Analysis on Application of License Plate Recognition System Technology

The rapid development of the modern automobile industry has brought great convenience to people's work and life, and it has also increased the pressure of traffic management. As a result, intelligence in traffic management has been put on the agenda, and the resulting intelligent transportation has gradually become the focus of industry research. Especially in recent years, the development of intelligent transportation has been very rapid.

The advantages of license plate recognition
In smart traffic applications, in addition to extracting vehicle information, it is of great importance to easily confirm the identity of the vehicle. Among many vehicle identification methods, license plate recognition has been widely recognized by the industry for its excellent performance in applications. Compared with other identification methods, license plate recognition has the following outstanding advantages.

1, uniqueness

The license plate is unique as a vehicle identity, and no two cars in the world use the same legal license plate. Although there are two vehicles in the world that use the same license plate number, the license plate colors are different.

2, the standardization of the license plate information formulation

As a vehicle identification mark, the license plate has a very standard coding method, which provides convenience for identification. For each type of license plate, its license plate format type is unique. For example, in an ordinary civilian car, the first character of the license plate is Chinese characters, and is the abbreviation of the province to which the vehicle belongs; the second character is a letter, and different letters Represents different cities. At the same time, different types of vehicles use what kind of color license plate also has strict regulations, such as military and police special vehicles are using white license plates, civilian cars are using blue license plates and so on.

3, the convenience of license plate image collection

The installation position of the license plate is relatively fixed, and it is installed at the fixed position of the front end and the rear end of the vehicle. It is very conspicuous, and it is also convenient for photographing and collecting.

License plate recognition system functions and components 1. The main functions of the license plate recognition system include:
· Vehicle number plate identification, including plate number color, letters, Chinese characters, numbers and other complete license plate information identification;

· Vehicle speed measurement function;

·Illegal vehicle identification alarms;

· Linkage control of vehicle identification information and vehicle information in the vehicle management office;

• Judgment monitoring of vehicle direction.

The license plate recognition system function module, including the license plate recognition system, mainly includes the following function modules: snapping control module, video acquisition module, number plate identification module, image and data interactive control module, and remote receiving.

2, the basic function of each module

1, capture control module. The main function of this module is to generate a capture control signal when a vehicle passes through, and pass the signal to a video capture module to notify the video capture module to perform image acquisition. This module is generally used only in the non-video triggered capture mode. In the video trigger capture mode, the capture control is generally controlled in the software, and there is no need to capture the control module.

2, video acquisition module. The main function of this module is to complete the collection of video images when video capture is required, and to provide images for subsequent identification through the image and data interaction control module.

3, number plate identification module. This module is the core module of the license plate recognition system. The main function is to identify the number plate provided by the video module, extract the license plate information, and complete the data transmission through the image and data interaction control module. In some application places, while transmitting license plate recognition results, it is also required to encode and compress the image and transmit the code stream to the remote receiving system. From the perspective of hardware design, the image compression and encoding are generally also in the identification module. Completed.

4, image and data interaction control module. The main function of this module is to complete the transmission of images and data, and at the same time realize the data exchange with the remote receiving module.

5, remote receiving module. The remote receiving module is mainly used to receive the recognition result information of the license plate recognition system and related pictures or code streams.

There are two ways to implement the license plate recognition system: one is the identification of still images and the other is the real-time identification of dynamic video streams.

The recognition efficiency of the static image recognition technology is greatly limited by the quality of the captured image. It is a single-frame image recognition. Currently, the product recognition speed in the market averages about 200 milliseconds. The dynamic video stream recognition technology has strong adaptability and fast recognition speed. It realizes the recognition of each frame of the video, increases the number of identification comparisons, and chooses the best license plate number. The key is that it is less affected by the quality of single-frame images. The time that the better products on the market can be identified on the PC can be achieved. Control within 10 milliseconds.

Vehicle license plate recognition is based on image segmentation and image recognition theory. It analyzes and processes images containing vehicle license plates to determine the position of the license plate in the image, and further extracts and recognizes text characters.

The identification steps are summarized as: license plate location, license plate extraction, character recognition. The three-step identification work complements each other, the efficiency of each step is high, and the overall recognition rate improves. The speed of recognition depends on the character recognition. The main application technology of character recognition is to compare and identify the sample database, that is, to establish a sample database for all characters. After the character is extracted, the characters are judged by comparing the sample database. The recognition process will generate The intermediate result values ​​such as the credibility and the inclination; the other is the character recognition technology based on the character structure knowledge, which can more effectively improve the recognition rate and accuracy, and have a stronger adaptability.

License plate recognition system main performance parameters

The advantages and disadvantages of the license plate recognition system are mainly determined by two key parameters: recognition rate and recognition speed.

1, recognition rate

Whether the license plate recognition system is practical or not, the most important indicator is the recognition rate. The internationally recognized recognition rate index requirement is that the correct recognition rate for the whole card 24 hours a day is 85% to 95%. The recognition rate statistics also have the following three ways:

1. Natural traffic recognition rate = total number of vehicle license plate numbers identified / total number of vehicles actually passed.

2. The identifiable license plate rate = the total number of license plate numbers correctly read/manually passed.

3, the whole card recognition accuracy rate = the total number of license plates correctly identified by the total number of cards / manual identification of the total number of license plate read.

The above indicators determine the recognition rate of license plate recognition.

2, recognition speed

The recognition speed determines whether the license plate recognition system can meet the real-time application requirements. At present, the recognition speed of international traffic technology is within 1 second. Of course, the faster the recognition speed, the better.

The license plate recognition system on the market recognizes an average speed of 200 milliseconds in practical applications, while a good license plate recognition system has reached 10 milliseconds, and the actual application recognition speed can reach an average of 40 milliseconds.

The main problems in license plate recognition system 1, camera technology needs to be improved

The major camera manufacturers have developed corresponding products for road monitoring and improved the specific requirements for traffic. However, due to the long-term outdoor use of cameras in road monitoring, they are greatly affected by the objective environment. Therefore, it is not only required that the camera shoots images with high definition but also requires strong adaptability of the camera.

2. The contradiction between the high resolution of the image and the recognition speed needs to be resolved

High-definition advantage is self-evident, but everything is two-sided, and in the license plate recognition system, mainly reflected as: HD pictures due to a wide coverage of the picture, there may be multiple license plate recognition issues in the picture. In addition, due to the large amount of data in high-definition, not only will the processing speed be too slow, but it will also be difficult to realize high-definition video stream identification due to excessive resource requirements.

3. The picture adaptability of the license plate recognition system needs to be strengthened

Current license plate recognition products require that the size of the identified license plate is fixed, while license plates that are too large and too small are generally not accurately identified. This poses the problem that some license plates cannot be identified in the event of video triggering. In addition, in some scene environments, due to the influence of external conditions, it is impossible to set the camera in the most reasonable position, resulting in varying degrees of deformation of the license plate in the picture.

4, the recognition of dirty license plates is not good

In practical applications, it is difficult to ensure that the license plate involved is not defaced. After the license plate is used for several years, it will inevitably cause pollution and wear, and it is difficult to ensure that all the vehicles running on the road surface are Standard and clean license plates, so in the actual environment, in the face of damaged old license plates, how to improve the recognition capabilities of the license plate recognition system is also a problem that needs to be solved.


Now the license plate recognition system has been widely used. There are a variety of products of the same type, and it can be said that “hundred flowers bloom together and one hundred schools of thought contend”. This article is only the author's understanding of the license plate recognition system and his own experience, to provide reference for industry peers.

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