With the increasing role of active-matrix LCDs in medical equipment, design issues are arising that relate to the physical environment in which the equipment operates and the image quality the user demands. Other issues relate to the special requirements of LCD integration and to user requirements for the equipment's operating life span.
To complicate matters, the medical market comprises many submarkets, with requirements ranging from small, low-resolution panels in rugged and mobile medical applications to large, high-resolution, high-contrast color or monochrome panels for medical diagnostics.
The small-panel category typically encompasses 3.5-inch QVGA to 12.1-inch SVGA types. Application examples include defibrillation, cardiographic and bedside monitoring equipment. Within this category, application requirements determine which panels are used and for what purpose.
For example, defibrillators designed for emergency medical treatment, air rescue or ambulatory use are exposed to adverse environmental conditions and typically do not require panels with high resolution, high color fidelity or fast response times. The demands are relatively simple because the information to be displayed is simple-maybe several scanned waveforms and some numerical content.
Even so, the displays need to be power-efficient and mechanically robust, with high reliability, high quality and a small footprint. Further, many display integrators place a great deal of value on long-term product support, secure supply, field-replaceable backlights and backward compatibility, to lessen redesign costs and minimize issues related to extended service support.
Harsh environments
In this class of products, displays can be subjected to temperature extremes, shock and vibration, moisture and variable ambient lighting. Those conditions mandate a true industrial-quality display and, possibly, the use of a clear protective cover that can safeguard the display from the rigors of daily use. Variable ambient-lighting conditions range from low-level indoor lighting to direct sunlight, in which case peak luminance and high contrast become very important.
When standard display performance is not sufficient, adding enhancements such as direct-bonded glass with anti-reflective coatings to provide protection and preserve contrast can help significantly. Another option is to use industrial-type transflective displays that combine both reflective and transmissive properties and that are suitable for performance in all ambient-lighting conditions.
Wide operating-temperature ranges and varying thermal-management conditions must also be addressed, since temperature ranges from -10 degrees C to 70 degrees C are not uncommon. For use at elevated temperatures, there are industrial-based displays that include high-clearing-point liquid-crystal material to combat the effect of solar loading in direct sunlight.
Applications for large, high-resolution displays include ultrasound, CAT scan, digital mammography, picture-archiving and communication systems, and 3-D imaging equipment. The displays used in this category typically measure 18.1 to 23 inches, with resolutions of 1,280 x 1,024 pixels to 2,560 x 2,048 pixels, and can be either color or monochrome. The displays must have high resolution, gray-scale capability, high contrast, high image uniformity, fast response times and very low pixel defects.
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High-resolution displays enhance medical imaging. Shown: a head detail on a multimegapixel NEC TFT LCD.
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Although the large displays are not typically subject to the same type of environmental conditions as the smaller displays, they do have to meet very challenging standards for optical performance, since they attempt to replace the long-used and proven cathode ray tube (CRT) technology with LCD technology.
Exacting requirements
In radiology, for example, thin frame perimeters are needed to better enable designs to be tiled or to allow two monitors to be positioned as close together as possible. Digital mammography displays demand the highest contrast and highest accuracy, whereas 5-megapixel displays should be "filmlike" and optimized to view in portrait orientation.
Two widely accepted standards organizations demand even more from display integrators: the European DIN (Deutsches Institut fur Normung eV) body and the American Association of Physicists in Medicine Task Group 18. Both organizations outline measures for acceptance testing, as well as quality control, to ensure consistent display performance over time.
The two organizations define acceptable parameters for resolution, noise, glare, color uniformity, geometric distortion, reflection, luminance response, luminance uniformity and viewing angle. One challenging specification requires luminance to be maintained at plus/minus 15 percent across the display's viewable image.
Another common medical-standards organization with relevance for LCDs is Digital Imaging and Communications in Medicine (Dicom). High-resolution LCDs use lookup tables to emulate the Dicom standard for CRT performance using de-interlacing, frame rate conversion and response compensation. Multiple video input formats, such as analog/digital, composite/noncomposite or interlaced/noninterlaced, are preferred.
Adding to the issues has been the recent implementation of the European Restriction of Hazardous Substances (RoHS) directive to eliminate the use environmentally toxic materials-including lead, chromium, cadmium, polybrominated biphenyls, polybrominated biphenylethers and mercury-in manufactured products. The RoHS directive has become a common concern among medical manufacturers, since it requires all electrical components sold or used in Europe to be compliant by July 2006.
Of particular concern is the dominant use in backlighting systems of cold-cathode fluorescent lamps and their associated mercury content. The best solution is to employ the latest LED backlighting technology.
Although design considerations for high-resolution displays differ greatly from those for low-resolution ones, there are some common areas of concern. Long-term product support and secure supply rank near the top, because slight design changes could necessitate costly requalification and tooling expenses.
Furthermore, backward compatibility, both in electrical and mechanical specifications, can save significant costs for products that need to evolve with the latest technology or for applications needing the same form factor for different high-resolution offerings.
For bedside monitoring and recovery room applications, price can be a key driver. Strong competition in emerging overseas markets is forcing equipment makers to provide more entry-level medical displays with lower performance but more attractive pricing.
Indeed, in some cases, the reason equipment makers succeed in these markets is because of capital-spending limitations.
Robert Dunhouse (cc.dunhouse@necelam.com), engineering manager, and Dean Collins (cc.collins@necelam.com), senior marketing manager, NEC Electronics America Inc.
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