How LPR works – The best tutorial for License Plate Recognition

Before decide for the best vehicle license plate recognition system (LPR) it's important to learn what factors you must take in consideration.

Be aware that LPR depends on good Optical Character Recognition (OCR) systems, so choosing good OCR services to work with is essential. 

There is no straightforward answer on which is the best camera or system for vehicle license plates recognition, it depends on your project's budget and what you really need and expect from your system.

LPR software ISS

See in this picture the example of SECUROS software used for license plate recognition. To learn more, visit the official ISS website at:

What are the options when choosing a vehicle license plate recognition system?

There are two ways to do this job of capturing and recognizing vehicle license plates, and you must decide which one is most appropriate for your case based on your budget and type of implementation.

It is possible to purchase all items separately and work on  a solution integration by installing the camera, software and accessories or buy a LPR camera that already brings everything integrated into a package.

  • License Plate Recognition integration
  • Usually with lower cost but harder to install and configure
  • Embedded License Plate Recognition System
  • Usually with higher cost, but easier to install and configure

In this article, I will talk about the first system. It is possible in this case to merge different technologies, IP cameras brands and LPR software to better meet the project's specification.

License Plate Recognition integration

If you choose this option to have your license plate recognition working correctly you need to understand what features a camera must have to allow the correct settings according to the project.

Manual integration requires cameras, protection boxes, switches, cables, installation accessories, as well as servers, software and specialized workers.

LPR System

It's necessary to install all the devices and adjust the IP camera and LPR software parameters. It's a really time consuming task

The diagram below shows an example of a traditional license plate recognition (LPR) system with its main components.

LPR System

In this model, the IP cameras must capture the image with enough resolution and quality so that the LPR server can use the optical character recognition (OCR) and store the information in a database. A monitoring station can display the images and the license plate recognition results.

IP camera for LPR

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IP camera for LPR

IP cameras are the best options for use in a system of license plate recognition system due to the features available on those devices, such as high resolution images, network protocols, etc. To learn more about IP cameras read the article: What are the IP camera advantages

Another great advantage in the use of IP cameras is the possibility of using embedded intelligence, there are some LPR software developers that have the option for two steps processing by using the IP camera to process part of the job and let the second part to be processed by the LPR server.

To learn more read the article: Video Analytics for CCTV cameras.

Switches for CCTV and LPR projects

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switch for CCTV

Switches are used to connect the IP cameras and other devices on the network. There are different types of switches and some models can be used for CCTV projects. Read more in the article: What is the best switch for CCTV.

Another important factor to consider in the switches for your project is Power over Ethernet (PoE). To learn more read : PoE Switch for IP cameras.

What are the characteristics of an IP camera for a license plate recognition system?

To obtain an image with enough quality for the LPR software do a good job, the camera must have a minimum image resolution.

Even analog cameras with traditional resolutions can be used for LPR, however it is recommended to use IP cameras with high resolutions.

Image resolution for license plate recognition

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CCTV resolution

A 2MP (Mega Pixel) also known as FUll HD or as 1080p resolution IP camera is enough for most vehicle license plate recognition projects, 4K IP cameras can also be used, but it's not required on most cases.

Varifocal lens for license plate recognition

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LPR lens adjustment

To correctly adjust the aperture, zoom and angle of the image, it is necessary to use varifocal lenses with infrared correction and focal length ranging from 5 to 50mm.

==> Watch the video below to learn more about lenses for CCTV

Shutter speed for LPR projects

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Shutter speed configuration for LPR

With the shutter speed control it is possible to "freeze" images that are in motion, this is very important, especially if the vehicle moves quickly in the are where the LPR system will be installed

==> Watch the video below with an example on how to adjust the shutter speed on the camera and fix the issue with blurry images.

Backlight compensation (HLC, BLC and WDR)

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Compensacão de luz para captura e reconhecimento de placas de veículos

Missing or excessive light can have a negative impact in the LPR process, Cameras with features such as HLC, BLC or WDR allow to improve the image quality.

Depending on the scenario it's possible to use different types of features according to the incidence of light, shadow, excessive light or reflections.

If you want to learn more about light compensation, watch this video that explains about BLC, WDR and Sens-UP. Keep in mind though that the Sens-UP is not very useful in low-light and motion scenarios, which may be the case with vehicles on a dark road. In scenarios like these it is important to use additional lighting, whether normal or infrared.

What brand or camera model should I buy for my license plate recognition system?

It is important to know that there is no specific camera for vehicle license plate recognition that is best for all situations, it will all depend on what is required for your specific project, you can choose different types of cameras and lenses that will work just fine as long as you understand the concepts.

Software for distance and angle calculation

You can use the IP Video System Design Tool  to calculate the distances and angles for your LPR project. 

With this software you can work with information such as distances from the camera to the vehicle, lens focal length, camera resolution, and more.

LPR distance
LPR software calculation

What should I consider during the installation of a LPR system?

Several factors will influence how well your system captures and recognizes the license plates, you must be aware of each one of them and make the necessary adjustments to optimize the system, here are some important details from in the ISS software manual .

Ideal visualization angle

ISS angle

For plate recognition software to work at the best possible rate, the vehicle plate image must be at a more perpendicular angle to the ground as shown in this photo.

Maximum visualization angle

ISS Angle

The license plate recognition software can accept a variation of angle as shown in the photo, keep in mind however that this variation may be different according to the model of the plate for each country. Always try to use the smallest angle variation.

Camera visualization angle

When installing the camera in an upright position as shown in the photo hold the maximum angle at 40 degrees so that the plate recognition software can operate efficiently, smaller angle is even better.

ISS LPR angle

When installing the camera in a side position as shown in the picture hold the maximum angle at 30º. Smaller angle is even better.

ISS lateral angle

Summary and conclusion on the installation of vehicle license plate recognition system

In summary to have a LPR working properly, follow the recommendations:

  • Choose the technology appropriate for your case (analog or IP);
  • IP cameras are most commonly used for plate recognition;
  • The camera resolution will depend on the project specification;
  • For most projects 2MP resolution is enough;
  • The cameras must have shutter control features;
  • Light compensation feature helps to have a better image;
  • It is necessary to choose the appropriate lens for the project;
  • It's very important to use the correct camera angles.
  • It is possible to integrate cameras and software from different manufacturers;
  • There are different manufacturers and LPR software.

Want to learn more ? 

If you want to become a professional CCTV installer or designer, take a look at the material available in the blog. Just click the links below:

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In another article I will talk about professional cameras with embedded LPR technology such as Hikvision, Vivotek and Genetec cameras. This type of camera facilitates the installation because it is already prepared with the technologies that were discussed here in this article

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