Setting up ANPR for your carpark or gantry

Automatic number plate recognition looks like a camera problem and is almost entirely a geometry problem. The best camera on the market, mounted at the wrong angle, will read plates worse than a modest camera mounted correctly.

Why ANPR cameras look broken

Point an ANPR camera at a scene and the image will appear unusably dark. That is not a fault. Number plates are retroreflective: they return far more light to the camera than the vehicle around them. An ANPR camera exposes for the plate and lets everything else fall to black.

The consequence is a legible plate at night, in rain, and against oncoming headlights, from a camera whose general-purpose picture is worthless. If you also want an image of the driver or the vehicle, that is a second camera. One device cannot do both jobs well, and any supplier who says otherwise is selling you a compromise.

Angle is everything

The camera wants to look at the plate as close to straight-on as the site permits. Every degree of horizontal or vertical offset stretches the characters, and past roughly thirty degrees the recognition rate falls off sharply.

This is why the mounting position should be surveyed before the camera is specified, rather than after. Sites where the only available mounting point is high on a wall looking down at a sharp angle are sites where ANPR will underperform regardless of budget.

Speed determines the design

At a barrier, vehicles are already slowing and the read zone can be short. At an open gantry with vehicles passing at thirty kilometres per hour, the plate must be in frame, in focus and correctly exposed for long enough to capture a clean frame. That means a longer read zone, a shorter shutter, and consequently more illumination.

Tell your installer the approach speed. The whole design works backwards from it.

What happens after the read

The plate becomes data. Whitelisted vehicles trigger the barrier and drive through without stopping. Unrecognised vehicles are logged with a timestamp and an image. Entry and exit reads are paired into a duration, which is what a billing system consumes.

The barrier integration is where most of the commissioning time goes, because the safety logic is site-specific: what happens when two vehicles arrive together, what happens when a vehicle reverses under a descending arm, where the induction loops are cut. Getting that sequencing right is the difference between an automated entrance and a liability.