Luminosity

Luminosity is a measurement of brightness.

1 In photometry and color imaging
2 In astronomy
- 2.1 Computing between brightness and luminosity
- 2.2 Computing between luminosity and magnitude
3 In scattering theory and accelerator physics
- 3.1 Elementary relations for luminosity

In photometry and color imaging

Main article: luminance

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’).

Main article: HSL color space

The "L" in HSL color space is sometimes said incorrectly 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.

The luminosity function a.k.a. luminous efficiency function describes the average visual sensitivity of the human eye to light of different wavelengths. There are two luminosity functions in common use. For everyday light levels, the photopic luminosity function best approximates the response of the human eye. For low light levels, the response of the human eye changes, and the scotopic curve applies.

In astronomy

In astronomy, luminosity is the amount of electromagnetic energy a body radiates per unit of time.

The luminosity of stars is measured in two forms: apparent (counting visible light only) and bolometric (total radiant energy); a bolometer is an instrument that measures radiant energy over a wide band by absorption and measurement of heating. When not qualified, luminosity means bolometric luminosity, which is measured in the SI units watts, or in terms of solar luminosities, $L_{\odot}$ ; that is, how many times as much energy the object radiates than the Sun, whose luminosity is 3.846×10²⁶ W.

Luminosity is an intrinsic measurable property independent of distance, and is appraised as absolute magnitude, corresponding to the apparent luminosity in visible light of a star as seen at the interstellar distance of 10 parsecs, or bolometric magnitude corresponding to bolometric luminosity. In contrast, apparent brightness is related to the distance by an inverse square law. Onto this brightness decrease from increased distance comes an extra linear decrease of brightness for interstellar "extinction" from intervening interstellar dust. Visible brightness is usually measured by apparent magnitude. Both absolute and apparent magnitudes are on an inverse logarithmic scale, where 5 magnitudes increase counterparts a 100:th part decrease in nonlogaritmic luminosity.

By measuring the width of certain absorption lines in the stellar spectrum, it is often possible to assign a certain luminosity class to a star without knowing its distance. Thus a fair measure of its absolute magnitude can be determined without knowing its distance nor the interstellar extinction, and instead the distance and extinction can be determined without measuring it directly through the yearly parallax. Since the stellar parallax is usually too small to be measured for many far away stars, this is a common method of determining distances.

In measuring star brightnesses, visible luminosity (not total luminosity at all wave lengths), apparent magnitude (visible brightness), and distance are interrelated parameters. If you know two, you can determine the third. Since the sun's luminosity is the standard, comparing these parameters with the sun's apparent magnitude and distance is the easiest way to remember how to convert between them.

Computing between brightness and luminosity

Point source S is radiating light equally in all directions. The amount passing through an area A varies with the distance of the surface from the light.

Imagine a point source of light of luminosity $L$ that radiates equally in all directions. A hollow sphere centered on the point would have its entire interior surface illuminated. As the radius increases, the surface area will also increase, and the constant luminosity has more surface area to illuminate, leading to a decrease in observed brightness.

$F = \frac{L}{A}$

where

$A$ is the area of the illuminated surface.

$F$ is the Flux of the illuminated surface.

The surface area of a sphere with radius r is $A = 4\pi r^2$ , so for stars and other point sources of light

$F = \frac{L}{4\pi r^2} \,$

where

$r$ is the distance from the observer to the light source.

It has been shown that the luminosity of a star $L$ (assuming the star is a black body, which is a good approximation) is also related to temperature $T$ and radius $R$ of the star by the equation:

$L = 4\pi R^2\sigma T^4 \,$

where

σ is the Stefan-Boltzmann constant 5.67 × 10⁻⁸ W·m^-2·K^-4

Dividing by the luminosity of the sun $L_{\odot}$ and cancelling constants, we obtain the relationship

$\frac{L}{L_{\odot}} = {\left ( \frac{R}{R_{\odot}} \right )}^2 {\left ( \frac{T}{T_{\odot}} \right )}^4$ .

For stars on the main sequence, luminosity is also related to mass:

$\frac{L}{L_{\odot}} \sim {\left ( \frac{M}{M_{\odot}} \right )}^{3.9}$

The magnitude of a star is a logarithmic scale of observed visible brightness. The apparent magnitude is the observed visible brightness from Earth, and the absolute magnitude is the apparent magnitude at a distance of 10 parsecs. Given a visible luminosity (not total luminosity), one can calculate the apparent magnitude of a star from a given distance:

$m_{\rm star}=m_{\rm sun}-2.5\log_{10}\left({ L_{\rm star} \over L_{\odot} } \cdot \left(\frac{ r_{\rm sun} }{ r_{\rm star} }\right)^2\right)$

where

m_star is the apparent magnitude of the star (a pure number)

m_sun is the apparent magnitude of the sun (also a pure number)

L_star is the visible luminosity of the star

$L_{\odot}$ is the solar visible luminosity

r_star is the distance to the star

r_sun is the distance to the sun

Or simplified, given m_sun = −26.73, dist_sun = 1.58 × 10⁻⁵ lyr:

m_star = − 2.72 − 2.5 · log(L_star/dist_star²)

Example

How bright would a star like the sun be from 4.3 light years away? (The distance to the next closest star system Alpha Centauri)

m_sun (@4.3lyr) = −2.72 − 2.5 · log(1/4.3²) = 0.45

0.45 magnitude would be a very bright star, but not quite as bright as Alpha Centauri.

Luminosity can also be calculated given a distance and apparent magnitude :

L_star/ $L_{\odot}$ = (dist_star/dist_sun)² · 10^{[(m_sun −m_star) · 0.4]}

L_star = 0.0813 · dist_star² · 10^{(−0.4 · m_star)} · $L_{\odot}$

Example

What is the luminosity of the star Sirius?

Sirius is 8.6 lyr distant, and magnitude −1.47.

L_Sirius = 0.0813 · 8.6² · 10^{−0.4·(−1.47)} = 23.3 × $L_{\odot}$

You can say that Sirius is 23 times brighter than the sun, or it radiates 23 suns.

A bright star with bolometric magnitude −10 has a luminosity of 10⁶ $L_{\odot}$ , whereas a dim star with bolometric magnitude +17 has luminosity of 10⁻⁵ $L_{\odot}$ . Note that absolute magnitude is directly related to luminosity, but apparent magnitude is also a function of distance. Since only apparent magnitude can be measured observationally, an estimate of distance is required to determine the luminosity of an object.

Computing between luminosity and magnitude

The difference in absolute magnitude is related to the stellar luminosity ratio according to:

$M_1 - M_2 = -2.5 \log_{10} {\frac{L_1}{L_2}}$

which makes by inversion:

$\frac{L_1}{L_2} = 10^{(M_2 - M_1)/2.5}.$

In scattering theory and accelerator physics

In scattering theory and accelerator physics, luminosity is the number of particles per unit area per unit time times the opacity of the target, usually expressed in either the cgs units cm⁻² s⁻¹ or b⁻¹ s⁻¹. The integrated luminosity is the integral of the luminosity with respect to time. The luminosity is an important value to characterize the performance of an accelerator.