In radiometry, the radiant flux of a light source is its emitted optical energy per unit time. That quantity, however, does not take into account the sensitivity of the human eye. For that, one has the photometric quantity luminous flux, specified with units of lumen (lm = cd sr), which takes into account the photopic response of the human eye. This is the spectral response for reasonably high illumination intensities, where color vision works well.
The luminous flux is frequently found as a specification of light sources which are used for illumination purposes – for example, of incandescent lamps, fluorescent lamps and lamps based on LEDs. It is a useful measure for how much a light source can contribute to the illumination of a room. For example, a 60-W incandescent lamp may generate a luminous flux of 900 lm, corresponding to a luminous efficacy of 15 lm/W. (A lamp based on light emitting diodes (LEDs) may produce the same luminous flux with an electric consumption well below 10 W because it has a much higher luminous efficacy.) A large living room requires several thousand lumen, corresponding to several of those lamps.
Although an incandescent lamp turns most of the consumed electrical power into thermal radiation, most of that radiation is in the infrared spectral region, where the luminosity function is zero, i.e., that light is not useful for illumination. It does not contribute to the luminous flux.
Relation to the Luminous Intensity and Illuminance
The luminous flux contains no information concerning in which direction or how uniformly the light is emitted. The luminous intensity of a light source is its luminous flux per unit solid angle. Only in the case of unidirectional radiation, which is approximately given for an ordinary light bulb, the luminous intensity of the light source (measured in candela) can be calculated as the luminous flux divided by <$4\pi$> sr, and the illuminance in some distance <$d$> from the light source will be the luminous flux divided by <$4\pi d^2$>. Light sources with directional emission can produce a much higher illuminance with the same level of luminous flux; this is particularly so for lasers.
Measurement of Luminous Flux
If the output of a light source is well directed, e.g. in the form of a laser beam, one can simply measure the luminous flux with an optical power meter. That method would not work, however, with a light source which emits in a wide range of directions – for example, a light emitting diode. In such cases, one may need to use an integrating sphere, which uniformly distributes the light and provides an illuminance of a photodetector which is proportional to the incoming luminous flux, and is largely independent of its spatial properties.
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