The rayleigh is a unit of photon flux, used to measure airglow (auroras, for example). It was first proposed in 1956 by D. M. Hunten, Franklin E. Roach, and J. W. Chamberlain. It is named for Robert John 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.