Research On Phase Masks For Extending Depth Of Field In Wavefront Coding Systems

More plentiful object information can be obtained by a larger depth-of-field(DOF) imaging system. The image of object with a larger DOF is more similar with a real scene. In the past half century, many researchers explore various methods to extend DOF of incoherent imaging systems. Wavefront coding(WFC) is recently a new optical-digital hybrid imaging technique. WFC systems produce phase modulation in pupil plane in order to record the coded images. The coded images are decoded to improve the performance of traditional imaging systems. In WFC systems, phase modulation is achieved by a phase mask. Thus, the key in WFC systems is how to design and optimize phase masks. Base on this, this dissertation focuses on the study of phase masks of WFC systems and hopefully more superior performance can be realized in extending DOF imaging.Asymmetrical phase masks for extending DOF are studied. The imaging properties of WFC systems with asymmetrical phase masks are analyzed in spatial and spatial frequency domains. The constraint condition of asymmetrical phase masks in design process is expressed. The model of imaging quality evaluation of WFC systems with asymmetrical phase masks is established. By considering coded and decoded properties of asymmetrical phase masks and image artifacts and noise, asymmetrical phase masks are studied. Based on the research results, four new asymmetrical phase masks are proposed. The superior imaging performance of the four phase masks in extending DOF is proved by comparing with previous phase masks.Conjugated asymmetrical phase masks for extending DOF are studied. The point spread function(PSF) of only an asymmetrical phase mask produces the shift in image plane. As a result, the image artifacts are introduced and the imaging quality is degraded. These will seriously affect the performance in extending DOF imaging. So, imaging method based on conjugated asymmetrical phase mask pair is proposed for extending DOF. This method can yield almost invariable defocused PSF and the performance of WFC systems is promoted. Additionally, this method can also correct the image artifacts.Radially symmetrical phase masks are studied for extending DOF imaging. The decoded images of radially symmetrical phase masks do not include image artifacts because the PSF is symmetrical. But, the defocus invariable characteristics will reduce. So, radially symmetrical phase masks are studied in order to promote defocus invariant imaging performance of WFC systems. Two new kinds of radially symmetrical phase masks are proposed. The proposed phase masks can achieve more stable PSF and modulation transfer function(MTF) in the different defocus positions.