Luminous Flux (Luminous Power)

Luminous flux is a radiant flux weighted to match the eye response of the “standard observer.” Its unit of measurement is the lumen. The lumen (lm) is the photometric equivalent of the watt, and it is defined as 1/683 W of radiant power at a frequency of 540 x 10¹² Hertz, or better at a wavelength of 555 nm.

LUMINOUS INTENSITY
The luminous intensity is the luminous flux emitted from a point source per unit solid angle into a given direction. The luminous intensity is measured in candela (the Latin word for “candle”). Together with the CIE photometric curve, the candela provides the weighting factor needed to convert between radiometric and photometric measurements. The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation with a frequency of 540 x 10¹² Hertz (λ = 555 nm) and has a radiant intensity of 1/683 W sr^-1 in that direction.



If a light source is isotropic, that is, its intensity does not vary with direction, the relationship between lumens and candelas is 1 cd = 4π lm. In other words, an isotropic source having a luminous intensity of 1 cd emits 4π lm into space, which just happens to be 4π sr. We can also state that 1 cd = 1 lm sr, analogous to the equivalent radiometric definition. If a source is not isotropic, the relationship between candelas and lumens is empirical. A fundamental method used to determine the total flux (lumens) is to measure the luminous intensity (candelas) in many directions using a goniophotometer, and then numerically integrate over the entire sphere. Because a steradian has a projected area of 1 m² at a distance of 1 m, a 1 cd light source will similarly produce 1 lm m^-2, (Figure 5.12).