Selecting an appropriate pitch is one of the most important aspects to ensure the system measurement range and
precision in grating projection principle of 3D profile measurement while the Fourier transform frequency spectrum is
separated completely. According to the concept of the equivalent wavelength, the basic requirement for measurement
system and the selection of projection grating pitch are discussed in this paper to avoid shadows and frequency spectrum
alias in a crossed-optical-axes system in Fourier Transform Profilometry (FTP). The influence of CCD sampling to the
FTP is also discussed to obtain CCD sampling condition. When the CCD sampling frequency is unchanged, it is
necessary to reduce the grating frequency which means to increase the grating pitch to satisfy the sampling condition.
Finally, the range of the grating pitch is determined, and the optimal grating pitch is obtained after the experiments.
The π phase-shifting technique have used early to remedy frequency aliasing in Fourier transform profilometry , two
deforming stripe of phase difference π is photographed and then subtracted to remove the influence of zero frequency
and high frequency. Due to background of measured object represent zero frequency of deforming stripe in frequency-domain, image processing based wavelet transform is described to separate background image through image wavelet decomposition, then original image and background image is subtracted to remove the influence of zero frequency, and so fundamental frequency extracted easily. The proposed method reserve merit of frequency domain phase demodulation with low equipment cost and fast convenience. The proposed method is tested through three-dimensional surface sensing experiment of arc welding pool.
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