Angular- selective window coatings: some theoretical and experimental studies

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University of Dar es Salaam
This dissertation proposes a potential solution to the problem of seasonal and diurnal control of radioactive throughput across windows. The variation of solar transmission with the sun's altitude can be controlled by window coatings with angular selective transmittance. A theoretical study, based on several model investigations, on angular selectivity of homogeneous films of Ag, Al and Cr was made. A computer simulation optimized the transmittance of single films and of 3- & 5-layer stacks including these metals and ZnS or SiO2 in the visible and near IR regions. The latter constructions have improved transmittance performance. It is demonstrated that more pronounced angular selectivity is attainable with five (than with three or single) layer coatings. In order to devise means to obtain enhanced angular selectivity, an initial experimental study investigated silver films with inclined columnar microstructures. These films, characterized by an off-normal optical axis, were fabricated by resistive evaporation at oblique angle of vapour incidence onto glass. Scanning electron microscopy revealed 3nm-resolved microstructures comprising distinct, more or less well defined, columns. Transmission electron microscopy and surface profilometry showed that the degree of inhomogeneity and porosity in the films increased with evaporation angle, a. Electron diffraction showed a decreasing crystallinity with increasing a. Effects of self-shadowing and ad-atom mobility are thought to be responsible for these features. Spectrophotametry yielded strongly angular selective transmittance. For the first time, a "Transition region", which defines the span of attainable angular selective transmittance, is defined and identified in silver films. Possible explanations for some of the salient optical features are discussed. The universality of the "tangent rule" for defining the columnar inclination angle, especially for a >_ 60°, is discredited.
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Windows, Angular window guards
Mbise, G. W. (1989) Angular- selective window coatings: some theoretical and experimental studies, Masters dissertation, University of Dar es Salaam. Available at (