Lambert (unit)
The lambert (symbol: L, La) is a non-SI unit of luminance. It is named after Johann Heinrich Lambert, a Swiss mathematician, physicist, and astronomer. It is defined as 1/π candela per square centimeter.
Specifically:
1 L = 1/π cd/cm2 = (104/π) cd/m2 ≈ 3183.1 cd/m2
The lambert is often used to specify the luminance of diffuse surfaces. Although it is not an SI unit, it remains in occasional use, particularly in fields such as illumination engineering and display technology. A related unit is the foot-lambert, which is used primarily in the United States.
It is important to note the conversion factor of 1/π in the definition. This is because the lambert is defined such that a perfectly diffusing surface with a radiance of 1 cd/cm2 emits a luminance of 1 lambert. This definition simplifies calculations involving perfectly diffusing surfaces.
Because the candela per square meter (cd/m2) is the SI unit of luminance, and is also referred to as the nit, the lambert is considered a deprecated unit. The nit is now the preferred unit for luminance measurements.