Fiber-optic backlights: svelte and efficient

March 21, 2002
By Gary Grotenhuis Vice President, Engineering Contrex Inc.

By Gary Grotenhuis
Vice President, Engineering
Contrex Inc.
Maple Grove, Minn.

Edited by Miles Budimir & Kenneth Korane

The operator interface display for the CX-1010 universal speed controller from Contrex Inc., Maple Grove, Minn., uses an LCD backlit by a fiber-optic panel. The display consists of 128 128 pixels formatted into 14 lines of information.

A typical fiber-optic panel is constructed from optical fibers extending from the panel and bundled into a brass ferrule. The bundle is connected to a remote light source; in this case, three green LEDs.

LCD panels are widely used as operator interfaces. But LCDs are passive displays, which means they don't generate any light. This means they must be backlit unless they work in extremely bright ambient light.

Older control panels often used an LED array as a backlight. Some arrays contained as many as 144 LEDs. The approach works reliably enough but consumes a lot of power and generates heat. Also, though the panel is readable, it can exhibit dark spots if the light doesn't diffuse properly. Fiber-optic backlight panels offer a welcome alternative.

The panels, manufactured by Lumitex Inc., Strongsville, Ohio, have up to eight layers of fiberoptic weave assembled together with double-sided adhesive. They are bright, giving off 30 fl (footlamberts) or better, and connect to an LED light source that is good for up to 100,000 hr of useful life.

The light sources use little current — as low as 10 mA at 2 Vdc, much less than electroluminescent (EL), cold-cathode fluorescent (CCFL), or LED arrays. This is because a fiberoptic panel needs only a few LEDs to illuminate an area up to 4 10 in. (100 250 mm). Moreover, there are no dark spots because light diffuses uniformly across the display.

The Contrex CX-1010 universal controller, for example, is now backlit by a single Lumitex fiberoptic panel distributing light from three green LEDs running on 60 mA (compared to 144 LEDs consuming 500 mA in the original LED-backlit display). The new version, of course, generates less heat. It is also more compact (4 7.5 in.) and easy to manufacture.

The form factor available through the compactness of the fiber-optic panels is appealing from the standpoint of both design and aesthetics. They can be just 0.013 in. (0.33 mm) thick, much thinner than an LED array. And the lower profile compared to an LED panel also simplifies assembly tasks.

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