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Luminous efficacy is a measure of how effectively a light source produces seen mild. It is the ratio of luminous flux to energy, measured in lumens per watt in the Worldwide System of Units (SI). Depending on context, the ability will be both the radiant flux of the supply's output, or it may be the entire energy (electric power, EcoLight chemical vitality, or others) consumed by the supply. Which sense of the time period is meant must often be inferred from the context, and is typically unclear. Not all wavelengths of mild are equally seen, or equally effective at stimulating human vision, due to the spectral sensitivity of the human eye