Rayleigh (unit)

{{Short description|Unit of photon flux}}

{{About|the unit of photon flux|the unit of acoustic impedance|Rayl}}

{{expert attention|Physics|date=September 2024|talk=Clarity of explanation}}

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.{{cite journal | author1 = Hunten, D. M. | author2-link = Franklin E. Roach | author2 = Roach, F. E. | author3 = Chamberlain, J. W. | year = 1956 | title = A photometric unit for the airglow and aurora | journal = Journal of Atmospheric and Terrestrial Physics | volume = 8 | pages = 345–346 | doi = 10.1016/0021-9169(56)90111-8 | issue = 6 | bibcode = 1956JATP....8..345H }} It is named for Robert Strutt, 4th Baron Rayleigh (1875–1947).{{cite journal | doi = 10.1364/AO.13.002160 | first = Doran J. | last = Baker | title = Rayleigh, the Unit for Light Radiance | journal = Applied Optics | volume = 13 | issue = 9 | pages = 2160–2163 | year = 1974 | url = http://www.opticsinfobase.org/abstract.cfm?URI=ao-13-9-2160 | pmid = 20134644 | bibcode = 1974ApOpt..13.2160B | url-access = subscription }} Its symbol is R (also used for the röntgen, an unrelated unit). SI prefixes are used with the rayleigh.

One rayleigh (1 R) is defined as a column emission rate of 10¹⁰ photons per square metre per column per second. A column is {{clarify span|one centimeter cross-section|how a cross-section (area) can be expressed in linear (length) units?|date=October 2024}}.{{cite web |url=https://folk.ntnu.no/espy/Second/Master_Student_Resources/Airglow/references/Baker_Rayleigh_ao-15-8-1966.pdf |title=The rayleigh: interpretation of the unit in terms of column emission rate or apparent radiance expressed in SI units |author1=Doran J. Baker |author2=Gerald J. Romick}} The rayleigh is a unit of an apparent emission rate, without allowances being 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, (with unit photon per square metre per second per steradian) and I (with unit rayleigh) is:{{dubious|date=September 2024}}

$$I\; =\; 4\backslash pi~\backslash mathrm\{sr\}\; \backslash times\; L\; \backslash times\; \backslash frac\{\backslash mathrm\{R\}\}\{\backslash mathrm\{10^\{10\}~photon\{\backslash cdot\}m^\{-2\}\{\backslash cdot\}s^\{-1\}\}\}$$

1 rayleigh can thus be expressed in SI units as either:

• 10¹⁰ photons s⁻¹ (m² column)⁻¹
• 1/4π 10¹⁰ photons s⁻¹ m⁻² sr⁻¹