ANGULAR SELECTIVE WINDOW COATINGS - THEORY AND EXPERIMENTS

This review is devoted to the angular selectivity that can be obtained in thin films prepared under conditions such that they contain inclin ed absorbing regions of sizes much smaller than the wavelength of visi ble light. The films are of considerable interest as window coatings f or energy-conscious architecture and, potentially, in the automotive s ector. The theoretical basis for modelling the optical properties is p resented, comprising rigorous bounds on the dielectric function, effec tive medium theories pertinent to different microgeometries and equati ons for treating the optics of anisotropic thin films. Experimental da ta are reported for films made by oblique-angle evaporation of Cr and for reactive and non-reactive oblique-angle sputtering of Cr, Al, Ti a nd W. The highest angular selectivity was obtained with evaporated Or, whereas the highest luminous transmittance, combined with some angula r selectivity, was found with reactively sputtered Al. Films made from Ti showed angular selectivity mainly in the infrared, whereas films m ade from W could display angular selective electrochromism. Samples of several types were subjected to elaborate theoretical analysis using effective-medium theories and it was seen that theory and experiment c ould be reconciled using plausible parameters to specify the microstru ctures of the films. Thus it appears that the angular, spectral and po larization dependences of obliquely deposited films can be understood, at least approximately, in terms of conceptually simple theoretical m odels.