Property Research On Wavefront Coding Imaging Systems For Extanding The Depth Of Field

In the large aperture photography, biological three-dimensional microscopic imaging, optical storage, machine vision, and many other practical applications, extending the depth of field of image systems is valuable, while maintaining image quality. Therefore, many methods to extend the depth of field have been presented. Owing to wavefront coding technique's unique characteristics, the method to extend the depth of field by using wavefront coding technique is a researching foreland.Wavefront coding technology is jointly optimized optical and digital imaging technology. A special-purpose phase mask alters or codes the received incoherent wave front in such a way that both the Optical Transfer Function(OTF) and the Point-Spread Function(PSF) do not change appreciably as a function of defocus, and meanwhile the optical transfer function has no regions of zero values within its passband. As a result, the intermediate image received by the detector is almost invariant but not clear, however, the clear sharp image can be restored from the intermediate image by digital processing. High performance imaging systems can use wavefront coding to extend the depth of field while keeping the light gathering and spatial resolution of the system constant. Defocus related aberrations that can be controlled with wavefront coding include spherical aberration, chromatic aberration, astigmatism, Petzval curvature, temperature related defocus, and assembly related defocus. Wavefront coding system has the predominant imaging capability which traditional optical imaging is absent. Consequently, the wavefront coding technique has a promising application prospect in many optical systems.There are essentially differences between Wavefront Coding imaging systems with the traditional optical systems in the imaging process and the image characteristics. In this paper, the Wigner distribution function, the Fresnel integral, and Cornu Spiral are used to explore and analyze the well-known properties of imaging systems for extending the depth of field in spatial and frequency domain, respectively. At the same time, a new method to optimize the pupil phase mask is proposed. This paper consists of four parts:(1)In the spatial domain, canonical projection of the associated pupil function's Wigner distribution function is used to show the insensitivity of the system's PSF to defocus-related aberrations. We analyze and compare the differences of the systems'point spread function when using several different phase plates'forms. The approximate expression of PSF in the presence of defocus aberration is derived for the system with a cubic phase mask. Based on this expression, the PSF characteristics are analyzed in terms of the boundaries, oscillations, and sensitivities to defocus.(2)In the frequency domain, the exact OTF of cubic-pm systems contains an extra term over that of the approximate OTF, which consists of four Fresnel integral and a scale factor. Spatial filtering properties of the cubic phase mask are been described. An expression for the usable spatial frequency bandwidth is now readily available and can be employed to undertake a comparative analysis of the performance of a wavefront coding system. While it is shown that the optical transfer function of Wavefront Coding systems is also insensitive to defocus through a geometrical description of the so-called Cornu spiral.(3)A new method to optimize the pupil phase mask is proposed: the angle in Hilbert space based method for pupil phase mask's optimization in wavefront coding system. This method makes the in-focus PSF and several defocus PSFs insensitive with defocus over a wide range of defocus parameter values, while considering the imaging restorability.(4)A doublet-wavefront coding system was designed based on a finite focused doublet system. The simulation results show that the phase mask can extend the depth of focus by an order of magnitude more than the Hopkins defocus criterion. The well-known properties of imaging system for extending the depth of focus are been explored and analyzed in spatial and frequency domain, respectively. Least squares digital filter and Wiener filter are used to restore the clear sharp image from the intermediate image. Better and more stable imaging quality over a large depth of focus is acquired with the optimized pupil phase mask.