Application of grayscale expansion for accuracy improvement in phase-measuring profilometry

Abstract

A method based on grayscale expansion for improving measurement accuracy in phase-measuring profilometry (PMP) is proposed. Digital light projector (DLP) has been widely used for projecting phase-shifting sinusoidal grating in PMP. However, due to its international standardization, DLP can only project at most 256 grayscales for all gray patterns. Moreover, the gray digitalization error may cause lower fidelity for the projected sinusoidal grating. In order to reduce the gray digitalization error, the sinusoidal grating can thus be designed as a 10-bit static pattern with 766 grayscales at most and then is split into three 8-bit static patterns to synthesize a repeated video. While this repeated video, rather than the static pattern, is projected onto the measured object by strictly controlling CCD camera integral time to be an integral multiple of three video refresh cycles, the captured deformed patterns are of 766 grayscales and can provide much more detailed information in phase calculation than those with 256 grayscales. Experimental results demonstrate the feasibility and validity of the proposed method. Both the measuring precision and the repeatability of PMP are improved efficiently.