Measuring License Plate Recognition Accuracy
When evaluating a License Plate Reader system to meet your needs, accuracy typically makes the top of the list of requirements.
This document identifies how to properly measure LPR performance accuracy using simple math and clarifying readable and nonreadable plate criteria.
License Plate Recognition systems (LPR) have been around for over 30 years. One misconception, but often falsely claimed, is the ability of LPR systems to read 100 percent of license plates 100 percent of the time. Although ideal, it is simply not possible. End-users and manufacturers often have different ways of stating their LPR system claims, which ultimately leads to customer dissatisfaction or mistrust about manufacturer’s accuracy claims. The savvy customer looking to buy an LPR system should be aware of the things outlined in this article.
Customers understandably want the highest levels of accuracy for a variety of reasons. Whether it be for throughput or helping officers keep
their eyes on the situation rather than typing plate numbers, accuracy plays a critical role in selecting the proper License PlateReader. Since accuracy is so critical we’re going to take a look at what factors contribute to LPR accuracy and how to avoid many of the mistakes companies make in selecting a solution.
Readable vs. Nonreadable License Plates
At Perceptics, we like to say, “If you can see it, we can read it.” Here are some obvious reasons a plate cannot be read:
• License plate missing from the vehicle
• License plate outside of the camera’s field of view
• Damaged license plates
- Portions of the plate are missing
- Plate is bent or otherwise distorted in a manner that alters the shape of characters that constitute origin or registration number
• Plates mounted in a manner inconsistent with the law of the state entered, i.e., plate is displayed in the rear window of a passenger vehicle
• Obstructed plates
- An object, such as a the ball of a trailer hitch, prevents clear view of origin or the registration number
- Any object that obscures the outline of origin or registration number, such as a license plate frame
Doing the Right Math
When evaluating a License Plate Reader system to meet your needs, accuracy is typically makes the top of the list of requirements. Unfortunately, not every LPR manufacturer measures accuracy the same way, so having a clear definition of accuracy is a good way to hold your LPR provider accountable. Below is a simple equation to help define accuracy.
Simple Accuracy Measurements:
No. of vehicles detected X 100 = % Vehicle Detection Rate
Actual No. of vehicles
No. of plates read X 100 = % Plate Read Accuracy
No. of Readable plates
But what constitutes a “plate read” and what is meant by “readable plates.” This is where your definition and your supplier’s definition may be worlds apart, which can translate into a bad LPR purchase experience.