twilight illumination

Does anyone know where I can obtain data on illumination levels
(lux) for civil twilight under perfect atmospheric conditions ?

I understand that 3-500 lux is about the range from sunset to
the end of civil twilight.

Regards
Graham

Graham asked:

Does anyone know where I can obtain data on illumination levels
(lux) for civil twilight under perfect atmospheric conditions ?


There's a nice plot of nominal ground illumination levels over a range
of Sun altitudes (from +90 to -18 degrees) on page 491 of _Explanatory
Supplement to the Astronomical Almanac_, P. Kenneth Seidelmann (editor),
1992; the page following gives the polynomial coefficients needed to
duplicate this curve. Of course, you'd be especially interested in the
range of values between 0 and -6 degrees, the interval defined as civil
twilight.

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Mark Gingrich San Leandro, California