Original article J. Optics (París), 1982, vol. 13, n° 5, pp. 257-262 C entro de I nvestigaciones O pticas Casilla de Correo 124, 1900 La Plata (Argentina) PSEUDOCOLORING METHOD FOR 3 CONTOURING H. J. RABAL, E. E. SICRE, N. BOLOGNINI, M. GARAVAGLIA M ots clés : Speckle Contour K ey words Speckle Contouring Méthode pour la pseudocoloration des contours d’un objet tridimensionnel RÉSUMÉ : Les propriétés d’interférence des diagrammes de gra- nularité en lumiére blanche obtenus avec un systéme optique non corrigé de l’aberration chromatique, sont appliquées á la codification en fausses couleurs des contours de niveaux d’égale profondeur d’un objet diffusant tridimensionnel. On enregistre d’abord les différentes images des niveaux de profondeur codifiées en couleur. En une seconde étape, on obtient une image faussement colorée en modifíant la position d’un filtre spatial. On peut obtenir d’ailleurs un contour quantitatif en balayant l’image faussement colorée avec un dispositif spectroscopique. SUMMARY : The interference properties of white light speckle pattern obtained with an optical system not corrected for chromatic aberration are applied to pseudocolor encode the depth level contours of a 3-D diffusing object. At a first step, a recording of the different encoded images of depth levels is done. In a second step, a reconstructed pseudocolored image is obtained, where different color assignements can be tuned by changing the position of a spatial filter slit. Also, a quantitative contouring can be made scanning the pseudocolored image plañe with a spectroscopic device. INTRODUCTION Speckle patterns produced by partially coherent white light have been theoretically and experimentally studied in the last few years [1], Namely, the statistical properties of white light speckle have been investigated in the image plañe [2-6] and in the diffraction field [7, 8]. The dependence of the speckle pattern contrast with surface roughness was established in connection with different parameters that characterize the optical system and the illuminating light : the amplitude point spread function, and the spatial and temporal coherences. K. Nakagawa and T. Asakura [9] have shown experimentally that the average contrast of the white light image speckle depends strongly on the surface roughness : for small valúes of r.m.s. roughness (lower than 0.5 pm) the relevant parameter is the spatial coherence, while for'large valúes of r.m.s. roughness the effect of the point spread function is more important. When the speckle pattern is observed at a defocused plañe, for an object roughness greater than 1 pm, the additional phase-noise distortion pro duced by defocusing does not affect much the speckle pattern contrast. On the other hand, for surface objects having a r.m.s. roughness lower than 0.3 pm, the speckle pattern contrast is affected by the phase- noise distortion due to defocusing [7]. In the present study, the interference properties of white light patterns are applied to pseudocolor encode depth level contours of a 3-D object. For this purpose, an optical imaging system affected by chro matic aberration with an exit pupil consisting in two identical slits, is employed. In this way, the white light speckle pattern results in pairs of tiny spectra. When both spectra of each pair intersect they do it for the same wavelength and interference Young’s fringes appear in the intersection región. Therefore, at a fixed distance from the imaging lens, for each object plañe there exists a coded image for a certain wavelength. This spatial codification of color information is decoded, in a further step, to obtain a depth level pseudocoloring of the 3-D object. On the other hand, if the rough surface is a plañe one, a study of the chromatic aberration of the optical system can be done.