Automatic Number Plate Recognition - Difficulties

Difficulties

There are a number of possible difficulties that the software must be able to cope with. These include:

• Poor image resolution, usually because the plate is too far away but sometimes resulting from the use of a low-quality camera.
• Blurry images, particularly motion blur.
• Poor lighting and low contrast due to overexposure, reflection or shadows.
• An object obscuring (part of) the plate, quite often a tow bar, or dirt on the plate.
• A different font, popular for vanity plates (some countries do not allow such plates, eliminating the problem).
• Circumvention techniques.
• Lack of coordination between countries or states. Two cars from different countries or states can have the same number but different design of the plate.

While some of these problems can be corrected within the software, it is primarily left to the hardware side of the system to work out solutions to these difficulties. Increasing the height of the camera may avoid problems with objects (such as other vehicles) obscuring the plate but introduces and increases other problems, such as the adjusting for the increased skew of the plate.

On some cars, tow bars may obscure one or two characters of the license plate. Bikes on bike racks can also obscure the number plate, though in some countries and jurisdictions, such as Victoria, Australia, "bike plates" are supposed to be fitted. Some small-scale systems allow for some errors in the license plate. When used for giving specific vehicles access to a barricaded area, the decision may be made to have an acceptable error rate of one character. This is because the likelihood of an unauthorized car having such a similar license plate is seen as quite small. However, this level of inaccuracy would not be acceptable in most applications of an ANPR system.