Optimizing the plate number recognition (notes)

The note is based on the short research by googling the internet and reading some paperwork.

The parameters need to be considered is

  1. Site environment
    1. Weather
    2. Day and night
    3. headlights
    4. Vehicle types

1 Hardware consideration

The parameter that need to take in consideration

  • Power of illumination/number of LEDs
  • Shutter type
  • Type of lens (fix or zoom)
NoItemSpec
1TypeIP Based Camera
2Lens>= 2 MP Color
3IR Illuminator>= 8 pieces dot matrix LED with wavelength 850nm
4Frame Rate >=25 frame per second

2 Camera Positioning

  • A good ANPR engine can read the plates from images taken in various condition.
  • However to optimize the recognition accuracy up to 95%, we need to positioning the camera correctly.
  • There are 2 type of camera positioning
    • Overhead
    • Lateral

2.1 Overhead

  • The best position for camera is on top of the road as per below image

  • a = 5 to 11 meters, b = 3 to 6 meters
  • vertical rotation angle is <= 40 degrees
  • horizontal rotation angle is 0 degree

2.2 Lateral

  • Lateral The 2nd option is near the road with a maximizing the straight angle and to ensure the camera position is 1 – 1.5 meters above the headlights

  • a = 1 to 1.5 meters, b <= 1 meter
  • vertical rotation angle is <= 25 degrees
  • horizontal rotation angle is 0 degree

2.3 Common Mistakes

  • Camera viewing the sun
  • Camera viewing sky and hill, should focus on the road
  • The sun light reflecting the plate

 

3 Images

  • The best size for pixel of the character is between 16 pixel to 50 pixel height

3.1 Common mistakes

  • Low Contrast – avoid overexposure

  • Overexposure – minimize contrast and control the brightness

  • High distortion – adjust the camera position with the correct rotation degree

  • Low spatial resolution – adjust zoom and focus

  • Blurred image – lower the shutter speed

 

4 Software Configuration

  • Zoom and Focus
  • Brightness Control
    • outdoor
    • indoor
    • Night mode
  • Contrast control

5 Safety

5.1 InfraRed illumination

  • Standard device equipped with an 850nm InfraRed illumination unit.
  • The human eye will not or slightly see this light coming from the LED’s.
  • Do not look into the illumination unit directly from close range or for more than 100 seconds.
  • Eyes can be damaged by not taking these precautions.

6 Product for reference

  1. Tattile Vega Smart Speed
  2. Carmen ANPR