Field Extending Technique Based On Wave-front Coding Theory

Larger depth of field (focus) is often the purpose of imaging system except of some special situation (such as close-up picture) because of its high imaging performance. Therefore, many methods to extend the depth of filed have been presented. Owing to WFC with its unique characteristics, the method to extend the depth of field by using wavefront coding (WFC) technique is the researching foreland.Wavefront coding (WFC) is the fundamental technique that the optical transfer function (OTF) or point spread function (PSF) is invariant, or substantially insensitive over a broad spatial region in the object distance by placing a special-purpose optical phase mask to encode the wavefront of the optical system, 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. By matching traditional optical design with digital image processing creatively, the depth of filed can be greatly increased, moreover, all defocus-related aberrations, such as spherical aberration, chromatic aberration, astigmatism, Ptzvel(field) curvature, distortion, and other defocus induced by the error of assembling and temperature change, can be corrected or minimized. WFC technique has the predominant imaging capability which traditional optical imaging system is absent. Consequently, the wavefront coding technique has a promising application prospect in all kinds of optical systems.In this paper, three different methods to optimize the pupil phase mask are proposed firstly: Simple Strehl Ratio Based Method for Pupil Phase Mask's Optimization in Wavefront Coding System; Simple OTF stability based method for pupil phase mask's optimization in wavefront coding system; Simple PSF based method for pupil phase mask's optimization in wavefront coding system. Above three methods are applied on a cemented doublet system separately which is chosen as the optical system for imaging simulation. The results of imaging simulations for the cemented doublet system with and without pupil phase mask are compared and analyzed. The results also illustrate that all of above three methods are simple as well as practical, and all pupil phase mask optimized by above three methods show novel properties both in improving the image quality and in extending the depth of focus.Whereafter, the asymptotic solution of point spread function for the wavefront coding system with cubic phase mask was derived for the first time with the ambiguity function andthe stationary phase method, based on which the boundary, bandwidth and oscillations of the point spread function were described, than the sensitivity of the wavefront coding system to defocus, astigmatism and coma are analyzed. A further knowledge of the imaging characteristics for wavefront coding system is acquired from spatial domain. Furthermore, the asymptotic expressions of ray aberrations are achieved by the ray tracing method in geometric optics for free form phase mask. According to the asymptotic expressions of ray aberrations, the spot diagram characteristics of anti-symmetric phase plates are analyzed, including the spot size, boundaries and ray intersection structures. The results are also compared with those derived from Fourier optics, which shows that they two are in great agreements. Through the spot diagram characteristics analysis, a deeper insight into the wavefront coding imaging systems can be obtained.Fisher Information (FI) is used as evaluate function to evaluate similarity of the PSFs of different object distance in practical wavefront coding (WFC) system for the first time. A WFC doublet system is designed based on an infinite focused doublet system with F# = 3,EFL=100mm. Simulative and experimental results show that the PSF of WFC doublet system is insensitive to the object distance, with the WFC, the field depth of the doublet system is greatly extended. The object distance region of sharply imaging for the WFC doublet system is from 5m to infinite, while that for traditional doublet system is from 10m to infinite.At the end of the paper, WFC technique applied on microscope imaging simulation is described. The same piece of phase mask is employed for 10X, 60X, 100X object lens to encode wavefront of system, the final clear sharp images is achieved by applying simple Wiener filers on the intermediate image formed by object lens. The simulative results of microscope with phase mask demonstrate that the depth of fields are extended to -20um~20um, -8um~8um, -0.5um～2um for 10X, 60X, and 100X separately, Furthermore, the ray aberrations of off-axis are minimized in some extent.