Overheating is a common problem both with the use of active and passive solar energy in thermal solar energy
systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during
stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar
gains in winter, but imply in most cases high energy needs for air conditioning in summer. A solution to such
problems might be provided by "smart" thermochromic coatings. A durable inorganic thermochromic material is
vanadium dioxide. At 68°C, VO2 undergoes a reversible crystal structural phase transition accompanied by a
strong variation in optical properties. By doping the material with tungsten, it is possible to lower the transition
temperature making it suitable as a window coating. In order to simulate the optical behaviour of multilayered
solar coatings, precise knowledge on the optical material properties is necessary. Experimental data reported in
the literature are rare and controversial. We determined the complex dielectric function for VO2:W by
spectroscopic UV-VIS-NIR ellipsometry above and below the transition temperature and subsequent point-by-point
analysis of the ellipsometric psi/delta data. For a validation, the solar reflectance, absorptance and
transmittance were measured by spectrophotometry in the visible range and in the near infrared range up to 2500
nm. The experimental reflectance spectra have been compared with the computer simulations based on the
determined optical material properties. Finally, we collected optical data in a more extended wavelength range by
digital infrared imaging to detect the switch in thermal emissivity of VO2:W at around 45°C.
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