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Transmission variation using scattering/transparent switching films

: Wilson, H.R.; Eck, W.


Solar energy materials and solar cells 31 (1993), S.197-214
ISSN: 0927-0248
Fraunhofer ISE ()
Mie scattering; multiple scattering; polymer dispersed liquid crystal; smart window; switching layer; thermotropic; transmission

Due to the fallibility of mechanical shading devices such as shutters or blinds, thin films with variable transmittance, which can be applied directly to a glazing surface, are being investigated for so-called "smart windows" or for applications together with transparent insulating materials. In the films discussed here, the transmittance is changed by varying the scattering properties, which depend on changes in the effective refractive index and/or size of microstructures. This may be achieved by orientation and relaxation of birefringent liquid crystal molecules within polymer-encapsulated droplets by applying an electric field, by reversible separation and mixing of two polymers with temperature variation or by exploiting the Christiansen effect (1). In this paper, the effect of varying experimentally accessible parameters on the transmittance change is first explored theoretically by applying Mie and multiple scattering theories. These results are complemented by measurements of t he spectral dependence of the transmittance and reflectance of the first two types of switching samples, i.e. polymer-dispersed liquid crystal (PDLC) films and thermotropic layers of polymer blends. Changes of almost 40 percent in the visual and solar direct-hemispherical transmittance were measured for the PDLC samples and of about 60 percent for the thermotropic layers investigated here.