Stilb (unit)

The stilb (sb) is the CGS unit of luminance for objects that are not self-luminous. It is equal to one candela per square centimeter or 10⁴ nits (candelas per square meter). The name was coined by the French physicist André Blondel around 1920.^[1] It comes from the Greek word stilbein (στίλβειν), meaning 'to glitter'.

It was in common use in Europe up to World War I. In North America self-explanatory terms such as candle per square inch and candle per square meter were more common.^[2] The unit has since largely been replaced by the SI unit: candela per square meter. The current national standard for SI in the United States discourages the use of the stilb.^[3]

Unit conversion

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

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

• Scotopic stilb

References

1. Parry Moon. "System of photometer concepts", in the Journal of the Optical Society of America, volume 32, number 6 (June 1942). – Page 355: "The lumen was proposed by Blondel in 1894 and is now universally accepted. The names, phot and stilb were likewise coined by Blondel (1921) and are in general use on the Continent."
2. American Standard Definitions of Electrical Terms, New York: American Institute of Electrical Engineers, 1941.
3. IEEE/ASTM SI 10-2002. American National Standard for Use of the International System of Units (SI): The Modern Metric System. New York: IEEE, 30 December 2002. See Section 3.3.3.

Further reading

• Stilb at A Dictionary of Units of Measurement, Russ Rowlett and the University of North Carolina at Chapel Hill. Accessed June 2008.
• Stilb at Sizes.com. Accessed June 2008.