DETERMINATION OF THE DIRECTIONAL SPECTRAL EMISSIVITY OF SOLIDS IN THE SPECTRAL RANGE FROM 2.5-MU-M TO 45-MU-M AT TEMPERATURES BETWEEN 80-DEGREES-C AND 350-DEGREES-C

The measuring set-up described allows the directional spectral emissivity to be determined in the wavelength range from 2.5 mum to 45 mum at temperatures between 80-degrees-C and 400-degrees-C. The samples must be opaque to radiation of the wavelength investigated. They must have a diameter of between 50 mm and 90 mm and be up to 10 mm thick. The uncertainty of the determination of the spectral emissivity amounts to 1.0%. To determine the spectral emissivity, the spectral radiances of black-body and sample surface are compared at known temperature using a Fourier spectrometer. The problematic determination of the sample surface temperature is carried out on the basis of Fourier's equation of thermal conduction, making use of the known thermal conductivity of the sample (which must be separately determined) and the calculated energy loss from the surface to the environment by conduction, convection and radiation. To check the measurement results, the vertical spectral emissivity was measured on three coated glass samples whose total emissivity had been measured in an international comparison. The total vertical emissivities calculated from the spectral values determined here show very good agreement with the values obtained in the international comparison and with other measurement values.