Series of measurements of the distribution of radiance over a clear sky made from locations at approximately 1000, 6000, and 14 000 ft elevation are reported. A theoretical relation is shown to correctly predict the observed variation in radiance with elevation angle in the 2–40 µ wavelength region, where radiation from the sky is mainly due to thermal emission by the atmosphere. In the 0.6–2 µ region where the radiation is mainly scattered sunlight the distribution is related to that which was observed simultaneously in the visible but forward scattering is relatively more important in the near infrared and the relative intensities in the two regions are not constant.
H. E. BENNETT, JEAN M. BENNETT, and MAX R. NAGEL, "Distribution of Infrared Radiance over a Clear Sky," J. Opt. Soc. Am. 50, 100-106 (1960)