We propose a method of measuring the optical thickness of stacked glass using an absolute distance measurement based on the principle optical balanced cross correlation. The optical balanced cross correlation system is mainly composed of a nonlinear crystal, which is used to generate two second-harmonic sub-pulses. The balanced cross correlation signal will be generated while the measurement pulses and the reference pulses are overlap by scanning the repetition rate. The signal is used as an error signal to lock the repetition rate of the laser by controlling the cavity length. In this study, multiple glasses are pasted one by one with different thickness and the thickness of each glass is measured with sub millimeter precision.
Three-dimensional (3D) shape measurement of fringe projection profilometry (FPP) with high speed and accuracy is required in many applications. The measurement speed of FPP system is mainly limited by the low frame rate of pattern projection. Although defocusing binary patterns can be used to increase the measurement speed greatly, it will decrease the measurement depth. To this end, a novel triangular pattern generation method is proposed in this paper. By designing and projecting binary strip patterns with focused projector, images of triangular patterns can be then acquired with high frame rate. Fast and accurate 3D shape measurement can be finally obtained by using phase-shifting triangular patterns. Compared with the traditional sinusoidal patterns, the proposed method can improve the projection rate more than 4 times. Moreover, the proposed method will not decrease the measurement depth. Experimental results are given to confirm the validity of the proposed method.
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