Luminous efficacy

Luminous efficacy is the ratio of luminous flux to input power, expressed in lumens per watt. It measures how effectively a source converts power into visible light, accounting for the eye’s wavelength sensitivity.

Luminous flux: The quantity of visible light emitted or received per unit time, weighted by human visual sensitivity. It is the numerator of luminous efficacy, measured in lumens.
CIE photopic luminous efficiency function: A standardized curve describing human photopic visual sensitivity across wavelengths. Its wavelength weights determine the lumens generated by spectral output.
Luminous efficacy of a source: The luminous flux emitted by a light source divided by its electrical input power. This practical measure includes the source’s electrical and optical conversion performance.
Radiant efficiency: The fraction of input power converted into emitted radiant power. It ignores human visual sensitivity, unlike luminous efficacy.
Radiant flux: The rate at which electromagnetic radiation carries energy, measured in watts. It supplies the radiometric power compared with visually weighted output.
CIE 1931 color space: A color space established by the CIE in 1931 from measured human color-matching responses. Its standard observer provides the photopic sensitivity basis used in photometry.
Luminous efficacy of a lamp: The luminous flux emitted by a lamp divided by the electrical power it consumes. It is a common product-level application of the ratio.
Electrical efficiency: The fraction of electrical input power converted into a specified useful output. It describes conversion as a fraction rather than visible lumens per watt.
Lumen: The SI unit of luminous flux, weighted according to the standard human photopic vision function. Lumens express the visible output used in the efficacy numerator.
Monochromatic radiation: Electromagnetic radiation concentrated at a single wavelength or within a very narrow wavelength band. For a single wavelength, efficacy follows directly from that wavelength’s visual sensitivity.

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