LPR software with Nx Witness: licence plate recognition that actually reads
Licence plate recognition is one of the few smart features that simply does what it promises, as long as the camera is placed for it. That is where most LPR projects are won or lost: not in the software, but in the angle, the distance and the light. Here is how LPR works with Nx Witness, and what we check before a camera goes up.
- Gates, barriers, search
- In-camera or server-side
- Measured on site
Short answer
What is LPR software, and how does it work with Nx Witness?
LPR (licence plate recognition, also called ANPR) software reads the plate of a passing vehicle and turns it into text you can search, log and act on. In Nx Witness the plate arrives as metadata on the video, so you can jump straight to the moment a car drove in, and use it in event rules, for example to open a barrier.
The recognition itself can run in the camera or on the server. Nx Witness supports in-camera analytics from several major camera brands as searchable metadata, and works with dedicated LPR plug-ins such as Plate Recognizer and NumberOk. Which route fits depends on the cameras you have and what you want to do with the plates.
Uses
What do organisations use LPR for?
Opening a gate or barrier
Known plates come in automatically, the rest uses the intercom. No more lost badges.
Searching by plate
Something happened and you know the plate. Jump to the moment it drove in instead of scrolling for an afternoon.
Logging who came and went
On logistics sites and construction sites this is often the real reason, not security.
An alert for a specific plate
A notification when a vehicle on a list enters, with a clear purpose and a short retention.
The camera
Why does LPR so often disappoint?
Because an overview camera does not read plates, however many megapixels it has. For LPR you need a camera placed for exactly that job, and four rules decide whether it works.
- Face the direction of travel, keep the angle small. The more oblique the camera, the less of the plate remains. Stay under about 30 degrees, horizontally and vertically.
- Enough pixels on the plate itself. Not the resolution of the image counts, but how many pixels fall across the width of the plate. That means zooming in on a lane, not a wide view.
- A short enough shutter time. A moving car on a long shutter gives a smear. At a barrier the car slows down; on a through road it doesn’t.
- Light that doesn’t work against you. Headlights at night and a low sun by day are the classics. The camera’s own infrared helps, because plates reflect it strongly.
We measure this on site before mounting. A camera that reads 80 percent of passages is not 80 percent good. It misses exactly the one you were looking for.
Architecture
In the camera or on the server?
In the camera costs no server capacity and the camera sends the plate along with the video. That suits a few lanes and works well with cameras that have a good built-in LPR engine. On the server, with a plug-in, you can use more camera types and often get more options for lists and integrations, at the cost of processing power. For a site with many lanes or specific integration needs, server-side is usually the better choice.
Either way, an event can trigger an action in Nx Witness: a bookmark on the video, a notification, or an HTTP request to a barrier or access control system. How far that goes in practice depends on the plug-in or camera, and we test it on your site before you rely on it.
GDPR
Is LPR allowed under the GDPR?
Licence plates are personal data, because they can be traced to a person. So you need a clear purpose, a retention period and a record of who can see the data. An allow list for residents, staff or suppliers is the common use. A list of plates that trigger an alert is something you only set up with a specific purpose, such as a previously established incident on your premises, a short retention and documented ownership. For large-scale use, a data protection impact assessment belongs in it. We set it up with that in mind, and your privacy officer has the final say.
Questions about LPR software
Can I use my existing cameras for LPR?
Sometimes. If a camera already looks straight at a lane with enough pixels on the plate, it can work. An overview camera never will. We check each position on site and tell you honestly what will and won’t read.
Can LPR open a barrier automatically?
Yes. A recognised plate can trigger an event in Nx Witness that sends a command to a barrier or access control system. We test the full chain on site, including what happens when the network or the plate reading fails.
Does LPR work at night?
With the right camera, yes. Plates reflect infrared strongly, so a camera with its own infrared often reads better at night than by day with a low sun. Headlights pointing into the camera are the main problem, and placement solves most of it.
What is the difference between ANPR and LPR?
None in practice. ANPR (automatic number plate recognition) is the common term in the UK, LPR (licence plate recognition) in the US and elsewhere.
Want to know whether LPR will work at your site?
Send us a photo of the entrance or describe the lanes. We tell you where a camera should go and what it will read, before anything is ordered.
A quick word
Questions about your situation? Call Eline.
Treats security and networking as one trade. Hands over a system the way she would want to find it. Describe your situation and you will hear straight away what we would do — with no obligation.
Eline Klaver · Security adviser and certified engineer