Measurement Of Near-Normal/Hemispherical Reflectance And Directional Emittance In The Mid-Infrared

Based on a FTIR spectrometer a measuring device for the determination of the spectral near-normal/hemispherical reflectance and the spectral directional emittance, respectively, of opaque, diffusely reflecting samples has been built up. The device is mainly used for the measuring of the spectral emittance of low-emitting selective solar absorber coatings. The measuring arrangement for hemispherical reflectance measurements with a gold coated integrating sphere is shown. The measuring geometry of the diffuse reflection attachment, the performance of the whole device as well as the calibration and evaluation of the measurements are described. Emphasis lies on the properties of the used integrating sphere and sphere design: on the angular response of the sphere and the influence of the limited reflectance and partly specularity of the sphere wall. It is shown that the integrating capacity of photometer spheres with an IR-active diffuse gold coating is partly reduced due to the relatively high deviation of the sphere wall reflectance from unity as well as due to the deterioration of the Lambertian behaviour of the wall indicated by a specular reflectance factor increasing with growing wave-lengths. Nevertheless the reflection method has some advantages over the radiometric measurement of the thermal radiation emitted by heated samples.