The apostilb is an obsolete unit of luminance.[1] The SI unit of luminance is the candela per square metre (cd/m2). In 1942 Parry Moon proposed to rename the apostilb the blondel, after the French physicist André Blondel.[2] 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
π asb = 1 cd/m2
Units of luminance
cd/m2 (SI unit)
≡ nit ≡ lm/m2/sr		 stilb (sb) (CGS unit)
≡ cd/cm2		 apostilb (asb)
≡ blondel		 bril skot (sk) lambert (L) foot-lambert (fL)
= 1 ⁄ π cd/ft2
1 cd/m2 = 1 10−4 π
≈ 3.142		 107 π
≈ 3.142×107		 103 π
≈ 3.142×103		 10−4 π
≈ 3.142×10−4		 0.30482 π
≈ 0.2919
1 sb = 104 1 104 π
≈ 3.142×104		 1011 π
≈ 3.142×1011		 107 π
≈ 3.142×107		 π
≈ 3.142		 30.482 π
≈ 2919
1 asb = 1 ⁄ π
≈ 0.3183		 10−4 ⁄ π
≈ 3.183×10−5		 1 107 103 10−4 0.30482
≈ 0.09290
1 bril = 10−7 ⁄ π
≈ 3.183×10−8		 10−11 ⁄ π
≈ 3.183×10−12		 10−7 1 10−4 10−11 0.30482×10−7
≈ 9.290×10−9
1 sk = 10−3 ⁄ π
≈ 3.183×10−4		 10−7 ⁄ π
≈ 3.183×10−8		 10−3 104 1 10−7 0.30482×10−3
≈ 9.290×10−5
1 L = 104 ⁄ π
≈ 3183		 1 ⁄ π
≈ 0.3183		 104 1011 107 1 0.30482×104
≈ 929.0
1 fL = 1 ⁄ 0.30482 ⁄ π
≈ 3.426		 1 ⁄ 30.482 ⁄ π
≈ 3.426×10−4		 1 ⁄ 0.30482
≈ 10.76		 107 ⁄ 0.30482
≈ 1.076×108		 103 ⁄ 0.30482
≈ 1.076×104		 10−4 ⁄ 0.30482
≈ 1.076×10−3		 1

Table of SI photometry quantities for reference

SI photometry quantities
Quantity Unit Dimension
[nb 1]		 Notes
Name Symbol[nb 2] Name Symbol
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 steradian) 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 (= lm/(sr⋅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 lumen per square metre lm/m2 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/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:
  1. ↑ 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.
  2. ↑ 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
  3. 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

See also

Other units of luminance:

References

  1. ↑ "IPS Perimetric Standards, 1978". Imaging and Perimetry Society. 1978.
  2. ↑ Moon, Parry (June 1942). "A system of photometric concepts". Journal of the Optical Society of America. 32 (6): 356. Bibcode:1942JOSA...32..348M. doi:10.1364/JOSA.32.000348. The unit is pi times the lumens per square meter per steradian. Such a name is impossibly cumbersome, so a new name is proposed, the blondel, after André Blondel who did pioneer work in photometric nomenclature.
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