29 September 2011 Comments Off on License Plate Recognition Cameras

License Plate Recognition Cameras

by Jennifer Spears – Business Solutions Manager

License Plate Recognition Cameras otherwise known as LPR cameras are meant to do one thing. Can you guess what it is???
You guessed it! They are made to capture license plates very VERY well.

If you aren’t familiar with LPR technology let me explain a few things.

These cameras are almost always a B&W camera only. Why you ask? Because you will get clearer low light images. And why is this important? Since a huge chunk of crime occurs at night we specifically optimize the cameras for this time period. There is also the problem with light reflection from the plates. When we filter out the visible light spectrum to avoid the reflection problem, black & white cameras must be used.

Even though the camera is B&W you won’t see things like you expect. A very dense filter is placed inside the housing in front of the lens that will block out almost all of the lighting coming to the camera except the specific wavelength of the infrared (IR) LEDS that are installed with the camera. So you will see the license plates practically glowing when they pass in the viewing area of the camera. Everything else is just very dark. This is NORMAL and not a problem with the camera.

Angle is VERY important when it comes to mounting LPR cameras. If you mount the camera at too severe of an angle (more than 30 degrees) then the IR will not be able to bounce off the license plate and get it back to the camera. It will bounce off and either you will get a very dark plate, or a super white plate. Neither one will help you if you are broken into. And remember this angle limitation is for the side angle and the top. So you can’t mount an LPR camera 20’ in the air and look straight down and expect it to get a plate.

Resolution and frames per second (FPS) are both important on the DVR that your LPR camera is going to be connected to. Resolution will directly correlate to how big you can look at the picture after it has been recorded. So if you are recording at the lowest 320X240 resolution you won’t be able to playback the video any higher than a quad screen or it will pixelate so badly you’ll never get a plate off of it. FPS correlates to how fast a vehicle can be moving and the DVR still have the ability to record the plate. 30FPS is what most TV shows record at, 24FPS is what most movies are recorded at. 1 to 5 FPS is what you are seeing when you watch REALLY bad footage on COPS of surveillance footage and a car is jumping through a parking lot instead of speeding through it. So you need a system that can do both higher resolution and FPS, at high settings to get very good footage, that you can prosecute from.

So the main points to remember are B&W is good, mount the camera no more than 30 degrees off center, and connect it into a DVR that can record the images big enough and fast enough so that the camera works for you instead of you working for the camera.

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