Composite-structured-light profilometry using greyscale expansion

Abstract

In traditional composite-structured-light profilometry, the N-frame carrier gratings and modulated phase-shifting sinusoidal gratings must share a 256-greyscale dynamic range, which is the standard for commercial digital light projectors (DLPs). Additionally, each of the demodulated phase-shift gratings must possess a 43-greyscale dynamic range. This decreases the greyscale dynamic range for each of the demodulated gratings and compresses the phase information. Consequently, measurement accuracy is a challenge, even for incomplete reconstructions. To solve this problem, we propose a new composite-structured-light profilometry using greyscale expansion. Specifically, a composite video is synthesised using three different purposefully designed patterns, each possessing a 256-greyscale dynamic range. These patterns are used as the composite-structured-light to replace the traditional static composite grating. After projecting the repetitive video onto a measured object and controlling the exposure time of the charge-coupled device camera, the deformed composite 766-greyscale pattern can be stably captured. Furthermore, after demodulation, the dynamic range of the deformed demodulated patterns can be expanded up to 229. Numerical and experimental results indicate that the proposed method can be used to divide the single 256-greyscale DLP standard and accurately reconstruct measured objects.