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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Brightness temperature ： ウィキペディア英語版 Brightness temperature Brightness temperature is the temperature a black body in thermal equilibrium with its surroundings would have to be to duplicate the observed intensity of a grey body object at a frequency $\backslash nu$. This concept is extensively used in radio astronomy and planetary science. The brightness temperature is not a temperature as ordinarily understood. It characterizes radiation, and depending on the mechanism of radiation can differ considerably from the physical temperature of a radiating body (though it is theoretically possible to construct a device which will heat up by a source of radiation with some brightness temperature to the actual temperature equal to brightness temperature). Nonthermal sources can have very high brightness temperatures. In pulsars the brightness temperature can reach 1026 K. For the radiation of a typical helium–neon laser with a power of 60 mW and a coherence length of 20 cm, focused in a spot with a diameter of 10 µm, the brightness temperature will be nearly . For a black body, Planck's law gives:〔Rybicki, George B., Lightman, Alan P., (2004) ''Radiative Processes in Astrophysics'', ISBN 978-0-471-82759-7〕〔 ::$I\_\backslash nu\; =\; \backslash frac\backslash frac\}-1\}$ where $I\_\backslash nu$ (the Intensity or Brightness) is the amount of energy emitted per unit surface area per unit time per unit solid angle and in the frequency range between $\backslash nu$ and $\backslash nu\; +\; d\backslash nu$; $T$ is the temperature of the black body; $h$ is Planck's constant; $\backslash nu$ is frequency; $c$ is the speed of light; and $k$ is Boltzmann's constant. For a grey body the spectral radiance is a portion of the black body radiance, determined by the emissivity $\backslash epsilon$. That makes the reciprocal of the brightness temperature: ::$T\_b^\; =\; \backslash frac\backslash ,\; \backslash text\backslash left(+\; \backslash frac\}-1\}\backslash right)$ At low frequency and high temperatures, when $h\backslash nu\; \backslash ll\; kT$, we can use the Rayleigh–Jeans law: ::$I\_\; =\; \backslash frac$ so that the brightness temperature can be simply written as: ::$T\_b=\backslash epsilon\; T\backslash ,$ In general, the brightness temperature is a function of $\backslash nu$, and only in the case of blackbody radiation it is the same at all frequencies. The brightness temperature can be used to calculate the spectral index of a body, in the case of non-thermal radiation. == Calculating by frequency == The brightness temperature of a source with known spectral radiance can be expressed as:〔(【引用サイトリンク】author= Jean-Pierre Macquart )〕 : $T\_b=\backslash frac\; \backslash ln^\backslash left(\; 1\; +\; \backslash frac\; \backslash right)$ When $h\backslash nu\; \backslash ll\; kT$ we can use the Rayleigh–Jeans law: : $T\_b=\backslash frac$ For narrowband radiation with very low relative spectral linewidth $\backslash Delta\backslash nu\; \backslash ll\; \backslash nu$ and known radiance $I$ we can calculate the brightness temperature as: : $T\_b=\backslash frac$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Brightness temperature」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.