Luminous flux

Luminous flux measures the rate at which visible light is emitted or received, weighted by the human eye’s sensitivity. Its SI unit is the lumen.

Radiant flux: Radiant flux is the rate at which electromagnetic energy is emitted, transferred, or received, measured in watts. It measures optical power without weighting wavelengths by human vision.
CIE 1931 color space: The CIE 1931 color spaces are standardized systems for describing color through tristimulus values. Its standard observer includes the photopic sensitivity function used in photometry.
Luminous flux measurement: Luminous flux measurement determines the total photometric output of a light source or optical system. It tests the quantity that lamp and luminaire specifications report in lumens.
Luminance: Luminance measures luminous intensity per projected area in a given direction, in candelas per square metre. It describes directional brightness, not total luminous output.
Luminous efficacy: Luminous efficacy is the ratio of luminous flux to radiant power, expressed in lumens per watt. It quantifies how much visible output a given optical power produces.
Photopic luminous efficiency function: The photopic luminous efficiency function describes the standard eye’s sensitivity to different wavelengths in bright conditions. It supplies the wavelength weights that turn radiant flux into luminous flux.
Integrating sphere: An integrating sphere is a hollow, diffusely reflective enclosure used to collect light from many directions. It captures a source’s total output for luminous-flux measurement.
Radiant intensity: Radiant intensity is radiant power emitted per unit solid angle in a given direction, measured in watts per steradian. It is the radiometric directional quantity corresponding to luminous intensity.
Photopic vision: Photopic vision is human vision under bright-light conditions, mediated mainly by cone cells. The standard weighting for luminous flux follows the eye’s bright-light sensitivity.
555 nanometers: 555 nanometers is the wavelength near the peak of human photopic visual sensitivity. The maximum photopic weighting is conventionally normalized at this wavelength.

Linked from 14 pages