Fiber optic technology delivers versatile LED-based illumination

Jan. 13, 2007
LEDs combined with fiber optic technology enable a huge variety of backlighting and other applications, according to Frank Byers and Walter Schulz of Lumitex.
LEDs have become the light source of preference, rapidly displacing cold cathode fluorescent (CCFL), electroluminescence (EL), vacuum fluorescent (VF) and other technologies in a wide range of backlighting and illumination applications.
Figure 1a Fiber optic technology has developed to make the most out of the brightest LEDs, spreading their light efficiently and flexibly, while minimizing power, circuitry and LED count. Fiber optic backlighting, in particular, is an increasingly popular way to differentiate product designs with light, increasing clearly-perceived customer value. Fiber optic panels couple with LEDs to provide more flexible, adaptable and cost-effective alternatives to such common technologies as molded plastic light pipes, EL, LED arrays, discrete (embedded) LEDs and CCFLs.
Figure 1b Lumitex is a company that engineers fiber optic lighting into unique, proprietary designs that provide light to sensitive applications – without customary problems of heat, electricity, EMI, RFI or ESD – in user-friendly forms that fit customer applications. Such lighting solutions offer significant advantages in design flexibility and adaptability, with low startup costs and rapid prototyping (in some cases, less than two weeks).
Figure 1c Lumitex offers a range of patented fiber optic technologies that provide RoHS compliant, custom backlighting solutions for membrane switches, keypads, overlays, LCDs, and other displays, as well as custom fiber optic devices, surgical and phototherapy lighting systems (see figures 1a, 1b, 1c).

For fiber optic technology, LEDs are the most ideal light source because they provide a focused point light source that can be easily coupled into the optical fiber. Fiber optic panels coupled with LEDs are also inexpensive, durable and have a long lifetime of more than 100,000 hours in most configurations. Commonly used colors are white, green, blue, aqua, amber, red, RGB and infrared. Lamp style (5mm or 3mm) round LEDs mate well with optical fiber panel technology.

Design factors to consider
Fiber optic technology is ideal for sensitive applications that require light without heat, electricity or EMI. In the case of unusual geometries, fiber optic lighting can backlight rectangles, ovals, circles, semi-circles and go around holes. For uniform area lighting, fiber optic backlighting can spread light from a single LED over a larger area than any other technology. This is particularly important for LCDs in low-power, hand-held devices and is superior to embedded LEDs when used in membrane switches and overlays.

In terms of complexity, EL and CCFL backlighting require an inverter, and LED arrays can consume more power, which may require circuit re-design. Fiber optic, LED-driven backlighting uses direct current, with no inverter or design changes necessary. When it comes to durability, the technology is virtually indestructible in most applications, and is completely RoHS compliant.

Membrane Switch Backlighting
Figure 2

Thin and flexible panels such as Lumitex’s UniGlo® product are ideal for membrane switches. The UniGlo panel is placed beneath the overlay and above the circuit board/metal dome assembly (see Fig. 2). There is sometimes a spacer layer that goes above the circuit board, around the domes and below the fiber optic panel. The flexible nature of the fiber optic panel preserves the tactile feel when the user presses through the overlay to actuate the domes. The fiber optic tail is routed, around any obstacles to the LED, which is designed into an appropriate space on the circuit board.

Figure 3 The remote control shown in Fig. 3 has conductive pads printed onto the underside of the fiber optic panel that, when integrated with rubber keypads or membrane switch overlays, function as a switch. This permits switches to be totally encapsulated, with an adhesive layer around the PC board traces to seal out contaminants that can cause switches to fail prematurely. This design provides a direct path for the light from the LED light source with no interference, enhancing the brightness, uniformity and overall look of the remote control.

LCD Backlighting

Done properly, backlighting an LCD does much more than just making it readable. Attention to detail, combined with the right illumination technology, can enable its information display to give the appliance a whole other dimension in term of aesthetics.

Figure 4a In high-end audio equipment manufactured by McInTosh (Binghamton, NY) for example, the backlight must do more than provide a high degree of light output. It’s important that it be perfectly uniform, consistent, and provide just the right color the manufacturer wants. The incandescent backlighting used for the large power output meters in their top-quality power amplifiers was causing a blue tint around the edges that became progressively more washed out toward the center (see Fig. 4A).
Figure 4b The manufacturer resolved this by using fiber optic panel backlighting, resulting in a perfectly uniform, brilliant blue (see Fig. 4B). The flexibility of the patented fiber optic panels – with their capability for color blending and die cut for clearance holes – also allowed the manufacturer to perfectly backlight the green logo and text on its audio/visual control center.
Figure 5 Another patented fiber optic backlighting technology developed by Lumitex uses optical fiber woven into cloth, which is layered into panels. This provides for optimal control of light emission across the panel surface. As shown in the typical panel construction diagram in Fig. 5, the optical fibers extend from the panel in cable form and are bundled into a brass ferrule (forming a “tail”) and highly polished. The ferrule is then connected to a remote light source. Tails can be preformed into 90 or 180 degree standard bends, with a minimum bend radius of 0.06 inches. In addition, fibers can be formed into complex shapes, including multiple bends, and connected to a board mounted LED. Medical Applications
Figure 6 In medical applications, fiber optic lighting can be used in the form of thin, flat, light-emitting panels (Fig. 6) woven from plastic optical fibers that are enclosed in plastic and mounted inside each bore tube in the MRI unit shown in Fig. 7 to provide an inert MRI comfort light. This provides a soft, cool and reassuring light that relaxes the patient in what can be a stressful procedure and helps medical professionals ensure correct and comfortable positioning – with no heat or electro-magnetic interference.
Figure 7 Fiber optics can also be used to provide just right color and mood for illuminating control panels for hospital beds. Other medical applications include switches and overlays, controls, and nameplate lighting and flexible light panels that help surgeons see during deep-cavity surgery. Full-Travel Keyboard & Computer Applications
Figure 8 For computer applications, thin plastic optical fiber backlighting devices can backlight full-travel keyboards and flat panel membrane switch keyboard overlays, while minimizing the number of LED light sources. A super-thin (0.013-inch/0.33mm) UniGlo™ fiber optic panel is placed behind the stainless steel keyboard backer that has a die-cut section behind each key for mounting of the key’s suspension system, as shown in Fig. 8.

The translucent keys are illuminated by the light from the panel shining through the die-cut hole for each key. When used to backlight flat panel membrane switch keyboard overlays, the UniGlo™ panel is placed between the translucent keyboard overlay and the membrane switch (metal or plastic domes, or non-tactile switches). A single-layer UniGlo™ panel adds only 0.2 to 0.8 ounces of additional snap-through pressure (depending on the type of dome being actuated).

Illumination/Indicator/Accent/Enhancement Lighting
Figure 9 Fiber optic lighting works uniquely well in humid environments, such as spas and hot tubs for accent, decorative and colorful splash lighting of the water stream. Because it is inert, it can be used safely in wet, humid environments (see Fig. 9). Additionally, the capability to quickly and cost effectively customize the lighting allows designers to try a number of different lighting techniques to get "just the right one".
Figure 10 Fiber optic lines of light can be used in self-service kiosks, such as a DVD kiosk manufactured by DVDPlay, Inc., Los Gatos, CA. About the size of an ATM, DVDPlay's automated movie rental kiosk is the world's first intelligent, Internet-connected DVD movie rental store. DVDPlay chose fiber optic lines of light from the Poly-Optical Division of Lumitex to provide brightly colored blinking enunciators above and below the Take (green) and Return (blue) slots on its new A55 model (Fig. 10). The blinking enunciators help guide customers through the rental process by clearly directing their attention to the right slot.

New (and Future) Applications
The versatility of LED-based fiber optic backlighting and illumination technology is such that applications are almost limitless, and new ones are cropping up all the time, making it popular with design houses that specialize in new product invention and design development. It is proving to be an effective solution for applications as diverse as backlighting the instrument panels in trucks, busses and in the cabs of off-road vehicles used in construction and agriculture to accent lighting for tanning beds to the illumination of the vests used in games of laser tag.