The apostilb is an old unit of luminance, which was withdrawn from use in 1978.[by whom?] The SI unit of luminance is the candela per square metre (cd/m2). In 1942 it was proposed[by whom?] to rename the apostilb the blondel, after the French physicist André Blondel. The symbol for the apostilb is asb.
The apostilb is defined in terms of another unit of luminance, the stilb (sb):
- 1 asb = 1/π · 10−4 sb
- 3.14 asb = 1 cd/m2
Other units of luminance:
|Luminous energy||Qv [nb 2]||lumen second||lm⋅s||TJ [nb 3]||Units are sometimes called talbots.|
|Luminous flux / Luminous power||Φv [nb 2]||lumen (= cd⋅sr)||lm||J [nb 3]||Luminous energy per unit time.|
|Luminous intensity||Iv||candela (= lm/sr)||cd||J [nb 3]||Luminous power per unit solid angle.|
|Luminance||Lv||candela per square metre||cd/m2||L−2J||Luminous power per unit solid angle per unit projected source area. Units are sometimes called nits.|
|Illuminance||Ev||lux (= lm/m2)||lx||L−2J||Luminous power incident on a surface.|
|Luminous exitance / Luminous emittance||Mv||lux||lx||L−2J||Luminous power emitted from a surface.|
|Luminous exposure||Hv||lux second||lx⋅s||L−2TJ|
|Luminous energy density||ωv||lumen second per cubic metre||lm⋅s⋅m−3||L−3TJ|
|Luminous efficacy||η [nb 2]||lumen per watt||lm/W||L−2M−1T3J||Ratio of luminous flux to radiant flux.|
|Luminous efficiency / Luminous coefficient||V||1|
|See also: SI · Photometry · Radiometry|
- Standards organizations recommend that photometric quantities be denoted with a suffix "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
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.