When choosing a camera for license plate capture there are many factors to take into consideration. Not just any CCTV security camera will do the job. Even with the right camera, you may not get the desired results without proper planning, placement and installation.
First things first, the “License Plate Cameras” I will be speaking about in this article are not to be confused with the “High Speed Traffic Cameras” commonly found on highways or intersections in many cities today.
Some of the higher end traffic cameras are also equipped with a license plate recognition system. Each time a car passes by the camera it captures an image of the plate. The camera then uses OCR (optical text recognition) to digitize the plate.
Then it stores the plate number as well as the lane the vehicle was traveling in, into a database.
This information can be an invaluable source of searchable, index-able information. As an example, if a child happened to be abducted, the authorities could search the database for a specific license plate number. If a match is found, it would pinpoint the location of a vehicle at a given time.
High Speed Traffic cameras are specifically designed to capture license plates at a high rate of speed, in variable lighting conditions, and are very expensive and costly to install and maintain.
The “License Plate Capture” cameras we will be talking about are much more affordable and are usually set up in a controlled environment, such as a guard gate for a community or front entry to home or office.
A few things to keep in mind when thinking about trying to capture licenses plates are, choose the right camera for the job, choose the right spot for installation and understand the capabilities and limitations of your equipment.
Let’s get started. The first thing we need to do is choose our camera. A standard fixed lens 700 TVL camera might be great for surveillance where you need a good overview of the surrounding area. What we need is a “Varifocal Lens”. That means that you can manually adjust the zoom and focus to a desired area when setting the camera up. This is not to be confused with a PTZ (Pan, Tilt, and Zoom) camera that can be moved and zoomed in real-time. These cameras are zoomed and focused at the time of installation and will always record at the pre-set position.
Many varifocal CCTV cameras have a field of view range between 2.8 to 12 mm. 2.8mm being a very wide view and 12mm being zoomed in to a very tight and narrow view. License plate cameras typically take it a step further with lenses in the 5mm to 50mm range, giving you the ability to zoom in tight on the license plate from long distances.
Next we need to think about camera placement. This not only means placement of the camera itself but the environment or traffic area where you are trying to capture the license plate image.
You probably don’t want to mount your camera up high on a building that is far away from your target. You also shouldn’t mount the camera perpendicular to the road where vehicles will be passing and always try to avoid trying to capturing license plates on vehicles traveling at high speeds.
In the graphic shown below, the distance from the camera to the license plate will make it very hard to get a good resolution capture, even with a varifocal lens.
You also drastically limit the ability to capture good quality video because, the license plate will only be in the camera field of view (shown as the yellow cone) for only a fraction of a second. This is due to the viewing angle and speed that the car is moving.
Depending on the camera and its IR distance, you may not even be able to see the plate at night if it’s too far away.
A bad viewing angle, long distance from the target and high speeds make this a less then optimal installation and almost guarantee you won’t be able to read the license plate if and when you have to.
Shown below is the optimal situation in which to setup and capture license plates.
As we review the setup you will notice we have a good viewing angle, short distance to the plate and a controlled environment with low speed traffic.
First you will notice that the camera is pole mounted, at license plate level and very close to the lane of travel. It is recommended that you mount your camera between 5’ and 15’ high and under 35’ away from the plate. Doing this assures that the vehicle will pass by the camera at close range and remain in the field of view (represented by the yellow cone) for a longer period of time.
You will also notice there is a speed bump controlling the vehicle speed allowing you to more accuracy determine where the car may be at a given point in time.
In addition to the license plate camera there is also another camera (represented by the blue cone) mounted on the corner of the guard house. This is part of a recommended 2-part solution. The second camera not only gives you a wider overview of the rest of the vehicle but also covers the license plate camera itself to avoid vandalism or someone attempting to tamper with it.
Here are a few license plate camera recommendations from TechPro security products and SecurityCameraKing.com.