Materials for energy-efficient windows: New coatings with spectral and angular selectivity

This lecture introduces materials for energy-efficient windows. The focus is on coatings with pronounced spectral selectivity, i.e., with optical properties which are qualitatively different in different parts of the 0.3-100 μm wavelength range, and/or with pronounced angular selectivity, i.e., with a transmittance which varies strongly with the incidence angle and which is different depending on which side of the surface normal the light is incident from. The coatings dealt with here have static properties and thus do not depend on external conditions such as irradiation intensity, temperature, applied electric voltage, etc. Another lecture will cover materials with dynamic properties and discuss chromogenic materials and their application in smart windows. Section II treats the radiation which prevails in our ambience; this discussion serves as a basis for introducing spectral and angular selectivity as means to accomplish energy efficiency in windows. Section III gives a birds-eye view on coating materials with spectral and angular selectivity. Emphasis is on principles for reaching selectivity, and, whenever possible, on the obtainable optical properties. Section IV is a case study on the spectral selectivity in sputter deposited ZnO:Al coatings. This material can combine luminous transmittance, infrared reflectance, ultraviolet absorptance, and electrical conductance. Section V is another case study dealing with angular selectivity in obliquely evaporated Cr. Both case studies include experimental data and discussions of the theoretical models which can explain these data. Further discussions on materials for energy efficient windows - particularly spectrally selective coatings - are given. 73 refs, 23 figs