Research On An Seamless Optical Butting Technology Based On Pyramid Reflector

In remote imaging systems, such as satellite camera and aviation camera, high ground resolution and large ground cover are required. It means that large format image sensor must be used to receive the large field of view (FOV). But monolithic image sensors at present are not large enough to achieve the requirement of some imaging systems. Thus, the format size of image sensors has become a limitation of the development of large FOV or high resolution remote imaging system. It has become a hot research to form a large FOV or high resolution imaging system using normal image sensors.This work is to investigate that how to achieve large FOV or high resolution imaging system with normal image sensors using optical butting methods. Several optical butting methods are analyzed and compared. To avoid their drawbacks a new optical butting system is proposed. In this system, a four-faceted mirror pyramid reflector is adopted to divide the FOV of an imaging system into four parts each of which is received by a corresponding image sensor which has normal format. A larger image can be obtained by combining these four images using image stitching algorithms. The overlapping pixels in each sub-image are used to remove the vignetting artifacts caused by the pyramid reflector. It makes the system has high speed and the final stitched image formed by the system keeps high signal-to-noise ratio.An equivalent optical system are set up for the optical butting system. Based on the optical system, the optical characteristics of the optical butting system are analyzed, the source of the vignetting artifacts and how to remove them are also detailed. An method named vignetting compensation is proposed to remove the vignetting artifacts using overlapping pixels in each sub-image to compensate each other. The signal-to-noise ratio (SNR) of this method is analized, it is proved that the vignetting compensation method keeps higher SNR comparing to other methods. Based on the physical optics and the Fourier optics, a theoretical model of imaging process is set up by considering the post image processing as a part of the whole imaging process. The influences of the lens splitter and the vignetting comperstion method to the imaging characteristics of the optical butting system are analized based on this imaging model.In order to prove the feasibility of this new method and understand the technology difficulties involved in designing and building an imaging system using this new method, a prototype camera was built. Besides the structure design, an image capturing module based on USB controlor and FPGA is detailed. Images obtained from the optical butting system should be processed and stitched to form a bigger image which has no vignetting artifacts and has no pixel loss. To do that, a image stitching strategy which combines vignetting compensation, anti-vignetting and image stitching method based on gradient is proposed. The images obtained from the prototype are processed using this image stitching strategy. Some results are present. Those resultant images have good quality. It proves the feasibility of the new optical butting method.The proposed optical butting method base on pyramid reflector is an effective solution to achieving large FOV or high resolution imaging system limited by the format size of the sensors. It is a method with compact structure, but without vignetting artifacts, no moving components. It can be applied to satellite and aviation imaging system.