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| '''Irradiance''' is the [[Power (physics)|power]] of [[electromagnetic radiation]] per unit [[area]] ([[radiative flux]]) incident on a surface. '''Radiant emittance''' or '''radiant exitance''' is the power per unit area radiated by a surface. The [[SI]] units for all of these quantities are [[watt]]s per square meter (W/m<sup>2</sup>), while the [[centimeter-gram-second|cgs]] units are [[erg]]s per [[square centimeter]] per [[second]] (erg·cm<sup>−2</sup>·s<sup>−1</sup>, often used in [[astronomy]]). These quantities are sometimes called [[Intensity (physics)|intensity]], but this usage leads to confusion with [[radiant intensity]], which has different units.
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| All of these quantities characterize the total amount of radiation present, at all [[Frequency#Frequency_of_waves|frequencies]]. It is also common to consider each frequency in the [[Electromagnetic spectrum|spectrum]] separately. When this is done for radiation incident on a surface, it is called '''spectral irradiance''', and has SI units W/m<sup>3</sup>, or commonly W·m<sup>−2</sup>·nm<sup>−1</sup>.
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| If a point source radiates light uniformly in all directions through a non-absorptive medium, then the irradiance decreases in proportion to the square of the distance from the object.
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| ==Technical details==
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| The irradiance of a monochromatic light wave in matter is given in terms of its electric field by
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| <ref name=griffiths>{{cite book|last=Griffiths|first=David J.|title=Introduction to electrodynamics|year=1999|publisher=Prentice-Hall|location=Upper Saddle River, NJ [u.a.]|isbn=0-13-805326-X|url=http://www.amazon.com/Introduction-Electrodynamics-3rd-David-Griffiths/dp/013805326X|edition=3. ed., reprint. with corr.}}</ref>
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| :<math>I \approx \frac{c \, n \, \epsilon_0}{2} |E|^2,</math>
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| where ''E'' is the [[complex number|complex]] [[amplitude]] of the wave's [[electric field]], ''n'' is the [[refractive index]] of the medium, <math>c</math> is the [[speed of light]] in [[vacuum]], and ϵ<sub>0</sub> is the [[vacuum permittivity]]. (This formula assumes that the magnetic susceptibility is negligible, i.e. <math>\mu_r \approx 1</math> where <math>\mu_r</math> is the magnetic permeability of the light transmitting media. This assumption is typically valid in transparent media in the optical frequency range.)
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| Irradiance is also the time average of the component of the [[Poynting vector]] perpendicular to the surface.
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| == Solar energy ==
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| The global irradiance on a horizontal surface on Earth consists of the direct irradiance ''E''<sub>dir</sub> and diffuse irradiance ''E''<sub>dif</sub>. On a tilted plane, there is another irradiance component: ''E''<sub>ref</sub>, which is the component that is reflected from the ground. The average ground reflection is about 20% of the global irradiance. Hence, the irradiance ''E''<sub>tilt</sub> on a tilted plane consists of three components: ''E''<sub>tilt</sub> = ''E''<sub>dir</sub> + ''E''<sub>dif</sub> + ''E''<sub>ref</sub>.<ref name=Quaschning>{{cite journal |last=Quaschning |first=Volker |title=Technology fundamentals—The sun as an energy resource |journal=Renewable Energy World |volume=6 |year=2003 |issue=5 |pages=90–93 |url=http://www.volker-quaschning.de/articles/fundamentals1/index_e.html}}</ref>
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| The [[integral]] of solar irradiance over a time period is ''solar irradiation'' or [[insolation]]. Irradiation is generally measured in J/m<sup>2</sup> and is represented by the symbol ''H''.<ref name=Quaschning/><ref>{{cite doi|10.1016/0038-092X(60)90062-1}}</ref>
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| {{SI radiometry units}}
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| == See also ==
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| * [[Radiometry]]
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| * [[Spectral flux density]]
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| * [[Photometry (optics)]] Main Photometry/Radiometry article—explains technical terms
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| * [[Radiance]]
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| * [[Albedo]]
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| * [[Illuminance]]
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| * [[Fluence]]
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| * [[Insolation]]
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| * [[Light diffusion]]
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| * [[PI curve]] (photosynthesis-irradiance curve)
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| * [[Solar azimuth angle]]
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| * [[Solar irradiation]]
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| * [[Solar irradiation#Solar_constant|Solar constant]]
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| * [[Solar noon]]
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| * [[Stefan–Boltzmann law]]
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| ==References==
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| <references/>
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| [[Category:Physical quantities]]
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| [[Category:Radiometry]]
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| [[ru:Облучённость (фотометрия)]]
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