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issue: November 2005 APPLIANCE Magazine European Edition

Display Technology
Shedding a New Light on Appliance Design


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This information was provided by Pelikon and was written by David Alppleyard, a consultant who specializes in the technology, energy and process industries

Appliance designers are being challenged to reconsider accepted practice with the emergence of new technologies that offer a thin and flexible illuminated display.

A printed, segmented electroluminescent display integrated in a microwave oven enables the unit to display only active cooking options while keeping unnecessary options unlit.

Advances in display technology are set to open up new avenues of exploration for appliance designers with the emergence of flexible film-based electroluminescent devices.

Limitations associated with existing display technologies such as LCDs and LEDs restrict design options as, being essentially made of layers of glass, they are both flat and rigid. Now, designers have the opportunity to use displays in innovative ways by integrating displays within appliances with the use of film-based displays. As the film-based displays are cost efficient to produce, and given that products with displays can retail for around 25 percent more than those without, the technology offers considerable commercial opportunities. Furthermore, they give designers another tool for product differentiation, another key commercial objective.

Flexible, printed, segmented electroluminescent displays can be used in applications such as multi-function remote controls.

Evolution of a New Technology

A thick-film or powder phosphor electroluminescent device is comprised of a light-emitting material in a dielectric matrix sandwiched between two conducting electrodes. The light-emitting component is phosphor, typically a zinc sulfide powder doped with manganese, while silver or graphite-loaded screen printable inks and indium tin oxide-a transparent conductive metal-are used as the electrodes. When an AC voltage is applied between the electrodes the emitter conducts current, the manganese ions are excited and give off light.

Varying the operating voltage and frequency controls, the brightness, and to some degree the color of the light with, for example, an increased voltage-boosting lamp, increases both brightness and shifts the color slightly toward the blue. Nominal voltage and frequency is 110 V and 400 Hz, but display operation is no longer tied to those figures. Phosphor electroluminescent devices can also be driven by low-voltage DC using inverters and inductors to generate the necessary AC voltages of 100 V to 300 V peak to peak, at frequencies of 50 Hz to 2,000 Hz.

Phosphor electroluminescence was first discovered in 1936, though it was not until the 1950s, when GTE Sylvania received a patent for an electroluminescent powder lamp, that the first practical devices emerged. However, lifetimes of only 500 hours limited the use of such devices until the 1980s, when new research revitalized the electroluminescent powder lamp. In 1990, Durel Corp. demonstrated a flexible electroluminescent phosphor device using a manufacturing technique that involved encapsulating the phosphor powder particles in an oxide coating and sandwiching the phosphor powder held in a dielectric matrix between two electrodes. Today, a typical display consists of many light-emitting phosphor-ceramic elements, each sandwiched between conductive electrodes, where one of the electrodes is transparent to allow light to escape.

Recently, work began on the development of a multi-segment display using phosphor lamp technology. In 2001, a commercially manufactured flexible plastic display technology, a process in which all the electronic display components are printed onto plastic film using a high-volume print process, was developed. Since then, flexible segmented electroluminescent displays have been in full production and used on domestic appliances and consumer electronics.

Design Edge and Advantage

At less than 0.3 mm, phosphor electroluminescent lamps are thin, although the displays are typically 0.5 mm. This dimension is increased if the display is mounted on a printed circuit board (PCB) and integral button mat. With such a display, it is possible to incorporate a full-width display within, say, a dishwasher, sited on a slide-out tray that is not visible until it is needed. The display can also be mounted on the thinnest of products and its use removes the need for light pipes and LEDs. Furthermore, phosphor-based displays can operate over a wide temperature range (-40ºC to +85ºC) and are serviceable to low temperatures, allowing use within freezer compartments, for example.

In addition, when applied to a flexible plastic substrate, electroluminescent displays are flat and fully flexible and can be bent around simple planar fixed curves. This allows the displays to be integrated into handles, doors or other curved surfaces. While not designed to make gross repeated flexure, the displays are rugged and can be flexed to allow button pressing through the display, having been qualified for up to 1 million button presses. This allows co-location of a metal dome button and a lit icon, allowing the product to be used like a touch screen, but at a lower cost.

The visual characteristics are another advantage as such displays do not require back-lighting and can incorporate displays and backlight lamp elements in the same component, all with similar performance to both LED and vacuum fluorescent displays (VFDs). With a wide viewing angle, such devices are also good indoors or in low light conditions. Despite this, the displays are energy efficient, making them suitable for battery powered, handheld devices. Furthermore, large display characters and segments up to 3 cm2 are possible along with displays up to 600 mm wide. The use of grey scale and animations within the display further enhance the product, although scrolling text is not currently possible as it is with LCDs.

In addition, as the illuminated layer is a surface emitter of broadband light, the film surface can be printed or have another printed film layer situated above it. Consequently, it is possible to have graphics or illustrations mounted on top of the illuminated sections, a situation which tends to blur other types of illuminated display, which are either sub-surface emitters or, in the case of LCDs, back-lit. The covering can be any color or texture, such as bronze, marble or even a wood effect, and a range of overlays can be used to give grey, black, purple, blue, green, and yellow when the display is off. It is also possible to have multiple colors within the same display, although RGB is not currently available.

Another aspect of the technology is that, due to its plastic film construction, the display can be die cut to any shape and can have holes through it to allow other components, such as buttons, knobs, sliders, or even other types of display, to be mounted through the display surface. The silk-screen printing process used for the manufacture of electroluminescent displays also allows the displays to be made at low costs and quickly, even in low volumes.

Fundamentally, electroluminescent displays allow complex information to be displayed simply and at low cost. The hide and reveal nature is suited to display multi-layered or tiered information, combining the benefits of a display and film keyboard in a consumer friendly interface. The display consequently avoids cluttered complex interfaces and unobtrusively brings high technology to the kitchen.

Design Considerations

Electroluminescent materials have been in development and application for the last 30 years and have been sufficiently characterized for most indoor products. Research has been conducted to investigate the behavior further and discover how to stabilize and balance the performance characteristics of blue and green light phosphors, while extending performance life.

Though electroluminescent technology is not yet suitable for permanent display devices, it is more than sufficient for most household appliances that require a display "on time" of up to 7,500 hours and a product life of 25 years. For applications where clocks or status needs to be shown continuously, the technology can be combined with more traditional display technologies, such as LED, to further extended display lifetime requirements. Also, the technology is not yet suitable for direct sunlight applications or applications that require an upper operating temperature of greater than +85°C.

In view of the design considerations, this is a technology that is capable of changing the way people think about and use displayed information. Certainly, the technology allows designers to integrate displays into products in ways that have not been previously seen.

 

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