Measurement of emittance and degree of polarisation of surfaces for solar energy converters
Dirk Labuhn, Stephan Kabelac
The basic equations for calculating radiation entropy and the major input functions to these equations, namely the spectral and directional distribution of radiation intensity and its degree of polarisation, are presented. Thermodynamic evaluation of energy conversion processes by means of the balance equations requires all involved energy and entropy fluxes to be known. The incoming radiation energy and entropy can be obtained from atmospheric models, but the properties of the outgoing radiation strongly depend on the optical properties of the surface. As the required optical properties usually are not available over the entire spectral range and for all directions, measurements are performed in order to improve the accuracy of the energy and entropy balances. An apparatus is presented, capable of measuring the spectral directional emittance and the degree of polarisation of the emitted radiation in the range of infrared wavelengths.