<title>Optical properties of thermotropic layers</title>

A number of different thermotropic materials are now known, which operate on the principle of a reversible change between a homogeneous, transparent state at lower temperatures and a heterogeneous, scattering state at higher temperatures. Depending on the switching temperature, which is between 20 and 100 degree(s)C for the samples investigated, these materials can be used to prevent overheating in different solar energy applications, e.g. overhead glazing, transparently insulated walls and flat-plate collectors. In this paper, such materials are characterized experimentally by the temperature dependence of the direct-direct and direct-hemispherical transmittance and reflectance spectra over the solar spectral range. The results from multiple scattering theory for Mie scatterers and models for scattering in random media are compared to determine their correctness in describing the material structure in its scattering state. On the basis of the results obtained (diameter of scattering center or typical correlation length for a refractive index domain and difference between the component dielectric constants), suggestions are made on the changes required for a further optimization of the solar radiation switching performance.