Research On Time-frequency Analysis Technology Applied In Optical 3-D Shape Measurement

Many physical quantity of an object can be reflected by changes of its shape. Optical 3-D shape measurement technology has been widely used in shape measurement because of its characteristics of non-contact, high accuracy, high speed and high automation. The Fourier transform profilometry as an active optical 3D shape measurement technology has been widely concerned because it need only one deformed grating image to make full-field analysis. However, conventional Fourier transform method is a global spectrum analytical method which is unsuitable to analyze non-stationary fringe signal that modulated by complex shape, and it is sensitive to noise.This paper not only review the shortcoming of the algorithm of Fourier transform profilometry, but also a class of time-frequency analysis algorithms for non-stationary signal has been researched. These methods are aim to improve the algorithm of Fourier transform profilometry, such as windowed Fourier transform, dilating Gabor transform and multiscale windowed Fourier transform. Then an improved window scale selection algorithm is proposed, the instantaneous frequency of the fringe pattern is obtained by detecting the ridge of the wavelet transform, and the pointwise analytical window width is controlled to keep the inside signal quasi-stationary. There is a better harmonization between the frequency resolution and space (time) resolution which make it more accurate to the fundamental frequency spectrum extraction. Even if the fringe signal is polluted by noise pollution, it still gets a good reconstruction results.Secondly, the application of improved window scale selection algorithm in time-frequency representation of non-stationary signal is researched and compared with existing time-frequency analysis algorithm applied in time-frequency representation. The result proved that the algorithm is at an advantage considering about both computational complexity and time-frequency concentration capability. At last, the application of popular empirical mode decomposition method in denoising of optical 3-D shape measurement is researched. Optical 3-D shape measurement is vulnerable to high-frequency background noise and CCD noise. Simulation result shows that the empirical mode decomposition method is at an advantage when denoising and improving SNR.