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Shedding light on illumination for ALPR

Shedding light on illumination for ALPR
One of the key factors to consider in ALPR is illumination. This article takes a closer look.
Automatic license plate recognition (ALPR) is now increasingly used as an effective law enforcement and access control tool. One of the key factors to consider in ALPR is illumination. This article takes a closer look.
ALPR detects and recognizes a vehicle’s unique identifier – its license plate. Today, a cutting-edge ALPR solution can even recognize the vehicle’s color, model and make. This makes ALPR popular in various applications, especially in law enforcement.
“In electronic toll collection, enforcement is based on license plate data. Law enforcement authorities use it to identify suspect or wanted vehicles, including vehicles for which owners do not pay leasing or insurance fees,” said Csaba Nagy-Amigo, Director at Adaptive Recognition. “Efficient speed enforcement is increasingly important as our lives have become faster in every sense of the word, and this includes traffic speed. As a result, approximately 1.35 million people die in road accidents related to speeding every year around the globe. Speed cameras manufactured by Adaptive Recognition have built-in ALPR components to identify offending vehicles and measure their speed. Authorities can quickly identify notorious speeders.”
More and more, ALPR has applications outside law enforcement as well.
“Recent advances in ALPR technology now allow them to be used for applications such as private parking lot billing, access credentials into gated communities or similar restricted locations, traffic analysis for marketing purposes (for example counting the number of unique vehicles that pass a location), and for venues and clubs to be able to quickly recognize incoming VIP guests to greet them by name,” said Eddie Reynolds, CEO, and Brian Karas, Stategic Advisor to iluminar.

Illumination challenges and solutions

To get maximal results, setting up an ALPR system properly becomes key. One major challenge in any ALPR deployment is reading plates correctly in challenging lighting environments.
“Vehicles present unique challenges for many surveillance applications, as they often have headlights or taillights which can cause exposure challenges within the scene. This is a particular problem for areas that employ front license plates, in which more vehicles use daytime running lights (DRL), which can cause cameras to under expose part of the image. Capturing plates at night with high accuracy has always been a challenge for the same reason,” Reynolds and Karas said.
Illumination, then, becomes critical in ALPR. Below we address some of the questions often asked of ALPR illumination.

Onboard or separate?

Should the user use the illuminator onboard the camera, or use a separate illuminator, for ALPR? This has to do with the user’s own use case and what objectives they want to achieve.
“The performance of onboard illumination in modern cameras ranges from very limited to moderately capable, so our guidance has always been to consider all factors in the project. As both vehicle-capture speeds and reliability needs increase, so also will the odds of a requirement for an external or supplemental high power illumination source,” Reynolds and Karas said.
“If the user only wants to read license plates, then the illuminator built-in into Vidar or Einar (cameras) should suffice. For other use cases, such as determining vehicle occupancy, external illumination would be required – this is the case with our PAX passenger counting camera, which takes images of vehicle occupants and counts them with the help of infrared wavelength light,” Nagy-Amigo said.

White light vs. infrared

There are two primary illumination methods for ALPR – white light and infrared – each with pros and cons.
“White light is not monochromatic, so it is perfectly suitable for identifying vehicle and license plate color at nighttime. But it may disturb the driver and therefore represents a road safety risk,” said Nagy-Amigo. “Infrared light does not disturb the human eye; it is barely or not visible, so does not represent a risk factor. It is monochromatic, therefore no use for identifying colors.”
Again, deciding which one to use depends on the use scenario. “Infrared (IR) illumination is by far the most recommended approach. Because the IR spectrum is generally invisible to humans, we can recommend high powered illuminators that will provide highly detailed images, without worrying about temporarily blinding drivers or affecting their vision,” Reynolds and Karas said. “White light illumination, at proper levels, is most common when ALPR systems are used to analyze vehicle color data in addition to plate numbers.”
Local regulations also play a role here. “In many countries, it is forbidden to use visible light to illuminate the vehicle and the plate so as not to disturb drivers. In some countries, on the other hand, white (visible) light is necessary to illuminate plates/vehicles. This is the case with countries where the character or background colors used in the plate have special significance,” Nagy-Amigo said. “In most countries, however, we need to use invisible light, in our case, infrared. This represents challenges as it is monochromatic, which makes it difficult to determine vehicle color in the case of images taken at nighttime.”

Consultation is key

That’s why having consultation with experts is key. “In every case, we ask clients about where the cameras are going to be installed. We use this information to decide on the optimum wavelength. There are of course rules of thumb we can apply, but it is always best to discuss the particularities of projects to determine the best possible configuration for illumination,” Nagy-Amigo said.
“It is recommended that system users consult a qualified system integrator to ensure they are being presented with the best and most applicable options. In any case, we would recommend a temporary real-world test of the proposed equipment at the user’s location, or in a similar location, to let them best assess what the performance of the system would be like for their specific scenario,” Reynolds and Karas said.

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