The Spectral Radiance of the Sun from 4μ to 5μ

The radiance of the solar disk was measured as a function of wavelength from 4.0 μ to 5.0 μ with a spectral radiometer system which was calibrated in terms of absolute radiance. The measurements were taken at an altitude of 31 km and, therefore, are relatively unaffected by the terrestrial atmosphere. The radiance values are presented at 0.1-μ intervals over the 1.0-μ range, and it is shown that the effective radiation temperature of the solar disk decreased from 5626°K ± 100°K at 4.0 μ to 5270°K ± 150°K at 5.0 μ. This result represents the only measurements which have been made of solar radiance vs wavelength beyond 2.5 μ so no direct comparison is available; however, the result fits with a single measurement which has been made at longer wavelengths.