IT TRENDS
Fourth wave in displays
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Brighter, lighter, monitors; giant TV screens that you can roll up; see-through walls; wallet photos that move. Anand Parthasarathy looks at the technology that makes these possible.
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Inkjet process was used to deposit the organic layers during fabrication of the world's largest OLED-based displayed screen.
FIRST THERE was the Cathode Ray Tube or CRT — still the technology behind the vast majority of computer monitors and television screens. Streams of electrons are accelerated by high voltage anodes, formed into three coloured beams by focusing electrodes and projected on a phosphorescent screen where they fuse to create the moving picture.
Then came the Liquid Crystal Display or LCD: Two sheets of polarising material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them.
Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light. For intense colour, LCD displays use what are known as Thin Film Transistors (TFT), where each sub-pixel or coloured dot in the display has its own active controlling transistor.
With the coming of home theatre systems, customers demanded larger and brighter screens up to 60 inches diagonally. The need was met by the Plasma display, an emissive flat panel, where light is created by phosphors excited by a plasma discharge between two flat panels of glass. Plasma displays use twice as much power as a comparable CRT television and were much heavier than the LCD screens.
None of these three technologies was perfect or easily scalable. Which is why the imaging industry in recent months is frantically dusting a technology that two scientists at Kodak first demonstrated in 1987: generating light with certain organic crystals, which had the property of electroluminescence. In other words, they brightened when excited by a jolt of electricity. However the very high excitation voltage requiring more than 100 volts — inhibited commercial development for many years. Today it has morphed into the Organic Light Emitting Diode (OLED) — an electronic device made by placing a series of organic thin films between two conductors.
When electrical current is applied, a bright light is emitted. OLED technology produces self-luminous displays that, unlike LCDs, do not require backlighting. These properties result in thin, very compact displays. The displays also have a wide viewing angle, up to 160 degrees and require very little power: between 2 and 10 volts.
Like LCDs, OLEDS come in both passive and active types. Passive-matrix displays are created by depositing the electrode material in a matrix of rows and columns, the OLED being formed at the intersection of a row and a column. The video image is obtained by rapid scanning through all rows and columns in about 1/60 of a second. Active matrix displays use thin film transistor (TFT) technology, with each OLED controlled by two transistors. For this reason the light output of each pixel is controlled continuously, instead of being pulsed once every refresh cycle, leading to brighter, sharper pictures.
While many companies have achieved prototypes of OLED displays, marketable products have come in fairly small sizes. Kodak and Sanyo produced the world's first active OLED displays for a digital camera — the Kodak EasyShare LS633 — last year, using 2.2 inch screens. Samsung and NEC are using the Kodak technology for passive OLED mobile phone displays.
But large home theatre-sized TV displays remained difficult to realize in OLED, inspite of the promise of drastically improved viewing experience. All that changed in May this year, when the Japan-based Seiko Epson, unveiled the world's largest OLED display to date — a 40-inch OLED TV.
The major technical challenge was in forming the organic layers on large sized TFT substrates. It was perhaps logical that a company better known for its range of inkjet printers should be the first to realize a large format OLED display. Its patented `micro piezo' technology of inkjet printing came in handy to `print' the OLED display elements on to the TFT by delivering the droplets to the precise position in the matrix and then applying a drying solvent. The beauty of this method is that OLED displays can now be essentially mass-produced and `printed' on a variety of substrates including plastic.
This opens the exciting possibility that if the plastic is flexible, one can, in theory, create TV screens that can be rolled up and carried away or hung on the wall like a picture. Indeed this is just one of the ideas that Epson's engineers are working on.
Other way-out ideas include `moving pictures' — that wallet photo of your family could well be a television-like micro-chunk of video. And entire walls can be fitted with ultra thin displays, much as one would affix wallpaper. A few cameras strategically placed, could then capture the scene outside and beam it on the wall to provide the illusion that the wall had suddenly become `see through'.
Bizarre? Perhaps, yes but clearly achievable, given the current road map of OLED. Epson is one of a handful of companies, which are racing to convert that large screen prototype into a mass-market TV product by 2007. The date has its logic; it will be nicely timed to ride the expected surge in television business before the 2008 Beijing Olympics. And even as the world prepares for this year's Olympic Games, hundreds of scientists in dozens of imaging companies are already looking four years ahead, in quest of their own version of Citius, Altius, Fortius — faster, higher, stronger ... or as they like to say in the display business: thinner, brighter, lighter.
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