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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 chargecoupled device (CCD)) works down
to 5 lux. Use a pinhole to get into candlecomparison 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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This document was uploaded on 11/02/2011 for the course AST 345 at SUNY Stony Brook.
 Fall '08
 Staff

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