☙

herschel

A unit of luminance proposed in 1942 by Parry Moon, corresponding to the blondel, but for radiometric measurements. It is doubtful if it was ever used.

Parry Moon.
A system of photometric concepts.
Journal of the Optical Society of America, volume 32, pages 348-362 (June 1942).

Page 356: “The unit is pi times the lumens per square meter per steradian....The corresponding name in the radiometric system is the herschel.”