Colorimetry is a physical science that deals with objective and quantitative ways of describing color. Since the impression of color is something subjective - what one person perceive as "light blue", may look "medium blue" to another - it is not quite clear how to describe and identify different colors. Colorimetry tries to connect the notion of a standard color sample with the physical concept of light as a form of energy. The trichromatic theory of color vision is based on the working of three types of cone receptors present in the retina.This theory was originated in the 18th century and thus also has long history. The most important feature of this theory is that it is possible to match all of the colors in the visible spectrum by appropriate mixing of three primary colors. Trichromacy is the basic principal of color reproduction methods such as those used in television, printing and photography. In trichromacy, the primary colors Red, Green, and Blue are processed and mixed by the human vision system to produce various types of colors in differing hues and shades. Color measuring instruments are designed to measure color in terms of reflectance.