The Research On Staggered Imaging Principle Of CMOS Image Sensor

CMOS image sensors have very extensive application prospects, but image resolution and dynamic range are the two key parameters that hinder its development. Therefore it's of important theory and actual meanings to study the methods of solving the two problems. In the paper, the operational principle, structures and developments both at home and abroad of CMOS image sensors are introduced. The factors influencing the characteristics of CMOS image sensors are also analyzed.Based on the above discussion, the half pixel offset staggered imaging principle that can be used in CMOS image sensors is presented. The principle composes four CMOS images taken staggeringly in half pixel offset in directions of horizontal, vertical and diagonal into a new reconstructed high resolution image. Theoretical analysis shows that the principle is correct. The image reconstruction algorithms are set up from the study of gray level matrix of the pixels. Corresponding calculations of the composed image gray level matrix show that the half pixel offset staggered imaging principle can fully utilize the detection ability of CMOS image devices and the resolution of the planar images sampled by CMOS image sensors can be raised by 2×2 times without improving the technique level. In order to prove the half pixel offset staggered imaging principle further, simulation experiment is done. MATLAB tools are used to process four images with half pixel offset between each two of them according to the presented algorithms, and completely accordant results to the theoretical analysis are obtained. Thus the validity of the half pixel offset staggered imaging principle and the reconstruction algorithms are fully proved.In the paper, a photodiode model that can be used in HSpice is built. Then the photon-voltage conversion characteristics of CMOS pixel circuit are analyzed. The analysis shows that the fixed integration time operational mode limits the dynamic range of CMOS image sensors greatly. In order to widen the dynamic range, a half pixel offset staggered ordinal exposure method is presented. The method changes the synchronous mode of the multi-exposure into sequential four exposures. The obtained four images also have half pixel offset in directions of horizontal, vertical and diagonal. They are composed into one image according to the above image reconstruction algorithms. Analysis shows that the exposure method can widen the dynamic range observably without changing CMOS pixel circuit. Use MATLAB tools to reconstruct the image from the four images obtained according to the half pixel offset staggered ordinal exposure method, and the results show that the dynamic range of the composed image is widened observably. So the method is proved to be correct.