Figure 16.1 (a) shows a schematic illustration of the human eye (Encyclopedia Britannica, 1994). The inside of the eyeball is clad by the retina, which is the light-sensitive part of the eye. The illustration also shows the fovea, a cone-rich central region of the retina which affords the high acuteness of central vision. Figure 16.1 (b) shows the cell structure of the retina including the light-sensitive rod cells and cone cells. Also shown are the ganglion cells and nerve fibers that transmit the visual information to the brain. Rod cells are more abundant and more light sensitive than cone cells. Rods are sensitive over the entire visible spectrum. There are three types of cone

cells, namely cone cells sensitive in the red, green, and blue spectral range. The cone cells are therefore denoted as the red-sensitive, green-sensitive, and blue-sensitive cones, or simply as the red, green, and blue cones.

Three different vision regimes are shown in Fig. 16.2 along with the receptors relevant to each of the regimes (Osram Sylvania, 2000). Photopic vision relates to human vision at high ambient light levels (e. g. during daylight conditions) when vision is mediated by the cones. The photopic vision regime applies to luminance levels > 3 cd/m2. Scotopic vision relates to human vision at low ambient light levels (e. g. at night) when vision is mediated by rods. Rods have a much higher sensitivity than the cones. However, the sense of color is essentially lost in the scotopic vision regime. At low light levels such as in a moonless night, objects lose their colors and only appear to have different gray levels. The scotopic vision regime applies to luminance levels < 0.003 cd/m2. Mesopic vision relates to light levels between the photopic and scotopic vision regime (0.003 cd/m2 < mesopic luminance < 3 cd/m2).

The approximate spectral sensitivity functions of the rods and three types or cones are shown in Fig. 16.3 (Dowling, 1987). Inspection of the figure reveals that night-time vision (scotopic vision) is weaker in the red spectral range and thus stronger in the blue spectral range as compared to day-time vision (photopic vision). The following discussion mostly relates to the photopic vision regime.