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In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.〔(Luminous exitance ) ''Drdrbill.com''〕 In SI derived units these are measured in lux (lx) or lumens per square metre (cd·sr·m−2). In the CGS system, the unit of illuminance is the phot, which is equal to . The foot-candle is a non-metric unit of illuminance that is used in photography.〔One phot = , according to http://www.unitconversion.org/unit_converter/illumination.html〕 Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word, such as to mean luminance. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light. The human eye is capable of seeing somewhat more than a 2 trillion-fold range: The presence of white objects is somewhat discernible under starlight, at , while at the bright end, it is possible to read large text at 108 lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages. ==Astronomy== In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band.〔(【引用サイトリンク】title=Radiometry and photmetry in astronomy FAQ, section 7 )〕 V-magnitudes can be converted to lux using the formula〔(【引用サイトリンク】title=Formulae for converting to and from astronomy-relevant units )〕 :, where ''E''v is the illuminance in lux, and ''M''v is the apparent magnitude. The reverse conversion is :. 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「Illuminance」の詳細全文を読む スポンサード リンク
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