The solar spectral irradiance outside the earth’s atmosphere was determined by Langley’s method of extrapolation to zero air mass, from measurements taken on Mount Lemmon at an elevation of 8025 ft near Tucson, Arizona, during October, 1951. The spectrum was produced and the energy scanned by a Leiss quartz double-monochromator, detected by a 1P21 photomultiplier, amplified, and presented on a strip chart recorder. About twenty-five spectra were recorded from sunrise to noon, with band widths ranging from 10 A at 3030 A to 170 A at 7000 A. The equipment was calibrated frequently by recording the spectrum of a standard tungsten lamp. Compared with earlier work performed in this field, our results agree best with those of Pettit. There is good agreement with the direct measurements from a rocket obtained by Purcell and Tousey in 1954 and with the Sacramento Peak ultraviolet observations by Stair and Johnston in 1955. The change of solar intensity with air mass showed that the attenuation of the atmosphere above Mount Lemmon was approximately 15% higher than that for a Rayleigh atmosphere in the region 3400 A to 4650 A, where there is no absorption due to ozone. A discussion is included which emphasizes the importance of clear and constant atmospheres which are necessary to obtain accurate values of solar spectral irradiance outside the earth’s atmosphere by the Langley method. The solar illuminance computed from spectral data was 12 700 lumens/ft2.
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