How to Read License Plates

[Donthitme]

What is the optimal resolution/fps for reading license plates?

Is the FPS more important than resolution?

Thanks

 

[kamkar]

Welcome to the forum Donthitme.

Resolution are good for you Americans as you have smaller plates / letters.
But that is under ideal circumstances.
When light levels fade a camera will expose a picture or a frame in a video longer, this introduce motion blur as the "target" move while the picture is being taken, so this will also make a plate impossible to read.

Having a higher FPS count like 60 FPS, this just mean the camera can not use a slower than 1:60 second exposure for each frame in the video. BUT ! 60 FOS are still very very slow in regard to anything faster than a baby crawling across a floor.
A rule of thumb i learned in photo class in school was if you take a picture of something moving ( i assume they meant humans unaided ) you need a exposure time of at least 1:250 second.
Razor sharp pictures of a race car you see, those are probably taken with a 1:XXxxx second exposure, there is a reason exposure timings on a camera go way over several thousands of a second.
Dash and action cameras have the downside of not having a aperture, so we can not use that to compensate for the level of light reaching the sensor, so the only 2 knobe such cameras have to use are exposure time and ISO ( sensitivity )
Action camera users some times use ND filters, which are like shades for the lens, to reduce the amount of light coming thru to the sensor and so being able to use a slow exposure time even in bright light, and so you can have some motion blur in your footage even if its taken in bright sunshine.
And apparently some degree of motion blur are " cinematic"

But a 30 FPS camera might still use the same exposure as a 60 FPS camera unless we are right there at the end of exposure timings, years ago i ran 2 identical cameras beside each other, one set to 60 FPS and the other to 30 FPS.
And after weeks of looking at their footage i came to the conclusion that it was extremely rare the 60 FOS camera got a capture the 30 FPS camera did not get, so since then i have just used 30 FPS footage.
Also as the bitrate are most often the same for both FPS settings, then using 60 FPS mean your total bitrate will be spread out over more frames, and so if you like in a thinner layer, so with 30 FPS the bitrate should be used better and so give a better image quality.

One thing i find most important for you guys over there is the 1 plate issue, so if you are in a 1 plate state, and that one plate are in the back of the car, well a oncoming car that side swipe you, your front camera will get nothing other than the make / model / color of the car.
So i think having 2 channel systems with a rear camera are a must, at least that camera will then have a theoretical chance of getting a plate capture of the plate in the back of the car.

BUT ! dont focus too much on plate capture, if you do you just stand a chance of getting let down no matter what camera you get, CUZ it is still very much a challenge.
I do use it myself in testing cameras, but really under good conditions just about any camera can do that ( Danish highway speeds of 80 KMH / 50 MPH ) and our large easy to read plate front and rear.
As soon as the light level fade, okay the not so good cameras fall by the wayside first, but it is not like it is several hours earlier in the day then a good camera do the same.

This video made wit the old Viofo A139 system ( 1440p resolution ) are right on the limit, you can also see that on the plates when there are trees along the road that make the light level lower and so the camera change to settings that accommodate that.

Add a splash of summer / more light and plates are captured much sharper as the camera can use faster exposure times.

 

[country_hick]

Read this link. It shows pixels per inch required for recognition.

Some ideas to consider before buying or installing a security system.

Before buying a security system the first thing you need to figure out how is many cameras of what resolution you need to capture what could happen. Be sure to look at the camera specifications. Some cameras are made that do not work in the extreme cold that winter brings to those of us up...

dashcamtalk.com

The problem with license plates is not merely motion blur and pixels per inch but reflectivity. At night license plates can glare so badly they can not be read by many dash cams. Some plates are made to be more reflective than plates from other areas.

 

[annafrost2390]

Thanks for advice!

 

[OldEastSider]

I'm testing the new Viofo WM1 as a front camera though it will probably end up as a rear or side camera because in my opinion it's average at best with the factory settings. However, when I turned off WDR, wifi, and audio, and set the loop rate to 10 minutes I noticed a remarkable increase in image quality. For the first time I could read rear license plates on interstate traffic and they were very clear (daytime only). I'm running the camera in 2K (2560x1440) at 30fps. Perhaps with several features turned off the processor wasn't working so hard resulting in a better image.

 

[kamkar]

At least a few years ago, having none essential things going on a dashcam would prompt it to loose on image quality, but if the bitrate actually drop, well you should be able to see that on the file properties.
But i have to admit i thought we was past those days.

 

[DAP]

Plate reading is also damaged by manufacturers prioritizing good looking pictures and low data rates over clear plate images. Increasing the aperture would capture more light, but at the expense of narrower depth of field, so a larger aperture is not really an option. So really, camera manufacturers have two knobs they can turn: exposure time and ISO setting. Since camera manufacturers want the best looking picture and lowest data rate, they want to minimize picture noise. The more light they capture, the less noise they get, less noise means they can more easily compress, so they opt for the longest exposure and lowest ISO to get a low noise picture. Even when you select 60 frames/second, some cameras will automatically drop to 30 frames/second before increasing the ISO in low light situations.
What needs to happen is to convince camera manufacturers that a short exposure is more important on a dash cam than a noise free picture. I'd much rather have a noisy picture where I could read a plate than a noiseless blurry mess.