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[a] |
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.[2]
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.[3]
One lux is one lumen per square metre.
- ^ The International System of Units (SI) (PDF) (9th ed.). Bureau International des Poids et Mesures. 2019. pp. 137–138.
- ^ ISO/CIE 23539:2023, CIE TC 2-93, Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.
- ^ Bryant, Robert H. "Lumens, Illuminance, Foot-candles and bright shiny beads…". The LED Light. Archived from the original on 2010-11-12. Retrieved Oct 4, 2010.
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