1577 0195-928X/01/0900-1577/0 © 2001 Plenum Publishing Corporation International Journal of Thermophysics, Vol. 22, No. 5, September 2001 (© 2001) The Spectral Directional Emissivity of Photovoltaic Surfaces 1 1 Paper presented at the Fourteenth Symposium on Thermophysical Properties, June 25–30, 2000, Boulder, Colorado, U.S.A. D. Labuhn 2 and S. Kabelac 3 , 4 2 OHB-System GmbH, Universita ¨tsallee 27–29, D-28359 Bremen, Germany. 3 Institut für Thermodynamik, Universität der Bundeswehr Hamburg, D-22039 Hamburg, Germany. 4 To whom correspondence should be addressed. E-mail: Kabelac@unibw-hamburg.de. Photovoltaic solar cells are used for the direct conversion of solar radiation to electric power. To evaluate the efficiency of this energy conversion process, all in- and outgoing fluxes in the thermodynamic balance equations for energy and entropy must be known. The spatial and spectral distribution of radiation energy intensities must be known to calculate the radiation energy fluxes. To calculate the entropy fluxes, additional information on the coherence properties of the radiation field is essential. This information is expressed by the degree of polarization. First results of measurements of the optical properties of a solar cell are presented. The calculation procedure to obtain the outgoing energy and entropy fluxes is described. The experimental apparatus introduced in this paper yields the spectral directional emissivity by comparing the sample radiation with the radiation from an isothermal cavity. The degree of polarization of the emitted radiation is determined by a retarder/polarizer set within the apparatus. Both quantities are measured in the infrared region for wavelengths between 4.0 and 20.0 mm. KEY WORDS: degree of polarization; emissivity; radiation entropy; solar cell. 1. INTRODUCTION The solar energy flux which reaches the terrestrial surface after passing through the atmosphere in the form of electromagnetic radiation is accompanied by an entropy flux. This entropy results, on the one hand, from the spatial divergence of the propagating electromagnetic wave and,