Automatic Number Plate Recognition systems work by capturing an image of the vehicle license plate of a vehicle approaching through the use of a camera. By using Optical Character Recognition (OCR) software built into the camera, the letters and numbers on the license plate are converted into a data stream which is then matched against a list of authorised vehicles within the access control system.
Most ANPR systems provide a range of between 6 to 10m line of sight.
How ANPR Systems Work
The camera takes a picture when it identifies a number plate using infrared technology
The camera isolates the number plate and adjusts the brightness and contrast of the image and segments it into individual characters
The pattern of each character is analysed to covert the picture into text
The output is sent over an IP or Serial connection to the access control system.
Where are Automatic Number Plate Recognition Solutions typically deployed?
Car Parking, such as airports and public venues which allow pre-booking, as the vehicle registration number can link the vehicle approaching the car park barrier to the associated booking.
Commercial sites where large people require short-term access to the site, for example, scheduled deliveries from suppliers that only need to access the site infrequently would not warrant having a tag or card issued to them on a permanent basis.
Visitor and VIP access to office sites and car parks normally use ANPR as the visitor can provide his number plate in advance to ensure swift access to the site, whilst allowing the site they are visiting to quickly identify whether the vehicle should be on site.
No transponders or tags required
The biggest advantage of an ANPR solution is that it does not require the distribution and installation of transponders, tags or cards, it uses something that all vehicles have – a number plate.
ANPR is similar to Long Range RFID/UHF Vehicle identification in that ANPR does not require the driver to slow down or find their card or credential in the same way that a vehicle barrier secured with a proximity card would, this eliminates queuing while providing access for authorised vehicles.
As there are no credentials, such as a tag or card that need to be issued to drivers, an ANPR solution can be very cost effective over the lifetime of the system, as the only administrative work is to maintain a list of authorised vehicles.
ANPR systems are also able to provide high levels of flexibility for vehicle access control, such as enabling access to specific VIP guests or visitors who require access for a limited period of time without needing to issue them with a card or tag.
Poor Visibility or Dirty Vehicles
Due to the way that ANPR works, any weather conditions which reduce visibility may mean that the ANPR camera cannot capture a good quality image of the vehicle, similarly, particularly dirty or muddy vehicles may obscure parts of the number plate making identification more difficult.
ANPR is susceptible to swapping number plates between vehicles and it is fairly easy to obtain a new set of number plates with an existing authorised vehicle registration number.
This could allow a would-be attacker to attach a duplicated set of authorised number plates to another vehicle, or substitute the number plates onto other vehicles, such as a van rather than a car to remove large quantities of property.
ANPR cameras are usually installed next to a barrier and face the direction of oncoming vehicles to capture the registration number. This poses a problem for motorcycles who only have rear facing number plates. In this instance, mounting the camera to capture the rear of the vehicle may cause issues with the amount of available light depending on the specific installation location and may prove to be less than optimal.
Complex Character Sets
As ANPR solutions work by converting an optical image into a data string, the camera must be able to accurately read and understand the number plate in front of it.
It is important to ensure that the camera is capable of reading multiple different country number plate formats, and also to ensure that it can process complex character sets such as Arabic or Cyrillic number plates sequentially within the camera, otherwise a number plate may not be read, causing traffic to queue or to aggravate the driver of the vehicle.
Images used with kind permission from Nedap Identification Systems.