Less is more

Less is moreThe DisplayTechnology Alliance recently held aworkshop onminiature displaysat CRL.Paul Gregg took along his magnifying glass to see what was happening     Tech specs… Densitron’s Personal Monitor, a lightweight, head mounted LCD, projects a high resolution colour video image directly on to the retina of the eye.
Sub-25mm LCDs are set to take advantage of IC production economies at the trailing edge, and find their way into consumer products in increasing quantities.
“The need to communicate with others, especially in groups, is leading to the commercial development of LCDs for use in personal mobile products,” said David Mentley of market research company, Stanford Resources. He cites projectors as an example: “Projectors are the cheapest way for groups to communicate.”
Miniature display production will only take off, and prices consequently fall, when a mass market application is found. Otherwise they will remain high cost devices for military and professional use. “The consumer market is the only one that will support millions of display units,” said Mentley.
Mentley was talking at a workshop meeting of The Display Technology Alliance, held recently at CRL near London, which discussed the status and application of miniature displays. The Display Technology Alliance comprises 20 organisations that are undertaking three-year collaborative projects supported by the government as part of the Technology Foresight initiative. Mentley said that the miniature LCD market, which is worth around $250m now, is expected to rise to $400m by 2004.
Polysilicon thin film transistor (TFT) active matrix technology is expected to open up this market. Polysilicon TFT screens are used in LC projection systems, and Piero Migliorato, who is a researcher at Cambridge University, said that polysilicon dominated the market for TFT LCD light valves in projectors (light valve size 33mm). But strong competition was developing in the use of single crystal silicon LCDs (light valve size 18mm). The control matrix of this kind of display is made using standard semiconductor techniques rather than depositing transistors on glass to make TFT types.
These single crystal displays could help transform mobile phones into personal communications systems, the development of miniature active matrix LCDs, measuring 4 x 5mm and used with a lens, will enable mobile phone makers to incorporate virtual displays into their phones. The user can then access large amounts of information, such as video, financial data and timetables.
Christophe Cugge of Motorola Semiconductors, whose company is developing these displays, said: “Imagine the power of having virtual, full colour 14in. images at your fingertips. Consider the impact these displays will have on today’s handheld mass market products, such as smart phones, pagers, smartcard readers, portable Web browsers and the like.”
As well as integrating miniature displays into products, they can be used in standalone mini-monitors, similar in function to virtual reality headsets, but not obscuring the whole field of view – miniature head-up displays.
A lightweight, head-mounted LCD that clips onto a pair of spectacles was demonstrated by Densitron Software’s managing director, Ivan Foldvari. The Personal Monitor projects a high resolution colour video image that appears in the viewer’s line of sight in a viewing angle that is comparable to viewing a 26in. monitor from two metres away.
The device is a monoscopic biocular display with a relatively narrow field of view. It can receive signals from any video source. These signals are converted in a separate controller unit which feeds the monitor block. This contains the display, a backlight and a driver for the LCD. A system of lenses and mirrors then projects the display image onto the retina of the eye.
Ferroelectric liquid crystal displays (FLCDs) were also a subject for discussion at the workshop.
The advantages of FLCDs over conventional active matrix twisted nematic displays has increasingly led to their use in miniature applications, where high resolution and fast speed are essential and can be used to get a grey scale from this inherently bistable technology.
Devices presently being developed are based upon single crystal silicon backplanes. They are similar to more conventional active matrix displays, except that the active matrix is formed on ordinary single crystal silicon chips with the FLCD built on top. Paul Surguy of display research company, MicroPix, said that FLCDs can be written with very high frame-rates, so that it is possible to produce colour on the display by using the colour sequential technique, rather than by using colour filters.
In this technique, colour is produced by writing a frame containing just the red parts of the picture and then flashing a red light (usually a LED), and then writing a frame of green information and flashing a green light, and finally a frame of blue information and flashing a blue light.
This is done at a high enough frame rate, so that the observer does not perceive flicker. As there are no colour filters, these displays are generally more light efficient, and give higher perceived resolution as the colour is not split into sub-pixels.
The same colour sequential principle can be applied to produce projection displays, where the flashing LEDs are replaced by a colour wheel synchronised to the display, and illuminated by a projection lamp. This colour wheel technology is already used in some projectors based on Texas Instruments’ digital micro-mirror device.
Since these silicon backplane displays are limited by the size and cost of silicon wafers that can be made with reasonable yield at reasonable costs, this technology is restricted to miniature displays, which can be used for projection and head mounted applications. Another advantage of FLCD is their low power capability. Once the image is written onto an FLCD, no power is required to maintain the picture – useful for graphical displays including timetables.

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