Overheating is a problem with the use of active/passive solar energy in thermal solar energy systems. A solution to these problems might be provided by a thermochromic material such as vanadium dioxide. In order to simulate the optical behaviour of multilayered coatings, knowledge on its optical properties is necessary. We determined point-by-point the dielectric function for VO2:W by ellipsometry. For validation, the solar spectra were measured by spectrophotometry. Such data have been compared with the computer simulations based on the determined optical properties. Finally, we collect optical data by infrared-imaging to detect the switch in emissivity of VO2:W at around 45°C.