A Medical Electronics Manufacturing Fall 1996 Feature
Advances in display technologies present medical device designers with numerous options.
Electronic displays can be thought of as the essential links between equipment and humans, supplying information, providing alarms, and indicating patient and equipment status. They can give access to historical information ranging from physiological data to payment records.
Careful implementation of the display function ensures that there are always enough details available for prompt and appropriate action, without introducing errors, confusion, or delays. This implementation involves both hardware and software issues, as well as important human factors issues, such as the capabilities of human vision and the ability of the viewer to detect and assimilate information while performing other tasks.
This field emission display allows complete 180° viewing angles both horizontally and vertically. Photo courtesy of PixTech, Santa Clara, CA.
Increasingly varied and specialized requirements have led to rapid developments in all aspects of displays, and have also contributed to the significant growth of the display hardware manufacturing industry.
The annual conference and exhibition of the Society for Information Display (SID), which brings together a wide range of important display producers, offers a convenient overview of the state of the industry and of display technology. Like the industry, the trade show is growing in both importance and size. The 1996 event in San Diego was the largest in its 27-year history; overflowing its usual venue, it was held for the first time in the city's convention center.
The conference revealed that recent technologies, such as flat-panel displays (FPDs), are slowly beginning to compete with cathode-ray tubes (CRTs), which have dominated the display industry in the past, and that several technologies now in development may soon produce major changes in display capability.
The SID conference also highlighted the increasing number of medical uses for displays, such as imaging for remote diagnostics. A discussion of these medical applications naturally begins with a look at displays in general, because the technology and components used for medical equipment displays are so similar to those used for other applications, such as electronic test equipment and vehicular cockpits. This similarity has benefited medical device designers, because groups such as the Advanced Research Projects Agency and the television and automotive industries have spent enormous research resources on display technology, ensuring rapid advances and the availability of low-cost components.
Basic Display Types
There are two fundamentally different types of displays: analog devices, such as CRTs, and digital devices, such as liquid crystal displays (LCDs), which are used in notebook computers and miniature televisions.
Of course, digital devices are preferred for displaying images from digital sources, and analog devices are preferred for analog image sources. The quality of the images produced by these displays is judged using factors such as definition, stability, and noise. The simple rule is that the best device for a particular application is the one that adds or subtracts nothing from the information that is to be viewed.
One quality factor, resolution, has different meanings depending on display type. For analog displays, resolution is the capability of the tube and system to show small details; if the space between data points is too small, adjacent portions of the image will blur together. Digital displays have fixed resolutions. Their screens are matrices of dots arranged in rows and columns; unlike analog displays, digital ones have no mechanism to select the space between dots. Thus, resolution is stated as the number of dots, or pixels, on the screen.
Display hardware can be grouped in four basic categories: alphanumeric, status, waveform, and imaging. Multipurpose computer-controlled displays designed to perform several display functions have been recently developed as well.
Numeric readouts were the first displays developed. Displays for electronic equipment began with pilot lights, simple pointers, and dials. Since that time, miniaturization of the basic components has made possible a wider variety of ways to indicate equipment status and function.
Even better utilization of space came with multifunction displays, in which the appearance of indicators could be programmed on small television-like screens, and then later on various types of flat-panel LCDs.
Developments in CRTs
Television sets have always used CRTs for displaying pictures. In the CRT, a scanning electron beam creates a glowing image on the phosphor screen. Standard component technologies have grown to support huge production volumes of television sets, making the CRT the most widely used and least expensive of the display methods.
When CRTs were first introduced, manufacturers often tried to build their own displays, buying the CRT and all of the other components that were needed to provide the tube with signal, power, and a stable operating environment. However, most firms quickly learned that building displays requires specialized design expertise. OEM suppliers also learned that it was profitable to integrate CRTs with electronics. These integrated systems, known as CRT monitors, could meet each customer's needs with only minor adjustments. Today, LCD monitors and even plasma display panel (PDP) monitors are beginning to enter specialized markets, replacing CRTs for applications that require thin profiles.
As miniaturization of electronic equipment continues, various approaches for reducing the interior space taken up by the display components are developing, such as integration of displays and controls, use of wide-deflection-angle CRTs or thin panels, and use of software that allows a display to be instantly changed to meet the task at hand.
There is increasing specialization in the use of CRT displays, such as in medical imaging. Products that use CRTs continue to be refined, with manufacturers adding features that simplify setup and operation of their products, reduce power consumption, or reduce manufacturing costs.
"We are building second-generation monitors," says Ken Compton, general manager of display monitor operations for OEM-supplier Clinton Electronics Corp. (Loves Park, IL). According to Compton, "These models have automatic power reduction to around 5 W in standby mode, and a user-operable button that restores the monitor to original factory settings."
Monitor designers are also pursuing new applications, such as products that are sunlight-readable.
Displays are also increasingly being designed to perform more than one function. For example, a waveform monitor or image display can also show users a list of equipment functions, or provide an instantly available and up-to-date electronic manual.
Another new development in CRTs is shutter-based sequential color. In these displays, electronic shutters are opened or closed rapidly to block viewing of an image. When combined with color filters, the shutters allow rapid sequences of red, blue, and green images, producing the visual effect of a full color image. The advantage of shutter technology is the ability to use a monochrome (white screen) CRT as the image source, avoiding the visible patterns that are created with discrete-dot color screens. This is particularly advantageous for small color screens, which can show small text and fine lines without degradation, and for tiny head-mounted CRT displays.
Other Popular Display Types
Rapidly gaining popularity are the FPDs, slim devices that offer some, but not all, of the picture quality of CRTs.
Of the FPDs, the active-matrix LCD (AMLCD) used in high-end laptop computers is the best known. AMLCDs and the less-expensive passive-matrix LCDs are backlit displays; they contain a white backlight that is always on, with the LCD element acting as a shutter. According to Tom Holzel, vice president of marketing and sales at PixTech (Santa Clara, CA), about 90% of all FPDs are composed of backlit LCDs.
Another type of LCD is the reflective, or passive, LCD used for wristwatches, gasoline pumps, and calculators. These are very inexpensive, but are too slow to show real-time motion and do not display well in low light.
All LCDs exhibit restricted viewing angles and temperature sensitivity and are more expensive than CRTs. However, flat panels can offer a medical device manufacturer a significant savings in space and power consumption.
New light-emitting FPDs, such as electroluminescent, plasma, vacuum fluorescent, and field emission displays, are now gaining popularity, and may produce technological solutions to some of the problems of backlit LCDs.
CRTs for Medical Imaging. According to several manufacturers and OEM users of displays, there is an increasing demand for a broader range of CRT-based medical imaging products. Radiological imaging displays are now a preferred means for examining x-ray images, and are essential for viewing images at remote diagnostic locations.
This high-resolution monitor is suited for diagnostic imaging applications. Photo courtesy of Data Ray, Westminster, CO.
"Radiologists are coming to depend upon electronic transmission, storage, and display of images," said Dean Scheff, president of Image Systems Corp. (Hopkins, MN), which manufactures a monitor for displaying x-ray images. "The CRT still produces an image that you can't get from any other display," he says, "and it provides sunlight readability and medical imaging at a reasonable cost."
"We are starting to see growth in the market," says Peter Hart, manufacturing engineer for Kodak Health Imaging (Dallas). "We use displays of 2000 x 2500 and 1200 x 1600 pixels for x-ray images, and an 1152 x 900 pixel monitor for quality control and patient demographic information," he says. "Our biggest concern is consistency among monitors. One of our workstations may have four or five monitors; all of the images must look the same. And radiologists depend on being able to make comparisons between images that are recent and those that are older."
Electroluminescent (EL) Devices.EL displays emit light by applying voltage across electrodes, energizing a thin phosphor layer to light an individual pixel. With the advantages of wide viewing angle, low EMI, and high brightness, EL devices are used in medical monitoring equipment by a number of manufacturers. For example, Protocol Systems (Beaverton, OR) replaced LCDs with EL displays in its patientmonitor products. "The advantage of LCDs was visibility in bright sunlight," says engineer Dick Van Horn of Protocol Systems, "but the new display has better resolution and the mean time between failures is very good."
Planar America (Hillsboro, OR) is another supplier of EL devices for medical manufacturers. According to Eric Petersen, business development manager at Planar, EL devices are well suited to the needs of medical manufacturers, especially when combined with custom enhancements such as antiglare features, backlighting for sharper contrast, and increases in the viewing angle.
Head-Mounted Displays (HMDs). Once the domain of helicopter pilots and occasional virtual-reality demonstrations, HMDs are being discovered as a viable alternative to conventional monitors. At least one firm is looking at HMDs as a way to improve the view and reduce distractions for surgeons. Unlike a conventional closed-circuit television monitor, the HMD is always in view and does not require head movements to be visible. Several display technologies are contending for use in this application, including EL, polysilicon LCD, and miniature CRT and shutter combinations.
High-Resolution Digital Imaging Applications. By trading speed for extremely high pixel density, dpiX (Palo Alto, CA) has developed display panels that can approach the quality of a printed page. Although the displays do not yet support gray-scale images, the technology is being used in some high-definition image-sensing applications.
According to dpiX product manager Chi Huang, "The objective is to create an all-digital medical imaging system that does not require film. This would allow reproduction and transmission of images at any place where they were needed, without image degradation."
New Display Technologies
Several new display types have also recently emerged, promising improved display capabilities for the future. These new technologies may soon find uses in medical devices as well.
Field Emission Devices (FEDs). Although they resemble CRTs in that they use electrons striking a phosphor screen to produce light, FEDs do not use a heated cathode to produce electrons as CRTs do. The FED's electrons are emitted from hundreds of tiny points under the influence of an electric field. In most other respects, FEDs, sometimes called cold cathode devices, resemble other flat displays; displayable dots are arranged in rows and columns, and the panel structure can be thin.
FEDs are typically configured with the emitters very close to the phosphor screen. This structure enables a very low voltage to produce desired electron emissions, although special phosphors must also be used. Because they are so close to the screen, the electrons need not be focused on the screen with a special device.
An FED structure that is currently under development uses conventional CRT phosphors. In this design, the screen is placed farther from the emitters, allowing higher voltages and conventional (high-voltage) phosphors to be used. An additional element is needed for focusing the electrons on the screen. This device is expected to offer the precision of a matrix display with the saturated colors of a CRT. Because the display is emissive, no backlighting will be needed, saving power when compared to current active-matrix LCD displays.
But the relatively small screen sizes of FEDs, which are typically less than 12 in., will make them more of a competitor to active-matrix LCDs than to CRTs in the near future.
Large-Screen Projection. A technique for integrating displayed information from several sources, large-screen projection can be an effective tool for presenting information to groups of people. The variety of projection display products is growing as costs are reduced by the demand for business presentations, home theaters, wide-screen televisions, and electronic cinemas. Higher resolution makes multifunction displays more practical, since larger amounts of data and greater detail can be shown. Until recently, projectors tended to be bulky, expensive, and maintenance intensive, but advances in digital projection technologies have made it worthwhile to revisit possible large-screen display ap- plications. Projectors can accurately reproduce a desktop computer screen, in color, at addressable resolutions up to 1024 x 768 pixels, with higher resolutions in development.
This 52-sq-in. patient-monitoring display is designed for easy interpretation from a distance. Photo courtesy of Planar America, Beaverton, OR.
Polysilicon technology for projectors is offered by most manufacturers of LCD panels, allowing fabrication of more efficient driver circuits along with the display device. Their improved optical efficiency results in brighter displays than previous designs. Texas Instruments (Dallas) is using this technology to produce a digital multimirror device.
In one recent polysilicon design, the LCD is fabricated as a layer on top of a conventional integrated circuit. This device, marketed by Sarif, Inc. (Camas, WA), permits the addressing circuits as well as the display drivers to be fabricated in a single device, offering performance and cost advantages over other approaches.
Plasma Display Panels (PDPs). Large, thin screens using PDP technology are emerging as an alternative to projection. Commercial PDP products are beginning to appear in the New York Stock Exchange, where space for computer displays is at a premium. Large size is easier to obtain than high resolution; most PDPs do not exceed 480 pixels vertically.
Other Liquid Crystal Technologies. A variation on plasma display technology, plasma addressed LCDs, originally developed by Tektronix (Forest Grove, OR) and Sony Corp. (Park Ridge, NJ), demonstrate impressive image quality, although the viewing angle is somewhat constrained.
Another promising new liquid crystal technology that does not require polarizers has recently appeared for the first time in a commercial implementation by Hitachi-Raychem (Sunnyvale, CA). By scattering light rather than using polarizers to obtain "black," this new material achieves high luminous efficiency. This permits the design of efficient projection displays that can produce bright images from a relatively small 100-W source.
This is a time of rapid growth for display technologies, as end-to-end digital systems take over virtually all electronic system functions. From miniature head-mounted displays to theater-sized projectors, the new displays are notable for their brightness, stability, accuracy, and freedom from artifacts, and these are attributes that should mean improvements for medical equipment.
The medical market also imposes some special demands upon OEM suppliers for long-term reliability, life-cycle cost, and after-sale support that may not apply to display applications in other industries.
According to Petersen at Planar, suppliers of medical displays as well as their OEM customers need to understand the special business needs of the medical industry. Because the time-to-market for a medical device is typically much longer than that for a display, a manufacturer may have to upgrade a device's display even before completing the development of the device.
Changing displays after a product enters the marketplace means that the device will have to be resubmitted to FDA for market approval. Therefore, it is more important for medical device manufacturers than for other types of manufacturers to choose not only the most appropriate display for their applications, but also a display design that will continue to be available for purchase for several years.
As the technology continues to improve, medical manufacturers can expect to enjoy greater display capabilities, and as the medical uses for displays continue to increase, the needs of medical manufacturers will become better understood and served.