The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cone cells and rod cells in an antagonistic manner. There is some overlap in the wavelengths of light to which the three types of cones (L for long-wave, M for medium-wave, and S for short-wave light) respond, so it is more efficient for the visual system to record "differences" between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels the cone photoreceptors are linked together to form three opposing color pairs: red versus green, blue versus yellow, and black versus white (the last type is achromatic and detects light-dark variation, or luminance). It was first proposed in 1892 by the German physiologist Ewald Hering.