Medical Electronics Manufacturing
Magazine
MEM Article Index
Medical Electronics Manufacturing Fall 1998
Display Technologies
Electroluminescent Technology
Jennifer Davis, global marketing services manager, Planar Systems
Trends in the medical electronics market show a move toward higher data density and an increase in portable, multifunction, and home healthcare equipment. These trends demand displays that exhibit the characteristics of high contrast image, wide viewing angle, and rugged design—all characteristics of electroluminescent (EL) displays.
EL displays are emissive. The display emits its own light, and therefore does not require a backlight or ambient light in order to be viewed. These displays consist of a luminescent phosphor layer sandwiched between transparent dielectric layers and a matrix of column and row electrodes (see Figure 1). A voltage applied to row and column electrodes causes the area of intersection (a pixel) to emit light. The EL structure also allows the displays to show graphics anywhere on the screen, unlike character displays that allow only for fixed placement of numbers or text.
Figure 1. Solid-state EL display architecture creates a flat, compact, reliable, and inherently rugged display with exceptionally fast response times (< 1 microsecond).
Electroluminescent displays are the only completely solid-state devices among the dominant flat-panel technologies. In the alternating-current thin-film electroluminescent displays, all electrodes, insulation, and luminescent layers are deposited on a single sheet of glass. A circuit board containing the drive and control electronics is connected to the back of the glass, making a flat, compact, and inherently rugged assembly using well-proven manufacturing processes. This compact assembly results in small, lightweight, and rugged components that fit the requirements of portable applications.
EL displays provide at-a-glance readability. The emissive nature of the technology generates a bright, high-contrast, crisp image. Contrast is defined as the difference in brightness between the lit and unlit pixel. Because each pixel is addressable and displayed in either a totally on or off state, a typical EL display exhibits contrast ratios of 48:1.
To maximize contrast, some EL displays incorporate a dark, light-absorbing layer to capture ambient light and eliminate blooming and haloing around adjacent pixels. This proprietary manufacturing process, called integral contrast enhancement (ICE), provides outstanding image clarity and a contrast ratio more than twice as great as most passive-matrix liquid crystal displays (LCDs) by absorbing the ambient light reflecting off the components, as well as the stray emission from lit pixels (see Figure 2). This high contrast makes EL displays easy to read in environments from low light to full sunlight, making them ideal for mobile medical emergency equipment and in-home health-care apparatus.
Figure 2. EL displays that use ICE technology absorb ambient light to deliver increased brightness and contrast.
The crisp image that results from the emissive design and ICE technology, coupled with EL's response time, makes EL suitable for medical electronics devices that require sharp, readable waveforms without float or image distortion.
A Metrax GmbH portable defibrillator uses a Planar Quarter VGA EL display because of the wide viewing angle, high contrast, fast response time, and durability.
Unlike cathode ray tubes (CRTs) or other flat-panel technology, the emissive characteristics of EL displays make them readable over a wide viewing angle (greater than 160° in all directions). This is critical for developing medical devices that enable technicians or physicians to monitor diagnostics from a distance or when time does not allow for elaborate equipment setup and adjustment.
Ruggedness and Reliability
EL's solid-state construction is suitable for portable applications that demand reliability under harsh and rugged conditions of weather and wear. EL displays have no visual performance degradation in high or low temperature conditions, humidity, and altitude. EL displays perform well in temperatures from –40° to 85°C. The solid-state construction provides the high shock and vibration tolerances required by portable medical equipment.
EL displays minimize both electromagnetic interference (EMI) susceptibility and emission. Minimizing EMI emissions ensures that the display does not generate electromagnetic signals that interfere with other equipment, including the instrumentation in which the display is mounted. This assurance increases the interface options available.
The emissive, solid-state nature of EL technology allows it to be easily integrated with touch screens. Touching or pushing directly on the glass of an EL display does not sacrifice image clarity or crispness, because there are no moving parts or liquid crystal. When a design uses a touch screen, it is not necessary to leave a space between it and the display; rather, the touch screen can be laminated directly onto an EL display, thereby leaving the visual performance unaffected.
EL displays have an extremely long life and an established tradition of reliability, partly because they do not require a backlight. Planar's EL displays, for instance, are field proven to retain more than 75% of their original luminance after more than 100,000 hours of operation in extreme conditions without having to replace any subcomponents. EL displays are rated for 50,000 hours mean time between failures.
As the trend toward mobile medical equipment continues, the demand for rugged, sunlight-readable, and space-efficient displays will become increasingly great.
