Credentials Explains: What is License Plate Recognition? – Part 1

by Rose Addison, Manager of Documentation & Training | Nov 29, 2017

License Plate Recognition (LPR) is an image-processing technology used to identify vehicles by their license plates. LPR was invented in 1976 at the Police Scientific Development Branch in the United Kingdom, but didn’t gain popularity until the 1990s when the technology became less expensive and more reliable.[1] LPR technology is used in many different applications, such as parking violations, access control, tolling, law enforcement, security and traffic control, and assisting with Amber Alerts.[2]

LPR usually functions in one of two ways. The first allows for the entire process to be performed in real-time. This means the license plate number, date and time, location, and any other pertinent information is processed within seconds. This information can be used at that time or transmitted to a remote computer/database for further processing if necessary. The second method typically handles much higher image volumes and requires the images to be transmitted to a remote computer/database location and scanned at a later point in time. This process often requires more computing power to analyze the high volume of scans.

License Plate Recognition Algorithms

Algorithms must be able to compensate for all the variables that can affect the LPR system's ability to produce an accurate read, such as time of day, weather, and varying camera angles. A system's illumination wavelengths can also have a direct impact on the resolution and accuracy of a read in these conditions.

Installing LPR cameras on law enforcement vehicles requires some initial consideration and planning. Using the right number of cameras and positioning them accurately for optimal results can prove challenging, given the various tasks and environments in play. For example, highway patrol vehicles require forward-looking cameras that span multiple lanes and are able to read license plates at very high speeds, while city patrol vehicles need shorter range and lower focal length cameras for capturing plates on parked cars. Parking lots with perpendicular parked cars often require specialized cameras with a very short focal length.

Most systems are flexible and can be configured with one to four cameras, which can be positioned based on the goals and requirements of a variety of in-field applications.