Research On 3D Laparoscopic Calibration Method Based On Light Field

Three dimensional light field imaging is a new technology in laparoscopic surgery,which can realize three-dimensional imaging through a single lens.The premise of 3D reconstruction of light field system is accurate calibration.Generally,the calibration of light field system depends on the parallax of system light field.Since the parallax of light field laparoscope is much smaller than that of traditional light field camera,the calibration of light field laparoscope is necessary but challenging.In this paper,a two-step combined calibration method is proposed.Different feature points are used in different calibration steps.In the first step,virtual objective is introduced to simplify the three-dimensional lfe imaging model,which makes the calibration process similar to the conventional camera.For the second step calibration,the relationship between the two feature points in the microlens image and the central subaperture image is derived to overcome the tiny light field parallax.The experimental results verify the effectiveness of the calibration method.The main work of this paper is as follows(1)This paper proposes a calibration method of light field system based on polar space.The existing calibration methods of light field system are limited by the imaging model proposed by researchers,and the calibration parameters are lack of actual physical significance,which can not guide 3D reconstruction well.In this paper,a calibration method of light field system based on polar space is designed to solve these problems.Based on the projection transformation model established by this method,the imaging formula derived is similar to the imaging formula of traditional camera calibration method in form,which is a closed form composed of scale factor,internal parameter matrix and external parameter matrix.It can be calibrated by solving matrix parameter method and nonlinear optimization method in traditional camera calibration algorithm.Finally,since the transformation matrix of the polar space calibration method is full rank,the three-dimensional information of the object point can be obtained by calculating the polar space coordinates and the parameter matrix of the object point.Experimental results show that this method can achieve basic 3D reconstruction.(2)This paper analyzes the structural reasons why the chromatic aberration of the laparoscopic optical system is much greater than that of the general optical field system,and puts forward a matching solution according to the characteristics of the chromatic aberration,so that the problem of large chromatic aberration of the laparoscopic system can be well improved.In this paper,the chromatic aberration model of general laparoscopic system is established to analyze the form and size of chromatic aberration in objective lens and eyepiece.Then,a new algorithm based on the mapping of subaperture image to original two-dimensional image is proposed based on the general algorithm of color aberration elimination of twodimensional image.Finally,experiments are designed and verified that the proposed chromatic aberration optimization algorithm can eliminate the chromatic aberration of laparoscopic light field images well.(3)This paper proposes a 3D reconstruction method,called line feature-based calibration method,which is suitable for the laparoscopic light field system.Most of the existing calibration methods of optical field system are based on optical field parallax calculation,which requires great depth variation of experimental data.Due to the structure of the laparoscope,its working depth is small,so the calibration algorithm based on the optical field parallax is difficult to be applied to the optical field system of the laparoscope.The calibration method based on line features proposed in this paper breaks through the limitation of optical field parallax,and calibrates the element image and central subaperture image through the distance between two object points,without requiring a large range of depth changes.In addition,a scheme for calculating and decomposing the parameters of the three thin lens model of the laparoscopic light field system is proposed,which can calibrate all the parameters related to 3D reconstruction.Experimental results show that the proposed calibration method performs well in the calibration of laparoscopic light field system.