An Etymological Dictionary of Astronomy and Astrophysics

A c.g.s. unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh (R) represents
the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second; or, in SI units, 795.775 x 10⁶ photons per square meter per steradian (m^-2·sr^-1). A dark night sky has a light intensity of roughly 250 R.

See also: In honor of the English mathematician and physicist Lord Rayleigh (1842-1919), surname of John William Strutt, Third Baron Rayleigh,
whose research ranged over several fields of physics.