Research On Noise Suppression In Three-Dimensional Display Of Digital Holography

Digital holographic three-dimensional display is recognized as one of the most promising three-dimensional display technologies, which can truly represent the real world. At present researches on holographic three-dimensional display are very active at home and abroad, which involve extensive ranges of digital holography. The current research status and existing problems are analyzed in three-dimensional display of digital holography at first. Then the factors affecting the clarity and quality of the reconstruction image are investigated. The zero-order and conjugate diffraction beam in the reconstruction image, statistical properties of speckle noise, optical suppression principles of speckle noise, digital image processing techiques on denoising are discussed. The optical systems are designed and set up to carry out experiments related.The main researching contents and innovative achievements are given as follows:1. Suppressions of the zero-order diffraction light spot and the conjugate image are investigated. The fundamental principles of digital holography and computer-generated holography are studied. Two kinds of typical coding techniques-Lohmann detour phase code and Burch code are digitally simulated, and the influence of two coding methods in the quality of the reconstruction image are investigated. For the zero-order diffraction light spot and the conjugate image that have seriously affected on the image reproduced, the phase shifting method is explored which can suppress this kind of noise more effectively. The optical experimental platform is designed and built. The spatial light modulator is used to achieve three-step phase shifting and four-step phase shifting. The major factors leading to the experimental error are analyzed.2. Phase shifting is merged into the Burch code to achieve a vivid three-dimensional display. The tomographic computer holography is adopted to implement three-dimensional holographic display. In order to obtain a vivid reconstruction image, the phase shifting method is proposed to integrate into the Burch code technology. The proposed method is demonstrated to be effective by images and performance parameters from the simulation experiment. For a colorful object field, three lasers with three different wavelengths are set and taken as light sources. The proposed method can produce a clear and full-color three-dimensional display of digital holography.3. A novel optical method to suppress speckle noise is presented which integrates variable wavelengths and a moving diffuser. The statistical properties of speckle noise are studied and dissected firstly. The denoising mechanism of common optical methods is discussed to a degree. Then the experimental scheme is proposed for every method above-mentioned, and the optical platform is configured. Optical techniques of suppression speckle noise are made. The test results and performance parameters are evaluated. Finally, according to the above research and experiments, a new optical speckle suppression method combining variable wavelengths with a moving diffuser is brought forward to suppress the noise. Reconstructed images and parameters from the experiment are compared with the ones of other methods. The proposed method is proved to be feasible and advantageous.4. A mixing method of optical preprocessing and the improved LEE algorithm is presented to suppress the speckle noise. At first denoising methods of digital image processing technology are investigated. Such methods are utilized to filter speckle noise in the reconstruction image of digital hologram. Then the Enhanced LEE filter is improved on the basis of the experimental results. For homogeneous areas, the size of a filtering window is expanded, or more pixels take part in denoising. Therefore, the ability to denoise can be enhanced. In the proposed method, the spatial light modulator with a good performance is employed to suppress noise initially. At the same time, the statistical characteristics of speckle noise are altered by the optical process. Nextly the improved LEE filtering algorithm suppresses speckle furtherly. 5. The method to cooperate the optical way of random phases with the improved NLM algorithm is used to suppress speckle noise effectively. According to the mechanism of NLM algorithm and the characteristics of speckle noise, the improvement is made for NLM algorithm. To accelerate the processing speed of NLM algorithm, measures are taken into the NLM algorithm. The optical method of suppression speckle is adopted to pretreat at the first stage. At the next stage, the improved NLM algorithm is used to continue denoising. Compared with other methods, the proposed method is more effective in reducing speckle noise and the images processed are more vivid from the view of the experimental results.