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
v t e		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
v
t
e
Quantity		Unit		Dimension ^{[nb 1]}	Notes
Name	Symbol^{[nb 2]}	Name	Symbol	Dimension ^{[nb 1]}	Notes
Luminous energy	$Q v$ ^{[nb 3]}	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called the talbot.
Luminous flux, luminous power	$Φ v$ ^{[nb 3]}	lumen (= candela steradian)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	$I v$	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	$L v$	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	$E v$	lux (= lumen per square metre)	lx (= lm/m²)	L⁻²⋅J	Luminous flux incident on a surface
Luminous exitance, luminous emittance	$M v$	lumen per square metre	lm/m²	L⁻²⋅J	Luminous flux emitted from a surface
Luminous exposure	$H v$	lux second	lx⋅s	L⁻²⋅T⋅J	Time-integrated illuminance
Luminous energy density	$ω v$	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: SI Photometry Radiometry

^ 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.
^ 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
^ ^a ^b ^c Alternative symbols sometimes seen: $W$ for luminous energy, $P$ or $F$ for luminous flux, and $ρ$ for luminous efficacy of a source.

Table of SI photometry quantities for reference

See also

References