Optical System Design And Analysis Of License Plate Recognition For Eliminating Keystone Distortion

Recent years,the intelligent transportation system has developed rapidly,and the license plate recognition optical systems have been widely used in places that require a wide range of high-definition image quality monitoring,such as: intersections,mixed lanes and other places.It is usually installed at an oblique downward at a certain angle,so the license plate photographed is deformed,that is,the license plate has keystone distortion,which affects the accurate recognition of the license plate character features.Therefore,this paper has developed a license plate recognition optical system that can correct keystone distortion.First,the keystone distortion during the tilt imaging of the license plate recognition lens is analyzed,and the expression of keystone distortion is derived.The relationship between keystone distortion and object height,object plane tilt angle,focal length and object distance is established.Compared with the existing correction methods of keystone distortion,a new method for correcting keystone distortion in tilted imaging optical system using a micro-mirror array is proposed,and the feasibility of this method is validated by simulation and experiments.For the optical system installed at a downward tilt of 12° and the working area was in the range of 16.56～60m,the overall design scheme of the optical system was determined applying the new method of correcting keystone distortion,and the design parameters of the optical system were calculated.The optimization of the system finally completed the full-field MTF of 0.3 and 0.35 in the visible and near-infrared spectrum at a spatial frequency of 110lp/mm,which can correct keystone distortion and be used day and night.Finally,the effect of the system on correcting keystone distortion is analyzed.When the system is tilted downward at 12°,the keystone distortion caused by tilted imaging is corrected more than 97.5%.The license plate recognition optical system capable of correcting keystone distortion designed in this paper has a very obvious effect on the correction of keystone distortion,and once again proves the feasibility of correcting the keystone distortion method,which is of great significance in practical engineering applications.