Why does ALPR resolution matter?

When comparing one license plate recognition (LPR) solution to the next, it is easy to get overwhelmed with the specs manufacturers throw at you: IP67, plates per minute, differential speed, MIL-STD, power consumption. It is easy to simply chalk these all up to specs, and focus on the wrong ones. While most specs do carry some level of importance, some are more important than others.

Let's take a moment to focus on resolution, more specifically the resolution of the LPR camera. It amazes me how "hidden" this spec is when it should be at the forefront. It is a fundamental and key indicator of quality. When buying consumer goods, we are quick to compare megapixels for still cameras and definition standards (NTSC, 720p, 1080i...) for TVs; these features are all about resolution!

Here's an analogy to put things into perspective. Assume a basketball game is being watched on two TVs side by side. You're watching it on your new 50" HDTV at 1080i (1080i video frame consists of 2 interlaced frames that add up to a total of 1080 horizontal lines). Next to you, your Dad has his 50" classic rear projection NTSC TV from 1990 (NTSC is what we had prior to HDTV, which also had 2 interlaced frames that add up to a total of 486 horizontal lines). So during any given point in the game, a player races down the court, you can clearly make out the name on his jersey on HDTV from the overview shot. At the same time your Dad's TV shows the same player, same play, same overhead shot, but you can't quite make out the name on the jersey, what's the difference?

The reason you can read one jersey image and not the other is because you simply have better definition or resolution on one versus the other. Seeing is believing, the two images below were taken with the same camera, same time of day, same condition, different resolutions. Can you read the extension on the phone, the numbers on each button, the model of the headset, or the brand of coffee? Anyone of these details may very well have been the license plate number of a wanted felon.

License plates are much more prone to visual challenges than a telephone or coffee cup found on a desk. They are exposed to dirt accumulation, snow and slush, dents and dings, obscured by trailer hitches, and are often seen at skewed angles. All these factors can hinder the reading of a plate, but better resolution and definition helps to decipher and read each of those plates. Another advantage is the ability to read plates further away; be it in the third lane of adjacent traffic, or across the median. Plates further away appear smaller, and hence the resolution comes into play again, just like the extension, the small plate in the distance can either be read or appear as just another blurr in the image.

All LPR solutions start with an image, this is the first step in a complex process. If you do not feed the LPR software engine good quality images, you can expect poor results. So next time you gloss over the specs, pay attention to the resolution of the IR or LPR camera, if it is not on the spec sheet you should be asking questions.