# Apostilb

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{{short description|Unit of luminance}}
The '''apostilb''' is an obsolete unit of [luminance](/source/luminance).<ref name=IPS>{{cite web |url=http://www.perimetry.org/index.php/publications/141-general/799-1978-ips-perimetry-standards |title=IPS Perimetric Standards, 1978 |publisher=Imaging and Perimetry Society |date=1978}}</ref> The [SI](/source/SI) unit of luminance is the [candela per square metre](/source/candela_per_square_metre) (cd/m<sup>2</sup>). In 1942 Parry Moon proposed to rename the apostilb the '''blondel''', after the French physicist [André Blondel](/source/Andr%C3%A9_Blondel).<ref>{{cite journal |first=Parry |last=Moon |title=A system of photometric concepts |journal=Journal of the Optical Society of America |volume=32 |issue=6 |page=356 |date=June 1942 |doi=10.1364/JOSA.32.000348 |quote=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.|bibcode=1942JOSA...32..348M }}</ref> The symbol for the apostilb is '''asb'''.

The apostilb is defined in terms of another unit of luminance, the [stilb](/source/Stilb_(unit)) (sb):
:1 asb = 1/{{pi}} ⋅ 10<sup>−4</sup> sb
:{{pi}} asb = 1 cd/m<sup>2</sup>
{{luminance conversions}}

==Table of SI photometry quantities for reference==
{{SI light units}}
==See also==
Other units of luminance:
<!-- in order of relevance in regard to apostilb -->
* [Lambert](/source/Lambert_(unit))&nbsp;(L)
* [Skot](/source/Skot_(unit))&nbsp;(sk)
* [Bril](/source/Bril_(unit))&nbsp;(bril)
* [Nit](/source/Nit_(unit))&nbsp;(nit)
* [Foot-lambert](/source/Foot-lambert)&nbsp;(fL)

==References==
{{reflist}}

Category:Units of luminance
Category:Centimetre–gram–second system of units

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Adapted from the Wikipedia article [Apostilb](https://en.wikipedia.org/wiki/Apostilb) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Apostilb?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.