UNIVERSAL ANTIREFLECTION COATINGS FOR SUBSTRATES FOR THE VISIBLE SPECTRAL REGION

It is possible to design normal-incidence antireflection coatings that reduce the reflectance of any substrate with a refractive index that lies in the range of 1.48 to 1.75. The performance of such coatings de pends on the width of the spectral region over which the reflectance i s to be suppressed, on the coating materials used for their constructi on, and on the overall optical thickness of the layer system. For exam ple, the calculated average spectral reflectance of a set of six diffe rent substrates with refractive indices 1.48, 1.55, 1.60, 1.65, 1.70, and 1.75, when coated with a 0.56-mu m-thick, eight-layer antireflecti on coating designed for the 0.40-0.80-mu m spectral region, was 0.34%. This is higher than the average reflectance that is attainable with a conventional antireflection coating of similar optical thicknesses de signed for a particular refractive index. However, it is an acceptable value for most applications. With the universal type of antireflectio n coating described, it is thus possible to coat a number of different refractive-index substrates in one deposition run, and this can resul t in considerable cost and time savings. (C) 1996 Optical Society of A merica