Rayleigh (unit)
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 Donald M. Hunten, Franklin E. Roach, and Joseph W. Chamberlain. It is named for Robert Strutt, 4th Baron Rayleigh. Its symbol is R. SI prefixes are used with the rayleigh.
One rayleigh is defined as a column emission rate of 1010 photons per square metre per column per second. Note that rayleigh is an apparent emission rate, as no allowances have been made for scattering or absorption. The night sky has an intensity of about 250 R, while auroras can reach values of 1000 kR.
The relationship between photon radiance, L, and I is
