What is "apostilb"

n. (context physics English) a unit of luminance, being the luminance of a perfectly diffusing surface emitting light at the rate of $pi^\{-1\}$ candela per square metre. Also known as a blondel.

The apostilb is an old unit of luminance, which was withdrawn from use in 1978. The SI unit of luminance is the candela per square metre (cd/m). In 1942 it was proposed to rename the apostilb the blondel, after the French physicist André Blondel. 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 sb 3.14 asb = 1 cd/m