Figure 2-2 is a schematic cross section of the retina indicating the layers and the names associated with each layer. Cone density decreases with increasing distance from the fovea. The candela per square foot is a unit of luminous energy leaving a surface and arriving at the retina. The rods must take over the visual process during low levels of illumination. While moving from a dark environment into a very light environment, the visual system experiences a change in sensitivity.
Light adaptation involves primarily the cone system, and usually takes less than a minute.
These multiple hookups result in very poor visual acuity. However, since each nerve ending serves a relatively large portion of the visual field, the rod system is more likely to detect changes such as motion or flicker. There is no color response with the rod system. Rods produce a black and white response, which is actually a reaction to variations in luminance. Moving from a very light environment into a dark environment results in a change in sensitivity of the visual system due to dark adaptation.