Lumen (unit)
| lumen | |
|---|---|
600-lumen LED light bulbs | |
| General information | |
| Unit system | SI |
| Unit of | luminous flux |
| Symbol | lm |
| Conversions | |
| 1 lm in ... | ... is equal to ... |
| SI base units | cd⋅sr |
The lumen (symbol: lm) is the SI unit of luminous flux, which quantifies the perceived power of visible light emitted by a source. Luminous flux differs from power (radiant flux), which encompasses all electromagnetic waves emitted, including non-visible ones such as thermal radiation (infrared). By contrast, 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.
The lumen is defined as equivalent to one candela-steradian (symbol cd·sr):
- 1 lm = 1 cd·sr.
A full sphere has a solid angle of 4π steradians (≈ 12.56637 sr), so an isotropic light source (that uniformly radiates in all directions) with a luminous intensity of one candela has a total luminous flux of
- 1 cd × 4π sr = 4π cd⋅sr = 4π lm ≈ 12.57 lm.
One lux is one lumen per square metre.