Lumen (symbol: LM) SI derived unit is a measure of the total amount of luminous flux emitted by the visible light source. The luminous flux is different from power (radian flux), which contains all emitted electromagnetic waves, while the luminous flux is weighted according to a model of a sensitivity of the human eye to various wavelengths (a "luminance function"). With a lux of lumens about one lux per square meter being lumens.