Thin-film electroluminescent (EL) displays were initially developed in the early 1980s as a high-performance alternative to monochrome passive LCD displays. EL displays were developed in the laboratories and commercialized by four companies: Planar Systems in the U.S., Finlux in Finland, Sharp Electronics, and later Nippon Denso, both in Japan. Initial applications for this technology were many and varied, but they were found to be particularly advantageous in portable medical equipment, industrial controls, commercial in-vehicle, and military applications. Displays produced with EL technology have ranged in size from a few inches to 18-in. diagonals in monochrome, grayscale and multicolor.

There are many performance features of the technology that have made EL a good choice for applications that require displays that are able to stand up to extreme conditions. These features have justified its longevity in the market, even in the face of the multi-billion dollar investments in other display technologies. This proven display technology continues to offer innovations and opportunities for designers seeking unique performance characteristics.

Small monochrome EL display

EL is an emissive technology, generating its own light, rather than modulating light from another source, and its display elements are defined with photolithography. The pixels have essentially perfect edge-definition, which provides a crisp clear image with what has been called “at-a-glance readability.” This readability feature is critical in medical, transportation and other applications that demand rapid response to critical information.

Electroluminescent displays use a very dark, non-reflecting back electrode structure, which maintain high contrast even in very high ambient light situations with relatively low power consumption. For applications requiring dimming for nighttime operation, wide dimming range via digital control and analog voltage input is also provided to reduce brightness and power consumption.

Planar QVGA multi-color electroluminescent display.

 Viewing angles for electroluminescent displays are unparalleled, exceeding 160 degrees in both horizontal and vertical angles with essentially no chromatic aberrations in the displayed image. All of these visual performance characteristics are consistent over a very long life due to the solid-state nature of the structure and the long life of the inorganic phosphors. It is common for EL displays to last over 11 years, even with continuous use.

The solid-state nature of EL displays and its electronics, result in operating temperature ranges of –50 DegC to 85 DegC. Unlike many other display technologies, EL displays are instant on at –50 DegC, with essentially full brightness with no warm up time and no requirement for expensive and power-hungry heaters.

Enlarge this picture Structure of multi-color thin film electroluminescent display

Mechanically, the displays have been designed for extreme ruggedness. Properly mounted, EL displays can withstand high shock loads, as well as severe vibration found in demanding applications. Typical specifications for shock exceed 100 Gs. The sealed, solid-state nature of the displays and electronics enable high reliability with MTBF specifications exceeding 50,000 hours. With this reliability, no lamps to replace or plastic films on the front surface to scratch, EL displays have a significantly lower installed cost than most other display technologies available today.

EL displays are typically available with digital, serial video data interfaces that are compatible with the majority of passive LCD controllers and embedded video controllers available on the market today. Many of the newer EL displays incorporate AMLCD type interfaces to make design easier. Most EL displays on the market today support both 3 V and 5 V logic levels. The displays are also designed for minimal system design impact in reaching current global EMI requirements. The electronics design of the majority of EL products has been recently refreshed to offer RoHS compliance. Input power is typically 12 VDC, with some units supporting wide input voltage levels for battery-powered portable and in-vehicle applications.

Only electroluminescent displays can be transparent or cut into curved shapes.

Traditionally, EL displays have been monochrome and available in sizes from 3.5-in. to 10.4-in. diagonals and resolutions from 160 x 80 (H x V) to VGA. EL displays commonly use an amber color generated by the phosphor. Planar Systems offers more than 16 different display families with over 75 variations with temperature range, interfaces and connector selections.

Planar has offered multi-color displays for several years in the VGA format. Recently it has developed a QVGA multi-color EL display. This product utilizes the conventional amber phosphor utilized in its monochrome displays, but with red and green color filters incorporated into the display stack. This results in the same basic display characteristics, but with color variations for highlighting of displayed information. For certain applications, this solution enables the continued use of EL’s visual performance, but with the added benefit of highlighting critical information. 

EL technology is also distinctive in its ability to create transparent displays. Since the thin-film stack is very thin, the typical rear aluminum electrode is replaced with another layer of transparent indium-tin-oxide (ITO) making the whole structure effectively transparent.

Because there are no liquids involved, the substrates can be fabricated in unique shapes, including ones with curves. Holes can also be drilled directly into the display or incorporated as part of the design. One application concept consists of displaying digital information on the transparent display over a mechanical gauge or other visible objects. Transparent displays share the same visual performance and rugged characteristics of all EL displays. The addition of transparency, and the ability to create unique display shapes, offers an unusual design option for appliance designers. This twist on a proven display technology creates a different and upscale look at one of the primary points that a consumer interfaces with an appliance or other consumer product. 

Despite a long track record of proving itself as a rugged and reliable display technology, EL displays continue to offer innovations. With continued investment in the EL platform by companies such as Planar Systems, appliance designers can find benefits not available in AMLCD and passive LCD modules.

For more information, email: info@planar.com