One of the most common requests in security camera system design is capturing license plates. Unfortunately, one of the most difficult things to do in any security camera system design is capturing license plates.
For years there has been a desire to capture video of vehicle license plates both sitting still and in motion. Traditional analog cameras have always had limitations in resolution. The standard U.S. license plate is 12 inches wide, 9 inches tall, and has 2 inches letters. In a video frame that is 12 feet tall and 9 feet wide, the letters would be 1/72 of the screen. That is very difficult to see even on a 600 line resolution camera. Since the addition of megapixel cameras to the market (with their improved resolution) that problem has been reduced significantly. However, the cost for a megapixel camera over an analog camera will be at least 2-3 times the cost.
Besides the resolution, the next obstacle is light. During the day this is not an issue. But when the sun goes down you need to rely on artificial light. Many outdoor cameras have infrared lighting built in. The problem with an infrared camera is that the camera is creating the light. Tags are retro reflective. In other words, a license plate returns light to its source. That results in the tag looking like a bright, blurry rectangle. However, using a standalone infrared lamp (placed about a foot away from the camera) and/or other ambient lighting can help with this issue.
If you have the budget, the technology exists to capture license plates very efficiently. License Plate Recognition (LPR) cameras have built in technologies to auto-correct lighting and actually identify the tag numbers. These cameras typically start around $1000. Mega pixel cameras are great for getting fine detail with a wider angle. That gives you the ability to capture the whole car and zoom in later to retrieve the tag. This too is a fairly costly option, around $1000 per camera after paying for the camera and the additional costs of an IP based system. (For more information about Analog vs. IP Camera Systems click here.) In practicality, the solution to capturing a license plate on video, in all conditions and 24 hours a day, is lots of light and a high resolution camera set to the tightest picture possible.