The Application of Infrared Illumination to ANPR

The Application of Infrared Illumination to ANPR

If a standard colour or monochrome camera was focused to read number plates it would have to contend with a huge variety of lighting conditions, daytime, night-time, sunlight, backlight, headlights, and so on. One configuration simply would not cope with all conditions, so there is a need to provide a constant level and direction of illumination irrespective of any other conditions. And so we come to the development of special cameras for continuous capture of number plate data.he camera must be sensitive to the infrared part of the spectrum, to at least 850 nanometres. Then it must be fitted with a filter to restrict the visible part of the spectrum. The lens would have a manual iris set fully open and the shutter speed set to 1/1000th second. Finally an infrared source must be fitted adjacent to the camera. Therefore, taking advantage of the retro-reflective characteristics of number plates, the illumination from the illuminator will be reflected directly back to the camera. Thus only infrared light will be seen without any visible light or other reflections or refractions. The picture will of course be black with no detail except for the number plate. The OCR software then takes care of converting the image to usable code. Note that this is the sort of image on the monitor both day and night

 

This then is the core of ANPR technology, but there are many other factors to be considered. The first of these are the selection of lens and the distance to view the vehicle. The size of a UK license plate on cars and commercial vehicles is approximately 510mm long x 110mm high. Motorcycles are different being approximately 255 x 200. However more significantly, the minimum height of the letters must be 79mm. The current UK font is Charles Wright, although there are some illegal formats seen. The size of the number plate and the actual characters will need to be of a certain size when seen by the camera for the OCR software to function. One line of thought is that the number plate should be 18% of the scene width; I prefer to consider the vertical height of the characters, which from previous research should be 3% minimum for a 400 line camera. This in fact equates very closely to the 18% screen width but is more logical when considering different shapes of number plates. (For instance when a car plate is 18% of the screen a motorcycle plate would only be 7 %.) Also note that motorcycles currently do not have to carry a front number plate, but this could change in the future.

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