Core Technology
If you scan a document into your PC and then open it in a word processor you cannot edit or alter it in any way. This is because it is simply one bitmap made up of thousands of individual pixels. However, there is software available, frequently a freebie with scanners that can convert these groups of pixels into characters. This is Optical Character Recognition (OCR), which scans each group of pixels and estimates whether or not it could be a letter and replaces the pixels with the ASCII* code for the letter. For instance, the ASCII code for the lower case ‘a’ is 01100001. So, the software scans the whole document and produces a page of letters exactly the same as though you had typed them in, which can be edited or manipulated in any way.
(*) American Standard Code for Information Interchange.
OCR is the fundamental technology used in ANPR and provides the capability to store and sort data. ANPR cameras need to be a special type and set up within certain important parameters as will be described later.
As a vehicle approaches the camera the software takes a series of ‘snapshots’ 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. This continues for a series of images according to the speed and position of the vehicle. The list is scanned for similarities and a ‘favourite’ selected to retain. The system would typically scan and compare 10-15 images, with 5 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.
This then, is the start of the ANPR capture and is totally dependent on the correct set up of camera, lens, illumination, angle of view and configuration. Get one wrong and you have a disappointed customer who won’t pay the bill.
Camera Positioning
Where the camera is positioned other than directly in the line of the approaching vehicle the ANPR provider must be consulted. Many systems will not function with more 30° of misalignment.
The positioning of the camera is a most important consideration for satisfactory operation of an ANPR system. This can vary the percentage of recognitions to number of vehicles from 30% or 40% to near on 100%.
Multi-national number plates
There are thousands overseas vehicles on the roads today, many of them with symbols and other labels incorporated into the plate. The ANPR system must be capable of reading all of these. The software should have a built-in list of such plate styles.
The ANPR database
Just capturing number plates and storing them is not much use by itself. The screenshot below shows an ANPR example review screen. The associated database should be able to provide much more information.
Searching should possible on several fields:
- Full plate.
- Part plate.
- Time.
- Date.
- Category.
- Notes added to image file.
Further functions could be:
- Counting vehicles in and out of premises, leaving a list of all vehicles on site.
- The length of time a vehicle is on site.
- Vehicle speed (from two cameras).
- Employee names can be associated with number plates and access allowed or denied to certain areas.
Example flow diagram for ANPR system
ANPR is the generic term generally used in the UK but other terms are common in other parts of the world. Such as; NPR (Number Plate Recognition), LPR (License Plate Recognition).
This article was written by Independent Consultant Mike Constant and published in CCTV Today Magazine. It first appeared on this website in 2009 and was updated by Robyn Sones in 2019
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