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The experimental assessment of the effects of non-Lambertian surfaces on integrating sphere measurements in the mid-IR

Integrating spheres are appropriate tools for the measurement of the near-normal/hemispherical reflectance and transmittance. The sphere walls have to be highly reflecting and should have a Lambertian BRDF in the interesting wavelength range. Diffusely reflecting gold-coatings are used in the mid infrared, which have a suitable surface roughness. Their surface properties influence the scattering behavior wavelength-dependent resulting usually in decreasing total integrating scattering with increasing wavelength and changing BRDF. We investigated two different spheres with the same dimension (200mm diameter) and design, but with different coating, which were attached to a Fourier-transform spectrometer (Bruker IFS 66) for measurements between 1.7μm and 17μm. The surfaces of the spheres were investigated by Scanning Electron Microscopy and angular-dependent reflectance measurements in the relevant spectral range. Samples with different surface roughness, but with the same coating were measured with both spheres. The correction of the results using formula derived by Hanssen yielded a very good agreement of the spectral reflectance.