Automatic license plate recognition (ALPR) has become the most important tool for identifying vehicles. To optimize its performance, the installer needs to have a proper understanding of how the system works and how it’s set up.
Automatic license plate recognition (ALPR) has become the most important tool for identifying vehicles and can be applied to various use scenarios, for example road monitoring, parking, and even crime prevention. However, there are still some challenges remaining, and to optimize its performance the installer needs to have a proper understanding of how the system works and how it’s set up.
ALPR optimization: Software vs. hardware
ALPR performance optimization can be done either via software or via hardware. In terms of software, optical character recognition (OCR) is used in today’s ALPR. It looks at the image of the plate, singles out the characters, and compares each character with a list of characters stored in the database. Or, in feature extraction, the software breaks each character into individual components and performs recognition based on a set of rules, for example two converging lines forming an angle with a third horizontal line in the middle constitute the letter “A.”
Some challenges in ALPR today include erroneous recognition of letters, for example “P” and “R,” that look similar. As such, the OCR software must be made strong and powerful enough to distinguish the differences. Certain license plates use specific colors or characters with different fonts and sizes; those nuances must be distinguished as well. For more sophisticated systems, they can recognize license plates from different countries or states. For example, two vehicles from different states with different license plate designs may share the same license plate number. The system must be able to tell them apart. License plates should be effectively recognized even if they are dirtied, soiled, or damaged.
In terms of hardware, it helps address other issues that affect the performance of ALPR. For example, illumination can be a problem at parking lots, and cameras now have superb wide dynamic range functions, able to recognize license plates even with the vehicle’s headlights fully on. For night-time surveillance, reading license plates can be difficult; with an IR illuminator the problem can be solved. If the transmission between the frontend ALPR and the backend server is temporarily disrupted, the edge device can still store vehicle information in the RAM and re-process that information afterwards.
Besides the aforementioned, the speed of a moving vehicle also presents a challenge for ALPR, which must be able to process license plates on vehicles traveling fast on roads or highways. Typically, an ALPR system should have enough computational capability to process vehicles traveling between 100 and 200 kilometers per hour.
Tips on ALPR Installation
Installation of ALPR should be reliant upon the area of monitoring, the angle of monitoring, and other unique features at the venue. As ALPR is mostly set up on roads or outside parking lots, the camera should be installed on a pole, which should be fixed on a plate flange. Space for transmission cable should be reserved. The foundation of the pole should preferably be hard concrete.
The pole should be set up firmly. It should be strong and can withstand wind, vibration, corrosion, and vandalism. As there are different types of poles, selection of it should be based on the actual use scenario.
Typically, the pole should have a height of 4 to 6 meters and thickness of 6 millimeters or above. The exterior of the pole should be coated with a layer of zinc via hot dip galvanization; the finishing of the pole should well blend into the environment. Various types of finishing should be available for users to choose from.
Cameras installed above a garage driveway should be no less than 6.5 meters from the ground. For cameras installed in an outside venue, its distance from the ground should not be less than 3 meters.
It is not advised that the camera be installed on a street lamp pole. But if it is, the pole should be non-shaky and non-corroded. The diameter of the pole should not be less than 250 millimeters, and the thickness should not be less than 4 millimeters. The equipment box should be placed low or on the ground. If an opening needs to be made on the pole its diameter should be less than 50 millimeters, and it should be welded properly after finish.
Coming Together
ALPR has become an important tool for vehicle identification and can now be applied to road monitoring, city surveillance, and crime prevention. Special challenges, for example erroneous reading of characters or recognition in low-light environments, can be addressed with an optimization of software and hardware. Proper installation is also required, and the installer needs to confirm the specific needs, demands, and requirements of the user. Only by having everything come together can the installation be successful and effective.