An emerging reflective display technology allows product designers to match the inactive part of the display to the housing of the product. (See image above.) Because the method relies on the displacement of dyed oil within a pixel, the display can be designed to show any one of an extensive range of colors simply by changing the color of the dye. It’s even possible to match a specific shade used as an identifying shade, as in a brand color or corporate logo color.

The ColorMatch displays are being developed by Liquavista, Eindhoven, The Netherlands, a new technology company spun off from Philips Research in April 2006. The first products that will be produced are simple segmented displays, but the line will be extended to active matrix displays at a later date. Initial sizes available will be small versions for small, portable applications, but Liquavista says the technology is ultimately scalable to both smaller and larger applications, everything from watches to large flat panel displays.

The color coordinating capability of electrowetting displays is only one of the technology’s virtues. Liquavista says that the highly reflective displays deliver superior brightness in all viewing conditions, produce a paper-like readability, have a wide viewing angle, and consume significantly less power, making them an attractive alternative to other technologies, both reflective and emissive.

How it works

Using different color displays in an application can serve as a way to easily convey different types of information, as in this hypothetical example, where different colors represent different time zones.

The Liquavista displays bear a conceptual similarity to other reflective displays in that material is moved within a cell to determine whether light is reflected or absorbed. But the similarities end there.

The concept of electrowetting in general involves the application of a voltage to modify the wetting properties of a material system to shift it between either a hydrophobic or hydrophilic state. (See water droplet images.) Researchers at Philips took this concept and exploited it to create a reflective electrowetting display technology.

The principle of the display can be seen in the illustration. (See Fig. 1.) It shows an optical stack that consists of a reflecting electrode at the bottom covered by a hydrophobic insulator. Over that is a layer of dyed oil, covered by a layer of water. The whole stack is sandwiched between glass or polymer substrates.

Fig. 1. Electrowetting display principle. The optical stack (from bottom) consist of a reflective electrode, hydrophobic insulator, layer of colored oil, and a layer of water, all sandwiched between either glass or polymer substrates. In equilibrium, the colored oil forms a continuous film layer between the water and the insulator, as seen at left in (a) and (c). In this state, the pixel shows the color of the oil. When a voltage difference is applied across the insulator, the water moves toward the insulator and pushes the colored oil aside, as shown at right in (b) and (d). When the colored oil is displaced, it exposes the underlying reflecting surface. The pixel now shows the color of the reflector.

In equilibrium, the dyed oil spreads out to form a continuous flat film between the insulator and the water because this is the lowest energy state of the system. Within a typical display pixel, measuring 200 microns or less, the surface tension force is more than 1,000 times greater than the gravitational force, making the oil stable in all directions.

When a voltage difference is applied across the hydrophobic insulator, an electrostatic element is introduced into the stack, which causes the water to move toward the insulator and push the oil aside. The balance of electrostatic forces and surface tension determines the degree to which the water moves the oil aside.

In this fashion, the optical properties of the stack can be continuously tuned between the colored OFF state and the white ON state, provided the pixel is small enough for the human eye to average its parts.

The images below the illustration show a typical oil retraction obtained for a group of pixels measuring 160 sq. microns. The oil retraction is sufficient to display about 80 percent of the pixel area as white space. Part of the electrode has been omitted in the upper right corner of each pixel to control oil motion. This ensures that all the oil moves to the same corner in each pixel, improving pixel homogeneity and display uniformity.

Performance

Given that the color of the display is based on the color of the dye used, virtually any color display is achievable.

The electrowetting reflective display delivers a number of attractive performance features. According to Liquavista, the displays offer superior brightness than other reflective technologies across a wide viewing angle. When the full-color versions become available, they will be much brighter than LCD counterparts because the electrowetting displays won’t require the polarizers that significantly reduce the light from LCDs.

The company says that the contrast ratio of an electrowetting display is also advantageous, approaching that of paper. Resolution is currently about 160 dpi, with further improvements expected. As resolution increases, so will pixel switching speed. Currently, about 5 milliseconds (on/off), the speed will improve, making the displays suitable for displaying video.

The multi-layer structure of the full-color version of the electrowetting display will also deliver a broader color gamut than other technologies, providing superior color rendering, according to Liquavista.

Even with bright light shone upon it, the ColorMatch display exhibits high viewability.

Another key advantage of the electrowetting approach is significantly reduced power consumption, because its high reflectivity maximizes use of ambient light, cutting the need for power-hungry backlighting. LCDs rely heavily on the backlighting, which can consume a large percentage of the power budget, depending on the application.

“In a typical mobile phone display, for example, the Liquavista display would consume only 10 percent the power that a comparably sized LCD would,” says Mark Gostick, CEO. “To put that in context, the display in a mobile phone consumes about 25 percent of the battery budget.”

So the Liquavista screen significantly reduces the portion of the battery budget needed for the display. The reason is that the greater reflectivity of the Liquavista display makes it less dependent on backlighting.

“An LCD  has a backlight because it is not a good reflective technology,” Gostick says.

He adds that the Liquavista technology also uses less power than emissive displays, both those that are in the field and under development. “Emissive displays are inherently power hungry,” he says.

The mechanical properties of electrowetting displays are also noteworthy. One is that cell gap thickness, which can be as small as 25 microns, does not significantly affect optical performance of the display, because the electric field is applied across the insulator, not across the entire cell gap. This characteristic eliminates the need for a frame to fix the cell gap, allowing thinner modules. The insensitivity to cell gap thickness will also permit the technology to be compatible with flexible displays, minimizing distortion during bending.

Roadmap

Water droplets on hydrophobic surface demonstrate electcrowetting concept. At left is droplet without voltage applied. At right, the droplet spreads out when a voltage difference is applied between the electrode in the water and a sub-surface electrode.

Liquavista will deploy its electrowetting display technology across three different product lines, each with a different time schedule.

The ColorMatch display will be deployed first. Consumer electronics products with this display should hit the market in the third quarter of 2008, according to Gostick. The ColorMatch is a monochromatic display so named because it exploits the technology’s ability to customize the color of the dye to virtually any color shade desired. The first products are simple segmented displays, with active-matrix versions to follow.

Prototypes of the ColorMatch display were exhibited at the Society for Information Displays exhibition held in May in Long Beach, Calif. One of the prototypes shown was a watch with a simple, black-and-white segmented display to demonstrate the high level of contrast that is achievable. The company also showed a 6-in. active matrix version of ColorMatch to demonstrate the product’s readability and scalability.

The ColorBright platform will add color filters to the ColorMatch architecture and use a black dye as an optical switch. This will allow the architecture to provide full color in similar fashion as an LCD, only with greatly improved brightness and color performance because it won’t need the standard optical enhancement layers found on color LCDs.

The manufacturing process for the ColorBright display will be very similar to that of an LCD, which will allow the ColorBright displays to be manufactured on standard LCD production lines, keeping costs down. The technology will also employ standard active-matrix backplanes, driver ICs, and color filters. Amplitude-modulated gray scales will also be added to the line to increase the breadth of applications. This platform should also become available late in 2008, Gostick says.

The third step in the roadmap is the ColorFull display, a multi-layer architecture that will stack three monochrome layers: cyan, magenta, and yellow. This approach will be a more expensive product, due to the extra layers. However it will provide several advantages, including a greatly enhanced color gamut. This structure will also enable the creation of any color in any area of the display and deliver a brighter, more optically efficient display. The launch of this platform is still a few years away.

Applications

Watch prototype using black Liquavista display shows ability to achieve high contrast.

Gostick says that smaller displays for portable applications will be the company’s first target market, given the burgeoning market for such devices. And the high daylight viewability of the technology makes applications used outdoors the easiest targets. Products like handheld GPS terminals would be high on the list, he notes. Mobile phones would be pursued sometime later, Gostick says, because the company does not yet have the production capacity for such high-volume products.

The technology’s scalability and outdoor viewability will also make it attractive for applications such as ATMs, kiosks, and vending machines.

Gostick notes that the high-switching speed of the technology makes it suited for video in all three platforms. At some point, it can even be used to create large, flat-screen televisions.

“There is no intrinsic size limit on our technology,” Gostick says. “Because the way we handle the backplane is similar to an LCD, we should, in theory, be able to scale this up to any size an LCD can go. However, we are going to focus on small, mobile applications initially, because that is where LCD technology is weakest in terms of outdoor performance and power consumption.”

For designers of indoor devices, including small and large appliances, and consumer electronics, the appeal of the Liquavista ColorMatch display will be the ability to match the display color to a housing or control panel.

“Designers needing a display can now avoid having a grey lump in the middle of their designs,” Gostick says. “They can make the display any color they want.”

He expects that capability to be particularly attractive to designers of consumer electronics, a segment where vibrant colors are increasingly being used to differentiate products.

For more information, email: info@liquavista.com