Research On Fast Autofocus Technique In Digital Holography

Digital holography technology can realize digital autofocus and achieve full-field quantitative high-resolution imaging,suitable for surface contour,thickness or refractive index measurement of samples,widely used in biomedical,MEMS / MOEMS,particle field and other imaging analysis.The whole process of digital holographic autofocus is completed in the computer,and the optimal reconstructed distance can be automatically determined according to the change of the actual position of the object to be measured during the measurement process,thereby providing a reliable and clear image for the measurement.Therefore,how to find the optimal reconstructed distance quickly and accurately is a key issue in digital holographic autofocus technology.The subject "Research on Fast Autofocus Technique in Digital Holographic " studies the fast and accurate autofocus technology for single-wavelength and dual-wavelength digital holographic structures.The main contents of the paper include:Firstly,based on the analysis of digital holographic imaging principle and numerical reconstruction principle,the angular spectrum method is adopted in research.Based on the principle of digital autofocus,the classical digital autofocus algorithm is analyzed.It provides a basic theory for subsequent research.Secondly,in view of cosine score autofocus algorithm,an autofocus algorithm based on the spectrum score method is proposed for single-wavelength digital holography.The algorithm uses the reconstructed two adjacent phase distribution to calculate in the frequency domain.The algorithm avoids the process of the traditional cosine score autofocus algorithm to convert the image from the frequency domain to the space domain,which greatly improves the autofocus efficiency.Thirdly,based on the analysis of the theory of dual-wavelength digital holography,an autofocus algorithm based on the correlation coefficient evaluation function is proposed.The algorithm calculates the amplitude correlation value of the image reconstructed from the two wavelengths at the same distance.Compared with the dual-wavelength diffraction difference autofocus algorithm,the focus evaluation function curve of proposed has better convergence while ensuring focus accuracy.Finally,based on the analysis of the theory of critical sampling technique,it is proposed to combine the critical sampling technique with dual-wavelength correlation coefficient autofocus algorithm,which change the size of the object wave field in the iterative process reduces the number of pixels involved in the calculation,which can greatly improve the autofocus efficiency.