An improved temporal phase unwrapping based on super-grayscale multi-frequency grating projection

Abstract

Multi-frequency temporal phase unwrapping (TPU) is an essential algorithm in phase-shifting profilometry, especially when measuring surface discontinuities and isolated objects. The unit-frequency phase with primary contour information guides the high-frequency wrapped phase with measured objects' detail information unwrapping. Ordinary, the projected digital grating is of 256-grayscale at most for the 256-grayscale standard of the projector. The digital error will inevitably be introduced into the projection grating, leading to the phase error in each wrapped phase calculation. Using unit-frequency phase with error will directly affect the low-frequency and high-frequency wrapped phases unwrapping, thus limiting the phase unwrapping accuracy to some extent. In this work, by skillfully using the time-division multiplexing characteristic of projector and the integral characteristic of CCD camera, a high accuracy temporal phase unwrapping method based on super-grayscale multi-frequency grating projection is proposed. The expected super-grayscale rather than traditional 256-grayscale multi-frequency gratings are purposefully designed to reduce the digital error initially. The captured realistic super-grayscale patterns can calculate the high-resolution phase information, so that the reliable unit-frequency phase can guide the low-frequency and high-frequency fringe order calculation more precisely, and improve the measurement accuracy initially. The experimental results verify the validity and practicability of the proposed method.