Lighting Design Glossary

Luminous Flux

(Term of photometry)

Luminous flux is photometrically weighted radiant flux (power).

At a frequency of 540 x 10¹² Hertz, it is defined as 683 lm/W of radiant flux. As with luminous intensity, the luminous flux of light with other wavelengths can be calculated using the CIE photometric curve.

The luminous flux is a very basic unit of measurement for light. If a uniform point light source of 1 cd luminous intensity (about the intensity of a normal wax candle!) is positioned at the center of a sphere of 1 m radius, then every area of 1 m² on the inside of that sphere will receive a luminous flux of 1 lm.
(This relation also holds when using other units like 1 ft radius and 1 ft² area.)

Since the surface area of a full sphere is 4*Pi times the square of the radius, a uniform point light source of 1 cd produces a total 12.57 lm of luminous flux.
Most real life light sources area not uniform though. This factor is therefore only useful for rough estimations and will normally give inaccurate results.

Standard unit of luminous flux is Lumen (lm).