Optimization Of Resolution Characteristics Of Digital Holographic Imaging For Dynamic Samples And Living Cells

As a novel imaging technique,digital holography has the advantages of real-time,non-contact,non-damage quantitative phase contrast imaging,and has been widely used in various fields,such as holographic interferometry,non-destructive testing,surface micro-displacement measurement,and object deformation analysis.Both in terms of the high-precision non-destructive measurement and the combination of microfluidic chip technique to obtain in-chip sample information,major research always focuses on the resolution and image quality of digital holographic reconstruction imaging.Only if imaging resolution and quality of digital holographic microscopy increase continuously,it is possible to further expand the application scope of digital holography.In this thesis,the research focuses on the improvement of resolution and image quality of digital holographic imaging for dynamic samples and living cells,and imaging experiments on epidermal cells on onions.The method of improvement of digital holographic resolution is studied.Firstly,the matching superposition method of sub-pixel shifting recognition is studied.Using a resolution test target as a sample,its sub-pixel shifting holograms are recorded with CCD via scanning motion of the resolution test target at a certain speed.Then,after the sub-pixel shifting holograms are processed by recognition and matching algorithm,the reconstruction imaging are performed.In addition,the image reconstructed by the sub-pixel shifting method is compared with that by the ordinary digital holography method.Secondly,improvement of the imaging resolution by spatio-temporal scanning digital holography by is studied,in which the spatio-temporal digital holograms are recorded with an area-array CCD via scanning the sample.A set of video holograms is recorded by translating the object along the horizontal x direction at a certain matching speed.By using Matlab,a column of pixels at the same x position in each frame of the video holograms is spliced sequentially in chronological order to form a spatio-temporal scanning digital hologram.Then,after performing a four-step phase shift processing on the spatio-temporal scanning holograms,the digital reconstruction of the four-step phase shifting digital holograms is performed.The experimental results with the method of spatio-temporal holographic imaging show that the resolution of the reconstructed amplitude image of the resolution test target is obviously improved along the scanning direction.Finally,the spatio-temporal scanning combined with the sub-pixel shifting method of recognition and matching superposition is employed to improve the image quality.In the experiment,the column pixels of video holograms being in the same x position are extracted and spliced column by column to form a spatio-temporal scanning digital hologram.In this way,a set of four spatio-temporal scanning holograms can be obtained by selecting four different x positions.After this set of four holograms are processed by using four-step phase shifting,the spatio-temporal scanning hologram with four-step phase-shifted is obtained.The spatio-temporal scanning holograms obtained with column pixels at different x positions should be the same in principle,but in fact,there are slight differences between them.Therefore,for the four-step phase-shifted spatio-temporal scanning digital holograms,recognition and matching superposition algorithm for the sub-pixel shifting is applied on them according to sub-pixel shifting between the holograms,and the holograms after recognition and matching superposition are digitally reproduced.The imaging results show that the method can further improve the imaging resolution and image quality beyond that of using spatio-temporal scanning.Furthermore,the above-mentioned methods of improving resolution is applied to the imaging of living onion epidermal cells,and the resolution characteristics of three methods in living cell imaging are compared.In the experiment,after the sample of living onion epidermal cells prepared from fresh onions,its holograms are recorded by using spatio-temporal scanning digital holography.Then,its spatio-temporal scanning digital hologram is obtained by splicing and four-step phase shift processing,and then its amplitude and phase images are obtained by numerical reconstruction.In addition,the amplitude and phase images of onion epidermal cells are also obtained by using spatio-temporal scanning combined with sub-pixel shifting method.The experimental results show that the method of spatio-temporal scanning combined with sub-pixel shifting can further improve imaging quality of onion cells.Moreover,the phase images of onion epidermal cells dynamically show the changes of cell nucleus and cytoplasmic tissues during the life course of onion cells,and the phenomenon of plasmolysis in the process of cytoplasmic dehydration of onion epidermal cells.