Lambert (unit)

Short description: Non-SI metric unit of luminance

The lambert (symbol L, la[citation needed] or Lb[citation needed]) 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).

## Definition

1 lambert (L) = [math]\displaystyle{ \frac{1}{\pi} }[/math] candela per square centimetre (0.3183 cd/cm2) or [math]\displaystyle{ \frac{10^4}{\pi} }[/math] cd m−2

## See also

Other units of luminance:

* Apostilb (asb)
* Blondel (blondel)
* Bril (bril)
* Nit (nit)
* Stilb (sb)
* Skot (sk)

SI photometry quantitiesv * d * e

Quantity  | Unit  | Dimension | Notes

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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 | L−2⋅J | 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−2⋅J | Luminous flux incident on a surface
Luminous exitance, luminous emittance  | Mv | lux  | lx  | L−2⋅J | Luminous flux emitted from a surface
Luminous exposure | Hv | lux second | lx⋅s  | L−2⋅T⋅J | Time-integrated illuminance
Luminous energy density  | ωv | lumen second per cubic metre  | lm⋅s/m3 | L−3⋅T⋅J |
Luminous efficacy (of radiation)  | K | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux
Luminous efficacy (of a source)  | η[nb 3] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | 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

1. ↑ 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
2. ↑ 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.
3. ↑ 3.0 3.1 3.2 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

## References

* "Lighting Design Glossary: Luminance" at Schorsch Lighting Design Knowledgebase

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