License Plate Recognition Systems

There are several applications where automatic license plate recognition can be used. The term "Complete License Plate Recognition System" does not have a definite meaning out of the context of the specific application.

Different applications may mean rather different License Plate Recognition Systems in terms of layout, hardware and technology, and even for the same applications manufacturers provide LPR systems with similar functionality but quite different structure.

Probably the most common LPR application are parking and access control. In the scope of these applications we can already define a common type of license plate recognition system with a typical hardware configuration and system layout.

As an example of a license plate recognition system, we will introduce here a very simple, still typical LPR system for access control. (Note, that in most cases the LPR system is only a part of an integrated access control system.)

The vehicle approaches the gate of the restricted area it wants to enter into. There is a barrier and a traffic light showing red stopping it. There is an inductive loop installed at the entrance in order to sense the arrival (and the presence) of the vehicle. There is also a CCTV camera mounted to monitor the entrance.

The inductive loop, the camera, the traffic light and the barrier are all connected to a control PC. On the control PC there is an access control application software running coordinating the operation of the access control system.

As the vehicle arrives the inductive loop senses its arrival and gives a signal to the control PC meaning: "car arrived". This signal is processed and understood by the access control application.

Automatic License Plate Recognition
Vehicle arrives to the entrance, the inductive loop gives a signal to the control PC

The access control application - via driving a frame grabber card - captures the video signal of the camera and creates a digital picture about the vehicle in the memory (RAM) of the control PC.

Automatic License Plate Recognition
Creating digitised picture about the vehicle

Having the digitised picture of the arrived vehicle in RAM, the access control application requests the license plate reader module to analyse the digitised picture and read the license plate number of the vehicle. After reading the license plate number the LPR module returns the read plate number in ASCII to the access control application.

Automatic License Plate Recognition
Reading the license plate from the digitised picture of the vehicle

The access control application takes the license number in ASCII and passes it to a database module. The database module checks the plate number against different lists of permissions, etc. and returns an "access permitted" or an "access denied" flag. Based on the flag the access control application opens the barrier and sets the traffic light to green - or not.

Also, the access control application may pass relevant data - like date and time of access - to the database module to store it in an access diary. After the vehicle left the gate (either by passing through or leaving) the system gets ready to start the entire process again for the next arriving vehicle.

Automatic License Plate Recognition
Allowing access and storing data in the diary