Automatic Number Plate Recognition (ANPR) systems have been widely deployed to enable law enforcement agencies to control and manage traffic on highways. The systems are capable of reading the license plates of vehicles moving at high speeds during day and night, and even in heavy traffic conditions or where lighting is poor.
Seeking to improve upon the performance of earlier designs, Madrid, Spain-based Lector Vision has recently introduced a new ANPR system dubbed the Traffic Eye. The system combines the best of off-the shelf hardware with custom hardware designed and developed by Lector Vision, and the company's own OCR Software Engine, resulting in a highly versatile system that can be rapidly tailored to identify license plates in any country in the world.
The Traffic Eye system itself illuminates traffic scenes using pulsed infrared light whilst capturing monochrome images of vehicles as well as scenic overviews using two separate cameras. Software running on an advanced processor in the system then detects where the vehicle plates lie in the monochrome images, and identifies the individual characters on the plates using an optical character recognition technique that employs an artificial neural network. Both the license plate number and the color overview of the scene are then correlated before being transmitted to a control center in real time.
According to Gonzalo Garcia Palacios, the Manager of Research and Development at Lector Vision, the cameras play an important role in the number plate recognition process, since the overall performance of the system is highly dependent on the quality of the images that are captured.
"In the past, specialized cameras were required for ANPR applications, but thanks to the performance of Point Grey's camera range, the new system can make use of off-the-shelf products," he said.
The first of the cameras in the Traffic Eye system - a Blackfly 1920 pixel x 1200 pixel GigE monochrome camera with a Sony Pregius IMX249 CMOS global shutter sensor and an IR filter -- captures images that are analyzed by the system software to determine the license plate of the vehicles. The second, a Blackfly 1920 pixel x 1200 pixel GigE color camera equipped with a Sony IMX249 CMOS sensor, captures an overview of the scene. Different cameras equipped with different CMOS sensors can be easily changed to better fit into different applications, e.g. monolane or multilane (up to 3 lanes at the same time) ANPR devices, Red Light Enforcement , and Average Speed enforcement.
While GigE cameras have been extensively used in the Traffic Eye system since its introduction in 2013, the modular nature of the processor and control hardware boards used in the system means that the company is not restricted to one camera interface. According to Palacios, when systems demand higher resolution, it is relatively straightforward to choose a camera with the appropriate sensor and then decide whether a GigE interface or a USB3 interface with a higher bandwidth would be more suitable.
Due to the fact that the intensity of sunlight varies according to its wavelength in the IR spectrum, the wavelength of light chosen to illuminate the traffic scene was an important design consideration.
Having determined the location of the characters on the plates in the images, the system then identifies the individual characters themselves. To do so, Lector Vision chose to deploy a software-based artificial neural network that was first trained to identify characters on license plates by being presented with many thousands of separate examples from a single country. The neural network then uses the examples to automatically infer rules from which it can identify new unknown characters from the images captured by the monochrome camera once the Traffic Eye system is deployed in the field.
Lector Vision is a hardware and software developer company applied to machine vision systems, namely Automatic Number Plate Reading. With a wide technical and commercial experience in the areas of ITS, car park, access control, video surveillance and machine vision, we develop product range solutions in Traffic Management, Access Control Vehicle, Parking Management, other security operations and logistics.