Using a photoelectric telephotometer and a calibration source, drift curves of the luminance of the sun along a diameter were obtained for different zenith distances of the sun on clear days. After extrapolation to zero air mass, the luminance B (0,0) at the center of the sun’s disk observed outside the earth’s atmosphere was obtained as 243 000±5000 candles/cm2. The data obtained for the radial distribution of luminance being comparatively less accurate, the standard limb darkening data of Abbot and others were used to give the average luminance B¯= 193 000±4000 candles/cm2 and the extraterrestrial total solar illumination Ē0= 12 200±300 lumens/ft2 for the mean solar distance. Previous determinations of Ē0 found in the literature are reviewed and compared with these results.
R. V. KARANDIKAR, "Luminance of the Sun," J. Opt. Soc. Am. 45, 483-488 (1955)