The Quest for a Perfect Display
By Mark Frauenfelder, Wed Apr 20 08:15:00 GMT 2005
Trade-offs are inevitable. But mobile display manufacturers are trying to beat the system, God bless them.
The price of mobility is less-than-perfect acoustics, difficult user interfaces and short battery life. However, displays have problems above and beyond the normal trade-offs you get with a tiny package. The problem isn't that mobile phone displays are hard to read: they are, chiefly because they're tiny, and until roll-out displays become standard equipment, it's a problem that's not going to go away. The real problem is that there aren't any display technologies that work equally well under all lighting conditions.
Almost every mobile device today uses an LCD display. The most common kind is a transmissive display, which works by shining a fluorescent backlight through a sandwich of glass and liquid crystal material. Transmissive displays are fine for use indoors, but when you step outside in the daytime, you have to take your sunglasses off to use them.
Other types of LCD displays use the light from the sun to help them shine. A reflective display uses a frontlight. When the frontlight happens to be the sun, the resulting image is good. But in a low light condition, a reflective display has to use its built-in frontlight, and the quality is worse than a transmissive display.
The third kind of LCD is a transflective display, which uses a backlight -- just like a transmissive display -- but can also use bright outdoor light to improve readability. An increasing number of mobile phones are using transflective displays (TFT LCDs) because it offers a compromise between transmissive and reflective displays.
But compromises like this are a challenge and an opportunity to startups and labs, where researchers are working on display technologies that offer long life, low power consumption, excellent color and contrast, and sunlight readability.
A number of companies are working on bistable displays (also called e-paper). Besides requiring next to no power, bistable displays offer excellent legibility in both very bright and medium-light conditions.
With a bistable display, the only time power is required is when the image on the display is changed. Even when the power is shut off, the image remains. Today, companies such as Boston's E-Ink, SiPix in Fremont, California, Dublin's Ntera, Nemoptic in France, and ZBD in Worcester, are selling bistable displays for a variety of applications, from e-book readers to electronic shelf edge labels to wristwatches. Nemoptic has developed a 32,000-color bistable nematic display that uses a grayscale monitor combined with an LCD color filter. And Kent Displays from Ohio, which supplies its cholesteric LCD display for a Panasonic e-book reader, now has a prototype of a color display.
However, Bistable displays typically have a very slow refresh rate. That's fine for an e-book, but the current crop of bistable displays isn't suitable for mobile phones which feature games and animation yet.
Organic light-emitting diode (OLED) displays can already be found in a few phones and cameras today. Unlike LCDs, which don't produce their own source of light, OLEDs generate light when a current is applied. They're better than LCDs on some counts -- such as lower power consumption and a faster refresh rate -- but they do an even worse job than LCDs in bright sunlight, because they don't crank out enough nits to do the job. (One nit equals 1 candela per square meter. A candela is a unit of luminous intensity. The flame from a candle emits about one candela, and a 100-Watt lightbulb emits about 120 candelas.) Also, OLED pigments tend to fade over time, giving them a shorter life than LCDs.
Inorganic electroluminescent display technologies hope to bridge the LCD and the OLED worlds, with a luminescent display that doesn't fade like an OLED. A company called iFire Technologies in Toronto claims to have developed a "thick-film" fabrication process for inorganic electroluminescent displays that could scale up to make displays more cheaply than typical LCD manufacturing methods. iFire is focusing on large screen sizes for now, because the brightness -- about 100 nits -- is not yet sufficient for outdoor use. (Direct sunlight displays need about 800 nits.)
The best near-term display advance could start showing up in phones in a matter of months. Qualcomm recently acquired Iridigm, which has developed a MEMS (Micro-Electro Mechanical Systems) based interferometric display that doesn't use pigments like OLEDs do. Instead, it uses iridescence to produce colors - in the the same way a butterfly's wings give off metallic looking hues. The display uses reflected light, so it will work well in sunlight, and which reduces power consumption. (It uses a built-in light only in dark environments.) Called an iModD display, Qualcomm will go into production with it this year.