Spectral Radiance of Sky and Terrain at Wavelengths between 1 and 20 μ III Terrain Measurements*

A mobile infrared spectrographic laboratory has been used to make absolute determinations of the apparent infrared spectral radiance of various types of terrain. The spectral distribution of the observed radiation indicates the predominance of reflected sun and sky radiation at wavelengths less than approximately 3 μ, and thermal emission at longer wavelengths. Examples are shown for a number of types of terrain observed in various locations and conditions. The transition between the sky and the ground is also illustrated by spectra taken near the horizon. The spectra of gypsum and silica sands show reststrahlen effects in the atmospheric window of wavelengths between 8 and 13 μ. Many of the features of the spectra of terrain can be explained, in some cases quantitatively, on the basis of known parameters of the particular background and its atmospheric environment.