Metrology of Appearance: Study of Speckle within very high angular resolution BRDF measurements

The quantitative measurement of the visual appearance of objects is a subject of increazing importance for the industry in many fields such as automotive, cosmetics, packaging, 3D printing or virtual reality. The measurement techniques are now based on the acquisition of the bidirectional reflectance (BRDF) performed with a goniospectrophotometers. BRDF is usually measured with incoherent light. However, instruments have now reached such an angular resolution that even in these conditions, interference effects appear in the measurements, producing a phenomenon of light flicker called speckle. In metrology, this flicker, which varies according to the illuminated area on a homogeneous surface, is considered as a disturbance with respect to the targeted measurand, the BRDF. This thesis focuses on this speckle phenomenon, and on the way it depends on the characteristics of the goniospectrophotometer used (in particular the ConDOR instrument of LNE-CNAM, very angularly resolved) and on the statistical properties of the surface studied.A state of the art on appearance metrology allows to understand the issues related to the measurement of reference quantities for appearance. It is then specified that the thorough study of gloss is at the origin of the measurements with very high angular resolution which made appear the speckle within the measurements. The theory of speckle then shows that its statistical behavior as a function of different configurations allows to characterize it, and we use the probability laws of distribution of luminance on the sensor to understand the different "orders" of speckle observed. The different variable parameters of the system, namely the divergence of the source, its spectral width, the choice and size of the illuminated area on the sample and the impact of the angular resolution of the detection are then studied. The results of the measurements are presented in Chapter 2. The angular resolution of the system in these measurements is fixed so as not to degrade the progress made by ConDOR. From these measurements comes the need to specify the impact of spatial and temporal coherence. Chapters 3 and 4 aim to provide a deeper understanding of the speckle effect within BRDF measurements. A first emphasis is put on the spatial coherence phenomenon in monochromatic light. Measurements in laser illumination allow to conclude on the role of the beam diameter: if it increases, the speckle grains are larger, the contrast is lower. The consistency of the results of these measurements with the speckle theory is ensured by empirical BRDF simulations corresponding to the different cases presented. The impact of polarization is also observed. The approach in monochromatic illumination is then completed by measurements and simulations in incoherent illumination, where the impact of the spectral width of the source is considered. The observed speckle patterns see the grains transform into radial streaks. The comparisons between measurements and theoretical simulations proved insufficient, so a simulation approach based on surface microtopography was implemented. The results of these simulations are conclusive. These simulations, accompanied by the generation of fictitious surfaces based on the microtopography measurements, are then used in chapter 5 as a basis for theoretical averaging to approach a mean function, which can be considered as the "true" BRDF that characterizes the entire surface, and not only the particular area measured. Once again, this average is compared to an averaging of measured signals.

metrology, appearance, gloss, radiometry, BRDF, speckle

Full document (FR) : HAL-04205764