Research On Enhancement Of Reconstructed Image And Applications Of Digital Holography

Digital holography as a method to use light interference phenomenon to record the amplitude and phase of the object reflected or transmitted light,and to reconstruct the image of the object by means of digital computing,its unique imaging method has many advantages,has been widely used.The speckle noise in digital holography reduces the quality of holograms and their reconstructed images.At present,there is no fundamental solution.This severely limits the further development and application of holograms.Non-coherent digital holography that does not rely on coherent light sources can theoretically avoid speckle noise,but practice has shown that speckle noise still exists.Therefore,whether researching coherent digital holography or incoherent digital holography,reducing speckle noise and enhanced reconstructed image are all very important research topics.This is directly related to the field in which holography can be applied.This dissertation begins with the basic theory of holography,briefly expounds the basic principles of holographic recording and reconstruction,and then discusses the digital discrete Fresnel holography,and proposes some methods to reduce the speckle noise existing in digital holography.The main research work and innovations of this article are as follows.(1)A new approach is proposed to suppress speckle noise in the digital hologram based on the theoretical model of speckle noise and the scale transformation characteristics of the Fourier transform.The method is composed of two steps: the first is to enhance the noisy digital holograms by Gamma correction,and the second step is to process the reconstructed images by filtering.It is demonstrated that the proposed method can effectively reduce speckle noise in the digital hologram.(2)A method using masks in spatial and frequency is proposed to improve the spatial resolution of the reconstructed image and decrease the speckle noise.In this method,two amplitude masks are used,one is used in spatial domain to obtain a number of sub-holograms from a whole digital hologram,and another is used in the images reconstructed from these sub-holograms to obtain exactly the object image.The ultimate image is obtained by average grey scale values of these object image.Several experiments have been carried out to validate the effectiveness of the proposed method.Experimental results prove the feasibility and effectiveness for improve the spatial resolution and decrease the speckle noise of the reconstructed image.(3)A method of image fusion and image filtering to improve the spatial resolution of image reconstructed from hologram is proposed.Firstly,a certain numbers of sub-holograms of equal size are captured from a hologram,and a certain numbers of reconstructed images are obtained by digitally reconstructed the sub-holograms,then the average of the gray values of these reproduced images is obtained.Finally,the reconstructed image with a preliminary noise reduction is filtered to obtain the final result.Experimental results show that this algorithm can significantly improve the spatial resolution of the reconstructed image while reducing the speckle noise.(4)A method using sub-hologram filtering is proposed to improve the spatial resolution of the reconstructed image and decrease the speckle noise.In this method,a number of sub-holograms are obtained from a whole digital hologram,and filter is used decrease the speckle noise in the averaged images reconstructed from these sub-holograms.Several experiments have been carried out to validate the effectiveness of the proposed method.Experimental results prove the feasibility and effectiveness for improve the spatial resolution and decrease the speckle noise of the reconstructed image.(5)For the problem that the image recognition technology cannot capture images at different distances from the camera at the same time,digital holography is used to acquire text images of different distance scenes,and then a universal character recognition method is used to identify effect of the method.The reconstructed images with different sizes and different background and different types of characters are compared.The experimental results show that holography has certain advantages for character recognition.(6)The application method of digital holography using SFDM algorithm is proposed on the detection of various parts of circuit production under the condition of using laser light source illumination.The reconstructed image of the circuit is obtained used the digital holography,and the edge information of each part in the reproduced image is extracted by edge detection method.The defects of the circuit can be determined from the edge detection result.The results show that this method can basically meet the circuit inspection requirements.