Importance of Sensor Size for LPR

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EagleEye7

Getting the hang of it
Jul 29, 2024
116
35
UK
Disclaimer: I am not experienced in LPR!

Context: I have an old Hik turret which I decommissioned, replaced with an EmpireTech, because it is 4MP on a 1/3, and thus performs terribly in low light. However, I wondered about repurposing it for something useful, instead of doing nothing.

It occurred to me that since with LPR, the aim is a very dark image, so that the plates aren't overexposed, can we get away with using less-than-ideal sensor sizes on an LPR cam? In this case, the camera would be covering a driveway type entrance, instead of a main road, so the camera would have vehicles driving either directly towards or away from it, at quite low speeds, possibly giving a bit more leeway on shutter adjustment etc for LPR...

Anyone using poor sensor size cameras for LPR's?
 
I am sure you are aware of my post on how focal length is more important than MP and focal length is even more important when it comes to LPR.

The aim of the LPR cam isn't the dark image, that is the consequence of running fast shutters to freeze frame. And running faster shutter speeds on less than ideal MP/sensor means you need even more IR light to capture the reflective properties of the plate.

With that said, for a driveway situation, as long as you can make the field of view such that it is not much larger than a vehicle and if the camera has enough IR to work with, you probably can use a less than ideal sensor, but an ideal MP/sensor ratio on a camera designed with the right focal length would be the better choice.

Only way to know is to test it!

But if your 4MP 1/" sensor is 2.8mm focal length and you are trying to read plates at 40 feet, no not a chance, at least not reliably.

Along other lines, I created this post when I found out my 3rd party LPR reader was able to read D1 resolution when my camera had rebooted and changed the stream to D1.
 
I am sure you are aware of my post on how focal length is more important than MP and focal length is even more important when it comes to LPR.
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Yes :)
From memory its a 2.8-12mm varifocal. So not a very long focal length at all by LPR standards, but the distance isn't too bad (40ft max, then the vehicles will get a lot closer, although they may start to turn sooner or later).

Only way to know is to test it!
Click to expand...
Exactly! I think I will be doing some testing at some point.

I just wanted some input on the idea basically - I had issues with this cam in the past because of the sensor size - it would not do well in low light at all, and worsened as you zoomed it in! However, I wondered if this would be less of an issue for LPR, due to reflectiveness of the plates - on my other regular cams at night, plates are normally totally overexposed and unreadable...

I guess increasing the shutter speed massively for LPR would amplify the low light issues - but I might still test it in my location, because I might be able to get away with slower-than-ideal shutter, since vehicles will be going directly towards / away from the cam at low speeds - also, there is probably a good bit of IR from other cams in the area.
 
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