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In physics, specifically photometry (optics), the lumen (symbol: lm) is the SI derived unit of luminous flux. It is the amount of light that falls on a unit area at unit distance from a source of one candela.
| SI light units [ }|action=edit}} Edit }] | ||||
|---|---|---|---|---|
| Quantity | SI unit | Symbol | Notes | |
| Luminous intensity | candela | cd | ||
| Luminous flux | lumen or ( candela · steradian ) | lm | ||
| Luminance | candela / square metre | cd/ m2 | ||
| Illuminance | lux or (lumen / square metre) | lx | ||
| Luminous efficacy | lumens per watt | lm/ w | ||
Physical quantity Optics