This guide focuses on specific details that need to be considered when capturing license plates. The focus is on the placement of the optical sensor, the camera settings themselves and the configuration of the event triggers.
Please note at this point that the camera settings must be adjusted according to the installation location, as lighting conditions may vary.
Perfect sensor placement is critical in order to get a clear image and readable number plates. While some parameters such as distance from the sensor to the number plate can be fine-tuned by zooming after installation, mounting height and angle between the sensor and travel direction of vehicles can only be adjusted by physical and cost-intensive re-arrangement. The sensor position has to be chosen in a way that passing vehicles are fully visible and can be captured throughout several frames of the video stream while making sure the number plates are large enough for the ANPR system to identify every single character.
We recommend mounting heights between 3 and 8 meters, therefore the suitable minimum capture distance ranges from 5 to 20 meters. Besides the vertical angle constraint, number plates should be visible with at least 250 pixels-per-meter (PPM), this constraint determines the minimum focal length (zoom) the sensor has to be set to.
| Mounting height [m] | Minimum capture distance [m] | Maximum capture distance [m] | Range of focal length [mm] |
|---|---|---|---|
Why between 3 and 8 meters of mounting height?
The lower bound of 3 meters is determined by rather practical reasons and not technical limitations. Sensors mounted lower than 3 meters are often prone to vandalism. Also, headlights from passing vehicles can lead to reflections on the sensor. The upper bound of 8 meters is determined by the resulting minimum capture distance of at least 14 meters for the needed camera resolution of 1920x1080p. License plates need to be visible with 250 pixel-per-meter (PPM).
As the compute module and sensor are mainly mounted on existing infrastructure such as traffic light poles, there are two general options to mount the sensors: side mounting or overhead mounting.
Overhead mounting
When positioning the sensorabove the vehicles, two lanes can be covered with one sensor.
Consider mounting height (1) and capture distance (2) which determine the vertical angle (3) between the sensor and the travel direction of the vehicle. The distance between the center of the lane (4) and the senosr determines the horizontal angle (5) between the sensor and the travel direction of the vehicle.
Side mounting
When mounting the sensor to the side of the road, two lanes can be covered, assuming the horizontal angle between the sensor and the travel direction of the vehicles is not exceeding 20°.
Position the sensor as close as possible to the side of the road to avoid a horizontal angle larger than 20°. Larger angles can lead to lower accuracy because parts of the number plate can become unreadable. While traveling directions (1) and (2) are the same for both vehicles, horizontal angle (3) is much larger than (4).
| Set up parameters | Recommended |
|---|---|
Pixels Per Meter is a measurement used to define the amount of potential image detail that a sensor offers at a given distance.
250 PPM (vehicle)
Using the camera parameters defined below ensures achieving the minimum required PPM value)
Tip: Use the Axis lens calculator or generic lens calculator.
Sensor mounting - distance to object center
5-20 meters Please consider that the zoom needs to be adjusted according to the capture distance.
Sensor mounting height
3-8 meters
Sensor mounting - vertical angle to the object
<40°
Note: setting the correct distance to vehicle and sensor mounting height should result in the correct vertical angle to the vehicle.
0° - 20°
3
5
20
4-12
4
7
19
5.4-12
5
9
18
6.6-12
6
10
18
10-12
7
12
18
11-12
8
14
17
12
Sensor mounting - horizontal angle to the object
