A 0.7% accurate formula is derived for the easy conversion of power spectral radiance Lλ in W cm-2 sr-1 µm-1to rayleigh spectral radiance Rλ in rayleigh/µm, Rλ = 2πλLλ × 1013, where the wavelength λ is in µm. The rationale for the rayleigh unit is discussed in terms of a photon rate factor and a solid angle factor. The latter is developed in terms of an equivalence theorem about optical receivers and extended sources, and the concept is extended to the computation of photon volume emission rates from altitude profiles of zenith radiance.
Doran J. Baker, "Rayleigh, the Unit for Light Radiance," Appl. Opt. 13, 2160-2163 (1974)