Multi-Frequency Fringe Projection Profilometry: Phase Error Suppression Based on Cycle Count Adjustment

Abstract

Fringe projection profilometry is one of the most widely used three-dimensional measurement techniques at present, in which phase is the key factor for the accuracy of dimensional measurements. Jumping errors may occur due to improper handling of truncation points in phase unwrapping. Meanwhile, projective dual-frequency grating has the shortcomings of a narrow measurement range and coarse projection fringe due to the requirements of an overlapping grid. To address the above problems, this paper puts forward an improved multi-frequency heterodyne phase unwrapping approach. Firstly, the phase principal values of three frequencies are obtained by the standard four-step phase-shifting approach, and two wrapped phases with lower frequencies are obtained through the dual-frequency heterodyne phase unwrapping approach. Then, the decimal part of the fringe order is again calculated using the dual-frequency heterodyne principle, and the actual value of the current decimal part is calculated from the phase principal values of the grating fringe corresponding to the fringe order. Then, a threshold is set according to the error of the phase principal value itself, and the differences between this threshold and the above calculated and theoretical values are compared. Finally, the absolute phase is corrected by adjusting the number of cycles according to the judgment results. Experiments show that the improved approach can achieve a correction rate of more than 96.8% for the jumping errors that occur in phase unwrapping, and it is also highly resistant to noise in the face of different noises. Furthermore, the approach can measure the three-dimensional morphology of objects with different surface morphologies, indicating the certain universality of the approach.