Posted by fauzi on Oct 17, 2010 in Article |

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:



2. 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
o Camera housing
o Camera
o Infrared focus
* (PU) Process unit
o 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
o 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
o It is necessary to install video and control wire for each one of the cameras
o Furthermore, it is necessary to provide power supply cable to the cameras, focus and Process Units
o 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)
o If the Process Units are installed directly on the lane it will be necessary to protect it with a weatherproof rack

*3º – High cost
o The wiring and the time of installation are mulplied
o The maintenance of the system is multiplied


3. 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


4. 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:
o It is only necessary to provide 220v, Ethernet network or serial communication to each equipment
o 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
o If one equipment falls, it is possible to replace it by another one

* It reduces the cost:
o The wiring is reduced
o The installation and start-up time is reduced
o The maintenance of the system is reduced


5. 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
o 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

o 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

o 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

o Type of cameras:

+ Interlace camera: The capture of the images is made in two steps. First uneven lines and later the even lines. 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. The use of this type of cameras is totally recommended because if the vehicle is in movement the number plate always appears focused

o 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



+ Daylight: It is perceived by the human eye. It allows to take images in which the vehicle is distinguished.



* 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:

o To control the light emitted by the focus
o 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 recognize each character of the number plate



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