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;

• the camera has the correct focal length settings. The table below provides an example of the correlation between viewing angle and sensor size, as well as lens focal length. A standard video surveillance calculator was used for calculations:

  Object distance, m	Object width, m	Focal length, mm	Viewing angle, degrees	Sensor size, inches
  3	4	4	85	1/3
  3	4	5	65	1/2
  3	3	5	65	1/3
  3	3	6	55	1/2
  7	4	8	40	1/3
  7	4	10	35	1/2
  7	3	10	35	1/3
  7	3	12	25	1/2
  11	4	13	28	1/3
  11	4	18	22	1/2
  11	3	18	22	1/2
  11	3	23	16	1/2
  15	4	18	22	1/3
  15	4	23	16	1/2
  15	3	23	16	1/3
  15	3	30	12	1/2

  
  

• the camera has a 1.0, 1.2, 1.3, 1.4, or 1.8 aperture lens;

• in order to provide 24-hour recognition in an area with changing lighting conditions, the camera should have infrared illumination (built-in or stand-alone IR projector);

• the camera has the correct exposure settings which depend on the level of the camera's inclination to the surface of the license plate, as well as on the vehicle speed. Approximate shutter speeds are shown in the table below:

  Exposure, s	Maximum vehicle speed in the surveillance area, km/h
  1/200	18
  1/250	22
  1/500	45
  1/750	68
  1/1000	90
  1/1500	136
  1/2000	181

  
  

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: