| Digital holographic measurement technology has been applied in the field of optical measurement,including micro-optical component detection,biological sample detection,MEMS device detection,high dynamic particle velocity measurement.Due to the high coherence of the laser light source,the laser produces scattered light waves on the optical component surface.Speckle noise is formed after random scattered light waves interfere and superimpose,which seriously affects the accuracy of the measurement system.Therefore,the speckle noise suppression method is studied in this paper.The main research contents include:(1)Basic theoretical research of digital holographic measurement system and analysis of speckle noise characteristics.In this paper,the recording,reproduction and measurement principles of digital holographic measurement system are studied.The causes of speckle noise and the basic theory of its production process are analyzed.According to the statistical optics theory,the mathematical model of the signal-to-noise ratio in the digital holographic measurement system is established,and the relevant factors affecting the signal-to-noise ratio of the system are simulated and analyzed.(2)From the perspective of improving the optical path,a digital holographic measurement system based on diffusion glass rotation is constructed.Simulation analyzes the relationship between the rotational speed of diffusion glass rotation and the fringe contrast,speckle contrast,image signal-to-noise ratio,and coherence time,respectively.From the simulation results,it can be seen that when the ground glass rotation period is equal to the detector exposure time,the reciprocal of the detector exposure time is the optimal rotating speed of the ground glass.The micro-lens array is selected as the tested object to carry out a control experiment with or without rotating frosted glass:After the addition of diffusion glass rotation,the speckle contrast decreased from 0.53 to 0.35,and the signal-to-noise ratio increased from 1.72 to 2.69.When the exposure time of the detector is 2×10-2s and the ground glass speed is 50 r/s,the optimal system signal-to-noise ratio can be obtained,indicating that the experimental results are consistent with the simulation results.(3)From the perspective of optimization reconstruction algorithm,a digital holographic speckle noise suppression algorithm based on window Fourier filtering is introduced.Simulation analysis window Fourier filter algorithm window size,threshold size,frequency range of the optimal parameters.By comparing the wrapping phase after noise suppression by different methods,it is found that this method can effectively suppress speckle noise and improve the 3D reconstruction accuracy of the digital holographic measurement system.(4)Aiming at the problem that the linear model used by the window Fourier filter cannot fully apply to the discontinuous region of the wrapped phase,which leads to the loss of high-frequency information,a new hybrid technique of WFF-BM3D is proposed.The phase diagram with speckle noise is simulated,and then the phase absolute error and signal-to-noise ratio are used to evaluate the effectiveness of the WFF-BM3D.Using a standard scratch plate as the object to be measured,the relative error of the obtained width is 1.76%,and the relative error of depth is 1.25%. |
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