DISPLAYS: Low-cost touch screen relies on optical waveguides

An inexpensive optical touch-screen technology developed at RPO (Acton, Australia) and dubbed Digital Waveguide Touch is intended for applications in low-cost mobile devices, as well as for higher-cost professional touch-screen applications.

Aug 1st, 2007
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An inexpensive optical touch-screen technology developed at RPO (Acton, Australia) and dubbed Digital Waveguide Touch is intended for applications in low-cost mobile devices, as well as for higher-cost professional touch-screen applications.1 The company is hoping that this new technology will spur the adoption of touch screens in mobile devices, which has been hampered by cost and performance issues of existing resistive and capacitive touch-screen technologies.

As displays on mobile devices become larger, space for hard input keys is becoming more limited. Also, convergence means that many mobile devices are performing more than one function (such as a cell phone that is a multimedia player, a diary, and a game player), necessitating more-complicated user input controls and increasing the pressure to put touch screens onto mobile devices.

It should be noted that other optical-based touch-screen technologies do exist, such as the frustrated-total-internal-reflection method being deployed on the Apple iPhone, which involves the use of a transparent screen with light rear-projected onto it. For touch sensing, a camera is placed behind the screen to detect the user interaction (the iPhone technology has “multitouch” capability).

However, most touch screens use conventional resistive or capacitive methods that use an overlay containing material layers that translate touch into a voltage or capacitance to identify the touch position. The required overlays reduce optical brightness by up to 20% and 10%, respectively, reducing battery life to obtain the needed display brightness. In addition, reflection from these overlays in an outdoor setting limits the display performance. And finally, the relatively high touch pressure required leads to scratched display screens and limits finger scrolling. Most resistive and capacitive touch screens cannot accept multiple simultaneous touch points (multitouch)-an important requirement for many applications, including game playing.

The RPO technology has no overlays, reducing reflections and preventing deleterious effects on screen brightness, picture definition, or color clarity. The waveguide technology allows simultaneous multiple touch input, and has zero-pressure touch to allow for finger scrolling and longer screen life. Manufacturing cost is also inherently lower compared to conventional touch-screen technologies, says RPO.

Free-space light beams

The technology works by distributing light from a low-power infrared semiconductor light source through several polymer optical waveguides to the bezel of a flat-panel display. The light is then projected through free space, just above the surface of the display top cover. Each light channel remains collimated and illuminates a reciprocal waveguide behind the opposing bezel on the display. Each waveguide channel is then fed independently to one or more pixels on a semiconductor light-sensor camera. Touch position is detected by the blocking of one or more light beams in each of the x and y dimensions of the display. The high spatial resolution of this technology allows the capture of “rich” touch information such as the size and shape of the object doing the touching: the technology can, for example, discern between a finger and a pen, or even between a thumb and a finger, opening up new application possibilities.

The polymer optical waveguides are manufactured through a noncontact process using photopatternable siloxane polymers, which are synthesized by a process that uses polymer deposition and photolithography tools developed for the liquid-crystal-display industry (see figure). The process allows the manufacture of sub-10-µm-resolution, telecom-grade optical waveguides that can withstand dry or humid environments at temperatures up to 200°C on large-area flexible or rigid substrates, at costs two to three orders of magnitude lower than can be achieved with inorganic-glass materials.


A touch screen contains flexible polymer optical waveguides. High-volume fabrication of the waveguide substrate will translate to low cost for mobile-display applications. (Courtesy of RPO)
Click here to enlarge image

RPO is working with several large customers and expects to see the technology deployed in the market by late 2008, says Ian Maxwell, the company’s founder and vice president of business development.

Gail Overton

REFERENCES

1. G. Atkins et al., Digital Waveguide Touch-A New Touch Screen Platform,” RPO white paper (April 19, 2007).

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