The luminous efficacy of radiation is luminous flux divided by total radiant flux, in , for the specified visual conditions. For nonzero radiant flux, it is a spectral weighted mean of spectral luminous efficacy. Specify the denominator: a lamp's luminous efficacy relative to electrical input power also includes conversion losses and is a different ratio. [ref]
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Luminous efficacy is a measurement that expresses how well a light source produces visible light. It is defined as the ratio of luminous flux (measured in lumens) to power (measured in watts) consumed by the light source. The unit of luminous efficacy is lumens per watt (lm/W). In simpler terms, luminous efficacy quantifies how efficient a light source is in converting electrical energy into visible light.