High dynamic range scanning technique

Measuring objects with a high variation range of surface reflectivity is challenging for any optical method: This paper addresses a high dynamic range scanning technique that can measure this type of object. It takes advantage of one merit of a phase-shifting algorithm: pixel-by-pixel phase retrieval. For each measurement, a sequence of fringe images with different exposures are taken: the brightest ones have good fringe quality in the darkest areas while the darkest ones have good fringe quality in the brightest areas. They are arranged from brighter to darker (i.e., from higher exposure to lower exposure). The final fringe images, used for phase retrieval, are produced pixel-by-pixel by choosing the brightest but unsaturated corresponding pixel from one exposure. A phase-shifting algorithm is employed to compute the phase, which can be further converted to coordinates. Our experiments demonstrate that the proposed technique can successfully measure objects with high dynamic range of surface reflectivity variation.