Lumen (unit)
The lumen (symbol: lm) is the unit of luminous flux, a measure of the perceived power of visible light emitted by a source, 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, this weighting is standardized by the CIE and ISO. One lux is one lumen per square metre.
| lumen | |
|---|---|
| Unit system | SI |
| Unit of | luminous flux |
| Symbol | lm |
| Conversions | |
| 1 lm in ... | ... is equal to ... |
| SI base units | cd⋅sr |
The lumen is defined in relation to the candela as
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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