The steps outlined in this document provide the principles to optimise any License Plate Recognition (LPR) installation.
2. Camera Selection
Dual Sensor camera are ideal for LPR. One sensor will have a narrower field of view, designed to capture the license plate. The other sensor will have a wider field of view, in order to provide an overview of the area.
A single sensor camera will be able to capture the license plate. However you will not have the overview of the areas to put the captured license plate into context.
3. Lens Selection
The following factors need to be considered for the correct lens combination:
Lens assigned to capture the License Plate
For the lens allocated to capture the image of the license plate, you will need to determine the exact distance between the camera and where the license plate is to be captured, along with the width of the road or entrance way.
Input the values into a Optical Planning Tool to determine the correct lens to capture the license plate.
The lens with the narrowest field of view that covers the area is desirable. A lens with a high focal length provides excellent close ups. This allows a lower resolution to be used and as a result, the images with be captured with a higher frame rate.
Lens assigned to provide an Overview
Once again you will need to use a Optical Planning Tool to determine which lens to select.
The “Overview” lens will be of a wider field of view. As a result it will have a shorter focal length.
Note: It may be necessary to adjust the focus of the lens. The focal point of MOBOTIX wide angle lens is 6m / 20ft to infinity. However the telephoto lenses have a focal point of 40m / 13ft.
The LPF (Low-Pass Filter) sensor is designed to cut down the glare from vechile’s headlights. The LPF sensor prevents all the headlights visible light and only lets the Infra Red (IR) through. This results in a high contrast between the license plate and the remainder of the image.
The LPF Sensor should be deployed on the side designated to capture the license plate (i.e. the one will the longer focal length)
4. Selecting and Siting IR Lighting
In order to provide a workable LPR solution good, consistent light is paramount. Without enough light, the license plate does not reflect enough light back. In which case the is not enough detail to make the license plate legible.
An IR Illuminator with a wavelength of 850 nm, should be deployed in an LPR setup. 850nm is the point of maximum sensitivity for MOBOTIX LPF sensors.
It is essential that the IR illuminator generates enough power to illuminate the license plate. The ideal solution is to ensure that the IR Illuninator can light up three times the distance required. This will mean that for a distance of 22m / 72 ft, a IR illuminator that can light up 60 m / 196ft is required.
The angle of the IR Beam should match the angle of the viewing lens. This prevents the following issues from occurring.
Light Dispersion, which occurs when the IR beam is to wide
Dark Spots / Over Exposed Spots, which occurs when the IR beam is too narrow.
License plates are constructed from highly reflective, that reflects light light back towards to direction of the light source being generated. As a result, the IR Illuminator need to be sited as close to and as perpendicular to the target area as possible. More oblique and further away the IR Illuminator is sited, less effective it is.
With the rear license plate, “White Out” is no a concern, as the light level is a lot lower then that of the headlights. However it does have its own inherent problems.
Maybe obscured by trailers, caravans, bike racks, etc.
Rear license plate sometime are in hard to read areas.
Mobotix now offers it’s own IR Lighting solution: MxIRLight
Weatherproof, high-performance infrared illuminator
Wavelength 860 nm
Supplied with power via network cable in conformity with the PoE+ standard (max. 19 W)
For MOBOTIX cameras with night sensor, e.g., M16
Six different beam angles ranging from 15° to 120°
Maximum range of 20 to 160 meters
Suitable for B036 to B237 lenses
5. Polarization Filters
A polarizing filter is often placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. In regards to LPR applications it allows the camera to be mounted behind glass or to be able to view the driver of the vehicle through the windscreen.
Polarization Filter - How to
6. Siting the Camera
Installation for an On-Coming Vehicle
The camera image should fill the complete width of the vehicle. A minimum of 20 pixels high need to be capture for an accurate reading.This means the minimum percentage the license plate needs to represent within the total field of view is:
640 x 480 (4%)
800 x 600 (3%)
1024 x 768 (2.5%)
1280 x 960 (2%)
The distance between the vehicle and the camera should between 5m-22m (10ft-72ft).
The height should be between 3m-6m (10ft-19ft)
In order to calculate the distance when mounting a camera, Pythagorean theory will need to be employed
a= Vertical Distance from vehicle to camera
b= Height the camera is mounted above the ground
c= True Distance from camera to vehicle
The camera’s vertical angle should not be greater then 30°.
Installation for an Offset Camera
The camera should be mounted no more then a 15° horizontal angle.
Sun Light, Seasons and Weather
LPR needs to function all the time, regardless of time of day, year or weather. To this end avoid East / West installations. Also the angle of the sun over the various seasons needs to be considered. The camera will also need to be configured to cope with all possible weather (such as rain, clouds, fog, etc.)
Signs and Other Lettering
It is possible the lettering on signs or any other object could provide a false reading. To prvent this from happening you will need to obscure these areas. This is done in the camera’s web browser interface under Setup menu > General Image Settings.
Distance and Resolution
Recording in lower resolution, such as VGA allows for a higher frame rate. This allows for a better channce to caputre a clear image of the licence plate.
Recommended maximum distacnes for VGA are:
MOBOTIX L43 Lens - 3m
MOBOTIX L65 Lens - 7m
MOBOTIX L135 Lens - 15m
With digital zoom, you can add about another 50% to the above distances
7. Configuring the Camera
The steps below are designed to optimize the ability for licence plates to be captured
Setup Menu > Exposure Settings > LPR Sensor
Disable Backlight Correction by setting it to “-10”. This turns of a hardware amplifier, that can causes image noise
Change the Exposure Control to Full Image. This allows the camera to reference the entire picture to avoid changes of exposure times while the car is moving
Deactivate Minimum Average Brightness. Turning this feature to “Off” will reduce the image noise
Disable “Night Improvement”. Turn off this software amplifier to reduce the image noise
In regards to the LPR Sensor, the Exposure time will be goverend by the amount of light illuminating the licience plate. The Max Exposure Time should no more than 1/250 sec to avoid blurrred images.
With the Overview Sensor, a Max Exposure Time of 1/30 secs or less is ideal
Setup Menu > General Image Settings
Adjust Noise Suppression in poorly lit environments. While Sharpness can be manipulated to improve character recognition on the licence plate.
8. Optimizing Frame Rate
Higher the frame rate of the camera, allows you to capture faster moving vechiles. Adjusting the frame rate should only be done after the camera is sited correctly and the camera is configured to capture good quality images of the target licence plates.
Setup Menu > General Image Settings
Lower the Resolution
Unless mission critical, display only one sensor at a time
Set the Image Quality to “Fast” or “Normal”
Setup Menu > Recording
- Turn off Full Image Recording