📌 Table of Contents
■ What is License Plate Recognition?
■ The Origin of License Plate Recognition
■ Factors Affecting the Accuracy of License Plate Recognition
■ Automatic License Plate Recognition Features and Applications
▲ Automated Parking Lot Management
▲ Access Control for Community and Building Parking
▲ Law Enforcement Applications
■ Disadvantages of License Plate Recognition Systems
What is License Plate Recognition?
A license plate recognition system is a technology that uses optical character recognition (OCR) to locate a vehicle in an image and read its license plate data. This is the basic operation of license plate recognition. For commercial value, the system must be automated, so it is generally called Automatic License Plate Recognition (ALPR) or Automatic Number Plate Recognition (ANPR).
The Origin of License Plate Recognition (LPR)
License plate recognition was invented in 1976 at the UK’s Police Scientific Development Branch. In 1979, EMI Electronics began to deploy the first prototype systems. Early tests were conducted on UK highways and the Dartford Tunnel. The first theft arrest using technology occurred in 1981, when a stolen car was identified. By the early 1990s, with the development of cheaper and more user-friendly software, automatic license plate recognition systems began to be more widely adopted.
Factors Affecting the Accuracy of License Plate Recognition
- Vehicle Speed: The camera settings required for high-speed and stationary vehicles differ significantly. If the shutter speed is too slow for a fast-moving vehicle, the image may be blurred, making it unreadable by the recognition software. A shutter speed of 1/500s is suitable for vehicles traveling at 70 km/h, while 1/250s can handle traffic speeds of 20 km/h. In slow-moving traffic or when the camera is close to being level with the license plate and the vehicle is at a specific angle, a fast shutter speed is not necessary.
- Ambient Lighting: Increasing image contrast is a crucial factor for the accuracy of license plate recognition. Higher contrast leads to a greater chance of correct recognition. Therefore, ensuring appropriate ambient lighting to create high-contrast images is the first step in setting up a license plate recognition system. Light that is too strong can overexpose the image, while light that is too dim can create excessive noise. Both are detrimental to contrast. The camera must adjust the shutter speed for day and night, and an infrared or white light must be added at night for light compensation. In general, black-and-white images offer higher accuracy than color images, so it’s recommended to use a monochrome camera for license plate recognition.
- Clear Images: Factors that affect image clarity, such as poor focus, motion blur, overexposure, and low contrast due to reflections or shadows, can significantly reduce recognition accuracy.
- Shooting Angle: License plate recognition has physical limitations. Low pixel count, extreme viewing angles, and large offset angles can lead to recognition errors. Commercially available recognition software typically accepts pixel ranges of about 16-60 pixels, horizontal and vertical angles of about 20-45°, and license plate offset angles of about 5-10°.
- Country-Specific License Plates: Different countries or states use different fonts, colors, and background colors for their license plates. These factors affect precision and accuracy, often requiring different recognition engines for different countries.
Automatic License Plate Recognition Features and Applications
License plate recognition is most widely used for automated parking lot management, community and building parking access control, and traffic law enforcement. Let’s look at the benefits it provides in these applications.
1. Automated Parking Lot Management
The advantages of using a license plate recognition system for parking lots are:
- Full automation reduces labor costs and increases competitiveness and profitability.
- Eliminates management loopholes and theft. The license plate number is the vehicle’s unique identifier, and with video and timestamped records of entry and exit, no vehicle can leave without paying. This makes it difficult for staff to profit from loopholes.
- Reduces costs and is eco-friendly by eliminating the need to repeatedly purchase large quantities of magnetic cards.
- Prevents customer inconvenience caused by lost proximity cards.
- Protects owners from losses due to duplicated proximity cards.
- Enhances convenience by allowing customers to enter and exit without having to roll down their windows to scan a card.
- Provides extensive parking data for management to analyze and formulate strategies.
📖 Further Reading：Make Sense of Parking Management Systems
2. Access Control for Community and Building Parking
The advantages of using a license plate recognition system for community and building parking access control are:
- Increases security by preventing unauthorized use of parking spaces and providing video and time-stamped records of all vehicle entries and exits.
- Eliminates the hassle of lost, duplicated, or lent remote controls and proximity cards, removing management blind spots.
- Saves money by eliminating the need to purchase magnetic keys and remote controls.
- Reduces waiting time with long-range recognition that opens gates while the vehicle is still approaching.
- Prevents the inconvenience of getting wet in the rain by eliminating the need to roll down the window to scan.
📖 Further Reading：Dual-Recognition Parking Management System
3. Law Enforcement Applications
License plate recognition is used by police forces worldwide for traffic enforcement, primarily in two ways:
- Fixed Camera Systems: These systems are integrated with existing road surveillance cameras or are specifically installed for license plate recognition. Applications include checking for legal vehicle status, tracking the paths of hit-and-run drivers, and monitoring for speeding violations. They can also be used for electronic toll collection based on distance traveled and classifying traffic flow for time-based tolls.
- Mobile Systems in Police Cars: Law enforcement agencies often use handheld license plate readers for static, time-specific checks. This method is passive and has limitations in speed, efficiency, and human resources. A mobile system installed in a police car overcomes these drawbacks. Since police cars are always on duty, a mobile recognition system can actively check the license plates of parked cars while the police car is driving. This allows for large-scale, continuous license plate scanning to identify illegal vehicles.
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Disadvantages of License Plate Recognition Systems
No system is perfect, and license plate recognition systems have some drawbacks. Conditions like dirty license plates, incorrect entry angles, or heavy rain can prevent recognition. However, the benefits of using license plate recognition for automated control far outweigh the disadvantages. These drawbacks can be mitigated by using RFID as a supplementary recognition method, which can make the system even more reliable, especially for company, community, and building parking lots with restricted access.
📖 Further Reading：Dual-Recognition Parking Management Host