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A spectrophotometer is described by means of which the spectral distribution of radiation from the sun and sky, or the sky alone, may be conveniently measured if the sky is either completely clear or completely overcast. The instrument is calibrated using a standard lamp, whose spectral energy distribution is known in the region 0.4 micron to 0.7 micron. A “fatigue-correcting” lamp, included in the optical system, enables the results to be expressed in energy units. Possible errors are discussed.

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