In both of these cases we're using one camera to cover two or three vehicle lanes. for both cases the requirements/constraints include:
- facilities are closed at sunset; daytime traffic is the consideration
- capture the full vehicle in color
- no control over vehicle position in the roadways (three to four lanes wide)
- vehicles typically traveling at 30 to 40 MPH
- with multiple vehicles one vehicle can block the view to plates of another vehicles
- client does not want two sets of cameras, one for vehicle capture and the other for plate capture; single camera must perform both functions (requires megapixel camera)
I have no photos to provide, but the interest is mostly in daytime video so lighting is not a challenge.
At one customer site we have Axis P1347 cameras (Axis has since replaced that with P1357). These are 5MP cameras that we set to the max of 12fps. Each camera covers two parking lot exit lanes. The exit point to the street is a traffic light intersection that is four lanes wide with stripes to define the four lanes. Each exit lanes is 12ft wide, so the license plates in two lanes are typically 12ft from each other.
At peak traffic times we have to capture the plate as the cars turns into the exit lane, as they stack up (sometimes 6 deep) at the light and one vehicle blocks any view of the plate on the vehicle ahead of it. So we need high pixel density because often the plate is at a 45° angle when it's visible.
The camera lens is set to an 8° field of view. Vehicles are 250ft from the camera at the farthest point, and 100ft from the camera at the closest point. There are two lanes from which they turn into the exit road from either side, at 120ft (to center of the lane) and 160ft. This gives us a pixel density of 180ppf at 100ft, and 73ppf at the farthest point. The pixel density is important because as I mentioned, as the vehicles turn into an exit lane, sometimes the best shot we get is a 45° angle of the plate. At that angle the plate is half its width, so the letters are same height but half their width. We need high pixel density to accurately capture the plate number.
The horizontal fields of view are 17ft at a 120ft distance, and 28ft at 200 ft the horizontal field of fiew of the camera is 28ft. That's wide enough to catch the plates as the vehicles turn in to the exit lanes.
This is an entry/exit road scenario where the vehicles can be traveling at 30 to 40 MPH. The roads in and out are three vehicles wide, and a single vehicle can be anywhere in the roadway. To get the best view of the plates the cameras must be 150ft to 250ft from the vehicle, looking right down a road from the side of the road. Posted (private roadway) speed limit is 10 MPH painted on the roadway, but that never happens. Cars always go faster.
Similar to Case One above, we capture at entry points into the roadways as the cars are at their slowest when turning in but accelerate rapidly. We had to wait for 12fps 5MP cameras because 3fps was not enough to catch all plates at their visible points.
Here we are using Axis P1347 cameras with Theia Sl940P (P for p-iris) telephoto lenses.
In this case the roadways run east-west, and so on cloudless days the sun shines right into the plates at an angle that blooms out the plate twice per day, such as around 10 AM for one direction and around 3PM for the other—but that time varies with time of year. We are still working to deal with that issue.
Requirements May Change
If the hours of operation change then we'll be addressing night video as well. That's a possibility and is what brought me to IPVM to check on related discussions.