Lambert (unit)

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/m ²).

Definition [ edit ]

1 lambert (L) = ${\frac {1}{\pi }}$ candela per square centimetre (0.3183 cd/cm ²) or ${\frac {10^{4}}{\pi }}$ cd m ^?2

Other units of luminance:

SI photometry quantities
v
t
e
Quantity		Unit		Dimension ^{[nb 1]}	Notes
Name	Symbol ^{[nb 2]}	Name	Symbol	Dimension ^{[nb 1]}	Notes
Luminous energy	$Q v$ ^{[nb 3]}	lumen second	lm ?s	T ? J	The lumen second is sometimes called the talbot .
Luminous flux , luminous power	$Φ v$ ^{[nb 3]}	lumen (= candela steradian )	lm (= cd?sr)	J	Luminous energy per unit time
Luminous intensity	$I v$	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	$L v$	candela per square metre	cd/m ² (= lm/(sr?m ²))	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	$E v$	lux (= lumen per square metre)	lx (= lm/m ²)	L ^?2? J	Luminous flux incident on a surface
Luminous exitance , luminous emittance	$M v$	lumen per square metre	lm/m ²	L ^?2? J	Luminous flux emitted from a surface
Luminous exposure	$H v$	lux second	lx?s	L ^?2? T ? J	Time-integrated illuminance
Luminous energy density	$ω v$	lumen second per cubic metre	lm?s/m ³	L ^?3? T ? J
Luminous efficacy (of radiation)	$K$	lumen per watt	lm/ W	M ^?1? L ^?2? T ³? 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? T ³? 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

^ 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.
^ 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
^ ^a ^b ^c Alternative symbols sometimes seen: $W$ for luminous energy, $P$ or $F$ for luminous flux, and $ρ$ for luminous efficacy of a source.