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In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light in terms of the electromagnetic field or photons. In SI units it is measured in W m−3, though it is more practical to measure it in W m−2 nm−1 (1 W m−2 nm−1 = 1 GW m−3 = 1 W mm−3) or W m−2 μm−1 (1 W m−2 μm−1 = 1 MW m−3). The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density. The terms used to describe spectral flux density vary between fields, sometimes including adjectives such as "electromagnetic" or "radiative", and sometimes dropping the word "density". Applications include: *Characterizing remote telescopically unresolved sources such as stars, observed from a specified observation point such as an observatory on earth. *Characterizing a natural electromagnetic radiative field at a point, measured there with an instrument that collects radiation from a whole sphere or hemisphere of remote sources. *Characterizing an artificial collimated electromagnetic radiative beam. ==Flux density received from an unresolvable "point source"== For the flux density received from a remote unresolvable "point source", the measuring instrument, usually telescopic, though not able to resolve any detail of the source itself, must be able to optically resolve enough details of the sky around the point source, so as to record radiation coming from it only, uncontaminated by radiation from other sources. In this case,〔Green, S.F., Jones, M.H., Burnell, S.J. (2004). ''An Introduction to the (Sun and Stars )'', Cambridge University Press, Cambridge UK, ISBN 0-521-83737-5, page 21.()〕 spectral flux density is the quantity that describes the rate at which energy transferred by electromagnetic radiation is received from that unresolved point source, per unit receiving area facing the source, per unit wavelength range. At any given wavelength ''λ'', the spectral flux density, ''Fλ'', can be determined by the following procedure: *An appropriate detector of cross-sectional area 1 m2 is pointed directly at the source of the radiation. *A narrow band-pass filter is placed in front of the detector so that only radiation whose wavelength lies within a very narrow range, Δ''λ'', centred on ''λ'', reaches the detector. *The rate at which EM energy is detected by the detector is measured. *This measured rate is then divided by Δ''λ'' to obtain the detected power per square metre per unit wavelength range. Spectral flux density is often used as the quantity on the ''y''-axis of a graph representing the spectrum of a light-source, such as a star. 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「Spectral flux density」の詳細全文を読む スポンサード リンク
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