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rayleigh

A unit of luminous flux used to measure air glow and aurora, first proposed in 1956.¹ Symbol, R. The SI prefixes are used with this unit. One rayleigh is 10¹⁰/4π quanta per square meter per second per steradian.

The night sky has a luminous intensity of about 250 rayleigh, while auroras can reach values of 1000 kilorayleigh.

The unit is named for Robert John Strutt (1875 – 1947), the young man to the right in the lovely photo by an unknown photographer. He became the fourth Lord Rayleigh. The older man is his father, John William Strutt, third Lord Rayleigh. The rayl is named for the third Lord Rayleigh.

The third and fourth Lord Rayleigh

Photo from Wikimedia Commons

Note that the raleigh is not an SI unit.

The reason for using photons per second instead of watts is that in gaseous photochemistry (the word photophysics may be more appropriate) the photon is conveniently treated statistically along with the concentrations of the other particles of the medium.

D. Baker.
Rayleigh, the unit for light radiance.
Applied Optics, vol. 13, pages 2160-2163 (1974).
Page 2160.

1. D. M. Hunter, F. E. Roach, and J. W. Chamberlaine.
A photometric unit for the airglow and aurora.
Journal of Atmospheric and Terrestrial Physics (GB) vol. 8, no. 6, page 345 (1956).
doi:10.1016/0021-9169(56)90111-8

resources

Joseph W. Chamberlain.
Physics of the Aurora and Airglow.
New York: Academic Press, 1961.
Appendix II.

D. Baker and G. Romick.
The rayleigh: interpretation of the unit in terms of column emission rates or apparent radiance expressed in SI units.
Applied Optics, vol. 15, pages 1966-1968 (1976).

Robert E. Huffman.
Atmospheric Ultraviolet Remote Sensing.
Boston: Academic Press, c1992.
Page 22+.