Research On Illumination System Of Fourier Ptychographic Microscopy

High-resolution and wide field-of-view(FOV)microscopic imaging plays an important role in biological and physical sciences.However,in conventional optical system,wide FOV and high-resolution are difficult to meet at the same time.Fourier ptychographic microscopy(FPM)is a computational imaging technique that combines wide FOV and high-resolution developed in recent years.It can reconstruct high-resolution complex images which beyond the diffraction limit of microscope by capturing a series of angle-varied illuminated images and then stitch them in the frequency domain.At the same time,it keeps the advantage of wide FOV of low magnification objective lens.The technique can be used in different fields like digital pathology imaging,biological microscopic imaging and industrial inspection et al.FPM only needs to replace the illumination part of microscope to a LED array.Compared with other super-resolution method,its device is simpler and inexpensive,and has application prospects.However,up to now,FPM is still affected by misalignment error of LED array,declined illumination intensity of off-axis LED,and the vignetting effect in optical system.In response to the above issues:this dissertation has carried out the follow works:(1)An FPM system with synthetic numerical aperture of 0.5 and FOV of 14.6mm²is designed.The influence of LED array misalignment error on image reconstruction quality is analyzed through simulation experiments.A misalignment analysis model is established in the spatial domain.The error tolerance ranges of the designed FPM system for shift error(?x,?y),rotation error(?)and height error(?h)are given as[±5?m,±5?m,±0.1°,±40?m].(2)In order to reduce the strict accuracy requirements in the process of LED array alignment,a calibration algorithm based on spatial constraints is proposed.Taking the spatial misalignment parameters[?x,?y,?,?h]as the solution spatial constraints,the particle swarm optimization algorithm is used for optimization.The compression factor is introduced to improve the convergence accuracy,and the natural selection process is introduced to improve the convergence speed.The calibration range of misalignment error is extended to[1500?m,1500?m,5°,1500?m].The calibration accurccay reaches[2?m,2?m,0.01°,2?m].The difficulty of LED array alignment is further reduced.(3)Aiming at the problems of large angle LED illumination intensity attenuation caused by direct illumination of LED array,a LED array illumination system based on telecentric lens is proposed.The telecentric characteristic of telecentric lens increases the illumination intensity attenuation coefficient of large angle led with illumination angle from I₀/I(?)=cos⁴?to I₀/I(?)=cos?.When the illumination angle is 25°and 50°,the illumination intensity is 1.34 times and 3.76 times higher than that of traditional LED array illuminating.The excellent plane wavefront of telecentric lens ensures that the illumination wavevector of the whole FOV is consistent,and the field of view of single image reconstruction is expanded from the traditional 700?m to 5.34mm.(4)The mechanism of the vignetting effect on imaging wavevector of full FOV is analyzed.The vignetting effect leads to different imaging wavevectors of different FOV,and then leads to the problems of artifact,information error and uneven intensity in the reconstruction images.A full FOV illumination modulation strategy based on convergence lens is proposed.By compensating the illumination angle of the edge FOV,the wavevector of the whole FOV is consistent,the artifacts disappear,and the intensity is uniform.This dissertation solves the issues in FPM system mentioned above,which can achieve lower alignment difficulty,more stable system,better image reconstruction quality and more flexible high-resolution image reconstruction process.It is of great significance to promote the clinical application of FPM.