The rayleigh is a unit of
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...
, used to measure faint light emitted in the sky, such as
airglow
Airglow is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffuse sky radiation, diffuse ...
and
aurora
An aurora ( aurorae or auroras),
also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...
s. 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 (1875–1947).
Its symbol is R (also used for the
röntgen, an unrelated unit).
SI prefix
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official st ...
es are used with the rayleigh.
One rayleigh (1 R) is defined as a column emission rate of 10
10 photons per
square metre
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter ( American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...
per column per
second
The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
. A column is .
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
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
, ''L'', (with unit photon per square metre per second per
steradian
The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. A solid angle in the fo ...
) and ''I'' (with unit rayleigh) is:
1 rayleigh can thus be expressed in SI units as either:
* 10
10 photons s
−1 (m
2 column)
−1
* 1/4π 10
10 photons s
−1 m
−2 sr
−1
References
{{reflist
Units of luminous flux