Luminous Flux is the rate of flow of light per unit of time, especially the flux of visible light expressed in lumens.

The luminous flux accounts for the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the eye’s response to different wavelengths. Thus, luminous flux could be termed as a weighted average of the Radiant Flux in the visible spectrum. It is a weighted average because the human eye does not respond equally to all visible wavelengths.

This weighting factor or the ratio of the total luminous flux to the radiant flux (luminous efficacy) allows for conversion of radiant flux to luminous flux at any wavelength in the visible band. Light outside the visible band does not effect the calculation of luminous flux.

Units and Measures

The lumen is the unit of luminous flux. One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.

Useful References

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