Saturday, January 11, 2020

Some quick thoughts on license plate reading.

Prob is, thanks to TV, movies and even media, people think they should be able to log plates passing with something like a Ring doorbell cam so more and more people seem to be wanting to do this like local governments are.  But that is probably out of your price range. If you just want to be able grab shots of plates coming through a gate or other narrow / slow moving space it is not that complicated. I've been able to pull plates of people coming up my driveway at 100 feet with fairly cheap cams.

When it comes to reading plates you need:

At least 7 pixels per inch at the target distance. (that is rez AND lens view angle)

In my post Doing the math on pixels per inch at a distance for your cam I pointed out the math involved in getting the min number of pixels to be able to make out a plate.

The camera focused for that distance

Most cams are fixed focus and have the lens glued in place so you can not adjust it much less swap it out. To give you and idea watch this video of a guy that swapping out the lens in a Wyze cam. Sadly higher priced brands do similar. You might want to look at my compare between static and auto focus versions of a some Reolink cams.

Enough contrast to make those pixels usable.

See this post on low light which helps get you a bit more.

And if the target is moving, a high enough frame rate and shutter speed to minimize motion blur and get a frame in the target range.

There is some good info here about frame rates and such needed.

And if you are looking to read them at night you need to think about reflections.

Nelly's Security has a nice post about this.

It should be obvious but no tracking or battery powered cams.

But if you need convincing read this.

Then there are plugin options

Like this one for Blue Iris that is on my eval to-do list.
OpenALPR which has and open API and cloud based services.
This post shows how to link Blue Iris and SmartThings through a learning AI system for alerts.

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