Bandpass Filters for Car Number Plate Recognition

The infrared (IR) light source and the bandpass filter (with FWHM of , , or ) are the core components used together in Automatic Number Plate Recognition (ANPR) systems to capture high-contrast, noise-free images of license plates regardless of the time of day or ambient lighting conditions.

The choice of 30nm, 40nm, or 50nm FWHM represents a trade-off between signal strength and noise rejection:

Wider FWHM (e.g., 50 nm): A wider band (e.g., 825 nm to 875 nm) allows more total light from the LED (which naturally emits over a small range) to reach the sensor. This is great for long-distance capture or for fast-moving vehicles that require a quick shutter speed.
Narrower FWHM (e.g., 30 nm): A narrower band (e.g., 835 nm to 865 nm) provides better ambient light rejection because it blocks more of the surrounding noise. This is useful in extremely high-ambient-light environments (like direct desert sunlight) but may slightly reduce the total signal, requiring a more powerful illuminator.