The Application Research Of Fourier Transform Profilometry

With the development of science & technology and application need, people hopes to get the 3-D information of more and more objects. So there have been a lot of new methods of optical measurement in the recent years. The Fourier transformation profilometry (FTP) based on fringe projection is a fundamental 3-D sensing method, including following merits: only fringe needed, full field analysis, and high precision. Especially in the recent years, with the development of computer hardware and software technique and high-resolution image grabber, FTP has become one of the most important and popular methods and take more and more researchers' attention.In this paper, we have researched the FTP, first we propose an improved FTP based on two-frequency grating projecting, which are involved in the problems of the phase unwrapping and the frequency selection to satisfy the two carrier frequencies no frequency aliasing. In addition, we use FTP analysis method to research the phase-shift errors in the phase-shift device, which is a new way to improve measuring accuracy. The main results can be summarized as following:1. In the FTP based on fringe projecting, measuring accuracy is involved in the equivalent wavelength of the projecting grating closely. Usually speaking, the smaller equivalent wavelength is, the higher measuring accuracy can be obtained. But it becomes more difficult to unwrap phases. So we bring forward a new method of improved FTP based on two-frequency grating projecting: We can get twowrapped-phase maps corresponding to different equivalent wavelengths form a frame of fringe figure respectively. First we unwrap the low-precision wrapped-phase corresponding to low frequency, and then with it as reference we unwrap the wrapped-phase corresponding to high frequency according to the relations between two frequencies of two-frequency grating. The new method has considered both measuring accuracy and phase unwrapping at the same time. It is simple and applied and is especially fit for being used to measure the discontinuous objects' shape.2. Aimed at the problem of the phase-shift errors existed in the phase-shift device of the phase measurement profilometry, we use the FTP method to analyze it. We analyze the standard N-step phase-shift fringe patterns captured by the PMP device by FTP method (Fourier transform in spatial domain, filtering in frequency domain and inverse Fourier transform). We get the phase difference between the adjacent fringe patterns, and then find the accurate error between the each practical shifting step and the standard shifting step by surface fitting, which can be used to calibrate the phase-shift value. By the way, we can improve the PMP measuring accuracy.