Design Nomenclature / Light & Optics
Lumen
Also called luminous flux, lm
- Light & Optics
- Systems & Spec
- 2D & 3D
This entry has not been translated yet and is shown in English.
The unit of luminous flux: total light power emitted in every direction, weighted by the eye's response. It says how much light a source makes and nothing about where any of it goes — a bare bulb and a tight spot of equal lumens do utterly different things to a wall.
In practice
The number on the box. Useful for comparing sources against each other, useless for predicting the exposure on a subject.
Not to be confused with
- CandelaLuminous intensity: lumens per steradian, light emitted in a given direction. It is the SI base unit for light, defined by fixing the luminous efficacy of 540 terahertz radiation at 683 lumens per watt. A fixture's beam is a map of candela against angle — which is what an IES file tabulates.
- LuxOne lumen per square metre — the unit of illuminance. Overcast daylight runs around 1,000 lux, an office 300 to 500, full sun past 100,000. The foot-candle is the same quantity per square foot, and one foot-candle is 10.76 lux.
- NitThe working name for candela per square metre, the unit of luminance. A phone in a dim room sits near 100 nits, a laptop outdoors pushes 500, HDR displays peak past 1,000, and the SDR reference white was standardized around 100. Every display brightness claim is quoted in these.