Automatic number plate recognition is a government surveillance method that relies on optical scanning equipment to read car number plates as the vehicles travel on public roads. Car number plates, issued by national licensing agencies such as the Driver and Vehicle Licensing Agency (DVLA) in the UK, are unique vehicle identifiers that relate a car owner, their address and vehicle to each other. Number plate recognition methods are typically used to track down criminals, road traffic offenders, to monitor road tax compliance and to find stolen vehicles.

Automatic number plate recognition (ANPR) is also known as automatic licence plate recognition (ALPR), automatic vehicle identification (AVI), car plate recognition (CPR) or licence plate recognition (LPR). In the UK, ANPR was first developed in the 1970s and rolled out over the UK from the M1 motorway and onwards to other major roads.

ANPR can also be used to monitor cars for tolling purposes. In London, for example, the congestion charge can be paid automatically, with cameras able to check cars automatically for their (non)-payment. Car number plates can be read both from the front or the back of the car.

So as to be able to be used throughout the whole 24 hours and in as many atmospheric conditions as possible, most ANPR systems use infrared technology. The technology used can also read number plates on cars driving at considerable speed.  A common complaint against ANPR systems is that they amount to state surveillance of citizens on a mass scale.

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The positioning of the ANPR camera is of utmost importance in order that the system operates effectively. Poor positioning can result in less than 50% recognition, whereas optimum positioning can result in the majority of plates being recognised.

Where ANPR is set up using a single camera at a barrier entrance, the camera should be located in a bollard of around 1m in height to allow direct viewing of the oncoming vehicle. Similarly, if a single camera is responsible for the surveillance of one lane of traffic, the ANPR unit should be pole-mounted at a distance of between 18 and 30m from the oncoming vehicles. The use of multiple cameras to cover multiple lanes, for example on a motorway, is a more complex operation.

CCTV cameras have long been an integral part of our town centres, but these are now being installed with ANPR technology. As a result, the cameras need to be adapted and re-positioned to allow optimum use of the technology. The shutter speed on a standard CCTV camera is 1/50sec, but ANPR cameras should have a shutter speed setting of 1/1000sec, though a shutter speed of 1/500sec would be adequate for traffic travelling at up to 40mph. Once the cameras have been fitted with ANPR their position needs to be checked to ensure an adequate percentage of recognitions.

Police vehicles installed with ANPR cameras usually have them swivel mounted, which means that the cameras can capture images to the front and sides of the vehicle.

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