure 8.6 The contrast sensitivity of the eye as a
function of field brightness. The smallest perceptible
difference in brightness between two adjacent fields
(
B) as a fraction of the field brightness B remains
quite constant for brightnesses above 1 millilambert
if the field is large. The dashed line indicates
the contrast sensitivity for a dark surrounded field.
(1 millilambert is approximately the brightness of a
perfect diffuser illuminated by 1 footcandle, i.e.,
1 foot-lambert.)

Figure 8.7 indicates the capability of the normal eye as a comparison
colorimeter. Again, the eye is poor at determining the absolute
wavelength of a color but quite good at determining a color match;
wavelength differences of a few nanometers are detectable under suit-
able conditions. The comments of the preceding paragraph regarding
dividing lines between test areas apply to color sensitivity as well.
The sensitivity of the eye to light is a function of the wavelength of
the light. Under normal conditions of illumination, the eye is most

Figure 8.7 Sensitivity of the eye
to color differences. The amount
by which two colors must differ
for the difference to be detectable
in a side-by-side comparison is
plotted as a function of the wave-
length. Some data indicates a
more uniform sensitivity of about
twice that shown here.