Physics:Apostilb
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The apostilb is a largely 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
Template:Luminance conversions
Applications
While now considered obsolete in perimetry,[1] as of 2026[update] Humphrey visual field analysers still specify the illumination in the system in apostilbs.[3]
Table of SI photometry quantities for reference
| Quantity | Unit | Dimension | Notes | |||||
|---|---|---|---|---|---|---|---|---|
| Name | Symbol[nb 1] | Name | Symbol | Symbol[nb 2] | ||||
| 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 steradians) | 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 | 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 | lux | lx | 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 | ||||||||
- ↑ 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
- ↑ 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.
- ↑ 3.0 3.1 3.2 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
- ↑ 1.0 1.1 "IPS Perimetric Standards, 1978". Imaging and Perimetry Society. 1978. https://www.perimetry.org/__static/bb20125638576b668c43afcf4656d75b/1978-ips-perimetry-standards.pdf?dl=1.
- ↑ Moon, Parry (June 1942). "A system of photometric concepts". Journal of the Optical Society of America 32 (6): 356. doi:10.1364/JOSA.32.000348. Bibcode: 1942JOSA...32..348M. "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.".
- ↑ "Zeiss Humphrey Field Analyzer 3". 2025. https://asset-downloads.zeiss.com/catalogs/download/med2/0b234617-c7bc-452a-b5cc-d4e1f2e3ca7c/HFA3_Brochure_2025_EN.pdf.
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