Designers are always looking for ways to make their products slimmer, particularly in portable applications such as mobile phones, where thinness has become the fashion and helps determine the acceptance of the product in the market. For designers thinking thin, the selection of illumination technology for keypad and decorative lighting plays an important role in realizing those design objectives.

Motorola is the acknowledged pioneer in the movement toward super slim phones with products such as the highly successful RAZR V3. The choice of Durel DFLX electroluminescent lamps contributed to the phone’s slender profile. The DFLX lamps, made by Durel Div. of Rogers Corp., Chandler, Ariz., provide the stylish blue backlighting for the V3’s precision-cut, chemically etched keypad. The DFLX lamps also deliver the white glow behind the keypad in Motorola’s new PEBL handset.

The DFLX lamp fits between the keypad and metal dome array without affecting tactile response.

Measuring only 0.1 mm thick, the DFLX lamps achieve both their thinness and flexibility through the absence of the thicker and stiffer polyester film associated with conventional EL lamps. The profile of the DFLX lamps permits them to fit easily between the keypad and the metal-dome array. This enables designers to place light directly under each key without affecting the tactile response felt during actuation of the keys.

The rubber composition of the lamps makes them extremely durable. They have been tested to more than 1 million actuations in keypad applications with no degradation. They can also be folded, flexed, and even pierced, while maintaining complete functionality. The ability to wrap the conformable lamps around complex surfaces and emit light in different directions provides designers the opportunity to create intricate, three-dimensional lighting designs.

Motorola PEBL keyboard has white backlighting from DFLX lamps.

“The big difference is that, with traditional lighting methods, designers must place the light somewhere around or between the keys,” says Mike Bessette, vice president of Durel. “With DFLX lamps, the light is placed exactly where it is needed, under the keys.” This approach to lighting also provides a very uniform light, with no “hot spots,” eliminating the need for light guides.

Like conventional EL lamps, DFLX lamps use phosphors that emit light in green, green-blue, sky blue, deep blue, and orange colors. However, phosphors can be blended to produce variations of these colors, including white. A much wider range of colors can be obtained by employing Rogers’ cascading overprint technology. A cascading overprint absorbs light rays from the basic phosphors, then re-emits them at a different wavelength, producing a color shift. Through this method, specific customer colors can created virtually anywhere across the color spectrum.

Powered by a small, IC driver, the DFLX lamps use very low current, typically less than 20 mA, and generate no heat. In addition to the lamps, Durel has developed a broad range of EL drivers as part of its matched, system-solution approach. The Durel keypad drivers utilize a patented three-port technology that is designed to reduce both audible and electrical noise.

Sidebar: Life vs. Light

When designing products with EL lighting, it is important to understand how EL life is measured. The light from an EL lamp does not fail catastrophically, the way a light bulb burns out. Instead, an EL lamp very gradually becomes dimmer over time. Since it never actually burns out, there is technically no end of life to an EL lamp. Therefore, the best way to evaluate EL life is to consider its “useful” life. And that is defined as the period of time that the EL provides enough light to effectively backlight the application. Another way to evaluate life is to consider the Time-to-Half Luminance, or the time it takes the luminance to decay to one-half its original value. An alternate, but less commonly used method to measure life is Time-to-Quarter Luminance, or the time it takes the luminance to decay to one-fourth its original value.

Designers should also be aware that there is a trade-off involved between brightness and THL. An increase in brightness results in a decrease in THL, and vice versa. Overall, the THL of a lamp depends greatly on the lamp’s color, the ambient environment (temperature, humidity) and the initial luminance intensity and associated drive conditions (voltage and frequency). Longer THL can be achieved by starting with a lower initial luminance level. Higher operating temperatures and humidity will result in lower THL. Bluer phosphor colors will have a lower THL than green and orange.