ANPR-TUTORIAL.COM
THE AUTOMATIC NUMBER PLATE RECOGNITION TUTORIAL
ANPR TECHNOLOGY
1o - Description2o - Parts of the generic system 3o - Evolution to All-in-One equipment 4o - Benefits of the All-in-One equipment 5o - How does it work?ANPR APPLICATIONS
6o - Car parks7o - Access control8o - Traffic control9o - Red light controlANPR TECHNOLOGY
1. Description, ANPR (Automatic number plate recognition) or LPR (License plate recognition)
It is the technical method of artificial vision (OCR) that allows the recognition of number plates in images of vehicles. Historically, it has been applied on security systems to control accesses of vehicles and car parks.
Nowadays, the ANPR technology has improved its reliability, some systems are able to offer recognition rates between 95 and 98%. Also, some ANPR equipment are able to recognize the number plate of vehicles that drive up tol 200km/h.
Generally, the ANPR technology can be bought in two modalities :
The ANPR engyne
The ANPR equipment (Hardware + recognition engine)
The ANPR engine can recognize the number plate directly from the images stored in a hard disk. This type of software allows to take good use of images that have been obtained from other systems like CCTV or cameras.
The ANPR equipment incorporates all the hardware necessary to capture the images of the vehicles and to recognize the number plate. Moreover, it incorporates the ANPR engine. The ANPR equipment are designed to offer the maximum reliability.
Depending on each case the ANPR equipment or ANPR engine will have different uses and advantages:
To optain high reliability
To recognize number plates directly from images stored in a hard disc
To make a good use of CCTV and camera systems
ANPR engine
It is not recommended
It is a habitual use
It is a habitual use
ANPR equipment
It is a habitual use
No
No
Go to top2. Parts of the generic system,
Capture Unit (CU) + Process Unit (PU)
Although there already exist some All-in-One equipment, the great majority of systems are still using "CU+PU" architecture:
(CU) Capture unit Camera housing
Camera
Infrared focus
(PU) Process unit Embeded computer Framegraber
Recognition engine
Operation of the system
The "UC+UP" system consists of two parts. The Capture Unit that takes the image of the vehicle, and the Process Unit that receive the image from the Capture Unit and makes the recognition of the number plate. The Process Units can control one or more Capture Units simultaneously.
The quality of the solution depends on each manufacturer, although generally the "CU+PU" architecture has disadvantages respect the ANPR All-in-One, they are the following ones:
1o - Nonrobust architecture The Capture Units controlled from the one only Process Unit will fall if the Process Unit does not work
2o - Complex installation and start-up It is necessary to install video and control wire for each one of the cameras
Furthermore, it is necessary to provide power supply cable to the cameras, focus and Process Units
If the distance between the lanes are too long, the signal of the cameras will not arrive with clearness to the Process Unit. For example: If we have to control 4 lanes gathered in groups of two and separated by a distance of 2 km, It will not be possible to control the 4 lanes with the same Process Unit. Sometimes the cost of the installation may increase because of the limitation of distances between the Capture Unit and the Process Unit (
click to view example)
If the Process Units are installed directly on the lane it will be necessary to protect it with a weatherproof rack
3o - High cost The wiring and the time of installation are mulplied
The maintenance of the system is multiplied
Go to top3. Evolution to All-in-One equipment
By contrast to the generic ANPR, the ANPR equipment integrates directly in the housing the camera, the processor, the communications and the power supply unit
Go to top4. Benefits of the ANPR equipment
The ANPR equipment with an All-in-One architecture deletes the disadvantages of the generic ANPR equipments, these are the following ones:
It is more simple: All the necessary elements for the ANPR process are integrated in the same housing. Only one device is necessary for each lane to be controlled. The equipment may be connected by Ethernet or serial communication with the client application
Modular architecture: If an equipment with All-in-One architecture does not work, its fall does not affect to the other lanes, because the Process Unit are deleted
Installation and start-up is easier: It is only necessary to provide 220v, Ethernet network or serial communication to each equipment
The instalation is as easy as to screw the equipment with the support, to identify ANPR equipment with an IP and to adjust the optics
If one equipment falls, it is possible to replace it by another one
It reduces the cost: The wiring is reduced
The installation and start-up time is reduced
The maintenance of the system is reduced
Go to top5. How does it work?
The ANPR process is divided into three steps. The detection of the vehicle, the capture of the images and the process of recognition. Next, we will detail step by step how it works and depending on each case what the advantages and disadvantages are
Detection of the vehicle. The first step is to take an image of the vehicle at the right time. Thus, the number plate of the vehicle will be visible in the image. Nowadays, three kind of triger control exist
Hardware triger: The ANPR equipment controls physically a sensor directly installed in the lane. Whenever a vehicle has been detected by the sensor, the ANPR equipment will know its presence, and then the process of the capture begins
Software triger: The ANPR equipment communicates with the client application, who physically controls a sensor directlly installed in the lane. Whenever a vehicle has been detected by the sensor, the client application knows the presence of the vehicle and communicates it to the ANPR equipment. At this moment the process of the capture begins
Free flow: The ANPR equipment does not need to receive signal from any external sensor. The ANPR equipment takes images continuously and it is able to detect the vehicles automatically
Capture of the images. Once the vehicle is detected, the following step is the capture of the vehicle. In order to take a right image, the following points will have to be considered
Type of cameras:
Interlace camera: The capture of the images is made in two steps. First uneven lines and later the even lines (
click to see example). This type of cameras are cheaper but its use is not recommended for ANPR because if the vehicle is in movement, the number plate appears defocused
Progressive cameras: The capture of imagenes is made at once (
click to see example). The use of this type of cameras is totally recommended because if the vehicle is in movement the number plate always appears focused
Type of light. Infrared light is used generally for the ANPR equipment. Although some ANPR equipment are able to use the daylight and the infrared light during the night.
Infrared light: The ANPR systems uses infrared light because the human eye cannot detect it without other devices. One infrared filter located in the camera allows to emphasize the number plate, but, in the other hand, the rest of elements of the image are darkened
Examples
Daylight: It is perceived by the human eye. It allows to take images in which the vehicle is distinguished.
Examples
Light management: The type of light is as important as how it is managed. It is possible to use any typical technique of photography, but the ANPR manufacturers have chosen two great ways:
To control the light emitted by the focus
To control the light that enters in the camera
Number plate recognition process. Each ANPR manufacturer has developed its own recognition algorithms, although, these are the main ones and the common ones
To locate and to isolate the number plate in the image
To correct the brightness and the contrast of the number plate
To separate each character of the number plate
To recognize each character of the number plate
ANPR APPLICATIONS
Go to top6. Car parks
In some countries like Spain or Greece, the law binds the recording of the number plate on the ticket. In this way, the number plate and the ticket number are linked. Thus, car parks improve their management.
These are some direct benefits:
Number plate register. The ticket number, day and the time, and the number plate are linked and registered in the management application
Finding a lost ticket. Thanks to the registry of the number plates it is possible to find a lost ticket and to receive the correct amount
Ticket interchange is avoided. It is possible to block the exit of a vehicle, if the number plate of the vehicle does not match with the number plate in the entrance ticket
Go to top7. Access control
The ANPR equipment has been used for the access control of vehicles, it was thought as one more tool that allows increasing the security. The client application could control users through personal cards, and the ANPR allows vehicles control.
Nowadays, the ANPR equipment are used for automatizing the accesses of vehicles.
These are the main advantages to incorporate ANPR equipment in access control:
Security increased: Integrating the ANPR technology in access control aplications together with the traditional control devices, allows vehicle and people control. Thanks to this convination the security is increased
Dynamic access of vehicles: Automatizing vehicle access is possible through ANPR equipment. If the data base knows the vehicle, the client application will open the barrier automatically. By contrast, if the data base does not know the vehicle, it will not open the barrier
Vehicle images: It is possible to store the image used for the ANPR equipment during the recognition process. It allows to have more information about the vehicle in the client application.
Go to top8. Traffic Control
Some ANPR equipment are able to recognize the number plate of vehicles that circulate up to 200 km/h with a reliability of 95%. Thanks to this, the use of ANPR equipment for traffic control has increased significantly in the last years.
These are some examples of ANPR application on traffic control:
Detecting vehicles in a black list
It is possible to control vehicles that are in search and capture, throught the installation of the ANPR equipment in the main accesses to cities, such as highways and roads.
Average speed control
The majority of speed control devices, such as radars or speed traps, control the vehicle instantaneous speed. With the ANPR equipment it is possible to control the average speed during an itinerary. By means of the installation of two ANPR equipment in different points in the same lane, it is possible to make two consecutive recognitions of the number plate and to calculate the average speed of the vehicle.
Traffic optimization
It is important to improve the vehicles mobility during rush hours and traffic jam.
The installation of ANPR equipment allows to know how many time a vehicle spends to coss an itinerary. This way, the average time can be informed.
Urban toll payment
This application solution has been succesfully experienced. This proves that the mobility in the central zones of the city improves considerably. Another possibility of urban tolls is to maintain a register of foreign vehicles using the public ways, without contributing in the road tax.
Toll enforcement
Although the use of ANPR equipment in tolls is a logical application, today, the ANPR equipment are only used to control the vehicles that circulate in the toll, as a data base that increases the security
Go to top9. Red light control
To jump a red light is a really dangerous infraction with consequences for others drivers and pedestrians.
Nowadays, it is possible to control when a vehicle jumps a red light. This systems combinate sensors and image capture equipment, such as cameras or video recorders.
The ANPR systems reinforce the red light control systems. Thanks to ANPR technology the fine process ca be done automatically.
Last update: 15/08/2006
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