Apostilb

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The apostilb is an old unit of luminance, which was withdrawn from use in 1978.[by whom?][citation needed] The SI unit of luminance is the candela per square metre (cd/m2).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

See also[edit]

Other units of luminance:

SI photometry units
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Luminous energy Qv [nb 2] lumen second lm⋅s T⋅J [nb 3] units are sometimes called talbots
Luminous flux Φv [nb 2] lumen (= cd⋅sr) lm J [nb 3] also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J [nb 3] an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2⋅J units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx L−2⋅J used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx L−2⋅J used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J
Luminous energy density ωv lumen second per metre3 lm⋅s⋅m−3 L−3⋅T⋅J
Luminous efficacy η [nb 2] lumen per watt lm/W M−1⋅L−2⋅T3⋅J ratio of luminous flux to radiant flux
Luminous efficiency V 1 also called luminous coefficient
See also: SI · Photometry · Radiometry
  1. ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
  2. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
  3. ^ a b c "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.
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