Medical Device Link.

DISPLAY TECHNOLOGIES

Front-of-Screen Performance in Flat-Panel Displays

Dale H. Maunu director of flat-panel display products, Mitsubishi Electric & Electronics USA

Figure 1. Sample image in which the gray scale and CR are more important than luminance.

Flat-panel displays are known primarily for their physical attributes: a flat screen, low power consumption, light weight, thin form factor, and durability. Recent developments in color thin-film-transistor (TFT) liquid crystal display (LCD) technology have elevated the front-of-screen performance such that it is superior to color cathode ray tubes (CRTs) in many applications. Although improvements have been made, it is worth noting that not all displays incorporate the latest technological advancements. In many cases, front-of-screen performance is a trade-off between various image parameters.

It is important to consider a display's front-of-screen performance when designing medical equipment. There are aspects of human-factors engineering to take into account, such as providing clear, legible, nonambiguous information to the user, as well as considering the user's perception of the accuracy of the information provided. If the image quality is low, the user will perceive the quality of the device to be low. The display represents the face of the company. The user's impression of the product and the company will be influenced by the quality of the image presented on the display.

This article explains each display parameter in terms of its importance, compares CRT and TFT-LCD performance, discusses necessary design considerations for achieving optimal front-of-screen performance using a color TFT LCD, and takes a look at trends in flat-panel display performance enhancements.

The primary display image quality parameters include the following:

• Luminance.
• Contrast ratio (CR).
• Color saturation and gamut.
• Gray scale.
• Response time.
• Viewing angle and direction.
• Image sticking.

Luminance and CR

Luminance, in display vernacular, is the measurable optical energy from the display and is measured in nits (candelas per square meter). Brightness is the observer's perception of luminance. Luminance is important only when other factors are also considered, such as ambient room illumination, color saturation, CR, screen reflectivity, and so on. To consider only a display's luminance is akin to considering only a stereo system's sound volume. Display performance cannot be summed up by a single factor.

Figure 2. When viewed at different vertical angles, most TFT LCDs look brighter from one direction and darker from another.

The appropriate amount of luminance depends primarily on the user's environment. For normal office use, it is commonplace to have from 100 to 300 nits. For use in direct sunlight, anywhere from 600 to 2000 nits might be required. However, what has to be considered for applications in direct sunlight is the reflectivity of the screen itself and the CR. With appropriate antireflective, antiglare coatings, 600 nits is acceptable for sunlit environments; without additional surface coatings, 1500 to 2000 nits would be needed. CRT-based computer displays are typically 100 nits, whereas home televisions are about 400 nits.

It is important to remember that a TFT LCD is a transmissive device, meaning that a light source behind the display, called a backlight, is always on. The LCD acts as a light valve, so even when the display tries to show a black image, the backlight will still try to shine through. If a display is capable of 2000 nits, and the CR is only 200:1, then black has a luminance of 10 nits. Most people would consider 10 nits to be gray, not black. The CR is the ratio of the luminance of white to the luminance of black. It is usually measured in a dark room and is always found on the data sheet. Having a high CR is a positive attribute, and it would be unusual for a customer to complain about a display having too high a CR. (See Figure 1.) While CRTs struggle to achieve a CR of 100:1, it is common for a TFT LCD to have a CR of 350:1. Recent commercially available TFT LCDs have achieved a CR of 500:1.

Displays are commonly used in both high and low ambient light environments. For example, displays are used in a hospital patient's room during both daytime and nighttime. In this case, a wide dimming range for the backlight is necessary and is accomplished by the backlight inverter. Typically, the inverter is not provided with the display and must be purchased separately.

Color Saturation and Gamut

Figure 3. The reverse-scan function enables developers to mount the display so that it is optimally oriented for the viewing direction of intended use.

Another consideration, with respect to luminance, is color saturation. Displays enhanced to achieve higher luminance often sacrifice color saturation. This is most easily seen with the color red, which will often appear pink on a display with poor color saturation. If it is important for red to be accurately represented on the screen, then a traditional computer display will not be adequate, and a display intended for non-PC applications would be more appropriate.

Color gamut is the range of color a display can reproduce and is commonly expressed as a percentage of NTSC. Although NTSC stands for National Television System Committee, which developed television standards for North America, in this instance, 100% of NTSC refers to the full range of color that can theoretically be displayed. The TFT LCD used in notebook-style computers often has a color gamut of 38–42% of NTSC. The TFT LCD used in computer monitors often achieves 50% of NTSC. Some TFT LCDs achieve comparable color gamut to a CRT, which is about 70% of NTSC. A display that can combine both high luminance and a high-percentage color gamut should provide the best image- reproduction capability.

Gray Scale and Response Time

In medical imaging, 10-bit gray scale is common, but most TFT LCDs only support 6-bit gray scale. It is possible to achieve 8-bit gray scale on a 6-bit panel using frame-rate modulation. This is achieved by applying a different shade to each successive scan to achieve a gray level that is not normally available. This is only useful if the image on the screen is static. The reason is that the LCD responds by averaging the applied video signal. Many LCDs have a response time of 25 to 50 milli- seconds for black to white to black. Intra-gray-scale response times are frequently over 80 milliseconds. The screen is usually updated 60 times per second, or every 16.67 milliseconds. If it takes about 40 milliseconds to fully change a pixel from one image to another, it will take more than two screen refreshes to achieve the desired image. If the image is moving as well, the effective refresh rate of the display to create and then erase an image becomes 80 milliseconds, or 12.5 Hz.

Despite the lengthy intra-gray-scale response time, a TFT LCD can support motion video because the images moving across the screen take advantage of the faster response time afforded to black-to-white-to-black transitions. Background images are commonly subtle gray-scale transitions, which are very hard to see. If frame-rate modulation is applied to a display to achieve more gray-scale resolution, the effective response time is reduced again. The best solution, so far, is to use an 8-bit display for motion video and apply frame-rate modulation to static images to achieve 9- or 10-bit gray-scale resolution.

Viewing Angle and Direction

Figure 4. Components of an AMLCD optical stack.

An advantage that a CRT holds over an LCD is in the viewing angle. A CRT is a lambertian burst emitter, meaning it emits light in all directions. An LCD, being a light valve, creates more collimated light, resulting in a narrow viewing angle. To give the display a wide viewing angle, it is necessary to apply enhancement films or create novel liquid-crystal structures that can evenly spread the light. Until very recently, wide-viewing-angle displays suffered from the slowest response times, and fast-response-time displays suffered from a narrow viewing angle. Some display manufacturers have introduced advanced displays that achieve both wide viewing angles and fast response times. The new technologies are first being introduced to the computer market and should be more commonplace during the next couple of years.

To complicate matters further, viewing direction must also be considered. Most TFT LCDs have an asymmetrical viewing cone. These LCDs are optimized for a wide horizontal viewing angle and a narrow vertical viewing angle. Furthermore, when viewed at different vertical angles, the display can look brighter from one direction and darker from another, as shown in Figure 2. TFT LCDs intended for non-PC applications will often feature a reverse-scan function. This function enables product designers to mount the display so that it is optimally oriented for the viewing direction of intended use, as shown in Figure 3.

Image Sticking

CRT displays are susceptible to image retention in the form of phosphor burn, permanent damage that results from applying the same image for an extended time, thereby aging the phosphor in the shape of the image. This results in image artifacts that make further use of the display difficult and impractical. A TFT LCD can exhibit a similar phenomenon called image sticking, although it is created differently. TFT pixels can become charged over time, resulting in an image that sticks on the screen. Because most non-PC displays have backlight lifetimes of 50,000 hours, designers should carefully test for whether a particular display is prone toward image sticking. Typically, a TFT display will be able to disperse this phenomenon within 2–5 seconds, and it usually requires the same image to be displayed for more than 2 hours for it to stick.

Ongoing Performance Enhancements

New technological developments continuously improve TFT-LCD performance. These improvements are in all of the areas mentioned above: luminance, CR, color gamut, gray scale, viewing angle, and response time. The techniques vary by manufacturer and range from simple refinements in existing technology—such as film-compensated twisted-nematic mode—to the addition of sophisticated electronics to enhance the image, to novel and exotic liquid-crystal (in-plane switching, multidomain vertically aligned) structures. As they mature, these techniques will yield excellent TFT-LCD performance.

The flat-panel display is clearly taking over as the preferred display in medical electronics, and for good reason. Continued refinements in front-of-screen performance are resulting in product features without compromises. Product designers that understand the trade-offs associated with flat-panel displays are better able to take advantage of key performance benefits: better luminance, CR, and color saturation. In addition, designers can minimize compromises in viewing angle and response time to create best-in-class products. The display is the face of your company, and therefore the careful selection and implementation of the appropriate display is critical for successful products.

Copyright © 2001 Medical Electronics Manufacturing