Video stream and scene requirements for the License Plate Recognition VT and its sub-tools

• Camera must be permanently installed

• In order to recognize state license plates on highways, camera must have the Global Shutter mode

• Frame size is at least 704*288 px

• Character height for IP cameras is at least 20-30 px, for analog cameras—at least 14-20 px, the stroke width is at least 2 px. If it is necessary to recognize two-line license plates, then these requirements are relevant for each line

• Focal length of the camera must be set correctly. The table below provides an example of the correlation between the viewing angle, sensor size, and lens focal length. For calculations, a standard video surveillance calculator was used:

  Expand the table with focal length settings

  
  

  Distance to the object, 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

• Сamera must have a lens with an aperture of f/1.0, f/1.2, f/1.3, f/1.4, or f/1.8

• Color space model: Y800, RGB32, RGB24, YUY2, or UYUY

• Сamera must have infrared illumination (embedded or stand-alone IR projector) to ensure round-the-clock recognition in an area with changing lighting conditions

• Shutter speed settings of the camera must be correctly set depending on the level of the camera's tilt to the license plate, as well as on the vehicle speed. The table below provides approximate shutter speeds for different vehicle speeds:

  Expand the table with shutter speed settings

  Shutter speed, 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

• Camera must have a photomatrix suitable for license plate recognition. When selecting a camera, we recommend you to rely on a comparison of the photomatrix characteristics obtained as a result of measurements in accordance with the EMVA 1288 standard. This standard was developed by the EMVA (European Machine Vision Association) and it is offered to video camera manufacturers as a way to unify product evaluation methods. Requirements for the photomatrix for the License Plate Recognition VT are described in the table below:

Recommendations for highways

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  Cameras must be mounted on L-shaped posts at the edge of the traffic lane or on arches above the middle of the lane
• Recommended installation height is 4-6 meters, maximum height is 20 meters
• Analog cameras have a relatively low resolution, so their surveillance area must not cover more than one traffic lane. IP cameras have a much higher resolution: their surveillance area can cover up to four traffic lanes
• The higher the camera is mounted, the higher the probability of capturing (and therefore detecting and recognizing) the license plates of vehicles moving at a short distance from each other (for example, during traffic jams). However, it is important to keep in mind that increasing the installation height leads to distortion of the license plate image, while the character size is almost at the minimum allowable level. In such cases, by reducing the vertical angle and changing the focal length (zoom), you can move the surveillance area to a greater distance so that the license plates can be recognized at the required installation height

• Maximum vehicle speed for correct license plate recognition is limited to 180 km/h

• Minimum allowable contrast in case of even staining of the license plate must be at least 20% (contrast visibility of characters relative to the background is 25 units with a 256-point scale)

• Maximum allowable uneven staining is not more than 12% (the ratio of the stained area to the total area of the license plate)

• There are no high-contrast objects in the frame (for example, billboards, trees, wire fences)—they cause an increase in the number of false positives during recognition

• Geometric proportions of the image must not differ by more than 10% from the geometric proportions of the state license plate itself

• For MPEG-4 and H.264 formats, the recommended distance between key frames is no more than 4-8 frames in a stable connection
• For surveillance in gateway conditions, camera is usually installed at the edge of the traffic lane. In this case:
  • Installation height must be above the headlights level
  • The distance between the installation site and the focal point must be at least three meters
• Camera tilt angles don't exceed the maximum values:
  

  Angle	Recommended value	Maximum value	Scheme
  Vertical angle (camera is tilted down to license plate)	18-20°	30°
  Horizontal angle (camera is tilted left/right towards license plate)	5-10°	20°
  Tilt of the license plate in relation to the road surface	0	5°	During recognition of a single-line license plate (with a minimum of six characters), the camera angle is checked by the "one-line rule": an imaginary horizontal line must cross both the first and last characters on the license plate: