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.
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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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