Designing Touch LCDs for Portable Devices

The enhanced functionality of portable devices has outpaced the user's ability to view information, execute commands, and easily navigate through comlicated menus. The solution is the integration of touch technology with a highly visible display.

By Bruce Devisser
Fujitsu Components America
Sunnyvale, CA

Portable electronic devices have made revolutionary changes in our daily lives. From mobile telephones to personal digital assistants (PDAs), these battery-powered marvels perform a wide range of useful applications. While telephones can get by with a numeric keypad and a few extra buttons, PDAs and related devices must convey more information and provide a more versatile control system. As a result, most rely on a liquid crystal display (LCD) screen with a touch panel to provide an intuitive, graphical man-made interface (MMI) commonly called a graphical user interface (GUI).

The trend to incorporate a touch GUI has expanded to encompass products that previously did not use an LCD and those that had LCDs but no touch interface. Examples include medical monitors, electronic control systems, and various instrumentation applications.

Typically, a popular point at which to introduce a GUI is at the inception of a new design. The evolution of a product design to a new model and the updating of a current design are also good points to consider adding the touch-GUI component. Regardless of the product stage, the design team will face many technology choices. But independent of technology, the optical-design goals of the product must remain the paramount concern.

Design for Touch

Original-equipment manufacturers (OEMs) continually strive to satisfy user demand for additional features, enhanced functionality, and ease of use. For most users, “ease of use” relates directly to the quality of the user interface. Since the primary user interface in the portable market segment is the touch display, marketers define a highly visible display as a key element. This translates into an engineering design requirement, and achieving it becomes one of the main project goals, especially considering that the LCD is often a top-cost item.

Designers select the LCD primarily based on modes of use, battery-life requirements, competitive goals relative to display appearance and performance, and—most certainly—cost. After the designers complete the lengthy and involved process of choosing the display, they must then select a touch panel. For many engineering-team members, this is a new experience and therefore a learning opportunity; for some others, it is an iteration in the process of life as a product-design engineer; and a small but growing group realize this will be an extension of the LCD criteria.

The growth of the portable-device market—as with any new or expanding product area—creates new design opportunities that engineers might not yet have experienced. This is one of the positive benefits of involvement in product design; the constantly changing set of challenges offers continuing opportunities to learn something new.

Application Requirements

Presenting adequately viewable information on a portable device’s display can be a significant challenge. High-ambient-light levels—from fluorescent or incandescent fixtures, sunlight coming through the window, reflected light from windows, or a combination of sources—can make it difficult or impossible to read a display. Whether the display is color or monochrome, marketplace acceptance depends heavily on the user’s ability to see the information on the screen.

LCD brightness levels can be increased through back and front lighting and other enhancements to achieve the desired readability. But when a touch panel is added to the LCD, the resulting optical stack can produce various effects, some of which may be interesting, but none are desirable. These typically include reduced light transmission, ambient and spot-lighting reflections, and distracting patterns produced by optical interference between the various layers.

The solution to these problems consists of choosing appropriate basic touch-panel components and adding various optical treatments that can provide significant improvement. Although any additional materials or processes inherently add cost—sometimes substantial cost—this must be weighed against the cost of product failure. The goal must be a product that performs to the designer’s requirements at a reasonable cost. Fortunately, designers can often achieve this without resorting to expensive methods.

Picking Parameters

The most popular type of touch panel for portable applications is the film-on-glass analog resistive panel. Resistive touch panels (RTPs) offer several choices of product properties, such as pen-and-finger or pen-only operation, tail length and position, and actuation force. For this discussion, we will focus on the choices that affect optical performance.

The RTP’s basic bottom-to-top-layer construction consists of a glass panel, an ITO resistive coating applied to the glass, spacer dots, an ITO resistive coating applied to the film, a flexible plastic film—usually polyethylene terephthalate (PET)—and a hardcoat applied to the film (Fig. 1). For many of these items, there are commonly available choices.

• Glass type: Normal (soda) glass or chemically strengthened (CS) glass.
• Glass thickness: Typically 0.7 or 1.1 mm for portable applications, although the available range extends from 0.55 to 1.8 mm.
• ITO: A variety of color effects are available.
• Hardcoat: Clear or anti-glare (AG) surface.

The optical transmissivity (or “transparency”) of the resulting combinations typically falls in the range of 78-82%, although higher values are available.

The glass choices are relatively simple: Thinner is better for weight and optical requirements as long as strength goals are met. Glass strength, however, is an area that is often not defined because it is difficult to accurately characterize the user environment, and glass strength ends up being evaluated empirically through user trials. Typically, LCD sizes under 4 in. use 0.7-mm glass, and other portable applications use 1.1-mm glass.

The ITO choices are not often a concern because vendor samples can be used to determine if there is any undesired coloration from the chosen product range. In the case where a specific color change is desired, it is very helpful to provide a working LCD sample to the RTP vendor. This will help the vendor select the most appropriate ITO coating to meet color goals for the specific LCD-RTP assembly.

The hardcoat is the protective finish applied to the exterior of the plastic-film layer, and there are two basic types: clear and anti-glare (AG). Clear hardcoat has a minimum effect on the LCD image, although there is always some measurable effect from any additional optical layer. Clear hardcoat has the disadvantage of first-surface glare, and it is scratched more easily than AG hardcoat. Clear hardcoat is the most prevalent choice for reflective applications because it does not scatter or diffuse the light. It is also becoming popular for transflective- and some transmissive-LCD uses.

AG hardcoat is used to reduce first-surface reflections, commonly termed glare. It is also referred to as “white shirt” or “mirror effect.” Optically, AG hardcoat scatters part of the light reflecting off the surface. The image on the OCD is also diffused, unfortunately, which the viewer perceives as reduced contrast. This effect increases significantly as the distance between the LCD surface and the RTP increases.

AG hardcoat also results in a reduced viewing angle, although this is apparent mainly at extreme viewing angles and is not a concern in most cases. AG hardcoat also provides the best resistance to scratching, which is an important consideration if a stylus is used as the primary input device. Finally, AG hardcoat can be effectively treated to embody antismudge (AS) properties. This can be valuable where oily fingerprints could build up and interfere with LCD visibility, as with automotive diagnostic tools.

Determining the Optimum Solution

Choosing the best design solution requires a consideration of several factors, so a well-defined specifications or requirements list will aid engineers immensely in making choices. Even those items not easily measured, such as “good user display readability outdoors,” are valuable criteria. No matter how much definition is provided, adequate time should be spent testing and reviewing samples with other decision-makers in the organization. No matter how well a product meets its specifications, if it does not satisfy user requirements, the design is not finished.

A good starting point is to determine up front whether a clear or AG hardcoat is required. This will save time because other more advanced choices depend upon this. A basic rule is that if reflections can realistically be avoided by positioning the device, then a clear hardcoat is the primary choice (Fig. 2). It also provides better viewing clarity than AG hardcoat for the same transmissivity value because the light is neither scattered nor diffused.

Under the Sun

Many portable electronic devices are used outdoors at least part of the time, so sunlight readability can be a major concern. Power budgets are typically very limited, so reflective and transflective LCDs are used almost exclusively, as are clear RTPs.

Recent developments in RTP materials have improved transmissivity beyond 90%, with 93% achieved in production. These gains were made possible through index matching (IM), also called anti-reflection (AR) technology.

Applying IM coatings to RTP surfaces significantly reduces internal reflections and results in improved visibility of the LCD surface (Fig. 3). The first-surface reflection from the RTP must still be minimized through positioning, but the overall improvement in usability can be quite impressive.

A few portable devices are designed for direct-sunlight readability, even if for a limited amount of time. These devices can utilize a strong backlight to achieve the high-lumen output required for sunlight readability, and therefore can use a circularly polarized (CP) RTP for optimum performance. The structure of the CP filter eliminates most external light reflection, has an AG hardcoat, and allows nearly 80% of the transmissive-mode LCD light to pass through, resulting in a clear display regardless of sun position.

Other Directions

The future promises further advances in materials and even higher performance. Weight and cost are always primary factors for portable devices, and emerging technologies may help designers create products that are lighter and less expensive.

Plastic technology has been in use for a relatively short period of time—although it can be found in several applications—so it has not yet developed enough to fulfill expectations. The plastic needs better transmissivity and durability, which will require more materials development.

One of the alternative plastic solutions is the film-on-film touch panel, which eliminates the relatively thick, plastic, back-panel support sheet. The LCD surface glass is used for support instead. Normally, this would be unacceptable because the touch pressure would alter the liquid-crystal layer thickness, which would change the image. LCDs can be made with an internal structure to prevent this deformation, which mades LCD glass support practical.

Also under development is surface-acoustic-wave (SAW) technology for portable devices. Previously only suited to stationary large-screen applications and comparatively quite expensive, Fujitsu Laboratories has announced the development of a miniaturized SAW-technology touch panel targeting the portable-device market (Fig. 4).

The portable-electronic- device market is changing rapidly as some products merge their functions (mobile telephones and PDAs) and new categories appear (electronic books). One fact remains certain: Touch input will remain a key factor in the user interface of these devices, especially as their displays become more sophisticated. Advances in touch-input technology are providing designers with a growing range of choices. As a result, we can expect to see smaller and lighter devices that work longer on better batteries and are also easier to view and use. We can expect end users to reward these improved designs by making them successful in the marketplace.