ABSTRACT
All materials have the capacity to absorb and emit the energy they receive from the sun. Absorption is a measure of how much incident radiation is received by a material. The emissivity of the surface of a material is a measure of its effectiveness in emitting energy. The emitted energy is in the form of long wave or thermal radiation. In scientific terms: Absorption refers to the ability of a material to absorb solar radiation; and Emissivity or emittance refers to the ability of a material to emit infrared radiation. The emission coefficient depends on the surface condition of the material, including its roughness, surface and oxide layers. The radiated power varies with temperature, being greater at higher temperatures. Most solar collectors are, in practice, operated at temperatures above that used to measure the emissivity factor, sometimes causing the amount of emitted radiation to be higher than expected.
