Speckle Reduction And Three-Dimensional Image Reconstruction Of Digital Holography

With the development of computer technique and other relative equipments, digital holography has attracted much more attention. Nowadays, the study of digital holography has focused on the theory and the algorithm of image information, as well as how to improve the quality of the reconstructed image and so on. Among them, the reconstruction of digital hologram is a key topic in the research of digital holography, which is maturely developed for the reconstruction of two-dimensional (2D) image. However, 2D image can not fully reflect the information of three-dimensional (3D) object, thus it is important to obtain the 3D information of an object by digital holography, which has wide applications in the fields such as industrial inspection, solid modelings, biomedicine, robot vision and so on.We take two steps in the work: firstly, reduce noise in the hologram; secondly, reconstruct 3D image using reconstruction algorithm. The final purpose of the paper is obtaining 3D shape of an object by digital holography and we elementarily probe into its application to 3D shape measurement.The dissertation consists of five chapters, they are as follows:In chapter one, we sum up the development of digital holography and reviews some important techniques for speckle reduction and 3D shape measurement.In chapter two, we discuss the basic theory of digital holography and focus on the lensless Fourier digital holography.In chapter three, we focus on reducing speckle noise of the image. Based on analyzing the cause of the speckle noise, two methods are proposed to reduce the speckle noise: the one is superposition intensity reconstruction image, the other is synthetic-aperture digital holography. In the end it is proposed to combine the two methods in order to obtain large and high quality hologram which is the basement of the next work.In chapter four, we also focus on reconstructing 3D image of digital hologram. 3D shape of an object is obtained based on studying relation between the phase and the coordination on the object. At first, numerically calculate the phase by Fresnel diffraction algorithm. Considering the fact that the obtained phase is wrapped on the interval (-Ï€,Ï€], we adopt two ways to obtain the correct phase. The one is Fourier transform phase unwrapping algorithm, the other is two-wavelength digital holographic contouring. The former has good antinoise quality and fast calculation speed, while it always fails for the object with large height steps or spatially isolated areas. The latter omits the phase unwrapping procedure and obtains continuous phase directly. It is simple in the operation procedure, however, it is hard to be carried out in the experiment because it requires laser with two near wavelengths to obtain arbitrary long synthetic wavelength.In chapter five, we give the conclusions and outlook.The novel idea of the paper is: at first, combine intensity image superposition with synthetic-aperture digital holography to obtain high quality and large digital hologram, then unwrap phase applying Fourier transform phase unwrapping algorithm and calculate the three-dimensional shape. Digital holography and 3D shape measurement are combined in this paper, which can not only enlarge the application of the digital holography but also make full use of the advantage of digital holography that contains all object information.