Flux is the measure of how much of something is flowing through unit area in unit time. Energy per square meter per second is an example. This can be measured in units of joules/meter2sec or J/(m2s). Since J/s is defined as a watt (W), we can write the flux as W/m2. So far I have been describing the intensity of light, which is the same as irradiance. That is great for a physicist, but for engineers dealing with illumination, it is not adequate.

The simplest way to proceed is to note that illumination engineers measure illuminance in units of lumens per square meter or lm/m2, and this flux is defined as a lux (lx).1 Therefore, the lumen, under certain conditions, is proportional to energy per ...

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