The behavior of a symmetrical nonabsorbing multilayer in the region where the component film thicknesses are small compared with the wavelength closely approximates the behavior of a homogeneous single film. The index of the equivalent film can be made to take any value intermediate to the index values of the constituent films; and by adding together multiple periods of the basic combination, the equivalent thickness can be made as large as desired. Such combinations are shown to have useful application in the construction of wide-range antireflection coatings for high-index infrared optical materials.
PETER H. BERNING, "Use of Equivalent Films in the Design of Infrared Multilayer Antireflection Coatings," J. Opt. Soc. Am. 52, 431-435 (1962)