Picking the Right Camera for License Plate Capture or LPR – IDIS Americas

Question

What camera do I need for a license plate capture or license plate recognition applications?

Answer

Unfortunately, there is no one-size-fits-all camera for license plate recognition. To determine the right camera for your application, you need to measure and understand your scene before you can pick the right camera:

1. How fast are cars moving through the scene? Is there a choke point?

Fast moving traffic typically requires a lower resolution, so that your framerate is high enough to capture plates at speed. If traffic slows at a gate, toll plaza, or the entrance to a parking lot, you can use a higher megapixel camera with a lower framerate to ensure high picture quality.

2. How wide is your field of view?

Measure the width of your target capture area. Understanding your capture area and camera distance from the capture area will help you understand both the pixel density you'll need to achieve and the appropriate lens and focal length for your application.

3. Where will you position your camera, relative to the field of view?

Ideally, your camera will have a straight view of the target (perpendicular to the target area). Keeping the angle straight:

Prevents depth of field issues as plates shear away from the camera into a less focused area. For example, If a camera is pointed down the center lane of a three-lane road, cars in the left and right lanes don't stay within the field of view as long as cars in the center lane.
Ensures that the pixel density remains consistent through the entire area of capture. As the field of view gets wider the pixel density drops.

4. How is the environment lit — are glare, car headlights, or poor lighting issues?

Poorly lit areas will result in a noisy image, and drops in framerate as the camera adjusts exposure time to compensate. Because cameras adjust exposure based on the overall brightness of the image, you should choose illuminators that illuminate not just the license plate but the entire field of view — especially in dimly lit areas or at night. You must be able to adjust the illumination source angle relative to the camera position to prevent light from reflecting off your target. This is why cameras with built-in IR are not the best choice for this application: it's much easier to change the angle of an independent illuminator than it is to move your camera.

For day time applications where the camera is facing the sun, you should consider whether or not Wide Dynamic range camera is appropriate.

Most locales require rear plates, which are much easier to capture. However, if you must capture front-side plates, then you must place the camera high enough that headlight glare will not be an issue or use a camera housing that incorporates a high contrast system.

Positioning your camera above headlights can pose a problem: at distance, headlight beams will reach high enough to cause glare, and, even at close range, cars may have fog lights or high beams on.
High contrast systems consist of a special glass filter called a "band bypass filter" that only allows a particular wavelength of IR light to pass, and an IR illuminator matched to the glass. These housing systems are expensive and hard to find, and once installed inside the only thing the camera will see are the plates and any reflected light in that wavelength. This works well, but not if you want to see the color of the car or the driver’s face as well.

5. What pixel density will you need to consistently capture plates?

The higher the pixel density you can achieve, the better, but the minimum pixel density you must achieve is determined by the user or system that will read plates. If using an OCR ANPR or PIPS system, the software provider will have minimum resolution requirements. If a person will manually read plates, minimum pixel density becomes more subjective. In general, we recommend a minimum pixel density of 60 pixels/foot (196 pixels/meter). While most people can read a normal USA plate at 45 pixels/foot (148 pixels/meter); however, some states (like Texas) have actually made the numbers and letters on their plates smaller than the 1” x 2 1⁄2” characters normally used on plates from other states in the US and Canada. The higher recommended pixel density provides a margin for error and increases real-world readability.

6. What kind of camera/lens should you use?

While a varifocal lens can help compensate for issues with distance and field of view, a fixed focal length lens will always provide a better image. In general, we recommend box cameras and housings with flat glass, preventing any prismatic lighting effects or focusing issues that could possibly occur with domes in the same application.