Optical Properties of Granular Materials: How to Model Coatings for Energy-Efficient Windows
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OPTICAL PROPERTIES OF GRANULAR MATERIALS: HOW TO MODEL COATINGS FOR ENERGY-EFFICIENT WINDOWS C.G. GRANQVIST Physics Department, Chalmers University of Technology and University of Gothenburg, S-412 96 Gothenburg, Sweden ABSTRACT I discuss coatings for energy efficient windows of several different kinds with a view to the possibility of modelling their optical properties by effective medium theory. Special attention is given to spectrally selective noble-metal-based coatings with induced transmission due to voids, angular selective metal coatings with oblique columnar microstructure, and tentative theoretical descriptions of chromogenic materials having dynamic radiative properties. I. INTRODUCTION Granular materials are of great scientific and technological importance [1-41. This paper reviews some work on granular coatings for applications to energy-efficient windows and discusses the use of effective medium theories for modelling optical properties. Energy efficiency and solar energy utilization is conveniently introduced by taking the radiative properties of our surrounding as a starting point [5,6]. Figure 1 illustrates four such prop-
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erties on a common logarithmic wavelength scale. Thermal radiationcan be described by multiplying the blackbody spectrum for the temperature of interest by an emittance less than unity. From the blackbody spectra in Fig. la, it is apparent that thermal radiation at temperatures of relevance to many solar-energy-related applications are confined to the wavelength range 2
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