Luminous efficacy is a measure of how properly a light supply produces visible gentle. It is the ratio of luminous flux to energy, measured in lumens per watt within the International System of Models (SI). Relying on context, the facility will be both the radiant flux of the supply's output, or it may be the overall energy (electric energy, chemical vitality, or others) consumed by the source. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, as a result of spectral sensitivity of the human eye