Luminosity

Luminosity has different meanings in several different fields of science.

In photometry and color imaging
In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.

In Adobe Photoshop's imaging operations, luminosity is the term used incorrectly to refer to the luma component of a color image signal; that is, a weighted sum of the nonlinear red, green, and blue signals. It seems to be calculated with the Rec. 601 luma co-efficients (Rec. 601: Luma (Y’) = 0.299 R’ + 0.587 G’ + 0.114 B’).

The "L" in HSL color space is sometimes said to stand for luminosity. "L" in this case is calculated as 1/2 (MAX + MIN), where MAX and MIN refer to the highest and lowest of the R'G'B' components to be converted into HSL color space.

Elementary relations for luminosity
The following relations hold
 * $$L = \rho v \,$$ (if the target is perfectly opaque)
 * $$\frac{dN}{dt} = L \sigma$$
 * $$\frac{d\sigma}{d\Omega} = \frac{1}{L} \frac{d^{2}N}{d\Omega dt}$$

where
 * $$L$$ is the Luminosity.
 * $$N$$ is the number of interactions.
 * $$\rho$$ is the number density of a particle beam, e.g. within a bunch.
 * $$\sigma$$ is the total cross section.
 * $$d\Omega$$ is the differential solid angle.
 * $$ \frac{d\sigma}{d\Omega}$$ is the differential cross section.

For an intersecting storage ring collider:
 * $$L = f n \frac{N_{1} N_{2}}{A}$$

where
 * $$f$$ is the revolution frequency
 * $$n$$ is the number of bunches in one beam in the storage ring.
 * $$N_{i}$$ is the number of particles in each beam
 * $$A$$ is the cross section of the beam.