Medical Device Link.

Originally Published MEM Fall 2004

DISPLAY TECHNOLOGY

Durable new displays that can be read in any light are built for easy integration.

Dave Hagan

For virtually every medical equipment application, the healthcare industry wants displays that are bright, crisp, and easy to read. Medical monitors and other instruments typically are not portable—at least not until recently. But new applications, including devices such as heart defibrillators, endoscopes, portable glucose monitors, and ultrasound imaging equipment, are incorporating displays that are truly designed for wireless, on-the-go utility.

Medical displays such as the one on this pump must be easy to read. Photo courtesy of HARVARD CLINICAL TECHNOLOGY (click to enlarge)

One recent trend in medical product applications is to integrate color displays and video functionality into mobile devices. This functionality allows medical practitioners to share visual patient information, such as charts, files, and color ultrasound images, and to view information anywhere. Video capability in devices lets a physician page through and review a series of files, and it provides access to full-motion video or still images. Video technology also makes it possible that, one day soon, healthcare providers will carry digitized and magnifiable x-ray images, computed tomography scans, and magnetic resonance images in a handheld device resembling a personal digital assistant (PDA).

Many forward-looking devices like these are on the drawing board. However, to make these innovations a reality, engineers must decide which type of display to use in each application. This decision is crucial, because the display is the key interface between the human user and the machine. It is the place where sophisticated data are made manifest for use. The visual quality of those data, whether video or other, affects user satisfaction with the device. But more important, it can make the difference between a correct and an incorrect interpretation of physiological condition, which ultimately could affect the health and well-being of the patient.

It is imperative, therefore, that design engineers take ample time to understand the display options available. Their choice affects not only the total bill of materials and how fast a product gets to market, but also how well the product is received by the end-user.

Designers should consider several important application questions at the outset of a development project, before deciding which displays to incorporate into their products. Matters for evaluation include the need for sunlight viewability, power consumption, ruggedness, and product size. By assessing these options, engineers are better able to compare different display components on the market. Few displays offer high reliability, high contrast ratios, and high brightness, along with shock resistance, an extensive operating-temperature range, and long backlight life, in a single package.

Transflective Displays

Images displayed on a portable device usually must be discernible both indoors and out. When this is so, the type of display panel selected is critical.

A new option designed to satisfy the need for universal viewability is known as transflective technology. It allows a display to be read effectively in any environment, from a quite dark or darkened area, such as an x-ray laboratory or a nighttime accident scene attended by an emergency medical technician, to an extremely well-lighted operating theater or area exposed to full sunlight.

A transflective liquid-crystal display (LCD) encompasses features of both reflective and transmissive display technologies. Transmissive displays rely entirely on a backlight for viewability, while the readability of reflective products depends completely upon the reflection of ambient light from the backplane of the LCD. Transflective technology, however, works with both reflected and transmitted light, so a display based on that principle is viewable in all conditions. Backlighted by white light-emitting diodes (LEDs), transflective products can be engineered for increased durability, allowing them to handle shock and vibration.

A key advantage of transflective display devices, beyond their viewability in all ambient light conditions, is their ability to use naturally available brightness more fully and thus minimize power consumption. Transflective displays provide such viewability without compromising the response speed response or the contrast ratio.

Products of this type are typically quarter video graphics array (QVGA; 320 x RGB x 240 pixels) and generally are made for the PDA industry. How-ever, these displays can be adapted and brightness-enhanced for medical applications. For example, a 3.5-in.-diagonal QVGA display is quite suitable for showing a good deal of information. Just as it works well in a PDA application for showing personal addresses and e-mail messages, it can work well in medical devices for exhibiting color imagery as well as text.

The primary consumer of power in any portable device is the display backlight. In many portable instruments, the backlight illumination is provided by cold-cathode fluorescent tubes (CCFTs), while newer products tend to use white LEDs for illumination.

Adding brightness to a transmissive display may improve display visibility, but it is at the cost of increased power consumption. The only way to address this greater power consumption, having assumed it as necessary, is either to accept a short operating interval between battery recharges or to add weight to the device in the form of more or larger batteries. Adding weight is a poor option for a portable product. Rather, it is tolerable only in movable products classed as transportable. By choosing a display technology that allows a portable device to be used in any kind of ambient light, engineers can avoid alternatives that entail unacceptable power-consumption compromises.

Emissive Displays

Another option available to engineers is the emissive display. Emissive types of displays are now used in portable medical devices. This category includes organic light-emitting diode (OLED) displays. OLEDs offer superlative viewing cones and the thinnest of displays; however, they do not perform better than transflective LCDs in terms of readability in a bright ambient, continuous operational lifetime, or power efficiency with full-motion video.

The emission level of such a display can be increased to compensate for too-bright ambient light, but at the cost of display lifetime and, again, greater power consumption. For many portable medical applications, weight and viewability are critical. These devices require display components that are not particularly susceptible to ambient brightness and that do not require additional illumination and power to perform dependably.

Length of Service

Display lifetime is an important evaluative factor. Thin-film-transistor (TFT) transflective LCDs compare favorably in terms of lifetime with other display technologies. The display electronics are rated at 50,000 hours mean time before failure (MTBF).

The effective lifetime of a TFT LCD is related to its backlight. The backlight's utilization factor determines the lifetime of the backlight, which is defined as the operational time it takes for the backlight to age to half brightness. Engineers using a 3.5-in. TFT with a white LED backlight can realize product lifetimes of more than 10,000 hours. This is 5 to 10 times the lifetime of an equivalent OLED solution.

Regardless of backlight lifetime, the fact is that, as an LCD with an LED backlight ages, the uniformity of the display's color rendition and image sharpness continues to be as good as it was when the display was new. Emissive displays, by contrast, age most rapidly in the localized screen areas where they most frequently present information. Their lifetime, depending upon the type of display and the driving conditions, may be as short as 2000, or even 1000, hours.

At that point in the emissive display's operational life, effects of the differential aging of the blue versus the red and the green phosphors are visible. Images are burned into the phosphors—a phenomenon called burn-in or ghosting. If engineers were to write a uniform white screen to the display, they might see an afterimage where the application's main menu, for instance, had previously been written. These afterimages—which are potentially serious trouble for a medical application where quality of image is critical—occur typically with emissive, but not liquid-crystal, displays.

Durability

Medical instruments must be designed for reliable long-term use. Healthcare providers cannot afford to replace products constantly or to have them fail while in use during patient care. Display manufacturers who focus on the medical market recognize this need. They have improved display technology while also basing product development on standardized electrical and mechanical platforms.

Just as a PDA or cell phone must have an extremely rugged display, so must portable medical devices. Medical practitioners are often in a rush, and while hurrying from one place to another can easily drop an instrument. In response to this challenge, medical equipment display manufacturers have developed durable products that are capable of withstanding shock and vibration.

The first step in ruggedizing an LCD display is to eliminate the CCFT backlight in favor of an LED backlight. White LEDs are inherently more rugged than CCFTs because there is nothing in the backlight assembly to break. The second step involves the glass panel. This panel can break as a result of deflection or because an impact force is applied to a corner of the glass sheet.

Manufacturers cannot themselves make the LCD panels stiffer to withstand any degree of deflection force, but an integrator can, by paying attention to the overall stiffness of the assembly. A touch screen, where appropriate, can add rigidity to the panel. Many resistive touch panels have a touch screen fabricated from a combination of plastic and glass, with the plastic layer on the outside protecting the display and enhancing durability.

LCD breakage generally is not a problem. Still, it falls to the designer to ensure that the panel's installation is stiff and that the display is positioned securely within the enclosure so that it cannot move. If the display were to be able to shift when the device was dropped, then a corner could strike the inside of the enclosure and cause the panel to break. No manufacturer specifies durability that can withstand a six-story drop, of course, but cell phones have been known to survive a fall of that magnitude with the battery dislodged but the LCD still working perfectly.

Touch Screen Integration

In many medical applications, touch screens are important. Some display manufacturers integrate resistive touch screens with their displays. This allows users to interact with the instrument through the display rather than through a keyboard—convenient for medical practitioners on the go. Touch screens can be used effectively with medical displays because these displays are engineered to offer superior contrast and brightness. Touch screen integration does introduce some degradation of brightness and contrast; however, it is minor and does not adversely affect end-product performance.

The ease with which a display can be integrated with other components is also important. Some display manufacturers provide further integration through the use of microcontrollers and system-on-chip (SoC) devices (see the sidebar "System-on-Chip Devices: Finding a Brain for a Portable Medical Device"). These displays can connect to SoCs without glue logic. The SoC might drive the touch screen and handle not only display processing but also text, graphics, and other computational jobs simultaneously. SoCs typically incorporate a hardy quantity of input and output, as well as several integrated onboard functions such as universal serial bus communication, universal asynchronous receiver-transmitters, timers, and interrupt processing.

LCD and SoC microprocessor technologies have become mainstays of portable instrument design. Coupling a highly reliable, bright, dependably viewable color LCD with an SoC provides medical equipment designers with a powerful foundation on which to base development of a more portable set of products.

Conclusion

Next-generation handheld medical devices will require high-resolution color displays that are bright, lightweight, less power-consuming, and easily integrated with other components. By understanding the display options available, design engineers can choose a display that meets design and application requirements, reduces the bill of materials, and delivers high user satisfaction.

Dave Hagan is senior product manager, display business unit, at Sharp Microelectronics of the Americas (Camas, WA). He can be reached at 360-834-2500.

Copyright ©2004 Medical Electronics Manufacturing