The significance of light scattering around the main beam in agonioreflectometer based measurement of spectral diffuse reflectance is discussed. This kind of scatter is always present in goniometric measurement systems that use a shaped measurement beam. Furthermore, errors due to this effect are a potential cause of discrepancies between goniometer and integrating-sphere based measurement methods. Procedures used to determine such errors and the required corrections are presented. At the Helsinki University of Technology, the determined wavelength-dependent correction in the worst case was as high as -1.6percent and could be reduced to -0.2percent with improvements in the optical setup of the gonioreflectometer. The measurement results for our diffuse reflectance reference materials with the appropriate corrections in each case are in good agreement, which gives a new view on some measurements reported earlier.
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