Three-dimensional profile measurements of objects with large height steps by frequency conversion fringe pattern scanning technology

To solve the problem that traditional technique is unable to measure three-dimensional profile for objects with spatially isolated surfaces, a new technique, which is a combination of phase shift fringe analysis and frequency conversion fringe pattern scanning, is proposed. A sequence of sinusoidal fringe patterns with a varying pitch is projected onto objects tested, and the temporal variation of the fringe signal is recorded with a charge couple device (CCD) camera. The phase of the temporal fringe signal is detected at each pixel by the phase shift technique, independently of the other pixels. The temporal frequency of the fringe signal estimated from the time slope of the unwrapped phase provides the information about the absolute surface heights of the objects. The experimental results indicate that the technique enables to measure object with large discontinuities and spatially isolated surfaces more successfully and accurately.