Practice

Reading number plates: what the camera has to manage

5 September 20263 min readcctvfinder

The van is gone, the recording is there, and where the number plate should be there is a grey smear. You can see there was a plate. You can even see that it is white. Six characters on it are not, and without those the police can do nothing. And the camera was 4K, standing only twenty metres away.

A number plate is a different task from a face

You recognise a face by coarse features: the outline, the hairline, the distance between the eyes, the shape of a chin. 125 px/m is enough for that, and 250 px/m for putting a name to it beyond doubt.

A number plate consists of characters about seven centimetres tall, and you have to tell them apart one by one. An 8 from a B, a 0 from an O. For that you need characters that are still several pixels wide in the picture, and that takes you quickly past 500 px/m.

A face you are allowed to guess at. A plate you have to read. That is the whole difference.

For tasks like this the standard has the level “Inspect” at 1000 px/m. It is meant for the finest features, serial numbers and security marks. In practice, watching an entrance puts you somewhere between those two worlds, depending on conditions.

What it takes, in numbers

The table shows arrangements that come into question for number plates. All at 3 metres of mounting height.

Arrangement	Width covered	Pixel density
10 m, 30° telephoto, Full HD	5.4 m	355 px/m
15 m, 30° telephoto, Full HD	8.1 m	238 px/m
10 m, 30° telephoto, 4K	5.4 m	710 px/m
20 m, 20° telephoto, 4K	7.1 m	543 px/m
25 m, 15° telephoto, 4K	6.6 m	582 px/m

Look at the middle column. Every one of these arrangements sees between five and eight metres of width. That is one lane, and nothing more. That is exactly the price of the pixel density, and exactly why the camera that is also supposed to show the whole yard fails.

Three things that decide it in practice

A camera of its own for the job. The yard needs a wide angle, the plate needs a telephoto. One camera for both delivers too little for both.
Look flat along the lane. A plate seen from steeply above is squashed in perspective, and the rectangle becomes a shallow trapezium. The camera belongs as far away and as low as you can manage.
Think about the direction of travel. A vehicle crossing the frame smears the plate at any exposure time. Head-on or from behind it stands still.

Then there is the light. Number plates are retroreflective, and at night that works in your favour: a camera with matching infrared illumination gets a bright plate against a dark background. In daylight against a low sun it is the other way round.

The practical way to an arrangement

Start with the lane, not with the camera. Measure how wide the area is that you have to cover reliably. For a single entrance that is often only three to four metres.

Enter that width together with the distance into the cctvfinder. It tells you which angle of view just covers that width, and what pixel density comes out of it. From there you can see straight away whether Full HD is enough or whether it has to be 4K.