Modulation Transfer Function Compensation Of Large F Number Optical Imaging System

The research,which is focused on high resolution optical imaging system,has become a hot topic in the field of space optical.For an image system with a fixed orbit height and lens size,it needs a long focal length to ensure the resolution.Hence,the optical system requires a relative small aperture,which equals a large F number,to realize a long focal length.However,a large F number leads to the increase of the diffraction effect and degradation of image.Therefore,the ground image process is a reasonable way that reduces or eliminates the degradation of image quality during the process of the image acquisition.As an image restoration method,modulation transfer function compensation(MTFC)can produce a detailed image which provides more accuracy,practicability and reliability.Based on the image chain,therefore,the research takes the modulation transfer function compensation as the main research object.The basic principle of MTFC is analyzed,and several common compensation algorithms are compared.Further,some corresponding noise suppression methods are demonstrated.And the modulation transfer function of large F number is experimentally verified.Finally,the restoration limit of the large F number system is studied by theoretical and experimental analyzes.It provides the basis for the later engineering production.The main contents of this thesis are arranged as follows:1.A degradation model is established according to the blur and distortion which happen during the image acquisition process.A large F number often leads to the image degradation.Based on the theory of point spread function,the influence of the large F number optical system on image quality is analyzed from three aspects of the modulation transfer function,the point spread function and the signal energy,and the degradation model of the large F number imaging system is established.2.Based on the large F number imaging system degradation model,the basic principle of MTFC technique is analyzed.The common methods of MTF measurement as well as subjective and objective image quality evaluation method are studied.It is pointed out that one should evaluate the image quality by the subjective and objective methods simultaneously.And then the restoration effect of different compensation methods are compared.Finally,the noise types and suppression methods of large F number images are analyzed.3.An experiment is also demonstrated on the modulation transfer function(MTF)test platform of the charge-coupled device(CCD)camera system.The images with different MTF values are generated with different F number,exposure time,and the focal length,and,then,restored.The study found that with the increase of F number,the MTF value at Nyquist frequency reduces accordingly.But after compensated the image quality,which is evaluated by the visual and objective the parameters,can be improved.4.The restoration limit of the large F number system is studied by theoretical and experimental analyzes.Although the MTFC can improve the image quality,which make the image more clearly,the method cannot work all the time.When either the spatial resolution is too high or the F number of the lens is higher than 35,the MTF of the restored image is still lower than 0.1.