The optical properties of heat-insulating ceramics made of microballoons of aluminum oxide

The effective absorption coefficient and the radiation diffusion coefficient are determined for two types of ceramics made of aluminum oxide microballoons with densities of $440$ kg/m$^3$ and $1100$ kg/m$^3$. The method is based on the experimental investigation of the normal hemispherical transmissivity of cylindrical samples of different thickness and the angular dependence of the directional hemispherical transmissivity of one of the samples for wavelengths of $0.63$, $1.15$, and $3.39\mu$m. The dependence of the bihemispherical reflectivity of a flat layer on its thickness is calculated from the data obtained.