Medical Device Link.

GUIDE TO DISPLAY TECHNOLOGIES

Medical devices are continuing to increase in sophistication and are being used in more varying conditions. Such conditions range from the clinical environment to outdoor use, day or night. Designing a device for diverse conditions makes the display selection more challenging. However, if successful, the device will result in a more-compelling product.

Display Surfaces and Legibility

Liquid-crystal displays (LCDs) have the distinct advantage over their cathode-ray tube (CRT) counterparts in that they do not have a shape that intrinsically reflects ambient light toward the user. Displays with compound spherical or cylindrical faceplates will reflect light toward the user.

Even with a flat faceplate, an LCD is not always legibile under high-ambient-light situations, such as outdoors, or even indoors near a window. Most LCDs utilize an antiglare (AG) surface treatment, which diffuses incident light. As a result, users cannot see reflections on the screen. However, AG treatments also reduce contrast. Indeed, if the luminance of the incident light is great enough, the screen will become opaque, and no image can be seen. The human visual system has a remarkably broad dynamic range. It can detect very low levels of light, such as 10-6 cd/m², yet can adapt to scene intensities of more than 108 cd/m2.1 The most common display devices operate over a very modest range, typically from just less than 1 cd/m² to about 300 cd/m². In a dimly lit environment, such as most indoor locations, this range of luminance has served fairly well. People have adapted by placing their display devices in locations that overcome inherent limitations to still be functional.

Technology Overview

Antireflective surface treatments improve legibility of liquid-crystal displays. (click to enlarge)

There are various approaches to improving legibility. Designers have increased luminance to overcome ambient light, applied antireflective (AR) surface treatments, and made transflective (transmissive and reflective) displays that use ambient light to illuminate a screen. In reality, a combination of these techniques is typically employed.

Increasing the luminance of an LCD requires an increase in the optical power of the backlight system. A modern, full-color TFT-LCD typically has a transmissivity of less than 5%. To achieve 300 cd/m², the backlight must generate about 6000 cd/m². Achieving luminance of greater than 1000 cd/m² requires a backlight that approaches 20,000 cd/m². This raises issues related to power consumption, and therefore heat dissipation. A backlight that is too hot ages prematurely through a combination of bulb decay and discoloring of the optical components. Fortunately, it is possible to create such a luminous display, and when combined with modern brightness enhancement films (BEF), the backlight is quite manageable and long lasting.

The use of AR surface treatments is becoming more common; indeed, many people have AR coatings on their eyeglasses. These coatings work very well, but they tend to be expensive and can be difficult to apply to an LCD. Although it is possible to treat a large piece of glass directly with an AR coating, it is not practical for an LCD because of the polarizing films that are placed on the glass. A separate sheet of glass can be AR treated and bonded to the front of the display, but that is commonly saved for applications where a vandal shield is required, because it increases the thickness of the display. The most desirable method of applying an AR treatment is to apply it directly to the polarizing film. However, there are some problems related to applying an AR treatment to large polarizing films. Currently, this approach is taken with displays that are 15 in. diagonal or smaller.

There are a couple of approaches to making a transflective display. One is to apply an external transflective film to the display between the panel and the backlight. This allows light that passes through the front of the display to be reflected back up into the display and added to the light coming from the backlight unit. This works fairly well, but because of the transmissivity of the display, very little light actually gets reflected.

Another approach is to add an inner mirror to the LCD. This is a matter of constructing many very small mirrors in between the two sheets of glass that make up the display. It is necessary to have one mirror for each subpixel. The downside is that this type of LCD is complicated to manufacture and therefore can be more expensive. This approach is also difficult to scale to larger sizes, although 8.4-in. displays are now available in transflective mode.

Visual Response and Adaptation

The issue of display selection for medical device designers can be quite complicated. Interestingly, luminance often plays a role in the usability of the device. One example is the relationship between luminance and response time. Users will react quicker to what they see as a function of its luminance, but with diminishing results.² The goal is to make the image that users need to see quickly much brighter than its surroundings.

Being able to notice an image is important, but it is also important to be able to resolve the details of the image being presented. Under normal conditions, a person has a visual angular acuity of one minute of arc.³ At a distance of 1 m, a person can resolve detail of about 0.29 mm—roughly the pixel pitch of a 15-in. extended graphics array (XGA) LCD. By increasing the luminance, a person can resolve the same level of detail at nearly twice the distance.

Increases in luminance must be considered carefully. Although the eye can resolve more detail when an image is sufficiently bright, the eye needs more time to adapt to a darker image than to a brighter image.¹ Therefore, a device should be capable of sensing the ambient environment and adjusting its luminance accordingly, or it should have an appropriately wide dimming range to accommodate varying conditions. It is not acceptable for the user to be blinded by an image that is too bright for its surroundings.

The amount of light that enters the eye is what is ultimately at question. A display is an illuminated surface, and the solid angle subtended by the pupil of the eye decreases by the square of the distance.³ So it is necessary to balance the size of an image with the available luminance as well as the amount of detail present in the image. In the field, the size of a display is fixed, and the distance from a user to the device is somewhat unpredictable, so luminance becomes a viable dimension of adjustment. A display with a very wide dimming range can be used not only in varying environmental conditions; the luminance can also be increased when it is necessary to gain users' attention to the specific information displayed.

Practical Guidelines

Developing medical devices incorporating an LCD is common. However, because devices are often used in variable environments, proper display selection is critical to a device's successful use (see Table I). The emergence of high-luminance backlights, AR surface treatments, and transflective technology gives designers options to overcome these challenges and provide a compelling product to the market.

Table I. The environment plays a key role ni the type of display used.