Passive solar heating – Energy production

Depending on whether it is opaque or transparent, a material will not have the same absorption and emission properties to a given radiation, although these two characteristics remain closely linked. Absorption is a phenomenon that takes place in the mass, and is a function of the thickness through which it passes. On the other hand, a material emits radiation by drawing on its internal store of energy, thus reducing its temperature.

Thus, the behavior of an opaque material is determined by its average absorption coefficient and its average emissivity at usual temperatures. These parameters qualify its surface as selective, a good reflector, cold or close to black. A transparent material, on the other hand, is characterized by its transmission coefficient, a function not only of the wavelength but also of the incidence of the radiation.