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ANPR works properly if the following camera installation requirements are met (for a list of supported countries see License plate recognition (VT) features and specifications):

• the camera should be installed in a horizontal position;

• in order to recognize state license plates on high-speed roads, the camera should have the Global Shutter mode;
• character height is at least 15px, stroke width is at least 2px;
• minimum allowable contrast for uniformly contaminated license plates should be at least 10% (contrasting visibility of characters relative to the background is 25 on a 256-point scale);
• maximum allowable non-uniform contamination is no more than 12% (the ratio of the contaminated area to the total area of the license plate);
• the geometric proportions of the image shall not deviate by more than 10% from the real geometric proportions of the license plate.

For stable ANPR operation, make sure that the image of the license plate is not:

• unequally lit;
  
• overexposed;
  
• blurred;
  
• distorted;
  
• interlaced;
  
• dirty.
  

Some example number plate images that should be recognized fully and properly: