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Encyclopedia > Luminosity
Look up Luminosity in
Wiktionary, the free dictionary.

Luminosity has different meanings in several different fields of science. Wikipedia does not have an article with this exact name. ... Wiktionary (a portmanteau of wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ...

## In photometry and color imaging GA_googleFillSlot("encyclopedia_square");

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. Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... In astronomy, photometry is the measurement of the flux or intensity of an astronomical objects electromagnetic radiation. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...

Main article: luma (video)

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’). As applied to video signals, luma represents the brightness in an image (the black and white or achromatic portion of the image). ... Adobe Photoshop, or simply Photoshop, is a graphics editor developed and published by Adobe Systems. ... As applied to video signals, luma represents the brightness in an image (the black and white or achromatic portion of the image). ...

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. The HSL color space, also called HLS or HSI, stands for Hue, Saturation, Lightness (also Luminance or Luminosity) / Intensity. ... The HSL color space, also called HLS or HSI, stands for Hue, Saturation, Lightness (also Luminance or Luminosity) / Intensity. ...

## In astronomy

In astronomy, luminosity is the amount of energy a body radiates per unit time. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ...

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 more energy the object radiates than the Sun, whose luminosity is 3.90×1026 W. Rendition of an imaging bolometer from Los Alamos National Laboratory A bolometer is a device for measuring incident electromagnetic radiation. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... For other uses, see Watt (disambiguation). ... The solar luminosity, , is a unit of luminosity (power emitted in the form of photons) conventionally used by astronomers to give the luminosities of stars. ... The Sun (Latin: ) is the star at the center of the Solar System. ...

Luminosity is an intrinsic constant independent of distance, and is measured as absolute magnitude corresponding to apparent luminosity, or bolometric magnitude corresponding to bolometric luminosity; in contrast, apparent brightness is related to distance with an inverse square relationship. Visible brightness is usually measured by apparent magnitude, which is a logarithmic scale. In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. ... The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ...

In measuring star brightnesses, visible luminosity (not total luminosity of entire wave length), 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. The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... Distance is a numerical description of how far apart objects are at any given moment in time. ...

### Computing between brightness and luminosity

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. A sphere is a symmetrical geometrical object. ...

$b = frac{L}{A}$

where

A is the area of the illuminated surface.

For stars and other point sources of light, A = 4πr2 so

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

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: As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ...

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

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 The Stefan-Boltzmann constant (also Stefans constant), denoted with a Greek letter &#963;, is a derivable physical constant, the constant of proportionality between the total energy radiated per unit surface area of a black body in unit time and the fourth power of the thermodynamic temperature, as per the... For other uses, see Watt (disambiguation). ...

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

For stars on the main sequence, luminosity is also related to mass: Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ...

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

It is easy to see that a star's luminosity, temperature, radius, and mass are all related.

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

where

mstar is the apparent magnitude of the star (a pure number)
msun is the apparent magnitude of the sun (also a pure number)
Lstar is the visible luminosity of the star
$L_{odot}$ is the solar visible luminosity
rstar is the distance to the star
rsun is the distance to the sun

Or simplified, given msun = −26.73, distsun = 1.58 × 10−5 lyr:

mstar = − 2.72 − 2.5 · log(Lstar/diststar2)

Example:

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

Also you can calculate the luminosity given a distance and apparent magnitude: Alpha Centauri (Î± Cen / Î± Centauri, also known as Rigil Kentaurus), is the brightest star system in the southern constellation of Centaurus. ...

Lstar/$L_{odot}$ = (diststar/distsun)2 · 10[(msun −mstar) · 0.4]
Lstar = 0.0813 · diststar2 · 10(−0.4 · mstar) · $L_{odot}$

Example:

What is the luminosity of the star Sirius? Sirius B redirects here. ...

Sirius is 8.6 lyr distant, and magnitude −1.47.
LSirius = 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. STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. ... The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ...

## 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. Scattering theory is a branch of physics and especially of quantum mechanics whose aim is the study of scattering events. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... Area is a physical quantity expressing the size of a part of a surface. ... A pocket watch, a device used to tell time Look up time in Wiktionary, the free dictionary. ... Look up opacity in Wiktionary, the free dictionary. ... This article or section is in need of attention from an expert on the subject. ... A centimetre (American spelling centimeter, symbol cm) is a unit of length that is equal to one hundredth of a metre, the current SI base unit of length. ... Look up second in Wiktionary, the free dictionary. ... A barn (symbol b) is a unit of area. ... The integral of f(x) from a to b is the area above the x-axis and below the curve y = f(x), minus the area below the x-axis and above the curve, for x in the interval [a,b]. Integration is a core concept of advanced mathematics, specifically...

### Elementary relations for luminosity

The following relations hold

$L = rho v ,$ (if the target is perfectly opaque)
$frac{dN}{dt} = L sigma$
$frac{dsigma}{dOmega} = frac{1}{L} frac{d^{2}N}{dOmega dt}$

where

L is the Luminosity.
N is the number of interactions.
ρ is the number density of a particle beam.
σ is the total cross section.
dΩ is the differential solid angle.
$frac{dsigma}{dOmega}$ is the differential cross section.

For an intersecting storage ring collider: In nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles. ... A differential can mean one of several things: Differential (mathematics) Differential (mechanics) Differential signaling is used to carry high speed digital signals. ... A solid angle is the three dimensional analog of the ordinary angle. ... In nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles. ...

$L = f n frac{N_{1} N_{2}}{A}$

where

f is the revolution frequency
n is the number of bunches in one beam in the storage ring.
Ni is the number of particles in each bunch
A is the cross section of the beam.

Results from FactBites:

 Luminosity - Wikipedia, the free encyclopedia (713 words) The SI unit for luminosity is candela per square metre. W. Luminosity is an intrinsic constant independent of distance, while in contrast apparent brightness observed is related to distance with an inverse square relationship. The integrated luminosity is the integral of the luminosity with respect to time.
More results at FactBites »

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