Apostilb

{{short description|Unit of luminance}}

The apostilb is an obsolete unit of luminance.{{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}} 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.{{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 }} The symbol for the apostilb is asb.

The apostilb is defined in terms of another unit of luminance, the stilb (sb):

:1 asb = 1/{{pi}} ⋅ 10⁻⁴ sb

:{{pi}} asb = 1 cd/m²

{{luminance conversions}}