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   Computes the earth's peak radiance. 
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   Form:
   rad = Radiance( lat, dateTime, year )
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   Inputs
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   lat             (n)           latitude (rad)
   dateTime        (6 or m)      [ year month day hour minute seconds ] or fraction
                                 of year
   year                          year if dateTime is fraction of a year.

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   Outputs
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   rad             (n,m)         Radiance values (W/m^2sr)

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