The variation with wavelength of thermal radiation properties of coated surfaces is important for solar energy absorbers and in other applications, and depends on the optical properties of the coating material and the substrate, and on the structure of the coating. Theoretical considerations indicate that, for homogeneous coatings, the good optical properties of semiconductors can be improved if the coating contains a high fraction of voids. This improvement has been shown by experimental particulate PbS coatings which are not homogeneous; the selective properties of particulate coatings are better than those calculated for solid coatings at the same coating weight. PbS coatings on aluminum substrates show absorptances for solar radiation of 0.9 and long wave emittance of 0.2 to 0.3. The feasibility of selective paints has been demonstrated; PbS can be mixed with a transparent binder, silicone, to make a paint which shows a good degree of selectivity, and absorptances of 0.9 and emittances less than 0.4 have been obtained.

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