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Medical Electronics Manufacturing Magazine
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Medical Electronics Manufacturing Fall 1998

Display Technologies

Field Emission Display Technology

Tom Holzel, vice president of marketing and sales, PixTech

Most medical instruments requiring a visual display have employed one of two main types of displays: mechanical gauges or electronic TV-type cathode ray tubes (CRTs). While gauges are still used in measuring pressure and temperature, displays requiring graphic information have historically been served by monochromatic, and lately, color CRTs. In the last five years, the flat-panel display has made rapid inroads into medical electronics. With the same frontal area as a 5-in. CRT, which is 11 in. deep and weighs 50 oz (including drive electronics), a typical flat-panel display is about 1/2 in. deep and weighs 10 oz.

Although similar to CRTs (left), FEDs are matrix-addressed.

The newest flat-panel display is the field emission display (FED). FED technology is similar in operation to CRTs in that phosphor is excited by a stream of electrons traveling through a vacuum. The FED, however, is matrix-addressed, one entire row at a time with millions of electron-emitting cathodes. These emitters are a fraction of a millimeter (or up to 5 mm in the case of high-voltage FEDs) away from the phosphor screen, and they are produced by cold cathode emission.

FED Characteristics

The performance characteristics of FEDs are well suited to medical imaging. If an instrument is not facing the viewer, or if the viewer moves about, displays with narrow viewing angles are difficult or impossible to read. At 60° off-axis, text is 10 times more difficult to read than at 0° off-axis if the text remains at the same brightness. If brightness or contrast drops with increased viewing angle, a display may become significantly more difficult to read at only modest off-axis angles. Because FEDs are emissive, they allow equal brightness at all angles.

Brightness. Most displays are adequate in normal (50–100 fc) room lighting. However, in dimly lit situations, such as a patient bedside at night, dim (reflective) displays are difficult to read. Most alarming, a dim display may be deceptively easy to misread. Because an FED is an emissive display that produces its own light, it can be dimmed continuously from full brightness to less than 0.05 fL. Outdoor instruments for emergency medical technicians require high brightness to compete with direct sunlight. This often requires the use of special contrast enhancement filters, such as 3M microlouver filters to generate contrast.

Speed. Display speed is the rate at which the image can be changed while maintaining image detail. Displays with inadequate response times will create image "smear" that can be confused with defective blood flow, or will hide jitter that can indicate instability or electrical interference. With a response time of 20 nanoseconds, FED technology produces smear-free video images.

Color Quality. FEDs use conventional TV phosphors. This is of particular importance in such areas as telemedicine. The ability of a display to show true flesh tones depends in large part on the colorimetry of the display. TV phosphors have been fine-tuned for decades to provide the most natural skin tones possible, and, although not yet widely used, are unchanged in some FEDs.

FEDs are CRTs in disguise. Thus, one drawback of FEDs is that the vacuum tubes do require maintenance. Although the FED is a vacuum tube, it has a spacing between cathode and anode of 0.2–5 mm (instead of many inches as in a CRT). Electrons stream from the cathode to the anode to illuminate phosphor. Rather than using a single beam that must be steered by a power-inefficient deflection system, FED technology uses a row-at-a-time matrix addressing scheme. FED technology provides a wide color gamut with continuous dimming and 8-bit gray scale. Its image is equally bright from any viewing angle, and power efficiency is high (from 3 to 40 lm/W, depending on voltage and phosphor).

Unlike CRTs, FEDs have no cathode heater, no deflection system, and no shadow mask. Because of the cold cathode emission, instant-on is available at wide temperature extremes (–40 to 85°C). FED technology is so new that although many medical display manufacturers have purchased sample quantities, only a few have incorporated them into devices. These applications include sonographs, x-ray imaging, and heart-rate monitors.

At the heart of the concept of FED technology is the idea of millions of tiny conical tips sitting beneath 1-µm gates. By applying a voltage differential between tip and gate (of, say, 80 V), the electric field at the tip is so high that instantaneous cold cathode emission occurs, resulting in a cloud of electrons hovering over the tip. Once these liberated electrons see the anode (which is charged to 400 V), they stream to it and strike the phosphor anode, producing light.

Achieving a high electric field strength with practical switching voltages (<100 V) requires a method for mass-producing gate holes in the 1-µm range. There are two basic approaches to FED design: low-voltage and high-voltage anodes. Emissions from the microtips of high-voltage anodes radiate in a roughly 60° cone. When these tips are very close to the anode (e.g., 0.2 mm), the spread of the emitted stream of electrons is small enough to result in a spot size (or pixel size) of 0.33 mm. Raising the anode voltage to obtain greater phosphor efficiency requires increasing the stand-off distance between anode and cathode to prevent arcing. The natural emission angle of the electrons produces a spot size that is too large, so this stream must be focused.

The low-voltage approach uses field sequential color operation to eliminate the registration problems—but at a cost. Field sequential color means that an entire screen image is individually painted in each of the three primary colors, one at a time. As each individual color is painted, only that color phosphor is grounded, so all electrons must strike it, and none can accidentally strike another color stripe. Accurately striking only the selected color is a problem with high-voltage FEDs. The swinging of this stream of electrons from one color stripe to another is easy at low voltages, but not practical above, for example, 2000–3000 V. Thus the disadvantages of low-voltage operation are that the highest phosphor efficiencies cannot be reached and the duty cycle is lower by two-thirds because only one color is on at a time. High-voltage operation is more power efficient than low-voltage operation, but it is more expensive to produce.

In practice, monochrome 5.2-in. (diagonal) FED displays currently are available with an optional 70-fL brightness after a 50% contrast enhancement filter. The brighter models run at 100–150 fL after filter.

FEDs produce gray scale by a number of different methods.

Frame Rate Control. Running at, for example 400 Hz, a 50% gray level can be obtained by alternating a white and a black field every other frame. A 25% gray level can be achieved by alternating one of four frames to white, or one out of 400 frames. This method is simple, allowing the use of digital on/off drivers, but the FED runs into flicker at low, and capacitive switching problems at high, frequency.

Pulse Width Modulation (PWM). PWM requires the column to switch off earlier than the row time to decrease the pixel brightness level. The advantage to this method is that when on, the tips are always operated at maximum voltage, but rate control delays can add up at short switching rates.

Voltage Modulation. This is the classic analog method of producing gray levels and gives a luminance response similar to that of a CRT. However, it requires accurate low-power drivers and very uniform tip response.

Current or Charge Control. This method corrects for tip nonuniformity but requires complex drivers to control the emitted charge.

Mixed-Mode Modulation. This is the method most display integrators use. Some gray scale is obtained from partial use of two or more of the above modes, thus avoiding the extreme conditions of any one method.

FED technology offers an array of display characteristics, ranging from efficient high-voltage focused versions to cost-effective low-voltage proximity focused iterations. Extracting electrons from microtips and modulating them with a G-2 gate provides flexibility and allows display designers to specify visual performance. Because of the simpler assembly, custom performance and special sizes are less costly to produce.