Reading License Plates at Night, Light Reflection / "overexposure". HID / Xenon, standard bulbs

reverend

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This was the best I've managed yet which was with EV turned all the way down to minus 3 but it makes for a dark video. Swings and roundabouts and I guess why we're starting to see HDR cams creep in now.


The BMW at 2:35 is interesting - maybe that's a retroreflective plate as it stands out a mile on the clip as different to all the others.

I think the night time cams work at night - I read something saying they're supposed to be able to take usable pics up to 120mph although whether that is at night is another matter.

That yellow filter in interesting and I'd be intrigued to hear how you get on please :)

Also I think the ANPR systems are IR based nowadays?
 
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arcticfire

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Colour temperature can effect clarity but that's more with how our eyes render detail under different lighting conditions, but with registration plates it's about dynamic range as they are essentially becoming highlights when our lights hits them and the detail is being blown out. Very similar to photography, I find a lot of similarities cross over between photography and dash cams.

A camera sensor's dynamic range will only be able to expose for a certain amount of the full dynamic range of the scene. The rest will either be overexposed or underexposed - or a combination of both.

It's like when you take a photo of someone on a bright sunny day with some nice fluffy clouds with a cheap camera. The camera will expose for the person and surroundings, but the sky which is much brighter will not show as nice and blue with fluffy clouds but will simply be white because the dynamic range of the camera isn't wide enough to expose for the darker ground scene and the brighter sky scene. More expensive cameras have a greater dynamic range, typically because their sensors are bigger and therefore the individual pixels are bigger and can collect more photons so to speak. This is also why more resolution is not necessarily better because you can introduce more noise into the image and also lose dynamic range.

Here's a good image to show how dynamic range works within the whole scene:

exposure.jpg



There's only really two solutions with the actual camera technology to fix blown highlights such as registration plates and make them readable:

1. Automatic switching metering (I don't even think this exists!)

I say "switching" rather than adjustable because light metering is always adjusting - but it's the type of meting which needs to switch and typically you have three types of light metering: Wide, Centre and Spot.

  • "Wide" assesses the whole scene and provides the best overall exposure. This is good for dash cams at night because the majority of the scene will be dark so it will shift the dynamic range of the device accordingly to pull back detail in the darker areas, however, this will cause more of the brighter parts of the scene to overexpose and in particular registration plates because their bright area only makes up for a small percentage of the overall coverage of the scene so the metering will not give it very much priority.
  • "Centre Weighted" assesses a pre-defined central area of the scene and exposes for this. Depending on how large or small the central area is this can be the best of both worlds. It may be large enough to include a good portion of darker areas and expose for this but as a brightly lit registration plate comes into the scene and more importantly this bright part of the scene comprises of a decent percentage of the metering areas it will shift the dynamic range accordingly so that brighter areas are better exposed whilst still trying to retain detail in the darker surrounding areas.
  • "Spot" which is basically like Centre Weighted except like the name suggests Spot metering uses only a very small area in the middle of the scene to expose. This should expose registration plates perfectly, but it will most definitely underexpose the rest of the scene and probably not far off pitch black.
The camera's metering system would have to be setup so that if it detects bright light in the central part of the scene then it will switch from wide to centre weighted and then onto spot should it get even brighter in the middle. This way you would have a nicely exposed night scene during general driving about, but as you come close to a plate then it will expose for this, but then you could end up missing some surrounding action as it will become underexposed. It's not really an ideal solution but it would be better than have a single fixed exposure type.


2. Wide Dynamic Range

Easier said than done to be properly effective, especially because of two key demands of customers with dashcams: Device size and Cost. Because as you can see from the image, the wider the dynamic range the more detail can be captured in highlights and shadows before it's lost. There's only so much you can do with software and other tricks to increase dynamic range (such as removing the gaps between pixels etc), but ultimately you need bigger pixels. This means a bigger sensor (especially if the resolution is going to increase) which means a bigger device and a bigger sensor will be far more expensive. I suspect dash cam sensors will be tiny but could you imagine an APS-C or full frame sensor in a dash cam? Would be lovely!

There's also keeping the resolution lower which helps but then the less pixels the less detail. 1296p will render more detail than 1080p (if the sensor and lens quality is the same for both - sensor quality also very important) but if they are both on the same size sensor then the 1080p should give better low light performance but at the expense of detail. But then as dynamic range decreases so does detail! lol So it's quite the balancing act and a proper wide dynamic range is a big ask. Dynamic range also decreases as the ISO goes up, so the more the camera has to ramp up the ISO so that it can cope with a darkening scene then the less dynamic range you'll have and subsequently less detail.



There is another factor which effects low light performance which is the lens speed, basically the max aperture it can go to or is fixed at. The wider the lens can open the more light it can take in, but then your depth of field becomes very shallow which typically isn't a problem on dashcams due to the massive wide angle. If you can let more light in then the scene in general will be brighter and therefore the metering can shift more towards exposing for the brighter scene and help reduce the blown reg plates. But as we have dash cams already using very fast lens speeds I don't think there's much more that can be done there.


I used to photography using a couple of D3 bodies which are full frame sensors and had lovely dynamic range. However, I gave up photography and just got a Sony A6000 which is compact yet has an APS-C sensor. It's smaller than the D3 sensor yet the dynamic range is higher at 13.1 Evs whereas the D3 is 12.2 Evs. This will just be due to technical advances though because the low light performace of the D3 still dwarfs the A6000, it's not far off double in fact. There's a lot of other factors inolved and it's not just sensor size, but it is usually the most important aspect.

I'm tempted to setup my A6000 on the dash and do a video test just to see the difference. I actually think the tech in some of these dash cams is very good but will be interesting to see.

Kind of rambled on there! lol
 

arcticfire

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Been out doing some testing with the Street Guardian SG9665GC and also my camera using the video function. Apologies in advance though, I had to set the camera to manual focus and I must have nudged the lens slightly when placing it up against the windscreen as it's gone out of focus slightly but the theory is still there for the purposes of the testing. My 4 year old son became ill today and I've also got a new born so I don't have much time for going out and redoing the tests. It's also worth noting that my camera has an electronic shutter and although it's got an APS-C sized sensor, my widest lens isn't very fast and has a max aperture of f/3.5 whereas dashcams are usually in the f1.8 and lower, sometimes even f/1.0. This makes a considerable difference with low light capability and will allow much lower ISO settings.

Firstly, I've also noticed that at night you can't read plates if in motion as they are just blurred. The other thing I've noticed (especially with the SG9665GC) is that reg plates blow out so they are not visible.

For the motion issue, increasing the shutter speed would help solve this to a certain extent, the only problem is that if you do that then you would also have to ramp up the ISO and the footage will become a little more grainy, but is that a compromise worth taking so as to be able to capture the all important registration number? I'm not sure what shutter speed the SG9665GC runs but it must adjust automatically depending on the light conditions as will the ISO I suspect. I guess it's the min. shutter allowed that's important and again we are back to the fine balance between overall visbility of the whole scene or being able to pick out certain details instead.

  • The first photo is of the SG9665GC and a reg plate which you can see is totally blurred when the car is in motion.
  • The second photo shows the Sony camera I used at 25fps with a shutter speed of 1/30th and a reasonable ISO for the dark conditions. All in manual mode. You can see the registraion plate is blurred.
  • The third photo shows the Sony camera set at 25fps but with a shutter speed of 1/125th and a much higher ISO. My focussing was knocked out slightly but you can clearly see the registration plate is legible and even more so if it were in focus. With a slightly higher ISO the surrounding scene would come up nicely also.

Street Guardian.jpg

Sony 25fps shutter 1-30th.jpg

Sony 25fps shutter 1-125th.jpg


I thought I would also see what would happen if I took my shutter back to 1/30th of a second and the ISO back to 1000 so the reg plates would be blurry but increase the frame capture rate to 50fps. Once again I had knocked the focus out so apologies but you can see that the reg plate is becoming legible again and it's more the focussing which has screwed things up. So could the SG9665GC be upgraded to say 60fps and perhaps the min shutter speed increased slightly? I don't know if it's technically possible or a firmware tweak or a hardware restriction but curious to know.

Sony 50fps shutter 1-30th.jpg




The next thing is the overexposed reg plates. I wondered if shifting the exposure would help so I did a few tests.

  • The first photo is a front white plate (which is worse than a rear yellow one but let's make this difficult!) at 0EV and the plate is completely blown
  • The second photo is now at -1EV and the plate is still blown but you are losing a lot of exposure in the surroundings
  • The third photo is now at -2EV and you are starting to get detail but not enough. The surroundings are becoming very dark now.
Street Guardian 0 EV.jpg

Street Guardian -1 EV.jpg

Street Guardian -2 EV.jpg

And here's a few photos when driving at -1EV and -2EV, the latter being really bad for visiblity. So the conclusion is that reducing the exposure doesn't help for the plates and will underexpose the rest of the scene considerably.

Street Guardian -1 EV Driving.jpg

Street Guardian -2 EV Driving.jpg




There is a solution though which allows you to keep the camera at 0EV for lovely night exposure whilst still being able to read the plate in front..... ;)



Hope this helps!
 

kamkar1

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Amasing it only took 1/125 second exposure to make plates more readable, and its not like the footage is that much darker.

I am not sure if dashcams can match that ISO setting in that picture, when i crank my nikon 5200D all the way up in ISO it get grainy a lot.
 

arcticfire

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They might struggle but when some of them are running an f/1.0 lens that's like four stops more than my f/3.5 so if my camera is using say ISO 800 then the dash cam could be at ISO100, although the smaller sensor size would probably counter this some what.

The Street Guardian is pretty damn good though at low light so I'd be very interested to see how it would handle a faster shutter speed.
 

jokiin

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The Street Guardian is pretty damn good though at low light so I'd be very interested to see how it would handle a faster shutter speed.
bigger advances will come from updated hardware, some of the product we will release later this year will be far more capable
 

kamkar1

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Bastard :p you cant go peaking my imagination like that just as i am about to head into bed :D
 

Sky Blue

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bigger advances will come from updated hardware, some of the product we will release later this year will be far more capable
Should we stop buying current product to get the upgraded HW later this year? @jokiin

Or will there be any offer for current product owners?
 
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Should we stop buying current product to get the upgraded HW later this year? @jokiin
What type / make / model of dashcam do you have in your car right now ?
 

Dashmellow

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There's that old saying about comparing Apples to Apples. The APS-C sensor is many times larger than those found in the typical dash cam and therefore comparing their performance regardless of lens speed is misleading and doesn't tell us anything practical as it applies to the cameras we use.

Sensor_sizes.jpg
 
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Sky Blue

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What type / make / model of dashcam do you have in your car right now ?
I just bought SG9665GC V2 two weeks ago for front and thinking to buy another for rear!
 
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I just bought SG9665GC V2 two weeks ago for front and thinking to buy another for rear!
Good choice.
Of course we all waitimg for newer models, but there are no ETA yet.
 

arcticfire

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There's that old saying about comparing Apples to Apples. The APS-C sensor is many times larger than those found in the typical dash cam and therefore comparing their performance regardless of lens speed is misleading and doesn't tell us anything practical as it applies to the cameras we use.

View attachment 21928

You just can't help yourself can you.
 

arcticfire

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As usual, you seem to immediately resort to petulant remarks when someone points out the obvious facts.
Have you read it properly or understood because the posts were about exposure and frame rate? I referenced that the camera I used has an APS-C sensor to explain why it could use such settings because a dash cam may not be able to due to the sensor size difference. It was not about putting an APS-C sensor in a dash cam, there was a tiny fantasifull reference to this so I don't know why you plucked only this bit out - well actually I do know. I can't be botherered replying to you properly because you are very clearly looking to antagonise people, or more specifically me and I don't have the same amount of time that you seem to have for big online arguments.
 

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