REFLECTANCE SOLAR ABSORPTIVITY AND THERMAL EMISSIVITY OF SIO2-COATED ALUMINUM

The reflectance, solar absorptivity (∝), and the total normal and hemispherical emissivity (∈N and ∈) of evaporated aluminum coated with Si02 films of various thicknesses were determined. High vacuum evaporation with an electron gun was used for preparing uv transparent undecomposed films of Si02up to thicknesses of more than 3.5 μ. Because of their hardness, chemical stability, and excellent adherence, evaporated Si02 films were found to be very suitable as protective layers for aluminum front surface mirrors, especially if high reflectance in the uv is required. ∝ of Si02-coated Al was determined to be about 11% and to be essentially independent of the Si02 thickness, whereas ∈N and ∈ increased with increasing oxide thickness, and reached values of 0.62 and 0.55, respectively, for a Si02 thickness of 3.75 μ. Films of this type are, therefore, suitable as surface layers for controlling the temperature of satellites in orbit. Ultraviolet irradiation in vacuum at one and five times the equivalent solar energy decreased the uv and visible reflectance of Si02-coated Al. The effect of this reflectance decrease on a/e and on the temperature of an orbiting satellite is discussed. © 1969 Optical Society of America.