According to Planck's Law, the wavelength of a radiation emitted from a surface is proportional to the temperature of the surface. These coatings do not alter the re-radiative properties of the plate metal, only enhance the absorption of the solar radiation. The thickness of the chemical coating is proportional to the selectivity of the surface. Despite the low relative cost and ease of application, chemical coatings are often undesirable because of the temperatures reached inside collectors which can cause a degradation in the chemical coatings. For example, black paint applied to the plate is considered to be a chemical coating and at high temperatures, the paint is likely to melt or burn off the surface, releasing volatile organic compounds to the environment.
Electroplated coatings are the most widely used coatings in the solar collector industry.
These coatings are applied to the absorber plate metal using traditional electroplating technology. Black Chrome, a common electroplated coating used in the manufacture of solar collectors is relatively stable, particularly in humid, tropical conditions. Vapour deposited coatings are not traditionally used in flat plate collectors, as there are a number of significant engineering problems which are yet to be overcome. However, they are used extensively in evacuated collectors, which utilise a partial vacuum , hence the term evacuated, such as the receivers in high temperature solar thermal systems. Oxide coatings were the first type of coating used in solar collectors.