AN ACTIVE VISION SYSTEM FOR 3D SURFACE COLOR MEASUREMENTS

摘要

Reliable colorimetric measurements can beachieved only if the measuring geometry is fixed or known. Unfortunately, when dealing with 3D objects, with large dimensions in space, like for instance monuments, geometry can hardly be controlled and new-concept instrumentation is needed to obtain results reproducible in different times and/or locations. To this end, a portable Stereo Active-Vision System (AVS) has been designed and built to fulfil the requirements for measuring colours of 3D surfaces. It allows both to measure its relative positions with respect to the tested surfaces and to the light sources, and to recover this measuring geometry carefully in successive campaigns. AVS is constituted by three aligned cameras. One camera (WA), equipped with a wide-angle lens, frames the whole region of interest at lower resolution, and drives the fixation of the other two ones on the subject. These cameras (TL) are equipped with long focal-length optics to frame only small portions of the scene at high resolution, and can rotate to fixate any point of the scene. Geometric measurements are carried out both by forward triangulation of the optical axes of the TLs cameras during fixation of an object and by three-dimensional (3D) reconstruction from its stereo pair of images. Colorimetric measurements are carried out by tristimulus and/or spectral filtering. The acquired and filtered images are converted then into CIE-Lab values. AVS acquires and relates geometric and colorimetric information of a 3D surface over areas of tens of square meters, and covers a working range of depths (approx2-8 meters). AVS performances have been assessed by laboratory tests and results are reported. Comparisons with other instruments and techniques commonly used in this field of application are discussed.