In the 3D measurement system based on light-source-stepping method, phase shift changes with the depth of the object changes. The phase distribution recovered by N-step phase shifting method is distorted and then the recovered 3D shape is distorted. Fujigaki proposed the whole-space tabulation method to obtain 3D shape without distortion. The moving stage is used to calibrate the system and then the whole-space look-up tables are constructed. It is inconvenient to calibrate the measurement system. We proposed a new method to calibrate the system without moving stage. Firstly, the camera is calibrated by Zhang’s method. Secondly, a reference plane with sparse marks is placed at different depth of measurement volume. The fringes are projected on the plane and the images are captured at the same time. Thirdly, the phase of different depth is worked out by improved Fourier transform profilometry method. The phase of marker is estimated by local curve fitting. The image coordinates of markers are detected. The projection matrix from reference plane to image plane is worked out from the image coordinates and world coordinates of markers and the inner parameters of camera. The 3D coordinates of every pixel of reference plane are worked out from their image coordinates and the projection matrix. Fourthly, a whole-space look-up table is constructed. This method does not need precise moving stage to calibrate the relationship between phase and 3D coordinates. The hardware requirement for system calibration is simplified. Experiments are carried out to verify the proposed method.
The methods based on spatial and frequency domain processing for resizing the reconstructed image of digital hologram are studied. The first kind of methods for resizing reconstructed image is to resample the hologram. The reconstructed image is reduced by up-sampling or interpolating the hologram and the reconstructed image is magnified by down-sampling hologram. But its viewing angle is reduced. An approach is proposed to preserve the viewing angle. The second kind of method for resizing reconstructed image is to resample selected region in frequency domain of hologram. The reconstructed image is resized by this method without changing its viewing angle.
An additional phase shift is produced when the 3D shape of dynamic scene is measured by phase shifting method. The phase distribution measured by traditional method is distorted and the recovered 3D shape is distorted as well. The Gerchberg iteration algorithm and the windowed Fourier assisted phase shifting method were proposed to reduce the distortion. The main ideal of these methods is to accurately estimate the phase shift between two fringe patterns. The accuracy of phase shift estimation is low near the edge where the frequency of fringe changes suddenly. We proposed a method to increase the accuracy of phase shift estimation. The edge of frequency sudden change is found out and then the fringe pattern is divided into different regions. The Gerchberg iteration algorithm is adopted to extend the fringe to outside of every region. The windowed Fourier analysis is adopted to estimate phase of every region and then the phase distribution of whole pattern is obtained. The phase shift between two fringe patterns is obtained by subtracting the estimated phase of first pattern from that of second pattern. Finally, the accurate phase distribution is obtained from the fringe patterns and the estimated phase shifts by least square method. The comparative experiments were carried out to evaluate the proposed method. The experimental results show that the accuracy of measured phase by proposed method is higher than that by windowed Fourier assisted phase shifting method.