How to choose and install license plate recognition (LPR) system? The overall performance is determined by many factors such as mounting angle, distance and shutter speed setting, which are critical for deploying an LPR camera. In the past years, license plate recognition application has becoming popular in video surveillance field such as traffic monitoring, packing lot access, law enforcement, automatic toll payment, etc.
To get a high confidence level of plate recognition, a lot of factors such as camera selection, suitable lens selection, artificial lighting and installation position shall be studied before placing a purchasing security cameras. Otherwise, a good result is unexpected. In this article, we will focus on the rules how to determine the installation position of camera and how to select suitable illuminator to obtain a clear image for license plate recognition, with SCR515PRO as an example. Also some key parameters setting of cameras for getting a clear image for LPR are recommended in the final section.
Taking SCR515PRO as an example, the camera’s built-in 7.5~50mm lens can capture a width of field about 13~16 ft.
To ensure an optimal license plate image, please adjust the lens under the conditions we suggest. (NOTE: The license plate image shall be 18% width of the screen for good license plate recognition.)
The recommended capture distance is 49-100 ft. (15-30m) for a reflective plate and 33-82 ft. (10-25m) for a non-reflective plate.
1. The camera height should be higher than 11.5 ft. (3.5 m)
2. The width of the license plate should occupy approximately 17~20% of the width of the screen.
3. Vertical angle should be NO less than 40 degrees
If the built-in IR light is not enough to get clear image on field installation, an additional illuminator is required. Here we point out two key items that shall be considered when selecting IR illuminator.
Assume the target distance = 6-8m and width of target = 1.72m; we can use the triangle formula below to calculate the required beam angle to cover the whole width.
Beam angle required= that means you have to select a illuminator with beam angle > 12∘.
In general, the projecting distance of an illuminator is specified at the condition of image captured under 1/50”(or 1/60”) shutter speed. However, the shutter speed usually is set to 1/1000” for license plate capturing, so we have to calculate the distance with this factor taken into consideration.
Good image quality is the key to succeed in LPR application, in which good contrast, no overexposure and no blur on image are three major factors that shall be strongly emphasized and followed.
(1). Choose “CATCH ALL” to enter the settings
(2). Refer to the table below to set best parameters, especially for LUMII and SHUTTER.
|SHUTTER||DAY SHUTTER 1/500 |
In camera web Setup page, Image > Exposure > Exposure Mode, select Manual Exposure.
Refer to the table below for recommended setting.
|Day Exposure Time||1/250|
|Night Exposure Time||1/250|
|Manual Lumii||0-5 (low is better to avoid noise)|
In camera web Setup page, select ANPR/LPR. Refer to the table below for recommended setting.
|Exposure Mode||Auto IRIS Mode|
|Manual Exposure||Time 1/250s, and higher is required for higher vehicle speed|
|Manual Gain||0-4 (low is better to avoid noise) |
Don’t choose Max Gain
|ICR Control||External mode (with default Alarm port=1, |
Active=LOW), implying the day/night is controlled by an external illuminator
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