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

SI photometry quantitiesv · d · e
Quantity		Unit		Dimension	Notes
Name	Symbol^{[nb 1]}	Name	Symbol	Symbol^{[nb 2]}	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 steradians)	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/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	E_v	lux (= lumen per square metre)	lx (= lm/m²)	L⁻²⋅J	Luminous flux incident on a surface
Luminous exitance, luminous emittance	M_v	lux	lx	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/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

↑ 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.
↑ ^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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[note-suffix-v-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

[note-dimension-symbol-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.

[note-alternative-symbol-photometric-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.

Lambert (Unit)

Definition

See also

References