Not all cameras provide good results!
Why use an ANPR Camera when people are now saying "a standard CCTV Camera and an OCR Engine are just as good"
The performance of any Automatic Number Plate Recognition (ANPR) system relies on the quality of the images from the camera and the optical character recognition software engine. No matter how powerful and accurate the software engine is, poor images from the camera will no doubt lead to errors resulting in mis-reads of number plates. Several factors need to be taken into consideration when specifying an ANPR Camera which will ensure that a good crisp clear image of the plate is captured.
The ability for capturing vehicles travelling at speed from a considerable distance can be challenging for the standard CCTV camera as there is a necessity for several functions to be configured so the camera can capture the image containing clear readable images.
Speed – to capture the plate image successfully at speed, this is anything over 10 mph, requires the camera to be working at a fast shutter speed of more than 2 milliseconds, the faster the vehicles speed the faster the shutter speed needs to be, typically for motorway speeds the shutter speed needs to be faster than 1 millisecond. A camera set up with a slow shutter speed of say 20ms will provide good clear images of the scene with plenty of light, however the plate will be blurred and unreadable.
Light – the camera needs sufficient light to be able to capture the plate in all conditions 24 hours a day. The effect of selecting a fast shutter speed for moving vehicles as described above reduces the amount of light reaching the camera sensor. To ensure there is enough light available to capture the plates 24/7 Infra-red (IR) illumination is used. There are typically two types of IR commonly used, flood IR and Pulsed IR. The main difference between the two types of IR is that the Pulsed IR is set up to work with Global Shutter cameras and is synchronised to flash with the camera frame negating the requirement for the IR illuminator to be ‘on’ all the time. This increases the amount of power that can be applied to the IR bulbs which increases the distances that can be achieved. Also, key to providing the right amount of light for the camera is by altering the Iris function of the lens. Ideally once set the Iris should not change, i.e. open or close automatically. Lenses with a manual Iris setting, which is a wheel turned by the installer is a usual way of setting this value, however a far better way if for the camera to be equipped with a remote-control lens allowing for the Iris function to be set and locked from off site via a software application. Similarly, it is of great advantage to be able to control the intensity of the IR illuminators from the Software application.
Focus - Once the light levels are set for the scene the focus and zoom of the lens requires adjusting to ensure the plate is the correct size in the image and in focus, again this can be carried out by having the remote-control lens using the software application.
Camera module – the camera module is very important and needs to be IR responsive and have all the necessary adjustable settings to enable set up for ANPR.
Poorly functioning cameras can prove to be very costly as they reduce the number of readable images of plates sent to the OCR engine and therefore the number of correctly captured read plates resulting in potentially a loss of earnings for car park management companies, skewed statistics, barriers and gates failing to open and missed opportunities for Law Enforcement to identify vehicles.
It is extremely important that cameras are configured correctly prior to leaving the factory, however with the correct tools, such as remote control lens and IR functions, easy shutter speed and gain control settings there is the ability for the operator to set the camera up easily and accurately.