Apostilb

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
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: SI Photometry Radiometry

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. 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:

• Lambert (L)
• Skot (sk)
• Bril (bril)
• Nit (nit)
• Foot-lambert (fL)

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.