Capturing a licence plate at night is one of the most challenging tasks a video surveillance system will ever be asked to do. How difficult you may wonder?
Well, exceedingly difficult: in fact, it’s probably easier to get facial recognition than it is to capture a licence plate.
No one would ever say that the task is impossible, but it’s fair to say that recording a licence plate clearly at night from any sort of distance is generally beyond the remit of most average home security cameras.
Most home security cameras will record number plate images up to a distance of around 25 feet – in other words, the average length of a standard driveway, but beyond that any clear identification is difficult.
A CCTV camera may well pick up number plate details at distances over 35 feet, but these images will often be blurred or pixelated, making them pretty useless for identification purposes.
Why is number plate recognition at night so problematic?
The night-time environment creates several challenges for all-but the most specialised video surveillance systems, particularly if the vehicle you are trying to identify is also moving when the image is captured.
The reasons for these difficulties are numerous, but basically a successful image capture depends upon the speed of the vehicle, the camera location, glare, and the lack of a camera whose sole task is number plate recognition.
The speed of a vehicle
The trick to capturing and freezing any moving image is to use the right combination of lens and shutter speed. The same rule applies in every form of photography whether that stills photography or video.
If the right combination of shutter speed and lens is not chosen you will simply end up with a blurred image. With video photography and image capture, the speed of the vehicle is paramount. If a vehicle is travelling at speeds over 20 miles an hour, then image capture will be compromised, unless you have access to a speciality video camera which can adjust shutter speeds to avoids image blur.
These ProVue 30fps cameras have shutter speeds and resolution to capture the plates of even fast-moving vehicles. If you do not have access to this type of equipment, however, then the best you can do is to train the camera on a targeted area where the vehicle will be forced to reduce speed to the required level.
The location of the camera
The location of the camera in relation to the travelling path of any vehicle you wish to monitor, and record is crucial to a successful image capture. If the location of the camera is at too acute an angle to the direction of vehicle travel, then no camera, even the most expensive, will successfully capture a readable image.
So where is the optimal place to locate a CCTV camera for licence plate recognition? Well, ideally the camera should be positioned is an area that allows it to record either the view of the vehicle’s plate driving towards or away from the property, or at an angle of less than 30 degrees.
The trick is to capture as perpendicular an image of the licence plate as possible.
How can you be sure that you have managed to get the best view of a vehicle for image recording purposes? Well, the easiest way is to check. Park the vehicle in the required capture location, then check the image from the camera position. If you can read the plate number, then the angle is OK. If you can’t, then change camera positions and check again until success is achieved.
The lack of a dedicated motion capture camera
A CCTV camera is a marvel of modern technology, but it isn’t a miracle worker. It can theoretically capture the licence plate details of a vehicle, but it won’t be able to view the driveway of a property and capture a still image of the driver at the same time.
You may wonder why this isn’t possible, but really the answer is quite straightforward. Each of these separate tasks requires a different angle or field of view, and that means the cameras capturing these different images will need different lenses of varying focal lengths.
A view of a property’s driveway and gardens will require a wide-angle view and corresponding lens. That same lens, however, will not be able to capture detailed images of a number plate or the driver of the vehicle. These images require a longer or telephoto lens to capture crisp images: the problem is that if you use such lenses, then number plate recognition will have to be sacrificed. The upshot of all of this is that if you are intent on capturing number plate images, then you will have to dedicate a single camera for that purpose and equip it with an appropriate lens.
Tring to capture an image of a vehicle number plate at night is fraught with inherent difficulties. A camera must strike the tricky balance between the low-level light conditions of the night sky and the bright, often intense, light from an approaching car’s headlights. Most cameras are not equipped for this.
Modern HD cameras will capture good quality images of number plates in daylight, but not at night as the glare form headlights makes image recognition practically impossible. A camera specifically designed for night-time number plate recognition can do this though. These cameras incorporate technology which all but eliminates glare.
How do you choose the right camera for night-time number plate recognition?
The trick to selecting the right camera for the job in hand is to accurately calculate the distances involved when capturing images.
Firstly, you will need to work out the distance from the camera location to the location where the car number plate image will be captured, and secondly, the horizontal viewing distance at the location of the number plate.
In essence what you will need to establish is the distance between the camera and the vehicle and the width of the vehicle to get the best usable image.
Because both distances will vary with individual requirements, the best solution is to invest in a camera with a manual zoom lens, you can then zoom the camera’s field of view in or out until the maximum horizontal distance is achieved.
As an example, most 2.8 to 12mm lenses will allow you to mount a camera up to 60 feet away from a number plate’s location and still be able to achieve a horizontal field of view of 18 feet. The actual maximum horizontal viewing distance recommended is dependent on the type of technology you are using.
Older technologies like analogue CCTV produces low resolutions, so only a small horizontal field of view is required. HD technologies require a much wider horizontal field of view to capture clear images of vehicle licence plates.