Optical Low-pass Theory And Application In Digital Imaging System

With the development of science and technology, digital imaging systems have found wide application in various fields. According to the theory of sampling, mixing phenomena are, unfortunately, inevitably caused by Moire fringe and false-color, due to the restrictions on the solid imaging sensor pixel size in the digital imaging system. In order to eliminate frequency mixing in digital imaging system, a series of optical low pass filters (OLPF) came into use. However, the optical low-pass filter theory, particularly, the embedded optical low-pass filter theory for digital imaging system has long been neglected and is still in its embryo phase.With respect to the necessity of optical low-pass filters for the digital imaging system, researches on the frequency mixing phenomena in the digital imaging system are, indeed, carried out to fill this gap. A National High Technology Research and Development Program (863 Program) titled "The research and development of optical low-pass filters used by digital camera /digital video"is granted by the Ministry of Science and Technology of the People's Republic of China (Grant No. 2004AA001019).Studies on filter theories of birefringent plate and phase grating are done from both spatial domain and frequency domain. Exploration of the relationship between MTF and factors such as the thickness, rotating angle and normal angle generates the MTF formula of one-dimensional birefringent plate. Through this formula, filter character of any type birefringent low-pass filter (BLF) can be seen clearly. The MTF formula and light intensity of one-dimensional phase grating with the number of cycle, distance between grating and solid imaging sensors, etching depth and etching duty cycle were analyzed deeply with the results simulated by Matlab software.Production of the matching theory between the pre-optical system and optical low pass filter is of great practical value because any kind of optical low-pass filter should be embedded in the optical system.A model of an OLPF-embedded digital imaging system was established and the matching curve of OLPF with pre-optical system was found for the first time. Moreover, the accuracy of the modeling and the theory was validated by the experiment. The matching problem between OLPF and the pre-optical system, was effectively solved through this model which, likewise, proved to be applicable to the screening and optimization of OLPF in real practical situations.The information theory was introduced into optical low-pass theory, in response to the rapid development of single lens reflex (SLR) and its high demand of imaging quality. An integral model of the digital imaging system and evaluation functions were, therefore, constructed whereby the optimal design of OLPF for 100% full-filled (corresponding black-and-white CCD) and 25% full-filled (corresponding red and blue pixel in color CCD or CMOS) were realized and the results were found much better than conventional pre-optimization. The defocus for optical system low-pass filter was also studied though this model.Finally, with regard to the optical low-pass filter theory and the reality in China, BLF was selected as a potentially industrialized product. BLF software was programmed for its intelligent and simplified use. An on-line real-time BLF optical lattice test device was also designed and implemented to serve the need of BLF industrialization, which filled a gap in the BLF test apparatus in China.