# Luminosity

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In psychophysics luminosity is the perceived brightness of an object as opposed to its objectively measured physical luminance, although of course these are usually related.

## In photometry and color imagingEdit

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.

Main article: HSL color space

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 luminosityEdit

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.

## ReferencesEdit

[[Category:Visual perception