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ay45c1-page7

# ay45c1-page7 - 1 1 lumen = light emitted by 60 cm2...

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Unformatted text preview: 1 1 lumen = light emitted by 60 cm2 blackbody at the melting point of platinum, 2044 K 1 lux = 1 lumen/m2  1 candle at 1 m A bright (mV = 0) star is 2:5  10 6 lux. A good VCR camera (with a charge-coupled device (CCD)) works down to 5 lux. Use a pinhole to get into candle-comparison range: L angle 1/2o (size of sun) pinhole image of sun. Area A' area A All light incident on area A appears spread out over an area A0 =  D2 =   L  1  1 rad 4 4 2 57:3 = L2  6:0  10 5 : 2 So brightness diminished by a factor (> 1) A0 = L2  6:0  10 5 : AA Taking L = 20 m = 2  103 cm, A = (:1 cm)2 = 10 2 cm2. This gives A0=A = 24000, which gets sun down to  5 candles at 1 m or 1 candle at 45 cm. So, by this and similar techniques, around time of Huygens (1650) people knew the \candlepower" of the Sun:  2 11 127000 candles  11AU = 127000  1:5  10 m m 1m = 2:8  1027 candles. 7 2 candles ...
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