Rayleigh (unit)
Encyclopedia
The rayleigh is a unit of photon
flux
, used to measure air glow (aurora
s, for example). It was first proposed in 1956 by D. M. Hunten, Franklin E. Roach, and J. W. Chamberlain. It is named for Robert John Strutt, 4th Baron Rayleigh
(1875–1947). Its symbol is R (also used for the röntgen
, an unrelated unit). SI prefix
es 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
. 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, (in units of photons per square metre per second per steradian
) and I (in units of rayleighs) is simply
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
, used to measure air glow (aurora
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
s, for example). It was first proposed in 1956 by D. M. Hunten, Franklin E. Roach, and J. W. Chamberlain. It is named for Robert John Strutt, 4th Baron Rayleigh
Robert John Strutt, 4th Baron Rayleigh
Robert John Strutt, 4th Baron Rayleigh was a British peer and physicist. He discovered "active nitrogen", was the first to distinguish the glow of the night sky....
(1875–1947). Its symbol is R (also used for the röntgen
Röntgen
The roentgen is a unit of measurement for exposure to ionizing radiation , and is named after the German physicist Wilhelm Röntgen...
, an unrelated unit). SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...
es are used with the rayleigh.
One rayleigh (1 R) is defined as a column emission rate of 1010 photons per square metre
Square metre
The square metre or square meter is the SI derived unit of area, with symbol m2 . It is defined as the area of a square whose sides measure exactly one metre...
per column per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
. 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, (in units of photons per square metre per second per steradian
Steradian
The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...
) and I (in units of rayleighs) is simply