Study On 3-D Shape Measurement Of Metal Surface By Digital Holographic Technology

Digital holography is a non-contact,high-precision measurement technique which has become an important research content in the field of precision measurement.Compared with traditional holography,digital holography can quantitatively analyze the structure information of three-dimensional or phase objects.However,due to the limitation of pixel size and light sensitive area of photoelectric converter in recording process,the digital hologram has a small recording range and low resolution,which seriously restricts the wide application of digital holography in high precision measurement.Aiming at the factors affecting the accuracy of digital holography,this paper analyzes the recording and reconstruction theory of off-axis digital holographic.According to the analysis,the best object angle and the shortest record distance of the off-axis digital holography under the certain spot size and field size are obtained.The denoising method based on frequency domain filtering is proposed and the relevant experiments are given,and the results of the reproduction are all satisfied with the resolution requirements.Then,the digital holographic phase reconstruction is theoretically analyzed.Aiming at the influence of fast phase change and noise on the phase reconstruction,a four directions least-square phase unwrapping algorithm based on shearing interferometry and second derivative of quality weight is proposed.The simulation results show that the optimized algorithm not only effectively restrain the noise and improve the undersampling caused by too fast phase shift,but also improve the stability of the algorithm itself.Influenced by the size of the photoelectric converter,the de-noising of the off-axis digital holography will restrict the resolution or the angle of view of the reproduced image.In order to achieve wide range and high resolution measurement based on off-axis digital holography,a phase matching and fusion algorithm based on block matching is proposed,which achieves wide range and high precision measurement of specular objects.Aiming at the problem of low success rate of metal object matching,this paper improves the original phase splicing fusion algorithm and proposes phase texture splicing algorithm based on block matching.By introducing the texture into the phase splicing algorithm,the speckle noise is effectively suppressed and the success rate of the stitching algorithm is improved.Finally,an off-axis Fresnel digital holography experimental system was set up,and the coordinates of the measurement results are calibrated.The following experiments are carried out respectively:(1)the instrument standard roughness block were measured.The experimental results show that the maximum deviation of the Ra values measured by digital holographic measurement and the nominal Ra value is 0.008?m.(2)The large-field three-dimensional topography measurement was carried out on the identification of gauge blocks.It is proved that the large scale three-dimensional morphology of the metal surface can be reconstructed successfully.