The effect of single and multilayer coatings on the reflectance of metals is discussed. Two multilayer film designs especially suitable for producing mirror coatings with low visible and high infrared reflectance are described. In the first design aluminum is coated with evaporated films of germanium and silicon monoxide, each film approximately one-quarter wavelength thick. In one example using this construction the visible reflectance is decreased to 2% and a steep rise in reflectance from 5% to 90% takes place from 0.7μ to 1.2μ. In a second more versatile construction aluminum is coated with two dielectric layers separated by a semitransparent metal film. In one example of this design, consisting of Al–SiO–Al–SiO, the low-reflectance region extends from 0.4μ to 1.5μ and more than 90% reflectance is obtained for all wavelengths longer than 3μ. These filter mirrors are useful for decreasing unwanted visible radiation in infrared equipment.
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