Your car has a license plate with a combination of letters and numbers. Maybe your splurged and have a "vanity" plate with your name or some pithy acronym. The technology for automatic number plate recognition provides a means to identify you. Assuming your have your plates on your car, that is. These techniques are optical and thus subject to many of the same limitations of facial recognition. Lighting, motion, visual noise, exposure, reflections and more can all cause license plate number recognition to be degraded and possibly fail. Typically, an error rate of one character is allowed and even expected. More than that and recognition fails.
That said, there are a number of reasons that automatic recognition of your license number might be quite difficult:
- Bright lights that shine on your plate may cause over-exposure, blooms and reflections. This is especially true with lights that are infra-red as they directly influence automatic exposure settings.
- A license plate mounted loosely, perhaps even on springs, might jostle just enough with the motion of your car to cause a blurry image of your license plate. Of course, this won't make a difference if, for example, you are stopped at in traffic.
- A plate that is mounted at an odd angle to the car might throw off geometrical analysis (by perspective distortion) and ranging (by distoring return signals from active microwave and laser emitters. Consider a license plate that is tilted slightly to one side and/or tilted slightly down or up. A downward tilt may cause radar reflections to bounce off the road before returning to the sensor. If a plate is tilted sideways it may cause radar relfections to bounce off other vehicles before returning to the sensor.
- Dirt. Mud. Road grime. These can be serious problems for plate number recognition.
- Something obscures the plate, such as your trailer hitch or bike rack.
- A plate cover that refracts near infra-red light would disrupt the contrast between the background (reflective) of your plate and the numbering (non-reflective). Please note that in many jurisdictions such covers are illegal or seriously frowned upon by the authorities.
- Sprays that reduce the reflectivity (clear matte) of the plate also may result in poor photo exposure.
- Clear sprays or coatings that contain "sparkly" particles for that pearlescent effect may scatter laser radar.