What are the user requirements in license plate recognition?
Source: William Pao, a&s International
Needless to say, license plate recognition
(LPR) has become an important technology with widespread applications. One way to assess an LPR system is to see whether it meets most of the user requirements.
So what are some of the requirements that users look for in LPR? To this, Lawson Noble, CTO of Vaxtor Recognition Technologies
, provided some insights.
A good LPR engine coupled with a fine-tuned camera
should deliver recognition accuracy of more than 98 percent. However, Noble cautioned there’s more to it than just a number. “The LPR system (engine) should be able to recognize plates of varying length (say 3 to 10 characters in most countries), be able to read plates in a Latin alphabet, in many cases be able to read plates in other languages such as Arabic, Korean, Thai and Chinese, be able to correct a plate using pre-defined syntax rules for each country – for example in the UK the characters 0 and O, and 1 and I, are written the same,” he said. “The only way to determine the correct plate text is to use the appropriate syntax.”
According to Noble, the system must be able to report, store and take action on all of the plates being processed in real time. “A typical modern intelligent camera can process more than 10 plates per second, PCs can handle many more – all transmitting to larger servers which should be able to receive several million plates per day,” he said.
Plates need to be stored at the camera head or at the processing PC for a short time and then transmitted to a back office application for longer term storage or further processing, Noble said. “Storage has become very affordable and in the case of an intelligent camera, a simple SD card
can store more than one million plate images and data,” he said. "However, in these days of GDPR, in a lot of cases plates should be auto-deleted (purged) after a fixed time.”
Comparing plate with multiple lists in real time
Next is the ability to check plates against multiple databases or hotlists as they are read. “In the case of law enforcement, site security, counter terror and border control the user will have access to multiple hotlists, for example stolen vehicles, vehicles with no tax, known criminals and wanted people. The application or back office should be able to compare the plate read with multiple databases in real time and sound or send alarms as appropriate,” Noble said.
Ability to open barriers or control LED signs
According to Noble, the system should be able to compare the read plate with several lists or databases of employees, night staff, senior management, known delivery vehicles or pre-booked visitors. “If a match is found then the system should be able to control standard relays such as the IP-based ADAM range to open barriers or gates by sending a timed pulse,” he said. “The plate data should also be available to display information on LED signs
such as: ‘Welcome Mr. Jones. Please report to security.’”
Ability to search
In the case of site security, car park or depot monitoring, the user or administrator may need to search the log files to look for a particular vehicle, Noble said. “Searches must not only allow for a complete plate search, for example search for ABC123, but also perform partial or complex searches such as: ‘Find all plates that contain ‘66’ that were seen by camera 23 on May 20, 2019 between 2 p.m. and 4 p.m.’ A list of candidates should be produced that can be further filtered,” he said.