Measurement and Calculation of the Directional Spectral Emissivity of Solid and Microstructured Surfaces

In today's world, a sound knowledge of the radiative properties of engineering surfaces is required for many engineering applications. Radiant energy transport is very important in space systems, high temperature heat exchangers, and any use of solar energy.For simplicity, in engineering, radiant energy transport is often described in terms of hemispherical and surface radiation properties integrated over all wavelengths. However, for a detailed description of radiant energy transport, the spectral and directional property values are needed. For example, the entropy of a radiation flux depends on the spectral directional distribution of the radiation. Therefore, one goal of our work is to measure directional spectral emissivities of engineering surfaces.

At our institute, an experimental setup for the measurement of directional spectral emissivities of solid surfaces at temperatures between 330 K and 500 K is being built. The directional distribution of the radiation can be investigated for polar angles between 0° and 70° and for azimuth angles between 0° and 90°. An FTIR spectrometer in the wavelength range between 4 µm and 24 µm is used for spectral detection of the radiation intensity. The system allows direct radiometric measurement of the emissivity. The emissivity is determined by comparing the radiation of the sample surface with the radiation of the installed blackbody radiator at the same temperature. The respective radiation under consideration is directed to the detector by means of a swivel mirror as shown in the figure. The sample chamber is evacuated to prevent convective heat transfer and cooled to reduce the ambient radiation.