The designations refer to the wavelengths associated with each cone system. According to this theory, normal color vision is a function of the three kinds of pigments present in the three classes of cones. Each of the four receptors has its own spectral sensitivity curve, which means that the human visual system is actually tetrachromatic rather than trichromatic. It is caused by a continuation of the photochemical process resulting in nerve impulses, which cause continued object perception for a short period of time. Afterimage effect is proportional to the luminance of the object.
Afterimage can be a negative or complementary process.
The original color of the object is replaced by its complementary color. Afterimage makes a series of still pictures appear to move, a phenomenon from which motion pictures are derived. The time sensitivity of the visual system is phenomenal. Stimulus Sensitivity Rods are a thousand times more sensitive to intensity than the cones are at low levels of illumination. Under scotopic vision, it is said that the rods can detect the flicker of a candle 14 miles away.