The apostilb is an old unit of luminance, which was withdrawn from use in 1978.[by whom?]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:
|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⋅m−3||L−3⋅T⋅J|
|Luminous efficacy||η [nb 3]||lumen per watt||lm/W||M−1⋅L−2⋅T3⋅J||Ratio of luminous flux to radiant flux or power consumption, depending on context|
|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 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
- 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.
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.