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The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI). Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. One lux is one lumen per square metre.
The lumen is defined in relation to the candela as

1 lm = 1 cd·sr.A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of

1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lm.

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