The rayleigh is a unit of photon flux, used to measure faint light emitted in the sky, such as airglow and auroras. It was first proposed in 1956 by , Franklin E. Roach, and . It is named for Robert Strutt, 4th Baron Rayleigh (1875–1947). Its symbol is R (also used for the röntgen, an unrelated unit). SI prefixes are used with the rayleigh. The relationship between photon radiance, L, (in units of photons per square metre per second per steradian) and I (in units of rayleighs) is