THERMAL EMISSION FROM A ROUGH-SURFACE - RAY OPTICS APPROACH

Surface roughness effects on thermal emission are studied with geometric optics. The surface consists of elements with known scattering and emission properties, and both the single and multiple scattering/emission is included. Heat conduction is not included, and therefore the present work is applicable only to surfaces with nearly static incident radiation. The method is applied to a rough surface obeying Gaussian statistics, whence it is completely determined by the standard deviation of height and the correlation length. Thermal radiation exchanged by facing surface elements enhances considerably the conversion of incident radiation into heat, and changes the directional characteristics of emission. Infrared measurements of the Moon are interpreted with the model, and a value of about 20 degrees is derived for the rms slope of its surface. Surface roughness is also shown to be the probable cause of the so called beaming of thermal radiation of the Moon toward the Sun.