Lumen is the measure of the power of light perceived by the human eye. It measures the total number of packets or quanta of light produced by a light source. This is the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions.

It is the SI unit of luminous flux which is energy within the range of frequencies we perceive as light. For example, a wax candle generates 13 lumens; a 100 watt bulb generates 1,200 lumens.

The lumen is defined in relation to the candela by

    1 lm = 1 cd•sr

Which means, if a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of exactly 4π lumens.

The number of candelas or lumens from a source also depends on the average visual sensitivity of the human eye to light of different wavelengths


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