The optical properties in the wavelength range from 300 to 2500 nm, affecting the solar energy gain of the glazing, are measured with a Varian Cary 5000 spectrometer, equipped with an integrating sphere and further custom-made accessories to enable the characterisation of light­scattering samples. All films show high transmission values both in the visible and the near infrared range, comparable to those of uncoated glass sheets as depicted in Fig.2a, thus proving to be appropriate candidates for the planned application. Light transmittance ranges from 0.90 to 0.92 and solar transmittance from 0.85 to 0.90, as calculated according to EN 410 [6].

The optical properties in middle and far infrared wavelength range (up to 50 pm), affecting the temperature-induced energy loss of the glazing, are measured with a Bruker Equinox 55 FTIR spectrometer. Both transmittance and reflectance measurements are carried out. Transmittance values mainly depend on the material thickness and vary from 0.06 for a 375 pm thick PMMA film to 0.25 for a 76 pm thick PET film, as calculated by weighting the measured spectra with the Planck’s black body radiation function at 283 K (Fig.2b). This partial infrared transparency, which distinguishes plastic films from glass panes, enhances the radiative heat transfer between inner and outer layers of the glazing and has therefore to be taken into account in combination with the emissivity while calculating the thermal insulation performance of the system.

Additional optical measurements are performed to evaluate the photo degradation of films upon irradiation with UV-light in the wavelength range from 300 to 400 nm. Films suspended in a glass unit are exposed to variable UV-light intensities, depending on the UV-transmittance of the outer pane (0.80^0.85 for a white-glass uncoated pane, 0.05^0.55 for a common low-e coated pane, as calculated according to EN 410).

Accelerated indoor tests are carried out similar to EN ISO 4892 [7]. Both UV-stabilized and not — stabilized films, screened by differently UV-light transmittive glazings, are exposed to our solar simulator, which reproduces spectral distribution and intensity of natural sunlight (Air Mass 1.5) in the corresponding wavelength range. First results are evaluated after 400 hours exposure: not-

stabilized films, with the expected exception of PMMA, already show significant changes in their transmittance spectra if not completely screened from ultraviolet radiation, proving to be not suitable for the planned application. The effectiveness of the UV-protective coating used as stabiliser (Fig.3) has now to be investigated over a longer period by both laboratory and outdoor tests.