Research On Scheimpflug-based 3D Imaging System

With the development of science and technology and the progress of industrial production level,the visual measurement technology to obtain the three-dimensional shape of an object from the two-dimensional image is playing an important role in more and more fields.At the same time,due to the diversity of the shape,size,roughness and texture characteristics of the measured object,the requirements for the measurement accuracy are also gradually improving.Traditional 3D imaging methods such as laser line structure light or fringe projection face structure light measurement system measuring range is conditioned by the lens depth of field,in the Z direction of the measuring range is small,in the case of high precision measurement will be unable to capture clear images etc.,affect the measurement precision,is difficult to meet the demand of measurement of complex workpiece.Especially with the rapid development of microelectronics manufacturing technology,a variety of micro components,such as semiconductor devices,optoelectronic components and MEMS,in the industry has been a large number of production,and is widely used in a variety of high-end electronic products and precision instruments industry.This requires high-precision 3D measurement of the object under high magnification and even microscopic optical path.The depth of field of a lens is inversely proportional to the square of its optical magnification.Therefore,the larger the imaging magnification is,the smaller the depth of field will be.Especially in the case of high magnification,the depth of field will become very small,and the part of the measured object that is not parallel to the imaging plane will produce virtual focus,thus affecting the measurement accuracy.The Scheimpflug lens has special imaging characteristics,when the image plane,lens plane and object plane intersect in a line,the whole object plane can be clearly imprinted and the depth of field can be expanded within a certain range.It has been widely used in photography,medical and other fields.At present,studies on the three-dimensional imaging of the Scheimpflug camera are rarely reportedIn this paper,the perspective distortion of circular calibration target and too small depth of field of traditional lens was deeply studied,and a Scheimpflug based conditional 3D imaging system was proposed.The main research contents and results are as follows:(1)This paper studied the imaging principle of the shift lens,the composition of the shift camera,and tested its effect on the depth of field expansion.This paper studied the calibration method of perspective lens under Scheimpflug condition.Based on the thick-lens imaging model and taking into full account the different light angles on both sides of the lens,the calculation formula of pupil center imaging method based on the thick-lens model was derived.The analytical formula of internal parameters was derived according to the homography matrix to complete the calibration of the lens with shifted axis.(2)This paper studied the distortion in camera calibration process,including lens radial distortion and tangential distortion.On this basis,a method was proposed to optimize the perspective distortion of Scheimpflug camera(especially the perspective bias in the condition of Scheimpflug when the dot target is used for calibration).Taking the standard circle in the world coordinate system as the reference,the objective function was established,and the perspective deviation caused by the use of dot target for calibration was optimized by the nonlinear optimization algorithm.The experimental results showed that this optimization calibration method can improve the camera calibration accuracy to a certain extent,and the error in space was reduced by about 80%.(3)Builded axis camera measurement system based on Scheimpflug condition,the system consists of an axis of the camera and a projector,on the basis of complete high precision of camera calibration,with the help of the projection of the virtual target to calibrate the projector,at last,system was calibrated and the phase auxiliary matching algorithm was used for 3D reconstruction.The experimental results showed that the Scheimpflug measuring system can realize the 3D precise imaging of the measured object,which proved the correctness of the shift ed axis calibration method and the reconstruction method of the camera.