Round robin of painted targets BRDF measurements

Several French research laboratories set up goniometers allowing BRDF measurements at different laser wavelengths in the infrared. On the effort of the Delegation Generale de l'Armement (DGA/STTC), a round robin set of painted targets BRDF measurements was undertaken, under the ONERA expertise. The laboratories participating to this round robin were the Aerospatiale Matra CCR Suresnes, the IPN SMA-Virgo Lyon, the Institut Fresnel Marseille, and the CEA DAM CESTA Le Barp. The goniometers of the four laboratories are firstly described. The targets studied are seven 5cm diameter painted disks of aluminum or steel, a spectralon reference sample, and a sandpaper sample. We have first demonstrated that the pollution of painted targets with dust has a very weak influence on the BRDF. Before and after each measurement series, the directional-hemispherical reflectance of the samples was measured at ONERA. The measurements have been achieved according to a protocol specifying the sample position and the laser probe size. Chosen wavelengths for the inter-comparison are 1.064 mum and 10.6 mum. For both wavelengths, the characteristics of the different goniometers are compared in term of noise and repeatability. The difference between the painted targets BRDF measured with the various devices are relatively limited at 1.06 mum, and mainly induced by speckle. More important differences are obtained at 10.6 mum, particularly for a BRDF measurement device using an absolute calibration method. In order to explain these differences, few hypotheses are advanced. Information on the absolute accuracy is obtained by the comparison of the measured directional-hemispherical reflectance and the one computed from BRDF measurements.