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Has anyone had any success with cameras that can resolve the license plates of vehicles that are near the bike?
What resolution will be needed if we want to be able to read license plate later on?
EDIT: Is resolution 1920x1080 enough for license plate resolution? Or is frame rate (interlaced vs. progressive; 30 fps vs 50fps vs 60fps) necessary; or is it the bitrate that is the most important?
Security cameras have a standard called "pixels per foot" (ppf). This means that at a certain distance, an object 1 foot across (about the size of a US license plate) will have a certain number of pixels. As the object moves further away, the number of pixels per foot decreases. As the object moves closer, the number of pixels per foot increases.
This is the correct way to measure whether you can read license plates or not, because it takes the camera lens field-of-view into account. This is particularly important when dealing with the GoPro since it has a maximum field-of-view of 170 degrees! This means its pixels are spread over a much larger area.
Conveniently, security cameras makers also have a standard for reading license plates of 40-45 pixels per foot.
Take a GoPro recording at 1920x1080 (aka 1080p) @ 170 degrees. Presumably you'd use this if mounted to the bike; the wider angle simply records more. Your resolution will drop to 45 ppf at 14.4 feet away. That is, beyond 14.4 feet away, it will get harder and harder to read license plates clearly.
A more thorough listing of a few different cameras: - GoPro 1920x1080 @ 170 degrees; 14.4 ft - GoPro 1920x1080 @ 127 degrees; 19.2 ft - GoPro 1920x1080 @ 90 degrees; 27.2 ft - GoPro 4k 3840x2160 @ 170 degrees; 28.8 ft - GoPro 4k 3840x2160 @ 127 degrees; 38.5 ft - GoPro 4k 3840x2160 @ 90 degrees; 54.3 ft - Novatek NT96650 G1W 1920x1080 @ 120 degrees; 20.4 ft - Mobius ActionCam 1920x1080 @ 116 degrees; 21.1 ft
The last two were selected because they were reviewed by Popular Mechanics as car dash cams, not because of their suitability for use on a bicycle.
More math! (oh noes!) A car travelling at 30 mph (relativistic) is travelling at 44 ft per second. That is, a car approaching you at 44 ft per second across a 30 ft distance (picked at random) will take 0.68 seconds to reach you. At 30 frames per second, that means you can record 20.5 frames. The more frames you get, the more likely that you get a clear one.
However, after laying all that math out, I'm not sure that 40 ppf is the minimum. Digital Image Forensics can recover an amazing amount of detail out of a blurry image, particularly because license plates are high contrast. Judge for yourself by searching for "digital forensics license plate"
I'm also not sure that 30 mph is a reasonable target to aim for; it seems a bit fast in most cases.
I recommend trying it out for yourself. Get a tape measure, borrow a camera, and record your license plate in 5 ft increments, and judge for yourself what seems like a effective distance is. Take angle into account; not everyone is going to be approaching the camera straight on; they could be approaching you from a cross street, so you might be viewing it at 45 degrees or so.
I'm interested in bicycle cameras at the moment, although for reasons other than reading number plates, and I have found several cameras to be available, which look pretty decent. There's a much wider choice in this market than there was a few years ago.
So while I can't say explicitly "such-and-such a camera will read a number plate at 20 paces" I'm happy to share what I've found so far.
The three cameras that I'm thinking seriously about are the GoPro Hero, the Garmin Virb and the Shimano Action camera. So any of these three might be worth a second look. As a start point, there is a cycling blog by a guy called DC Rainmaker, which has reviews and sample footage of all three cameras, which I have found helpful. You might be able to determine from the sample footage whether the resolution is "enough" for your needs.
You imply that you'd be looking for something helmet-mounted. I know that all three of these cameras can be attached to the frame (which is what I'm looking for), so it is probably best to double-check that suitable attachments exist. From what I've found, the area of attachments can become as complicated as the camera itself.
Over and above these, a couple of years ago I did buy a camera for my commute, which I attached to my helmet using cable ties. That was a Dogcam Bullet and whilst it worked after a fashion, there were issues with things like image stabilisation, white balance etc which made the solution less than ideal. This would have been about three years ago and you got the impression that the technology just wasn't quite there yet. If you're interested in this type of solution (it was certainly cheaper than the ones I'm looking at now) I posted my thoughts, and some sample footage, as an answer to this question a while back.
I don't think this is really an answer for you but hopefully it will give you some tips as regards where to start looking.
Here's footage of my commute to work in Manhattan. It was filmed using a Polaroid Cube camera. I stuck it onto my handlebars using the Polaroid-made handlebars mount.
YouTube allows you to view the video in various resolutions using the "gear" dropdown. Even in 1080p reading license plates can be a dodgy proposition. I say it depends majorly on lighting (sun from behind - good, from the front - bad). Obviously at night things are quite different as well.
My old JVC Addixion (XA1) recording at 1080P (30FPS) doesn't get the trick done at any useful distance. There are a number of higher resolution cameras coming out now and I'm thinking about upgrading for just that reason. I'm currently leaning towards the GoPro Hero4 Silver because it'll record 2.7k at 30fps, and it's not as expensive as the couple of 4k cameras that just came onto the market.
If money was no object, I'd probably go for the new Sony FDR-X1000V/W, although they don't say what the battery life is on that guy yet, which could be a deal-breaker.
Update: I picked up a GoPro Hero4 Silver a few weeks ago, and recording at 2.7k (2704x1520) 30fps I can pick out license plate numbers regularly in daylight. At nighttime they're still not very readable because the bright license plate reflecting my head light washes out the dark numbers.
In my experience, resolution doesn't really matter, as long as you're in the territory of High Defniition.
What has much more effect is the overall brightness of the scene, which directly affects the "sensor speed"
I've got a gopro3 and during a sunny day I could make out the screws holding a licence plate on a car when they're doing 100+ km hour in the other direction. At night I could stop within touching distance behind a stopped car and still not read the plate. Most of this is due to too much light from bike lights, and the rest of the scene being dark leading to oversaturation on the lit areas.
A secondary technique is for you the cyclist to read off the licence plate, so that the audio microphone in the camera can hear you. Its one more thing to think about at a busy moment though.
Don't count on a camera to save you from anything, its merely supporting information for after the fact of an accident. Your best defense is not to get in an accident, so bike defensively and anticipate problems.
Note - gopro makes a bad choice for a proof-camera because there's no date/time stamp on the images.
I have a Contour Roam2 which I use at 720p and it gets registration numbers about 6 times out of 10 which isn't bad at all. The usual reason for them not being clear is vibration on my bike (typically due to a poor road surface) but I'm used to reading the registration numbers out aloud now.
Edit: I often wipe the lens when I'm stopped at lights etc if it's wet outside to keep it as clear as possible.
A Mobius Action Cam w/wide angle lens at 720p might do the job. I have no experience with the Mobius but have played around with earlier '808' key fob cameras and discovered a wide angle lens was the way to go to read license plates when used as a dash cam. Techmoan has a very detailed video review of the Mobius.
These key fob cameras are small, cheap and lightweight. I've had good success walking around with one velcroed to the bill of a baseball cap. Haven't tried it on my bike yet.
