Tip #4 – Top ANPR performance I: Detection

The Guide “10 Tips For Best Free Flow ANPR Deployment” aims to provide a useful list of characteristics and features of ANPR cameras to be used in free-flow, auto-triggered and high-speed traffic control applications.

Definition: Vehicle Detection Rate

Overall performance of an ANPR camera is computed by multiplying the Detection Rate by the Recognition Rate.

Detection rate is measured by the number of vehicles (plates) detected by the ANPR camera over the Total number of vehicles going through the camera’s field of view.

Detection Rates range is 50% to 99%, depending on the selected ANPR solution.

Motion vs Plate Detection

Standard ANPR cameras with video triggering detect a vehicle by means of motion detection techniques. Motion detection consists on analysing the differences between video frames. When a difference occurs, this technique considers that there is a vehicle and it captures an image to read the license plate.

This technique has so main drawbacks:

• Illumination: Performance is sensible to illumination variations, and it has false detections caused by shadows of a vehicle travelling in the adjacent lane, or even by shadows of trees next to a road. Shadows create false detections and undermine the performance of the ANPR solution.

• Miss Detection: In addition to false detections, some ANPR cameras are affected by miss detections. This is a more critical issue, as it means that not all vehicles will be detected. Miss detections can occur by a variety of factors. But mainly because the motion detection algorithm, running in the ANPR camera, does not perceive a sufficient change on opacity in a sequence of images. Sometimes this affects the overall performance of the ANPR solution, to an extent of just detecting 50% to 80% of vehicles.

Atalaya ANPR cameras implement an innovative license plate detection technique (instead of motion detection).

Basically, it consists on continuously capturing and analysing images to detect a license plate pattern. When there is a license plate pattern it means that a vehicle is in the field.

This approach avoids the false detections that are common in an ANPR camera based on motion detection. And it achieves vehicle detection rates over 99%. Meaning that nearly every vehicle (full traffic) is detected by the Atalaya ANPR camera.