In this paper, we develop a binocular three-dimensional measurement system using a Dammann grating. A laser diode and a Dammann grating are employed to generate a regular and square laser spot array. Dammann array illuminator is placed between two cameras and narrowband-pass filters are embedded in the project lens to eliminate the interference of background light. During the measurement, a series of laser spot arrays are projected toward the target object and captured by two cameras simultaneously. Similar to stereo vision of human eyes, stereo matching will be performed to search the homologous spot which is a pair of image points resulting from the same object point. At first, the sub-pixel coordinates of the laser spots are extracted from the stereo images. Then stereo matching is easily performed based on a fact that laser spots with the same diffraction order are homologous ones. Because the system has been calibrated before measurement, single frame three-dimensional point cloud can be obtained using the disparity of homologous points by triangulation methods. Finally, three-dimensional point clouds belong to different frame which represent different view of the object will be registered to build up an integral three-dimensional object using ICP algorithm. On one hand, this setup is small enough to meet the portable outdoor applications. On the other hand, measurement accuracy of this system is better than 0.3 mm which can meet the measurement accuracy requirements in most situations.
Optical encoders and laser interferometers are two primary solutions in nanometer metrology. As the precision of encoders depends on the uniformity of grating pitches, it is essential to evaluate pitches accurately. We use a CCD image sensor to acquire grating image for evaluating the pitches with high precision. Digital image correlation technique is applied to filter out the noises. We propose three methods for determining the pitches of grating with peak positions of correlation coefficients. Numerical simulation indicated the average of pitch deviations from the true pitch and the pitch variations are less than 0.02 pixel and 0.1 pixel for these three methods when the ideal grating image is added with salt and pepper noise, speckle noise, and Gaussian noise. Experimental results demonstrated that our method can measure the pitch of the grating accurately, for example, our home-made grating with 20μm period has 475nm peak-to-valley uniformity with 40nm standard deviation during 35mm range. Another measurement illustrated that our home-made grating has 40nm peak-to-valley uniformity with 10nm standard deviation. This work verified that our lab can fabricate high-accuracy gratings which should be interesting for practical application in optical encoders.
A Dammann grating coupling method and corresponding coupling condition are proposed. The different diffractive efficiency of the traditional 3x3 Dammann grating are analyzed by using both the scalar diffraction theory and the Fourier Modal Method (FMM). One new 3x3 Dammann grating is designed by using FMM in association with the simulated annealing optimization (SAO), and its diffraction efficiency and uniformity are about 40% and 15%, respectively. The new 3x3 grating and one traditional 64x64 Dammann grating are then coupled and analyzed numerically. The diffractive efficiency and the uniformity of the final 192x192 dense spots array are 30% and 18%, respectively. As the reference object, one 121x121 beam-splitting grating is designed, whose uniformity and overall diffractive efficiency are 50% and 85%, respectively. Numerical results show that the Dammann grating coupling method is more economical and applicable way to generate dense spots array.
This paper summarized our work on three-dimensional optical technologies using Dammann gratings, e.g., threedimnensional Dammann gratings, three dimensional imaging using a Dammann grating, etc.. We developed threedimensional Dammann grating which can produce three-dimensional array with equal distance and equal intensity with a high-numerical-aperture lens. As we know, a lens usually has a single focal point. Fresnel zone plate can generate several axial focal points, but the intensity between them is unequal. By introducing the concept of Dammann grating into the circular phase plate, we invented Dammann zone plate(DZP) which can produce a series of axial focal points with equal intensity. Combining DZP with a Dammann grating, three-dimensional Dammann array will be generated, which is highly interesting for various applications. We also built a three–dimensional measuring system using a Dammann grating, with two cameras as the right eye and right eye, respectively. We used a 64×64 Dammann grating for generation of a square array of light spots for parallel capturing the three-dimensional profile of an object. The two cameras and the illuminating part are packaged together. After scanning the object, its three-dimensional profile will be obtained. Experimental results demonstrated the effectiveness of this technique.
In this paper we propose a 3D measurement system based on a structured light with a special pattern. The structured light is projected on an object by a projector. Two calibrated digital cameras are used to capture the images of projected area on the object separately. Artificial corner points generated by the structured light are detected. We defined a simple but effective feature parameter of the corner points. The corner points are clustered into several groups by adopting k-means cluster to analyze the feature parameter of corner points. We propose a stereo matching method using the cluster result and geometric constraint. A sequence of images is captured to enhance the measuring resolution. According to triangulation, the 3D point clouds are obtained from the pair of images. Experiment result demonstrated the feasibility of this 3D measuring system. The system with the advantages of high measuring precision and good robustness is highly attractive for applications in 3D measurement, 3D display and so on.