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How to assess ALPR image quality – and why it matters

Processing speed and the quality and functionality of infrared and color cameras can make the difference between a suspect caught and one who slips by


Sponsored by Jenoptik

By Laura Neitzel, Police1 BrandFocus Staff

Occasionally, my work and my home life intersect. On a recent Sunday afternoon, I was on a routine weekly trip to Costco when I drove up on a police incident. At least eight police cars, lights on, blocked a major intersection where a blue sedan sat idle. According to the police department’s Twitter feed, what I came across on that beautiful, sunny day was a homicide. A named suspect in a white Ford Fusion with factory trim is being sought.

ALPR image quality can make a difference between an accurate vehicle match or a crime that goes unsolved.
ALPR image quality can make a difference between an accurate vehicle match or a crime that goes unsolved. (Adobe)

As someone who has written or edited stories on the value of automatic license plate readers, I immediately wondered if there was an ALPR at the intersection and whether it could have captured the murder and helped identify the suspect. While as a private citizen I can’t speak to the investigation, I can speak to the desire to get a murder suspect off the streets and behind bars as soon as possible.

I’m sure the police department has the same goals, and I hope that they have an ALPR at that intersection – and a high-quality one, at that.

Variations in image quality

Several features that determine image quality and license plate readability can make the difference between an ALPR that can capture a clear image of a fleeing murder suspect’s vehicle and automatically send an alert when it’s spotted – and one that misses the mark. Here are some of the key differences:

1.  Processing speed

Much like my 2017 MacBook Pro, an ALPR with insufficient processing speed will be challenged to keep up with multiple applications running at the same time.

It’s annoying when my Mac can’t stream Netflix without stuttering, but no one dies. The lack of processing speed, however, could be a huge detriment in law enforcement when lives are at stake.

Ideally, an ALPR should capture readable images of 100% of vehicles passing the camera so the suspect doesn’t slip past. However, many lower-cost ALPRs simply lack the processing speed to capture a clear image quickly – if they can catch it at all.

“The problem with a slower-speed processor is that by the time the camera detects a vehicle and determines that it needs to take a picture, the vehicle may have traveled out of range,” said Finbarr O’Carroll, Jenoptik’s president of the Smart Mobility Division in the Americas. “That’s why you need a processor in the camera that is fast and capable.”

Jenoptik’s VECTOR LPR camera is purpose-built for reading license plates and can capture images of up to three lanes of vehicles traveling at up to 180 miles per hour – virtually ensuring that a suspect can’t outrun the camera.

Processing speed solves for one part of the challenge – capturing the image before the vehicle goes out of frame. Another challenge is to capture a clear image of the license plate itself, regardless of time of day or lighting conditions.

2.  The infrared camera

There’s a reason most newspapers and magazines are printed in black text on a white background – the high contrast makes the text easier to read. That’s the purpose of infrared (IR) cameras as well: to register the contrast between the license plate background and the alphanumeric characters.

“When you bounce the IR light off where the vehicle’s going through, it’s going to reflect the white of the plate and give you a nice contrast between the characters on the license plate and the white background,” said O’Carroll. “To do that, you use a black and white camera with IR light and bounce that against a reflective plate.”

ALPR cameras capture a high-contrast image using infrared light, generally a zoomed-in image of the rear license plate. The downside is that by being so zoomed into the license plate, you miss context; the vehicle could be traveling on any road in any part of the country.

For more clues, you need both the license plate clarity of an IR camera as well as the contextual clues that a broader image range and a color image can provide.

3.  The color camera

“The contextual image, captured with a color camera, gives you a whole picture of the whole vehicle on the highway,” said O’Carroll. “During the day, advanced ALPR systems operate with a camera that provides a color image that helps identify vehicle make, model and color, along with scene context.”

However, with lower-cost camera systems, the infrared camera image is not supported during the day and the color image is not supported at night. You have either a better look at the license plate – without scene context – or the scene context without the boost in license plate clarity provided by the infrared camera.

During the day, the readability of the license plate on the color image can be further degraded by weather and lighting conditions.

The best of both worlds

Many lower-cost camera systems automatically switch from color images during the day to infrared images at night. However, Jenoptik’s VECTOR camera system is optimized to work both day and night, including on unlit roads. The color camera can automatically switch to night mode, optimizing capture for low-light conditions and the monochrome camera can provide black and white images, even for fast-moving vehicles.

Having the color camera and the IR camera working both day and night ensures consistent image quality of both the ALPR and scene context, regardless of weather or lighting conditions.

In fact, to test the performance of the VECTOR compared to two lower-cost alternatives from other manufacturers, Jenoptik conducted traffic image capture tests over a 7-day, 24-hour-per-day period. Results confirmed that the VECTOR captured 14,644 images compared to 1,897 and 1,055 for the other camera systems. The vehicle capture rate (accuracy of the match) showed capture rates as low as 13% in the lower-cost ALPR cameras, where VECTOR achieved a vehicle capture rate in excess of 99%.

By making the best possible images available to the police, Jenoptik significantly increases the probability of a positive match. In my book, making a positive match means getting a suspect off the street before another life is lost.

Visit Jenoptik for more information.

Read next: How to take the guesswork out of license plate identification

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