New technology developed at NDSU uses an extensible array of light-emitting diodes (LEDs) as a means of sensing incident light levels. This light-sensing LED array can be used to construct highly-dynamic backlights for displays, as well as large-format synthetic aperture cameras.

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A hand casts a shadow on elements of a prototype light-sensing LED array. Elements in the shadow are illuminated, and those in the light are off.
The principle behind sensing light with an LED is to drive the LED as a capacitor, the discharge from which can be accelerated by light. The LED is reverse-biased, then the bias is removed and the LED is allowed to discharge. By measuring the time it takes for the LED capacitor to fully discharge, the level of light incident on that LED can be calculated. By alternating between modes, the LED array can be used to both display (emit light) and sense light.

A light-sensing LED array can be placed behind a liquid crystal display (LCD) to provide a highly-dynamic backlight. In sensing mode, the array can create a “light map” for the entire display, and then command individual LED elements to glow brighter where more backlight is needed and dimmer where little backlight is needed.

The LCD elements in front of each LED in the array can also be used as shutters, controlling the angle and amount of light that is allowed to pass through. By using these LCD “shutters”, a software-controlled synthetic aperture camera can be created.

There are a number of benefits to this new technology. For one thing, using an LED array that both emits and senses light allows the construction of a large format display backlight that automatically adjusts to changing ambient lighting conditions. The LED array can be used to sense reflected light and construct an image of objects passing in front of the array, allowing the creation of a large-format synthetic aperture camera. Finally, the construction of the LED array is extensible to virtually any size, meaning that an entire billboard, information display, or wall can be covered with the light-sensing array.

This technology is available for licensing. For more information, visit www.greendesignbriefs.com.