Development of vehicle-license number recognition system using real-time image processing and its application to travel-time measurement

In order to recognize vehicle license numbers in real time, a CCD (charge-coupled device) camera with electronic shutter and a hierarchical computer system with a general-purpose microprocessor and pipelined image processors have been developed. Much quicker extraction of the vehicle-license plate on the front-view image is realized by optimization of parameters using Taguchi´s method. Distinct images outdoors can be obtained using a suitable control of the lens-diaphragm and continuous lighting via floodlight equipment. The results of field tests show that recognition rates of about 90% in the daytime and about 65% at night were attained. The processing time per vehicle is about 150 ms on average