License plate recognition is a surveillance system used primarily by law enforcement to capture the license plate information of moving cars. It does this through optical character recognition of plates using cameras that generally are designed especially for this purpose, though existing close circuit cameras are sometimes used. A common situation involving license plate cameras is at an intersection where they are used to make sure red lights are adhered to. Violators are sent a ticket through the mail based on a photo taken by the system. Infrared lights are commonly used in conjunction with the camera for surveillance at night and a flash is sometimes incorporated, partly for clarity and partly as a visible cue of the offense.
The Software Component
A software component is used first to enhance the image taken of the license plate and then to extract alphanumerics from the plate. Six primary items have been identified with regards to software correctly reading a license plate. The first is that the software must localize the plate in the picture. The second is that the software must compensate for the angle of the plate and dimensions must be adjusted. Next, the brightness and contrast of the image must be corrected. The individual characters of the plate must then be identified. Optical character recognition now comes into play. Finally, the characters are checked against a database. Two versions of this system exist. In the first approach, the entire process occurs on site, while in the second images are transmitted to a central location for processing of the optical character recognition.
In addition to fixed cameras, mobile units exist. Actually, smaller cameras that can read license plates at high speeds and smaller processing units that can fit in the trunk of a car have led to an increase in mobile applications. There are, however, several challenges facing mobile applications. The ability to produce an accurate reading is contingent upon the angle of the camera and the vehicle in question, the weather, and the time of day. Highway police cameras must span several lanes and be able to read plates at high speeds while city police cameras require lower focal cameras for capturing the plates of parked cars. Also, certain states have rear license plates only and in these instances a forward facing camera will not do. Advanced systems have therefore been developed that utilize several cameras on a single police vehicle. The cameras are positioned and repositioned as needed.
Challenges and Controversy
One of the biggest challenges facing optical recognition is the identification of characters on the license plate because each state has an average of twelve different license plate designs and they all must be recognized by the system. The color of the plate and raised or flat letters are simply two of many variables that can affect proper character recognition. A system must also keep up with changes or additions to a state’s license plates. Given this, license plate recognition systems must be very versatile. These systems are not without controversy. Privacy issues have arisen due to usage of information attained based on a vehicle’s license plate number and misidentification of drivers are a common concern. Despite this, these systems seem poised to stay and increase in usage.
Types of License Plate Cameras
There are actually two versions of the license plate camera: those that connect to a database and those that do not. While the former are used by law enforcement, the latter is ideal for those who simply want to monitor traffic within a designated area. These license plate cameras, which are cheaper in cost, are hooked up to a DVR and the surveillance footage is thereby recorded for review. Herein lies the difference between the two types. The surveillance footage must be reviewed in the second version, as there is no database to reference. Despite this, the second type of camera has its usefulness. It is often utilized by homeowners and gated communities to keep track of who enters and exits their neighborhood. It is an effective deterrent of potential crime in this way. It becomes not only effective but an affordable deterrent with the difference in cost.