Medical Electronics Manufacturing
Magazine
MEM Article Index
Medical Electronics Manufacturing Fall 1998
Display Technologies
Passive-Matrix LCDs
Michael Petera, director of advanced display technologies, Three-Five Systems
The latest trend in passive-matrix liquid crystal displays (LCDs) is the integration of gray-scale imagery into a variety of display formats. Although LCDs without gray scale are used in many medical applications, the ability to provide gray-scale imagery on a passive-matrix display has been a significant development. This technology allows the passive-matrix LCD to be used in medical display applications that previously had been reserved exclusively for more expensive active-matrix display technologies.
In today's cost-conscious and information-sensitive healthcare marketplace, medical devices that can automate manual tasks, enable clinicians to make faster, better-informed decisions, or display up-to-the-minute patient data are invaluable keys to success. Many medical devices rely on a display or user-interface module to relay information to the user. Passive-matrix display technology can be implemented with gray-scale features in devices such as patient monitors, glucometers, portable imaging systems, and many other handheld personal digital assistant devices.
Using modern LCD drive techniques, passive-matrix display technology is now effectively emulating the look of thin-film transistor (TFT) displays. In most cases, gray-scale-capable, passive-matrix displays are approximately 33% of the cost of a comparable TFT active-matrix LCD display unit.
In general, monochrome display formats for passive-matrix displays feature 16 levels of gray. This resolution of gray levels is adequate for viewing most natural images. With 16 levels of gray, Windows-style display formats can be easily generated, making this well-known graphical user-interface available to display system users.
Figure 1. An x-ray image using a Windows-type GUI.
Figure 1 shows x-ray and images with a Windows-type of graphical user-interface (GUI). The screen resolution is an industry standard 1/4 VGA display (320 columns by 240 rows of pixels). These examples show the applicability of the medical imaging device in a Web browser type of application. Handheld display applications with 1/4 VGA display formats are becoming more prevalent. With a pixel size of 0.29 mm, 1/4 VGA displays feature an active diagonal display size of 5 in., which is suitable for handheld devices.
The true power of gray scale is to bring depth to the passive-matrix display environment. Until now, typical monochrome 1/4 VGA displays have featured four levels of gray. The method of generating these gray levels has resulted in significant flicker, low viewing angle, and reduced contrast ratio.
Liquid crystal active drive (LCaD) technology, an electronic method of simultaneously driving multiple rows of a passive-matrix display, provides multiple levels of gray, improved contrast ratio, improved viewing angle, and low power.
Using an advanced voltage generation algorithm, amplitude modulation is used to create the root-mean-square (rms) gray-scale voltage at each pixel. Because the voltage at each pixel is unchanged during the display update cycle, amplitude modulation is the preferred method of gray-scale image generation over frame-rate control or pixel dithering. Frame-rate control (alternating a pixel's on and off state over a number of display update cycles) manifests itself in flickering gray levels and reduces the display contrast. Pixel dithering (newsprint gray scale) results in reduced image quality because multiple pixels within a display area are used to create the look of gray scale.
Since the LCaD method spends more time updating a pixel than does the standard addressing mode, lower column voltage can be used. In turn, the lower column voltage reduces cross-talk due to lower intercolumn pixel voltage, increases the contrast ratio and viewing angle because more time is spent updating the pixel, and lowers the power necessary to drive the display.
Compared to standard addressing, the row drive sequence of LCaD addressing uses three times the amount of time to update each pixel. In the future, reflective color display technology—the integration of color filters into passive-matrix display construction—will become a low-power, low-cost alternative to active-matrix technology.
