Tip #1 – Resolution of ANPR camera

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.

OCR providers usually suggest a minimum of 3 pixels per 1 cm to achieve high license plate recognition rates (above 95%). Considering an European standard car or truck license plate with a character stroke of 1cm, to fully cover 4 meters of the road (3,5 meters lane width plus 0,5m overlapping) it would be required an image sensor with minimum resolution of 400cm x 3pixel/cm = 1.200 pixel.

On the opposite, if an ANPR camera with lower resolution is used (for instance 752x480), the following takes place:

Poor Recognition Rate: To cover 4 meters of the road, the resolution of the license plate image will be: 752pixel / 400cm = 1,88 pixel/cm. With this resolution it will not be possible to reach a decent license plate recognition rate (OCR software providers suggest to use a minimum resolution of 2,7 pix/cm to achieve 95% recognition rates).

ANPR on 2 meters: To achieve a 2,7 pix/cm resolution with an image sensor of 752x480 pixels, the lane width that is covered is: 752pix / 2,7pix/cm = 250 cm. Therefore, only the central part of the lane will be covered with the target resolution of 2,7 pix/cm. Vehicles travelling on the sides will be missed.

Sensitivity in Non-ideal operating conditions: License plate detection and recognition can underperform when a 752x480 image sensor is used. Furthermore, ANPR in non ideal operation conditions can suddenly decline. Non ideal operation conditions include dust in the lens, challenging meteorological conditions such as rain, snow, fog, etc. These circumstances can be overcome if a better resolution (megapixel) is used, because there is more information, more pixels per centimetre.

Imagsa’s Atalaya ANPR cameras use sensors of 2048x1024 pixels resolution. Hence, one Atalaya camera fully covers 7,5 meters of road width attaining the target resolution of 2,7 pix/cm. With one Atalaya ANPR camera, one installation and one investment is possible to simultaneously detect multiple vehicles on 2 Lanes of traffic.