12.9. Eye spectral response There are two basic types of retinal
photo-receptors: cones, responding to bright-light conditions,
and rods, responding to low-intensity light. Depending on their
spectral sensitivity, the former belong to either L (long-
wavelengths sensitive), M (mid-wavelengths sensitive) or S
(short-wavelengths sensitive) cones. By combining their separate
inputs, the brain creates colors. The cones are concentrated in
the center of the retina (fovea), where they are as small as ~2μ
in diameter. That puts the angular size of the smallest
individual foveal cones at ~1/3 arc minute. Cones become larger
- up to about four times - toward outer areas of the retina.
Therefore, eye spectral response is directly related, and
influenced by illuminance levels (light intensity) to which it
is exposed. Illuminance level determines the level of activity
of cones and rods, and with it main characteristics of human
vision (FIG. 248). Note that there is no general agreement on
the illuminance level that effect