I am not sure how much this helps but so far, this is my understanding/testing :
- the dedicated LPC cameras are normal cameras plus a low pass filter and some adjusting of the shutter/iris . This way , the "normal" light gets filtered out and only the IR gets in, thus the dark image with only the plate reflected back
- i got one of those filters from a Photo shop and with the proper camera - where you can adjust exposure ( easier to find in IP world but i would guess CVI/TVI also ) - it does work like so
- next problem is getting some IR light on the plate to reflect back to the lens. This happens either with a very narrow angle IR light and adjusted angle of the lens+illuminator or with a "special" bullet camera where the LEDs for some reason give a very narrow light. Here obviously the angles need to be as low as possible ( horizontal lines ) so the target would ideally be a bit far a way and you use a longer narrower lens.
So, in my view, these are the main factors :
1 adjust exposure low = dark image so you exclude most text from vans and etc.
2 low pass filter = only IR gets in the camera
3 angle of IR -> Plate -> lens as low as posible so the plate "shines" back ( either a "good bullet" or adjusting the IR and camera bracket )
In the end it is just like shining a mirror into someone's eye ... all about angles. Hope i made myself understood, otherwise ... sorry.
PS On Milestone's website there is a manual for their ANPR software and that has a section about the angles thing and how to improve reading
PS2 i remember someone telling me of a Hik bullet that had the "right angles" for this, i would guess a longer range one with a 5-50 lens or similar