Improving the performance of the NIST five axis goniospectrometer for measurements of bidirectional reflectance distribution function

The five axis goniospectrometer at the National Institute of Standards and Technology (NIST) measures the spectral reflectance of colored samples over a wide range of illumination and viewing angles. This capability is important for the colorimetric characterization of complex materials, such as gonioapparent coatings or retroreflective surfaces. To improve the efficiency of the goniometer, a broad-band source with a matrix-based stray-light corrected CCD based spectrometer was implemented. This new configuration offers a significant reduction in the measurement time allowing for the complete characterization of the goniodistribution of complex materials. Shorter measurement time reduces the load on the precise mechanical assembly, to ensure high angular accuracy over time. Special care was taken to extend the effective dynamic range of measured intensities in the multichannel detection mode to the values of 10(6) - 10(7) needed for the characterization of colored samples. The expanded uncertainty of the measured Bidirectional Reflectance Distribution Function (BRDF) for this new setup is about 0.5 % (k = 2) which is comparable to the uncertainty levels of the instrument operating with monochromatic illumination and a silicon photodiode. To validate the new system configuration, the measured BRDF or spectral reflectance factors (R) of test samples were compared with different instruments and we found an agreement of about 0.5 %.