Distortion Calibration Of Wide-angle Lens And Three Dimensional Image Reconstruction Technology In Monocular Photography For Endoscope

Endoscope has an extremely widely use in industrial detection,medical diagnosis,and minimally invasive surgery.Endoscope is a special imaging system,in order to obtain real and effective information from the internal scene of human body or machine according to images.Most endoscopes have at least the following two problems currently,causing the distortion of the acquired image information,which brings inconvenience for application.On the one hand,the use of wide-angle lens increase the viewing angle under the limited probe diameter of the endoscope,but using wide-angle lens can cause the distortion of image field of view.On the other hand,the obtained two-dimensional images can not reflect the three-dimensional structures of the scene effectively.This paper studies the two problems to improve imaging performance of endoscopes,making them more convenient and effective to use.In order to calibrate the distortion of the images captured by wide-angle lens of endoscopes,the existing methods need to determine the distortion model through a plenty of characteristic points or lines.However,it is complex and difficult to obtain accurate distortion model.A distortion calibration method for wide-angle lens is proposed based on phase analysis of phase-shifting fringe-patterns.At first,four standard cosine fringe-patterns with phase shift step of ?/2 are used as calibration templates,which are shown on a large-size Liquid Crystal Display screen,and then captured by the camera with wide-angle lens to obtain four distorted fringe-patterns.A four-step phase-shifting method is employed to obtain the phase distribution of the radial distorted fringe-patterns.According to the characteristic of almost no distortion in the central area of the image captured by wide-angle lens,the phase distribution of radial undistorted fringe-pattern,which is as a benchmark for computing radial distorted phase,can be acquired by performing numerical fitting by the central undistorted phase value of the distorted image.It means that the radial distorted phase distribution is computed by subtracting the phase distribution of radial distorted fringe-pattern from the phase distribution of radial undistorted fringe-pattern.Finally,the distorted phase is transformed into the actual distorted variables.There is no need to establish any kind of image distortion model by lots of characteristic points or lines.The radial distortion variable at each point of the distorted image can be determined by the proposed method.Experimental results show that the proposed method is simple,effective,and has high automation.Furthermore,each wide-angle lens only needs to be calibrated at once,and can realize to calibrated image of any scene,making it hopeful to widely use.Three-dimensional image reconstruction of objects under the endoscopes has been received people's attention.The existing stereo endoscopes include dual camera endoscope(also known as binocular stereo vision endoscope),structured light projection endoscope,etc.The probe diameter of these endoscopes are generally larger than that of conventional endoscopes,which makes them inconvenient to use and not be widely used.Three dimensional image reconstruction technology in monocular photography based on photometric stereo only needs a camera,which is similar to the structure of commonly industrial endoscopes,and not lead to the increase of the diameter body of the endoscopes.Therefore,this paper carried out the research of 3D image reconstruction technology in monocular photography based on photometric stereo.In the experiment,a black sphere is used to calibrate the direction vector of the incident light.In 3D image reconstruction,at first,the surface normals of the object are calculated according to the calibrated direction vector of the incident light and the brightness value of the image;then the surface normals of the object are transformed into gradients which change with the height of the object surface;finally,the three-dimensional height of the object surface is calculated by using the Frankot-Chelleppa method according to the gradients.First of all,the ordinary lens is used to realize 3D image reconstruction based on photometric stereo,and the influence of the number of lighting sources on the reconstruction effect is studied.Then,3D reconstruction based on close-range photometric stereo under the wide-angle lens is carried out.The experimental results prove the feasibility of 3D image reconstruction with a single camera in the near-field under the wide-angle lens,which is expected to be applied in the stereo endoscopes.