Study On Extended Depth Of Field Optical Imaging System

Extending the depth of field (DOF) of incoherent optical imaging systems has been an active research topic for many years, because the extended depth of field (EDOF) system can be used in many fields, such as machine vision, biological or medical micrology, and optical remote sensing, etc.Welford proposed to use an annual pupil to extend the DOF in 1960. From then on, many methods have been developed, for example, power-absorbing apodizer, image fusion of a series of images, specific designed lens with image post-processing, phase plate in the pupil with image post-processing, 3D holography, etc. Now most researches concentrate on the method: introducing a phase plate into the standard optical incoherent imaging system and employing the digital image restoration technique. In the optical stage, phase plate in the pupil alters the imaging wave front to get a focus-independent intermediate image. During the image processing stage, the middle image is processed by a focus-independent digital filter to get an EDOF image.In this paper, the theory of the EDOF imaging system was developed based on the scalar diffraction theory. Then we studied the performances of the imaging system employing the cubic phase plate (CPM) and the exponential phase plate (EPM). A customized optimization model of phase plate for extended depth of field is introduced based on the various requirement of different EDOF optical imaging system. The requirement contains customized DOF, resolution and minimum modulation transfer function value. In the optimization model, the integration of the Fisher information of the OTF in the designed range is employed as the optimization target function, while insuring the related MTF values bigger than the minimum MTF value. Then we optimized the CPM of the wavefront coding system and the EPM for several practical requirements.A radially symmetric quartic-polynomial phase plate was designed based on the defocus invariance of the point spread function of the incoherent extended depth of field optical imaging system and stationary phase method. A set of optimized parameters of the phase plate was given based on the customized optimization model. A novel circular pure phase plate was developed,which is divided into several annular equal-area zones. The zones alternately eliminate the influence of defocuses beside the Gauss object plane. The depth of field of the new system with the plate can be more than 4 times greater than that of the original system by adjusting the parameters of the phase plate. The merits and demerits of the EDOF imaging system employing the rectangular phase plate and the radially symmetric phase plate were summarized in this paper.The influence of the primary aberrations on the imaging performance of the EDOF imaging system was studied. The values of the aberrations were confined in the tolerance range. The EDOF image restoration filter was developed based on the frequency domain filtering theory and the OTFs of the EDOF imaging system. The restoration filter can effectively restore all the images captured by the imaging device in the DOF range. Finally, we designed several EDOF imaging systems employing the phase plates mentioned in this paper in the optical design software Zemax.