A nonabsorbing periodically stratified medium (abbreviated PSM) acts as a band-reflecting filter. Within a high-reflectance band, the reflectance tends to 100 percent in the limit as the number of periods increases indefinitely. A high-reflectance band is usually approximately centered on any wavelength such that the combined optical thickness of the layers constituting one period equals an integral number of half-waves. The integer in question is called the order of the high-reflectance band. Certain orders may be suppressed.
A heat-reflecting filter can be made up of PSM’s having some unsuppressed high-reflectance bands in the infrared and only suppressed bands in the visible. The longer the wavelengths reflected by the first order, the greater the number of orders in the visible which must be suppressed. Present heat-reflecting PSM’s have only a single order suppressed. A method is described for designing a PSM which has two successive orders suppressed.
L. IVAN EPSTEIN, "Improvements in Heat-Reflecting Filters," J. Opt. Soc. Am. 45, 360-362 (1955)