Luminosity has different meanings in several different fields of science.
In photometry and color imaging
photometry, "luminosity" is sometimes incorrectly used to refer to luminance, which is the density of luminous intensityin a given direction. The SIunit for luminance is candelaper square metre.
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 spaceis 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.
astronomy, luminosity is the amount of energy a body radiates per unit time.
The luminosity of stars is measured in two forms: apparent (counting visible light only) and bolometric (total radiant energy); a
bolometeris 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 SIunits watts, or in terms of solar luminosities, ; 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 constant independent of distance, and is measured 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 parallax is usually too small to be measured for many faraway 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 distanceare 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
Imagine a point source of light of luminosity that radiates equally in all directions. A hollow
spherecentered 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.
:where: is the area of the illuminated surface.
For stars and other point sources of light, so : where: is the distance from the observer to the light source.
It has been shown that the luminosity of a star (assuming the star is a
black body, which is a good approximation) is also related to temperature and radius of the star by the equation::where:σ is the Stefan-Boltzmann constant5.67e|−8 W·m-2·K-4
Dividing by the luminosity of the sun and cancelling constants, we obtain the relationship
For stars on the
main sequence, luminosity is also related to mass::
The magnitude of a star is a logarithmic scale of observed visible brightness. The
apparent magnitudeis the observed visible brightness from Earth, and the absolute magnitudeis the apparent magnitudeat a distance of 10 parsecs.Given a visible luminosity (not total luminosity), one can calculate the apparent magnitudeof a star from a given distance:
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: is the solar visible luminosity:"r"star is the distance to the star:"r"sun 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::Lstar/ = (diststar/distsun)2 · 10 [(msun −mstar) · 0.4] :Lstar = 0.0813 · diststar2 · 10(−0.4 · mstar) · Example:
What is the luminosity of the star
Sirius?:Sirius is 8.6 lyr distant, and magnitude −1.47.:LSirius = 0.0813 · 8.62 · 10−0.4·(−1.47) = 23.3 × :You can say that Sirius is 23 times brighter than the sun, or it radiates 23 suns.
starwith bolometric magnitude −10 has a luminosity of 106 , whereas a dim star with bolometric magnitude +17 has luminosity of 10−5 . Note that absolute magnitudeis directly related to luminosity, but apparent magnitudeis 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::
which makes by inversion:
In scattering theory and accelerator physics
scattering theoryand accelerator physics, luminosity is the number of particles per unit areaper unit timetimes the opacity of the target, usually expressed in either the cgsunits cm-2 s-1 or b-1 s-1. The integrated luminosity is the integralof the luminosity with respect to time. The luminosity is an important value to characterize the performance of an accelerator.
Elementary relations for luminosity
The following relations hold: (if the target is perfectly opaque): : where: is the Luminosity.: is the number of interactions.: is the number density of a particle beam.: is the total cross section.: is the
differential solid angle.: is the differential cross section.
For an intersecting storage ring collider::
where: is the revolution frequency: is the number of bunches in one beam in the storage ring.: is the number of particles in each bunch: is the cross section of the beam.
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Look at other dictionaries:
Luminosity — Lu mi*nos i*ty, n. The quality or state of being luminous; luminousness. [1913 Webster] … The Collaborative International Dictionary of English
luminosity — (n.) 1630s, quality of being luminous, from Fr. luminosité or else a native formation from LUMINOUS (Cf. luminous) + ITY (Cf. ity). In astronomy, intrinsic brightness of a heavenly body (as distinguished from apparent magnitude, which diminishes… … Etymology dictionary
luminosity — ► NOUN ▪ luminous quality … English terms dictionary
luminosity — [lo͞o΄mənäs′ə tē] n. [ML luminositas] 1. the quality or condition of being luminous 2. pl. luminosities something luminous 3. brightness … English World dictionary
luminosity — [[t]lu͟ːmɪnɒ̱sɪti[/t]] 1) N UNCOUNT The luminosity of a star or sun is how bright it is. [TECHNICAL] For a few years its luminosity flared up to about 10,000 times the present day luminosity of the Sun. 2) N UNCOUNT You can talk about the… … English dictionary
luminosity — /looh meuh nos i tee/, n., pl. luminosities. 1. luminance (def. 2). 2. the quality of being intellectually brilliant, enlightened, inspired, etc.: The luminosity of his poetry is unequaled. 3. something luminous. 4. Astron. the brightness of a… … Universalium
luminosity — noun a) the state of being luminous, or a luminous object; brilliance or radiance b) the ratio of luminous flux to radiant flux at the same wavelength; the luminosity factor … Wiktionary
luminosity — skaistis statusas T sritis chemija apibrėžtis Šviečiančio ar šviesą sklaidančio paviršiaus šviesos ryškumas tam tikra kryptimi (Cd/m²). atitikmenys: angl. brightness; luminance; luminosity rus. яркость … Chemijos terminų aiškinamasis žodynas
luminosity — skaistis statusas T sritis fizika atitikmenys: angl. brightness; luminance; luminosity vok. Helligkeit, f; Leuchtdichte, f rus. яркость, f pranc. brillance, f; luminance, f … Fizikos terminų žodynas
luminosity — skaistis statusas T sritis ekologija ir aplinkotyra apibrėžtis Medžiagos skaidrumą apibūdinantis dydis, lygus spinduliuotės srauto, kritimo kryptimi perėjusio per vienetinio storio medžiagos sluoksnį, ir į tą medžiagą kritusio srauto dalmeniui.… … Ekologijos terminų aiškinamasis žodynas