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.
In order to achieve best license plate recognition rate, it is required a high-resolution (Tip #1, Tip #2), contrasted and optimally illuminated image of the license plate.
Some variables affecting to the illumination of the license plate are:
- Global Variables: overall scene illumination, meteorological conditions, day-night time, sunrise, etc.
- Local Variables: reflective and non-reflective plates, physical conditions of plate (dirty, broken, ...), local shadows on the plate, etc.
These illumination challenges in an outdoor application affect the recognition performance of the ANPR. These challenges cannot be solved with standard cameras, with no smart and dynamic control of the exposure time of every caption. This is due to the physical fact that silicon imagers have limitations on their dynamic range.
Thus, it is very important that the camera adapts the exposure time of the image sensor to these variable illumination conditions.
The Multi-Exposure technique is an approach to overcome these global and local illumination variations affecting the license plate region. It consists on capturing the image of the license plate several times with different exposure times. Therefore, it is possible to get an image of the plate with a good illumination. Then the OCR engine will correctly recognize the number plate.
Imagsa’s ANPR cameras implement an innovative auto-adaptive multi exposure technique at high-speed (250 FPS), achieving a good illumination of the plate image regardless of local and global conditions. Even non-reflective license plates can be detected with this technique. The clue is having a high-speed sensor and a smart control of it.
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