Data on display: the cathode-ray tube, which dominated display technology for half a century, has given way to flat-panel displays. All current flat-panel display technologies involve a degree of compromise, but new concepts leaving the laboratory promise better visual images and lower power consumption. (Technology).

As military operations become ever more complex and faster-paced, combatants will require quick and easy access to ever-increasing amounts of data, a requirement which will demand the best which display technology can provide.

Ask the end user what features are desirable in a new display, and the response is likely to be:

* high resolution * fast response * legible under all lighting conditions from sunlight to near-darkness * light weight * rugged * minimal power consumption * low cost.

Anyone who has gone shopping for a new computer monitor will realise just how optimistic such specifications are, but display designers are determined to rise to the challenge, offering a range of technologies which will meet most conceivable demands and finally see the traditional cathode-ray tube (CRT) transformed into a museum exhibit.

Ultra Electronics used 1280 x 1028 pixel Fujitsu solid-state colour screens in 1995 to create a prototype console for naval applications. Displayed for the first time at a defence exhibition that year, it was positioned so that visitors could see its minimal thickness compared with conventional consoles based on cathode-ray tube monitors. At the time, it seemed the stuff of science fiction, but at the recent DSEi exhibition in England, Armada had to look hard to find an exhibitor who was showing old-style CRT-based displays.

The cathode-ray tube is bulky, heavy and power-hungry, but despite these drawbacks, still has some advantages over more modern display technologies. For a start, the technology is both simple and mature, so the hardware is relatively easy to manufacture and fairly inexpensive. A CRT provides excellent resolution, fast response time, superb colour saturation and a wide dynamic range.

The earliest type of flat-screen solid-state display was the Liquid Crystal Display (LCD). This uses an electric field to alter the chemical properties of each element in the display, changing that element's light-absorbing properties. This display panel is backlit, and the individual display elements either block this light or allow it to pass, effectively acting as optical switches. Although the inherent technology is monochrome, filters can be used to colour the output from the individual elements, creating a full-colour image.

The drive technology used to apply controlling voltages to the elements can be passive or active, but the active method is now the most common, since it gives a faster response and higher picture quality.

FLC 

Ferroelectric Liquid Crystal (FLC) displays are similar in general concept to LCDs. They have a high resolution, wide viewing angle and a fast response. However, they have no greyscale capability, so cannot be used to show TV-style imagery.

The earliest head-up displays (Hud) had relatively narrow fields of view, and could display only cursive (stroke) symbology. Today's users want the widest field of view and the ability to display raster-scanned imagery. The image-generating component of a Hud is normally a cathode-ray tube, but work is under way to develop flat-panel devices which could match the brightness and contrast ratio of the `tube. Hud manufacturer Flight Visions in the United States has given Brimar and CRL Opto a subcontract to develop a flat-panel `light engine' that could eventually replace the CRT in Huds. This consists of a CRL Opto high-resolution monochrome LCD, display drive and control electronics and a high-brightness electrodeless backlight.

Thin film electroluminescence (Tfel) displays exploit materials such as zinc sulfide that emit fluorescence when voltage is applied to them. A Tfel display consists of a thin film luminescent layer sandwiched between transparent dielectric layers and a matrix of row and column electrodes. Individual pixels are lit by voltages delivered by an electronic control circuit via intersecting rows and columns.

The unit is lightweight, slim, and has a low power consumption the level of which is proportional to the desired refresh rate. The image has a high contrast, high resolution and a fast response, and can be viewed from a wide range of angles. While the contrast ratio is only around 20:1, the image does not `wash out' in high ambient light levels (a problem to which LCD displays are prone). Efficiency is fairly low, and the 75 to 200 V power levels needed drive up the cost of the display.

Tfel flat panel display technology is particularly well suited for use in military applications because of its ability to operate over a very wide temperature range, its long life, ruggedness and excellent image quality over a wide viewing angle. The latter feature allows viewing-at-a-glance, making Tfels suitable for use on weapon platforms where the user may only be able to spare a few moments to consult the display. It was this `instant availability' of the image which led Computing Devices Canada to select in 1996 a thin film electroluminescent display developed by Planar for use in the next generation flir system to be installed in the M1A2 Abrams tank.

DMD 

The US Air Force Common Large Area Display Set (Clads) programme was begun in 1995 to develop modern displays able to replace aging tube systems on aircraft such as the E-3 Sentry and E-8 Jstars, also at ground command and control units. Various potential solutions were tested between 1996 and 1998, and in September 1999 Raytheon announced that it had been awarded a $ 29 million US Air Force contract to supply aircraft-specific variations of its 21-inch Digital Ruggedized Display, a unit based on Texas Instruments Digital Micromirror Device (DMD) technology.

A DMD is a micromechanical silicon chip containing hundreds of thousands of tiny, movable aluminium mirrors, plus electronic logic, memory and control circuitry. Computer controlled signals cause the DMD's mirrors to move, and the pattern of light reflected from the chip creates a high-quality image that can be projected, printed or displayed.

The DMD, invented in 1987, was an outgrowth from earlier work carried out by Texas Instruments on micromechanical, analog light modulators known as Deformable Mirror Devices. In 1989 TI was one of the original four companies selected for the projection display technology portion of the US Defense Advanced Research Projects Agency High-Definition Display programme. In 1992 the company demonstrated a DMD-based system that met current resolution standards.

The Clad contract called for the delivery of up to 1071 displays with deliveries starting during the fourth quarter of 1999. These will be form, fit, and function replacements for the existing CRT workstation displays. The Department of Defense (DOD) has estimated that the total requirement for Clad displays could be 15,000, and the adoption of the new units could produce DoD-wide savings of more than $100 million per year

Kaiser Electronics is currently under contract to Boeing to provide a projection Digital Expandable Color Display for the F/A-18E/F. In addition, Kaiser Electronics is under contract to Lock heed Martin Aeronautical Systems to provide an 8 x 8-inch projection display for the F-22.

Kaiser Electronics recently delivered the first Projection Primary Multi Function Display to Lockheed Martin for the F-22 program. The unit is a smart 8 x 8inch high performance rear projection display using reflective micro LCD devices. A single optical engine combined with slightly modified folded optics can be used for displays ranging in size from 5 inches square to 32 inches diagonal, says the company.

Flattened CRT

The search for a display that could rival the quality of the CRT has lead to a new concept which is essentially a flattened version of the CRT. Field Emission Displays (Fed) consist of a rectangular matrix of cold-cathode field emission devices facing a phosphorcoated transparent plate. The space between these two elements is evacuated, making the unit effectively a flattened CRT in which each pixel in the image is served by its own cathode. As their name implies, these individual cathodes do not require to be heated in order to liberate electrons. The array is X-Y addressable, allowing individual cathodes or groups of cathodes to release electrons that are accelerated towards the phosphor, which is given a positive voltage to make it the anode of the device. As in a CRT, the arrival of the accelerated electrons at the phosphor causes the latter to emit light. Feds produce high brightness over the full range of color, but could require only one-tenth to one-half the power of a conventional liquid crystal display.

Organic Compounds

The LED concept received an intriguing new twist with the development of organic compounds which, when spread into ultra thin films, emit light when subjected to a voltage. The resulting organic light-emitting diodes (Oled) could be the breakthrough needed to create bright, lightweight displays.

Oleds have a very fast response time, greatly superior to that of LCDs, and their luminance is directly linear with electrical current, giving good grey-scale capability. They generate light only as required, so require less electrical power than LCDs.

At present, display longevity is limited and varies with colour. A recent report by Stanford Resources notes that in one Kodak/Sanyo prototype, the red-emitting elements have a projected lifetime of 100,000 hours, but the corresponding figures from green and blue are only 10,000 hours and 5000 hours respectively. Several companies are now reporting lifetimes of more than 10,000 hours for Oled displays and in a few cases of more than 50,000 hours, and further improvements are expected. Lifetime is dependant on temperature, so operating conditions must typically be kept below 60 degrees C. By the end of the decade, full-color Oled-based displays are expected to have replaced active matrix LCDs as the dominant display technology.

Electronic Paper

Many displays used for control purposes are not required to show imagery, but only alphanumerics and simple graphics. Currently, such units use rigid panels, but could be revolutionised by the techniques currently being devised to create electronic paper. In the late 1980s, Xerox developed a display technology known as gyricon, which uses tiny bichromal beads embedded in a flexible sheet. When a voltage is applied to them, the individual beads change state in a manner similar to that of the elements in an LCD screen. A similar concept is being developed by E Ink, while Nanomat has developed pixel elements that change colour when exposed to an electrical signal.

The image on an LCD has to be refreshed around 30 times per second, but the pixel elements in electronic paper would be bi-stable, maintaining their on or off state without being refreshed, and thus storing an image without being powered.

Although electronic paper has not yet arrived, the US Army will soon have a novel display unit which uses bi-stable technology. In 1999, Honeywell, Kent Displays, Darpa and the US Army Soldier Systems Center teamed to develop the military e-book (mil-e-book) for the soldier of the future. Existing field operational displays are power-hungry, and their battery packs provide only a few hours of display life. The new mil-e-book will be useable for weeks, allowing soldiers in the field to view maps, orders, troop movements and other documents.

The mil-e-book uses reflective Cholesteric Liquid Crystal Display (ChLCD) technology, under development at Kent Displays, to hold a high-resolution colour image on the display screen for an indefinite period of time without applied power. Power is only needed when the display is being updated with new information. The display has a reflected luminance approaching 40 per cent. It reflects near-infrared radiation as well as visible light, so can be read in starlight conditions using night vision goggles.

Plasma

Briefing rooms and command-and-control facilities need large displays able to be viewed by large numbers of users. Cathode-ray tubes are not suitable for such applications, since their weight and bulk rises dramatically with screen size. LCDs are equally ill-suited, since panels of 19 inches (0.48 m) or larger can only be created by joining individual smaller panels together.

In the business world, computer-driven projection units are used to create large-screen images, but small command centres and command vehicles are likely to need flat-screen displays. One possible solution is to use Plasma Display Panels, which consist of an addressable matrix of very small florescent elements.

The first examples were monochrome and produced reddish/orange characters against a dark background. Most recent patterns use individual elements which can glow red, green and/or blue, creating a colour image, but at the price of requiring a complex system of drivers. The matrix is relatively coarse, with pixels typically spaced around one millimetre apart - around five times that of an LCD or Fed - so the image quality is low. Although large-screen displays of 40 inches (1.0 m) or more in size are available, the image resolution is poor - typically 640 x 480. Power consumption is high (hundreds of watts), but peak brightness is only some 30 per cent of that from a cathode-ray tube.

Light emitting diodes are widely used as indicator lights on personal-computer hardware. Originally available only in ,red, these were developed to create yellow and green light, and more recently blue light. Once reliable blue Leds became available - early examples had a short life and poor efficiency - it became possible to create full-colour large-screen displays based on arrays of diodes.

Creating a large-screen display able to provide high-quality images challenges most existing technologies, but several novel concepts could provide a solution. At a conference held in Britain last year, Adrian Travis from the University of Cambridge described how wedge displays could create large-area images. Light injected into the thick end of a wedge-shaped glass plate bounces up the wedge, and if the angle of the wedge is chosen correctly, some light escapes at each bounce. The location of these points of exit are controllable, depending on the angle of entry, so can be used to create an image. This principle could be used to create compact image projectors.

Another approach described at the same meeting involves the use of Feds. Printable Field Emitters has developed a material which consists of a screen printable ink of conductive particles in a polymer matrix. This technology can provide 200,000 emitters per square millimetre. These could be driven by technology already developed by plasma panel display manufacturers.

Oled is another technology which could be manufactured using printing techniques. Cambridge Display Technology has already developed light-emitting polymers which are compatible with ink-jet printing.

Helmet Mounted

Helmet-mounted displays (HMD) initially entered service on aircraft and helicopters for use as weapon-aiming aids. Early examples included the Honeywell Integrated Helmet And Display Sighting System on the AH-64 Apache, and the Arsenal Zh-3YM-1 HMD used to aim the R-73 (AA-11 `Archer') air-to-air missiles of the MiG-29 Fulcrum. All next-generation fighters will have HMDs.

The US Army's Land Warrior system will probably use an HMD to combine the data from the wearer's computer and communications subsystem, presenting sensor-derived graphics, digital maps and textual information. The soldier's Integrated Helmet Assembly Subsystem will also incorporate an image intensifier for night vision. Although some studies have been carried out on tiny display units which could be incorporated into the sight of a rifle or other personal weapon, most integrated systems for the individual soldier being studied around the world are also expected to specify HMDs.

Until recently, the only available image-generating device able to provide high-resolution high-brightness displays for HMD applications was the CRT. One-inch or even half-inch tubes are small and light enough for HMD applications, but like their larger counterparts, these require high voltages and enough power to warm the cathode.

Two techniques show promise as ways of displaying large amounts of data with a unit of very small size. One is microdisplays; the other is direct retinal projection.

Microdisplays are high-resolution displays that are viewed close to the eye with the aid of lenses, creating the virtual image of a much larger display positioned some distance away. This image can be superimposed on the outside world via see-through optics. Microdisplays require the generation of a large number of pixels in a small package. In practice, users will want at least VGA graphics (640 x 480 pixels), but preferably up to 1920 x 1080 pixels.

Getting the required number of electrical control signals to such a small array will not be easy. The most likely solution will be to integrate the display-drive circuitry onto the same substrate as the pixel generators, allowing the unit to accept data serially then transfer it to the pixels one row at a time.

Small image sources suitable for use in head-mounted displays have been developed under Darpa's high definition systems and head mounted display programmes. These projects evaluated hardware including a Planar 640 x 480 active matrix electroluminescent display for the Land Warrior programme and a Kopin 1280 x 1024 high brightness active matrix liquid crystal display for the HMS of the RAH-66 Comanche helicopter. The latter was a particularly demanding requirement which specified a 1280 x 1024 resolution, 1650 ft-lumens of brightness, and a contrast ratio of 80:1.

Under a Phase III Small Business Innovation Research grant from the US Air Force, eMagin Corporation is developing high-resolution active matrix organic light emitting diode (Amoled) microdisplays for incorporation into military helmet-mounted displays. The company is developing a 1280 x 1024 able to provide high brightness plus high energy efficiencies.

At the Society Information Display (Sid) Conference in May 1999, eMagin unveiled a 0.77-inch (2 cm) microdisplay. This had 1.3 million pixels and demonstrated real time video with 256 gray shades. Power consumption was less than 400 mW, and the display had a brightness of 200 cd/m2.

Retinal Projection

In a retinal projection system, the image is `painted' directly onto the retina of the user's eye. A system of moving mirrors steers the output of a modulated light source through a raster-scan pattern, building up an image which appears to be about an arm's length away.

The Virtual Retinal Display (VRD) was invented in 1991 at the Human Interface Technology Lab of the University of Washington. Microvision (now Microvision BAE Systems) was granted an exclusive license to commercialise the VRD technology. By 1999 it had delivered prototypes, primarily to defence and aerospace customers. Potential military applications include simulator displays, cockpit displays and wearable displays for dismounted troops.

The light source used in VRD hardware is either a super-bright Led or a laser, the choice for any specific application depends on whether the user needs to see the image superimposed onto the real world. Injecting light directly into the eye raises safety concerns, but Microvision has taken the advice of authorities such as Dr David L. Sliney of the US Army (the principal author of the ANSI standard on the safe use of lasers) and Professor John Marshall of the St. Thomas Hospital in London. These experts concluded that the light levels used for VRD are less than 1/100th of the acceptable retinal exposure, says the company.

Another approach to creating a virtual display is the Linus system being developed by Celsius Tech Electronics of Sweden. This projects towards the user the individual lines which make up a raster-scanned image. The scanning takes place only in the horizontal plane, so the individual lines are projected one by one in the same location. The user wears a special pair of glasses which deflect the successive lines of the picture by an ever-increasing amount, so that a full image is built up. For a monochrome system, the line-display unit (LDU) projects lines made up of a single row of pixels. If colour is needed, the line will be made up of three rows of pixels. The company has tested cathode-ray tube and Led-based patterns of the LDU.

Depth

Tomorrow's displays will use depth as a parameter. This is already being done by the New Zealand company Deep Video Imaging, which has demonstrated colour displays which consist of two LCD panels spaced a short distance apart so as to present two planes to the operator.

In the longer-term, some displays will become three-dimensional. This is already being demonstrated by prototypes such as the electronic sand table which forms the central feature of a virtual world constructed at Mitre's Visualization Lab in Bedford, Massachusetts. The surface of the table stereoscopically displays terrain, showing land contours, natural features, buildings, vehicles, and even shifting weather conditions. Objects above the terrain, such as aircraft, appear to above the surface of the table. If a radar is activated in the modelled area, its coverage can be shown as a dome, allowing aircraft routes to be devised which will avoid detection.

In short

* "Solid-state displays have largely replaced the traditional but bulky cathode-ray tube"

* "Newer techniques could replace the currently-popular liquid-crystal and electroluminescent displays"

* "Organic light-emitting diode (Oled) displays could dominate the market by the end of the decade - if problems with operating life can be solved"

* "Virtual displays - in some cases projected directly into the operator's eye - could allow small and compact hardware to create the illusion of a large traditional display screen."