Complete guide to license plate recognition (LPR) systems

What is automatic license plate recognition (ALPR)?

Automatic license plate recognition (ALPR), also known as automatic number plate recognition (ANPR) and license plate recognition (LPR) is a technology that uses cameras and software to capture and read license plate information, including characters and numbers. 

The reason for its many names is due to different regions around the world referring to the technology slightly differently:

• ALPR: Used mostly by North Americans, particularly in the US.
• ANPR: Used mostly in European and Asian countries.
• LPR: Used to describe any technology that can read license plate data.

Law enforcement, schools, hospitals, businesses and more use license plate recognition technology to identify vehicles to locate one of interest, such as stolen or wanted vehicles, check if it is authorized to be on-site and more. 

The cameras are often mounted on vehicles, poles, walls and vehicle entry barriers, which allows the LPR systems to automatically capture license plate numbers that come into view, along with the date, time and location information. This license plate recognition data, including photographs of the vehicle and its occupants, is then uploaded to the system’s server for record-keeping and further action if needed.

How does a license plate recognition system function?

License plate number reader technology uses a specialized hardware camera that communicates with a connected software program to recognize and record the details found on the number plates of vehicles.

LPR cameras must be far more accurate than the average commercial security camera, with the hardware needing to capture clear images of moving vehicles in around 20 milliseconds or less.

These cameras must consider several additional factors that could lead to an obscured image, such as: 

• Resolution
• Headlight compensation (glare reduction)
• Dynamic range
• Shutter speeds

While some forms of LPR technology house both LPR cameras and the optical character recognition software (OCR) used to decode the information within the same unit, these configurations are generally far less accurate than those that use two dedicated units for each task.

License plate detection and vehicle recognition: A step-by-step guide

A commercial LPR system is generally permanent in its design involving LPR cameras installed at key points throughout a parking lot, city or property, plus software that can decipher the license plate capture. To understand how an LPR system operates in practice, here is a step-by-step guide to the key technical processes involved:

1. Framing the license plate capture

When a vehicle passes through an LPR camera’s field of view, the device will identify the position of the number plate in relation to the entire image. This process is known as localization and allows the system to focus only on the license plate, ignoring any other data in the picture.

2. Sizing the license plate capture

An LPR camera may capture an image at an undesirable angle as a vehicle passes. In this case, a software algorithm will apply specialized adjustments and angular corrections to create an image that is easier for the OCR software to read.

3. Normalization

In some instances, shadows and shading can affect the quality of an image captured by a license plate reader camera. To account for this, various filters will be applied to read the number plate text. Some LPR systems will take multiple pictures quickly, blending them all to create a much clearer composite image for the OCR software to analyze.

4. Character segmentation

In this step, the LPR system will separate the individual characters on the license plate. This process involves finding the space between each character and using this data to discern individual letters and numbers. With fixed-spaced fonts, this process is simple, though variable-spaced fonts can be more challenging for the software to read successfully.

5. Optical character recognition

With each character separated, the license plate recognition software is ready to produce a final image. Several OCR techniques are used in this step, including edge tracing, pattern matching and pixel repetition.

In some cases, certain characters can be hard to distinguish from each other (0’s and o’s, 1’s and I’s, etc.), and so two images may be produced to be further analyzed.

6. Geometric analysis

The final step will compare the OCR-checked image(s) against the specific license plate rules for the given region, confirming that the software has been effective. This process helps to clear up any confusion between similar-looking characters and contributes to a higher confidence level in the final result.

What to look for in an automatic LPR system

If you’re looking to procure a license plate recognition system, there are a few key things you should look out for.

Type of camera system

Internet protocol (IP) bullet cameras are generally chosen as the most effective tools for use in an LPR system. They capture high-definition images at a long range and are suitably robust for use in outdoor environments to help reduce criminal activity.

In addition, high-speed types of security cameras like bullet LPR IP cameras can communicate with connected license plate recognition software and analytics solutions via a dedicated private network address, ensuring reliably fast processing times with no interference from outside sources.

Image and video capture

Effective LPR systems will capture both still images and videos to help operators investigate incidents. High-quality still images can be used to identify key details like text and defining features of passing vehicles, while videos can add extra contextual information like movements inside the car, witnesses outside and whether the person was driving erratically.

Customizable settings

License plates come in various shapes, sizes and colors. This means that an effective license plate detection system must not only be capable of capturing clear images at breakneck speeds, but it must also be able to successfully differentiate between distinct backgrounds, texts and color combinations.

There are several ways that license plate reader technology can achieve such precision. For example, wide dynamic range settings for the camera enable correct exposure across several frames; adjustable shutter speeds reduce blur in different light conditions; headlight compensation can better navigate glare when operated at night.

Make, model and context analytics

Modern LPR systems can secure extra information about passing vehicles like the make, model and color of the car to help operators identify subjects more easily. In cases where exact license plate details are hard to make out, this extra information can help to move investigations forward.

Some license plate detection systems can also offer extra contextual information like the direction of travel, the region the car originates from and how many people were in the vehicle, as well as optimize images where plates are blurred, slanted, damaged or partially obscured.

High-performance software

To capture and process license plate readings, the license plate recognition software needs to be advanced enough to process data at a high rate of speed without affecting accuracy. A documented accuracy rate of 95% or over is considered an advanced solution. It also needs to be a platform that is easy to use and, if possible, integrate with wider security systems for enhanced situational awareness and effective physical security operations.

On-premise and cloud

Options to run plate number recognition systems either over the internet or as part of a closed network make it easy to adjust solutions for different needs. Low-security, high-traffic sites can be run through cloud systems to support remote accessibility and alerts, while high-security installations can use on-premise servers to mitigate the risk of data breaches.

Considerations when installing a license plate recognition system

Alongside a suitable video recorder security camera capable of capturing clear images in all lighting conditions, a permanent LPR system must be designed with a few environmental factors in mind. This will ensure that the specific hardware and software combination chosen can produce accurate data at a reliable rate.

The problems with infrared and night vision

In any other video security system, infrared and night vision capabilities can be beneficial for capturing footage in low-light conditions. However, these technologies can cause a few issues regarding license plate readings.

The highly reflective paint used in license plate manufacturing will reflect infrared light straight toward the source, resulting in an unreadable image when used as part of an LPR system. This is why many license plate recognition cameras instead use a full spectrum light source, as this produces a much less reflective ambient light that’s far less likely to affect the clarity of the final image.

Positioning a license plate recognition camera

An LPR camera should be positioned at an angle less than 30 degrees above the intended target area to increase the chance of capturing a usable license plate image and allow varifocal lens cameras to zoom as needed. If the camera is too high, license plates can often be blocked by the bumpers and other design features found on specific models of cars and trucks.

Additionally, it is challenging for the human brain to distinguish characters positioned at an angle greater than 30 degrees. Even a clear image from this position may be complex for a human operator to comprehend reasonably.

Influencing vehicle speeds

The slower a moving vehicle passes by an LPR camera, the more likely the resulting image is clear and easily identifiable. This is due to the camera being able to considerably lower its shutter speed, keeping the shutter open for a more extended period to reduce motion blur.

License plate scanners should ideally be installed near a stop sign or a clearly defined junction. This way, the vehicles that pass by the camera will travel at a much more optimal speed for the LPR camera to take a clear image.

Use cases for license plate scanner and recognition technology

The most common application of license plate recognition systems involves monitoring vehicles in areas that regularly experience high traffic volumes; this includes commercial businesses, government buildings and public parking lots or garages.

An automatic license plate recognition solution is ideal in these situations. It can integrate with LPR cameras, such as Pelco IP cameras for license plate recognition, to reliably utilize video streams and access control networks to capture footage in free-flow environments.

Toll collection points

Many road networks operate unmanned toll collection points, in which vehicles are expected to pay for access each time they use a particular stretch of road. Using an ALPR system here allows authorities and police officers to ensure that appropriate payments are made and assess the volume of vehicle traffic at key toll areas.

An ALPR tollbooth also allows authorities to issue email-based invoices, meaning vehicles using the service are not required to stop each time a toll is issued. Instead, they can pay by mail at a later date.

Law enforcement and traffic monitoring

License plate recognition technology is also frequently used by law enforcement, federal agencies and even Homeland Security across the US, helping locate and identify criminal activity and persons of interest.

LPR systems are ideal in areas with frequent traffic violations and issues with speeding. These systems can work side-by-side with traffic monitoring cameras to decrease the frequency of such offenses. They can also identify stolen vehicles, producing real-time evidence and historical data that can be used in a criminal investigation.

License plate recognition parking enforcement

With the assistance of an LPR, parking lot security systems can be more efficient, assisting staff to monitor large parking garages and, in turn, allowing management teams to optimize their available workforce better through automation.

Additionally, fines and penalties for inaccurate ticket payments or overextended stays can be automatically issued from an integrated parking management system, and customer disputes can be settled fairly and reasonably with the assistance of a connected video security network.

Unauthorized parking

Locations that deal with unauthorized parking, such as retail parks, industrial estates and private roads, can use ALPR systems to identify offending vehicles and issue fines or written warnings.

LPR vehicle identification solutions can also help alert authorities of suspicious activity in such locations and be integrated with additional security hardware, such as analog CCTV systems and access control networks to protect private property from potential intruders and unauthorized persons.

The benefits of license plate recognition and scanner systems

Improved security

In addition to providing authorities and private property owners with a method of identifying potential trespassers, automatic license plate recognition systems can effectively deter crime.

If an ALPR system is visible to passing vehicles, simply knowing that a recording system is in operation can be enough to deter criminal behavior. Law enforcement agencies can also use ALPR data to check or search for suspicious vehicles.

The use of ALPR software carries benefits to smaller security operations too. For instance, if a vehicle has been previously banned from private premises, this technology can be used as an identification tool to support any further action against the individual in question.

Real-time imaging

Another great benefit of modern ALPR technology is its ability to provide real-time imaging to private security teams and other relevant authorities. In the past, license plate reading and analysis would take considerable time before producing a usable result, leading to penalty notices and fines taking far longer to be issued and eventually resolved.

However, with ALPR technology, reading and analyzing license plate information can be carried out almost instantaneously, with the resulting data being checked against an official database to locate vehicles and owners much more efficiently.

Journey time analysis and traffic management

License plate reader systems have quickly become vital for authorities and highway agencies to record and analyze public road systems. By setting up nodes along certain stretches of road, teams can gather valuable data concerning vehicle speeds, congestion and other important metrics.

The data collected can be used to crack down on traffic violations, plan more effective road network infrastructure and help cities allow for better route planning to be used by traffic administrators.

Due to the automatic nature of this technology, installing and using such systems doesn’t significantly strain human or financial resources, freeing up departments to divert their attention to other aspects of their operations.

License plate detection and scanner limitations

As is expected with any technology, there are a few limitations regarding license plate recognition systems. However, these issues can be mitigated through deploying a more comprehensive integrated security system.

Additional equipment

In many cases, a license plate scanner will need to be installed alongside dedicated security cameras, such as compliant ONVIF cameras, and a centralized video recorder, as this will allow the system to send recorded data and receive necessary directives.

While a standalone LPR camera system may seem more bang for your buck, a multi-faceted integrated security system is less likely to produce a license plate reader error, and offers more advanced security features and analytics.

How far can a license plate scanner camera read?

Though the maximum effective range for a license plate recognition system will vary depending on the specific hardware, the best LPR systems can generally discern usable number plate data from 50 to 100 feet.

Video quality will also factor into this equation, with the most popular LPR IP dome and bullet cameras configured to record in 1080p high definition. In addition, there are some newer LPR cameras capable of recording 4K quality footage. However, these devices will require a larger hard drive or dedicated server to store the more extensive data files produced by this video quality.

Final thoughts: is LPR technology right for your business?

License plate recognition systems provide several important benefits to public, private and government agencies. However, these systems are best used with related security technologies and well-planned infrastructure.

ALPR technology has been shown to aid in automating essential services, such as parking management, traffic control and law enforcement. It proves its value in the continued development of more advanced city camera systems, including working in tandem with the growing artificial intelligence and video analytics industries. 

LPR camera technology is a smart investment for many businesses where vehicle traffic can raise security or safety concerns and help streamline security operations and management when implemented using the best practices outlined in this guide.