Young's Trichromatic Theory
According to Young's theory, three types of cones exist, each sensitive to a particular pigment-rythrolabe (red), chlorolabe (green), and cyanolabe (blue). The three classes have a spectral sensitivity that peak at, blue (440-450 nm), green (535-555 nm), and red (570-590 nm). Although the peak wave lengths are different, the three classes of cones have overlapping spectral sensitivity. This occurs because there is a secondary band of absorption that allows sensitivity of all visual pigments to fall off sharply on the wave length side of the peak but less sharply on the short wave length side of the peak.