The lambert (symbol L, la or Lb) is a non-SI metric unit of luminance named for Johann Heinrich Lambert (1728–1777), a Swiss mathematician, physicist and astronomer. A related unit of luminance, the foot-lambert, is used in the lighting, cinema and flight simulation industries. The SI unit is the candela per square metre (cd/m2).
Other units of luminance:
|Name||Symbol[nb 1]||Name||Symbol||Symbol[nb 2]|
|Luminous energy||Qv[nb 3]||lumen second||lm⋅s||T J||The lumen second is sometimes called the talbot.|
|Luminous flux, luminous power||Φv[nb 3]||lumen (= candela steradians)||lm (= cd⋅sr)||J||Luminous energy per unit time|
|Luminous intensity||Iv||candela (= lumen per steradian)||cd (= lm/sr)||J||Luminous flux per unit solid angle|
|Luminance||Lv||candela per square metre||cd/m2 (= lm/(sr⋅m2))||L−2J||Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.|
|Illuminance||Ev||lux (= lumen per square metre)||lx (= lm/m2)||L−2J||Luminous flux incident on a surface|
|Luminous exitance, luminous emittance||Mv||lumen per square metre||lm/m2||L−2J||Luminous flux emitted from a surface|
|Luminous exposure||Hv||lux second||lx⋅s||L−2T J||Time-integrated illuminance|
|Luminous energy density||ωv||lumen second per cubic metre||lm⋅s/m3||L−3T J|
|Luminous efficacy (of radiation)||K||lumen per watt||lm/W||M−1L−2T3J||Ratio of luminous flux to radiant flux|
|Luminous efficacy (of a source)||η[nb 3]||lumen per watt||lm/W||M−1L−2T3J||Ratio of luminous flux to power consumption|
|Luminous efficiency, luminous coefficient||V||1||Luminous efficacy normalized by the maximum possible efficacy|
|See also: SI · Photometry · Radiometry|
- Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
- The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.