In the past 30 years, the progress in optical engineering, computer science and electronic techniques have made the endoscopy an invaluable tool in both internal clinics and surgical operation. As its applications increase exponentially, it has even become a specialized division in the clinical medicine. In order to obtain a larger field of view inside a small and narrow gastrointestinal tract, an endoscope is usually equipped with a wide-angle lens (Fish eye lens). Thus, an acquired image is often with certain degree of shape distortion. The distortion gets even more serious as the objects extend outward from the center of the lens in radial. This paper discusses the effect of such distortion and the correction of the effect. By using a calibration pattern, the nonlinear distortion is corrected with a simple mathematic model for the endoscope image. Once the endoscopic lens is calibrated, the same mathematic model can be utilized repeatedly for the images captured by that endoscope. After capturing the calibration pattern using an endoscopic instrument, digital image processing techniques are applied to extract the calibration pattern from the distorted image. We propose a second order mathematic model and consider the parameters of optical lens. The coordinates of each dot in the calibration pattern are the input to the mathematic model for the correction of endoscope images. The experimental results show that the correction method is effective. For example, by comparing to the original calibration pattern image, the average errors in area calculation are 76.46% and 4.68% for the distorted and the corrected images, respectively.
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