ALPR CONSIDERATIONS
An ALPR system worthy for consideration should not have difficulty with either brightness, as in direct sunlight, or darkness where there is no ambient illumination. IR illumination is used for the sole purpose of capturing the best possible images for ALPR processing, Kennedy said.
“Looking at the specifications of equipment offered as license plate cameras, it can be seen that some have LED illumination that is always on, while others may have it pulsed," Kennedy said. "By being pulsed, they are several times brighter than leaving them in the always-on position, and when pulsed they are on probably 5 percent of the time, which gives them an extraordinary life cycle.“
Some suppliers offer cameras that are aimed at low-cost security applications and are repurposed to capture license plates. These cameras have always-ON LED illumination and incorporate only a small number of LEDs, which generally have a life span of less than three years, Kennedy said. “In addition, many of these lower-cost devices need to keep a fan constantly running within the camera enclosure because of the heat problem generated from LEDs which are always ON. This prevents a truly weatherproof housing.”
Pulsing the LEDs requires circuitry and logic that guarantee the pulse takes place when the shutter is open, Kennedy said. “In addition to pulsing LEDs for longer life and greater illumination, the brightness of each pulse can be altered by varying the pulse width, and this is synchronized with the amount of time the shutter is open; the ‘shutter open' time is varied in a repeatable pattern, and this provides the ALPR engine images of varying contrasts from which to make a positive read. This is no simple task, but reaps great benefits regarding the overall ALPR accuracy, as well as eliminating many of the missed or unqualified plates encountered by camera systems of lesser quality.“
Distance is another item of concern in purchasing. Lower-price cameras can claim a useable distance of up to 40 or 50 feet, while higher-end cameras can be used at distances greater than 100 feet. However, the further the camera is from the license plate, the weaker the IR illumination that reaches the plate, Kennedy warned. As illumination drops off, the shutter must stay open longer to allow enough of that light to be reflected back to the image sensor, opening a window for motion blur.
The speed of the vehicle is another consideration to account for. For parking applications, a camera that can read the license plate of a vehicle traveling in excess of 100 miles per hour, in any light, at any time of the day or in any climate may seem like overkill, Kennedy said. However, it also means the “red line” will not even be approached by most users, guaranteeing a long life expectancy for the equipment.