Hello, basis is you fire IR at the plate within a fixed IR range, I won't go into finer detail as not best in open environment, the retro reflective nature of the plate reflects this IR back. The camera is a normally CCD but has a special cut pass filter on it (again not for open forum) the ANPR engine also requires the video feed to be of a specified shutter speed, frame rate and quality for accurate reads. This is open environment specific.
However you can use ANPR just on standard CCTV however for reliability and accuracy obviously a tailored product is better.
If you want more in depth detail and workings by all means shout me up anytime