We've got your number: the days when automatic number-plate recognition was confined to high security sites and slow-moving vehicles are over. Now, local authorities and police forces are using the technology on multi-lane roads with fast-moving traffic. Mike Constant brings us up to date with the latest technology.(ANPR)

Automatic Number Plate Recognition (ANPR) is becoming a growth sector for installation companies, Unlike other high profile areas--such as network video systems which have yet to realise their potential--ANPR technology is completely within the scope and knowledge of CCTV companies. But there are pitfalls for the unwary and a few new terms and technical points to come to grips with.

ANPR is the generic term generally used in the UK but other terms such as NPR (Number Plate Recognition) and LPR (License Plate Recognition) are common in other parts of the world.

Let's start by considering ANPR for a single lane using a dedicated camera and go on to discuss some other applications--such as multi-lane systems and those in town centres with colour cameras.

The principle behind ANPR is relatively simple. If you electronically scan a document into your PC and then open it in a word processing programme, you cannot edit or alter it in any way. This is because it is simply one bitmap made up of thousands of individual pixels.

There is software available, however (frequently a freebie with scanners), which can convert these groups of pixels into characters. This is Optical Character Recognition (OCR) which scans each group of pixels, estimates whether or not it could be a letter and replaces the pixels with the ASCII code (American Standard Code for Information Interchange) for the letter. For example, the ASCII code for the lower case 'a' is 01100001. The software scans the whole document and produces a page of letters exactly the same as if you had typed them in. The result can then be edited or manipulated in any way.

This is the fundamental technology used in number plate recognition and provides the capability to store and sort data. As a vehicle approaches the camera, the software takes a series of 'snapshots' via a frame or image grabber and stores them in a file. When the number plate is of sufficient size for the OCR software, the frame is scanned and the registration number is converted to ASCII code and held in a list. The list is then scanned for similarities and a 'favourite' is retained. The system would typically scan and compare 10-15 images, with five being considered the minimum for high accuracy. Note that this is the principle of the software we are describing; some systems only take one image at a certain position,

Frame-grabbers are a particular area where you really pay for what you get. The main criterion is the speed at which images can be captured. For instance,…