Modulated light triggering for traffic applications

The use of asynchronous triggering has always provided several key advantages in vehicle imaging applications. The most important of these is the ability to maintain more consistent positioning of the vehicle -- and its number/license plate -- within the image.

This is in contrast to systems that use free-running cameras -- with or without an associated trigger function. These systems typically must hunt through a large stream of images looking for usable vehicle images, or use synchronous triggering whereby the camera runs continuously, but only outputs an image during the next available frame after a trigger is received. Unfortunately, depending on vehicle movement and camera frame rates, the location of vehicle plates within designated "good" images may vary significantly when using either of these methods. This can significantly affect the focus, the pixel density, and other factors impacting automated readability by OCR software.

Asynchronous triggering, in which the camera immediately captures an image precisely when a passing vehicle triggers it, is the only way to ensure the best possible focus, angle and pixel density on the vehicle plate in every image captured. Building on years of experience in machine vision applications, JAI has incorporated asynchronous triggering technology into all of its traffic cameras and subsystems.

A new generation of triggering technology
In the past, triggered systems have required integration with ground loops, laser detectors, or other infrastructural elements to detect vehicles as they enter the imaging zone. While this provides precise positioning information, it also adds complexity and cost to an overall installation.

Today, the market is demanding self-contained "all-in-one" systems that can eliminate the need for roadbed excavation, large plaza structures, and other costly elements. This requirement has led JAI to develop a new generation of self-triggering technology that can be mounted above the roadway, either within the same enclosure as the camera, or alongside it as a separate unit.

JAI's modulated light trigger for license plate recognition cameras uses a patented approach to accurately self trigger VISCAM AIO systems with precise positioning and without the processing overhead of software-based triggerless systems.

JAI's unique system uses modulated IR light directed at the camera's field of view. Light reflecting off passing vehicles is detected by the system which, based on signal strength and location within the frame, can tell the camera if an image needs to be captured.

The result is a "virtual trigger line" of invisible light, providing reliable vehicle detection and the same precise positioning of vehicles within the frame needed for high automated read rates, but without any ground loops or other roadbed infrastructure.

In addition to triggering the camera, the same technology can be used to measure the ambient lighting of the scene and adjust the camera's exposure settings.

For more information or for a demonstration of this modulated light triggering technology, please contact JAI.

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