Study Of Influence Caused By Nonlinearity Of CCD In FTP

With the development of computer hardware and software technique and high-resolution image grabber, three dimensional surface measurement has been widely used for machine vision, industry monitoring, biomedicine, real object visualization etc. Today, It has been a hot spot in metrology science field because it has the characteristic of speediness, high precision and no contact. Fourier Transformation Profilometry (FTP) based on fringe projection has become one of the most important and the most active 3-D sensing methods and has paid more and more researchers' attention in Optical 3-D profilometry. It has follow merits: only one fringe pattern needed, full field analysis, and high precision. Because FTP needs calculating Fourier Transformation and filtering, so frequency overlapping , the precision of measurement and phase unwrap are key questions in application.Based on the nonlinearity of CCD and FTP theories, this paper analysis the problem of frequency overlapping in FTP caused by the nonlinearity and sampling of CCD. According to the analysis of high frequency produced by the nonlinearity of CCD, we have given a simple physical explain and detailed analytical deduction. Because of the influence ofthe nonlinearity, there are the second and the third order frequency in the frequency domain of the deformed fringe pattern. Thus, because of the sampling shortage will induce the frequency overlapping between the first order frequency and the second and the third or other higher order frequency in another set of frequency distribution. It affects the measurement precision of FTP. When the sampling frequency is comparative greatly, the first frequency will be separated with the second and the third order frequency or other higher order frequency, thus it can improve the measurement precision of FTP. The main result of study can be summarized as following:1, We have analyzed the question of overlapping spectrum because of the nonlinearity of CCD, and given the simple physical explain and detailed analytical deduction. The overlapping spectrum caused by the nonlinearity of CCD has been analyzed through simulation of computer. By denser sampling the measurement precision of FTP can be improved obviously.2, After analyzed the relation between the second and the third order nonlinearity of CCD and.the second and the third order frequency, we have given the severe theoretical analysis and deduction of formulate to decide the sampling frequency. The analysis has been indicated: due to the effect of nonlinearity, sampling condition must be satisfied and sampling frequency must be increased to avoid overlapping spectrum and to ensure the precision of measurement.