ANPR & Number Plate Recognition Systems

Specifying an ANPR system

An ANPR install is a chain, and every link has to hold: a camera aimed down the lane, an illuminator that suits the plate rather than the scene, a recorder that can receive and search the events, storage sized for the retention you promised, and — where the plate opens something — a barrier or access controller taking the relay. The part most specifications get wrong is where the intelligence sits. It is in the camera, not the box in the cabinet: a recorder specification that mentions ANPR describes it as camera-side analytics, which needs a compatible camera with the function enabled. A recorder on its own will never read a plate.

That is why the recorder still matters. What you need from it is a recorder that accepts the plate events the camera generates, holds them against the footage and lets you search on them later — so check that its own specification lists camera-side ANPR, or plate events, before you build a system around it. Size the channel count for where the system will end up, not where it starts, then check the PoE budget against the cameras you intend to hang off it, because an ANPR camera carrying its own illuminator is not a light load.

Then there is geometry, which decides whether any of it works. Our lens calculator works to the DORI figures of IEC 62676-4, in px/m: identify a stranger at 250, recognise a known face at 125, observe the event at 62, detect movement at 25. Those are PERSON thresholds, and a plate is a harder target than a face: OCR wants something like 100 to 130 pixels across the 520 mm plate, which is 200 to 250 px/m AT THE PLATE — the Identify end of the scale, not Recognise. Work the lens, the standoff and the lane width out against that figure before the ladder comes out. Angle matters as much: aim down a lane rather than across it, because the further the plate is skewed from square-on, the fewer characters the OCR gets. Night is the other half of the problem. UK number plates are retroreflective — they throw light straight back at its source — so a general-purpose IR camera lighting the whole scene blows the plate to white and reads nothing, which is why a genuine ANPR camera pairs narrow-beam IR with a shutter fast enough to freeze a moving vehicle.

Two things to settle before the order. First, compliance: reading number plates is processing personal data under UK GDPR, so the ICO expects a data protection impact assessment before deployment, the minimum number of cameras for the stated purpose, and signage that says cameras are in use and names who operates them. Their ANPR guidance at ico.org.uk is the document to read, and on a client site it is worth agreeing in writing who the data controller is. Second, sourcing: we are a trade distributor, not an installer, and an ANPR camera is a special-order line rather than a shelf item. The recorders, PoE switches, surveillance storage and cable an ANPR install needs are held here; the camera we source to the lane. Send us the standoff, the lane width and the mounting position and we will specify against them.