With the development of digital signal processing (DSP) technology, medical monitors have evolved from the old analog versions, into digital multiparameter monitors that can track many different vital signs at once, with the added advantages of miniaturization and portability. Devices such as defibrillators, cardiac monitors, vital signs monitors, and full-clinical-parameter bedside and portable monitors require many capabilities to be integrated into a single display and demand that the information displayed be clear, crisp, and easily readable. Key to their effectiveness is the liquid crystal display (LCD) that displays the critical information they provide.

Medical devices present a special set of requirements for the LCD. The LCD needs to provide high contrast, high resolution images, and data, as well as resistance to glare in high-ambient-light environments such as ambulatory surgical units, emergency rooms, operating rooms, and intensive care units. Portable medical devices must provide a bright, long lasting image when powered by batteries. Displays incorporated into devices used in emergency vehicles must be able to withstand extreme temperatures, shock, and vibration.

Active Matrix TFT (thin-film-transistor) LCDs can handle the high level of content required by medical displays. They provide integrated, all-in-one monitoring capabilities that can show waveforms, color-coded prioritized alarms and menu functions with online help features, while providing exceptionally high contrast, color saturation, and luminance, as well as a range of performance enhancements.

Improving the human-machine interface

TFT LCDs have become the defacto standard in medical displays, displacing the large, bulky, heat-generating CRT (cathode ray tube) monitors that were used for many years. Most medical diagnostic devices utilize TFT LCDs in the size range of 6.5 in. to 19 in. diagonal, and these displays have been incorporating a number of technological advancements that enable them to display more information and make it easier for medical professionals to read, view, and interpret.

Today's medical monitor LCDs are thinner, more lightweight and brighter than ever before. Increased brightness - up to 1,500 cd/m² (or “nits”) - with higher contrast ratios enable displays to provide better legibility and image clarity in high ambient light environments where surface reflections can degrade contrast by reflecting the ambient light. Another tool is an anti-reflective (AR) surface. This is a vacuum deposited thin-film, anti-reflective optical coating on the front polarizer for reducing specular reflections from the surfaces of optical substrates that dramatically improves high-ambient-light legibility and image clarity. The AR coating reduces the surface reflectivity of the LCD panel to ~1%; compared to the 6 to 12% reflectance offered by a normal anti-glare surface, without the need for a high-power backlight, no increase in heat dissipation, and no decrease in luminance or contrast. There are a number of anti-reflective optical coatings that can be applied by Value-Added Resellers (VARs) after the LCD has been manufactured and shipped. However, an AR coating that is integrated into the LCD at the factory where it is manufactured is generally considered to be more reliable.

A super high-bright LCD in the range of 1,000-1,500 nits with a 700:1 contrast ratio, plus AR surface coating will remain clearly legible over a wide operating temperature range (-20° - 70°C) in all high-ambient-light environments from the Emergency Room to the OR.

Another technological advance that increases ease of readability is wide viewing angles. A technology known as In-Plane Switching, or IPS, aligns the liquid crystal cells in a horizontal direction so that the crystal molecules move parallel to the panel plane instead of perpendicular to it, reducing the amount of light scattering in the matrix, which can achieve a viewing angle of 170°. And, since accurate color is critical in medical displays, IPS also solves the color shifting problems that commonly exist in traditional twisted nematic (TN) displays. TN displays, the most common consumer display type, suffer from limited viewing angles, and colors can shift when viewed off-perpendicular. In the vertical direction, colors will shift so much that they will invert (or reverse) past a certain angle. IPS ensures that colors will be accurate when viewed from any angle.

Putting more Information on the screen

With all of the complex information that medical monitors need to display, there can be a problem with getting it all on a single screen in a legible format. For example, if menu or information bars take up one half of the screen, there may not be enough room left on the other half of the screen for waveforms tracking heartbeat, blood pressure, pulse oximetry, respiration, etc. This problem can be solved by a wide format display having a 16:9 ratio (the same aspect ratio common to HDTVs and the new standard for laptop and desktop PC monitors) that expands the usable display area, meets the high definition display format, and allows more information to be displayed on the screen. New WXGA/WVGA wide format TFT displays are available in sizes from 4.3 in. to 17.5 in., with resolutions of 800 × 480 on small screens to 1,280 × 800 on the larger screens. These will become more and more prevalent in the next generation of medical equipment.

LCDs with touch screens are becoming more and more popular in many applications, including medical displays. A touch-screen display provides the ability for the user to make choices by simply touching icons or graphical buttons on the screen. There are several technologies available, including 4-, 5- and 8-wire resistive, surface capacitive, projected capacitive, and surface acoustic wave (or SAW). Optrex can integrate a standard 4-wire resistive touch screen at the factory. Resistive touch screens offer a number of features, including high reliability and durability, high accuracy, multiple input modes such as finger or stylus, high transmittance (>80%), and a wide operating temperature range.

Optrex also has developed a surface-capacitive touch switch based on technology experience gained in the smart phone market that detects variations in position and command and responds to fingertip input control without requiring the application of any pressure. Reduced reflectivity and higher transparency of the image displayed through the touch panel results in improved visibility and allows for a thinner touch screen in front of the LCD (as thin as 0.5 mm - 0.7 mm thick in the viewing area). These capacitive touch screens provide superior optical performance with excellent endurance and reliability over long periods of extensive use, and are the preferred choice for highly demanding applications such as medical displays. This new technology is being integrated into smaller displays at present (3.0 in. and 3.5 in. diagonal), with larger displays (5.7 in. and 6.5 in.) under development. Ultimately, in the next generation of medical equipment, touch screen technology will be increasingly integrated into everything from bedside patient monitors to the thermostats controlling the temperature in hospital rooms.

Dealing with EMI

The old analog monitors had a number of restrictions, including sensitivity to electrical interference. Electromagnetic interference (or EMI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit.

A traditional CMOS interface requires a minimum of six data lines for each R, G, B primary color, for a total of 18 data lines. Each line will be driven by a 3.3 V voltage. It is a huge voltage swing, and generates problems with EMI. To address this problem, LCD manufacturers are offering displays with Low Voltage Differential Signaling (LVDS) interfaces, a low-power, low-noise differential technology for high speed transmission. Optimized for point-to-point configurations in telecom, datacom, peripherals, and displays, LVDS delivers the bandwidth necessary for driving large data rates over PCB and cable with low power and low EMI.

A complementary technique is Reduced-Swing Differential Signaling (RSDS), integrated inside the display as the interface between the timing controller and the column drivers. RSDS is a signaling standard that defines the output characteristics of a transmitter and inputs of a receiver along with the protocol for a chip-to-chip interface between flat panel timing controllers and column drivers. The RSDS bus provides reduced bus width, low power dissipation, low EMI generation, high noise rejection (to maintain signal image) and high throughput (which enables high-resolution display).

New developments in backlighting LCDs

LCDs need to be backlit, and the quality of the displayed image is heavily dependent on the backlighting unit (BLU) and the power supply driving it. For many years, TFT LCDs were backlit by cold cathode fluorescent lamps (CCFLs) powered by DC-AC inverters. But in the last few years, more and more TFT LCDs are using HB (high brightness) LED-based BLUs powered by LED driver circuits.

HB-LED backlights offer numerous advantages over CCFL BLUs, including no warm-up time at low temperatures, lower power consumption, higher dimming ratios, and because they do not require high-voltage and high-frequency inverter circuits, significantly reduced electro-magnetic interference (EMI). And since LED backlights contain no mercury, these displays offer a more environmentally friendly or “green” option. LEDs are less fragile and more reliable than CCFLs, and won't degrade if operated for long periods of time, such as the 24/7 operation demanded of a bedside patient monitor. Average lifetime is 60,000 hours, and the wide range dimming capability of LEDs offers a valuable advantage for medical displays, which must be legible in bright daylight as well as in the subdued lighting environment of nighttime operation. That is why more and more LCD manufacturers such as Optrex are offering a wide range of LED-backlit TFT LCDs in display sizes ranging from 2.0- to 15.0- in. diagonal, with a variety of resolutions from which to choose.

Another advancement that been integrated into LED-backlit TFT LCDs by Optrex is Natural Color Matrix (NCM) technology, a patented color transformation algorithm implemented in the hardware that provides real-time on-the-fly processing to precisely match the colors specified in a data source for exceptionally vivid color reproduction. The difference NCM brings to LCD applications is immediately apparent to the eye in the vivid color renditions provided by the technology, as well as crisp text and artifact-free motion video.