Even though the size of motherglass continues to grow, monolithic displays can only be made so large. Tiled displays offer the perfect solution for large-area displays that need to be seen at great distances and in high-ambient-lighting conditions. Here is an overview of tiled technology.
by Anthony C. Lowe
THE GENERATIONS of flat-panel production lines are moving inexorably toward double-digit figures, and the dimensions of Gen 10 motherglass will measure 2850 ´ 3050 mm. At the time of this writing, the size of one-up "mine's bigger than yours" technology demonstrators had reached 104 in. for AMLCDs and 108 in. for PDPs. Although these panels are extremely impressive as demonstrations of the skill of their creators, they are not, nor are they likely to become, products for sale in volume. These large motherglass sizes are better employed in making multiple panels up to 65 in. on the diagonal, although panels up to 82 in. on the diagonal or more are now being marketed in small volumes.
What fuels the demand for large and very large displays? Two markets drive most of the demand: digital-signage networks (DSNs) and out-of-home electronic media. They overlap to some extent, and a plethora of alternative names for them exists. Simply stated, DSNs provide a mixture of information, advertisement, and entertainment content that can be specifically targeted at an audience, the size of which can vary from hundreds at a time down to one individual. Out-of-home electronic media covers everything from large-screen TVs in bars and clubs to digital cinema and giant stadium and live-event screens.
The worldwide DSN market is already worth more than $1 billion with an annual growth of about 30%. The technology has moved on from the old electromechanical signage systems once used in airports and train stations. Although they were expensive to build and maintain and could display only the simplest character information, such as arrival and departure times, customers liked them because they were extremely legible from large distances in bright-ambient-lighting conditions. In this respect, they worked better than many of the displays that have replaced them and, importantly, they did not suffer from image burn-in, points worth noting during the following discussion.
Screen Technology, Ltd.
Screen Technology's 5 ´ 4-tile 80-in.-diagonal 1020 ´ 608-pixel ITrans™ display operating at a luminance of 2000 cd/m2 in direct indoor sunlight at the Grafton Centre, Cambridge, U.K., during the 2006 World Cup. The inset shows the very bright environment in which the display was situated (the centre has a glass roof).
The key growth areas for DSN will be in the retail and transport sectors – high streets, train stations, airports, shopping malls, and inside individual stores. There are "image" and hard financial reasons for this growth. Having the latest signage technology will enable retailers to establish an upmarket brand image. On the financial side, recent trials have demonstrated sales increases from 10% to more than 100% when targeted digital advertisements were used in place of conventional poster or billboard advertising. Content is easy to produce digitally and even easier to distribute via wired or wireless networks. However, success will require an end-to-end solution involving content sourcing and/or creation, content distribution, software systems to manage and target the feed to the displays, and the displays themselves.
Displays are just one part of the total signage system. At present, most of the displays are monolithic (i.e., a single panel) and will have diagonals in the range from 40 to 60 in., with a minority being over 80 in. on the diagonal. Moreover, to satisfy content and legibility requirements, displays will have to be bright, full color, and fast enough to show video in brightly lit environments. In addition to the retail and transport markets, these displays can serve the professional markets, such as control-room displays, which require lower luminous output, and architectural displays, where displays are integrated into the structure of the building to provide information or ambience effects – these will have different requirements in terms of viewability, color, and speed.
However, even an 80-in.-diagonal display will appear small and hard to read if viewed from a distance of many meters. Signage can be made more effective and much more content-rich if the display surface can be expanded so it is optimized for the viewing distance in a particular mall, store, airport, or station in which it will be used. The solution to this problem is tiling, where several individual displays are arranged in an array and used as one large logical display. Tiling offers the potential to create displays that, for all intents and purposes, are unlimited in size and can have any aspect ratio that is a multiple of the tile dimensions.
Classic examples of tiling in the past start from the old electromechanical displays mentioned above, in which each character was a tile inserted into an aperture in a front panel. The first electronic tiled displays were single-character dot-matrix twisted-nematic (TN) liquid-crystal-display (LCD) modules in which each dot was directly driven and illuminated with an intense backlight. They had the advan-tage of color and high contrast, but because of their method of construction, they were expen-sive and suitable only for high-cost installations. In the mid-1980s, these were followed by the JumboTron™, an array of flood-gun cathode-ray-tube (CRT) displays that achieved extremely high brightness and, at the time, was the tech-nology used for the largest displays. However, light-emitting diodes (LEDs) have become the dominant technology for the largest and bright-est displays, thanks to the advent of manufacturing methods that drastically reduced the cost of making large arrays of LEDs.
But electronic signage is developing rapidly from being dominated by only the largest of displays into applications richer in content and directed at smaller audiences. These applications will still require displays larger than the largest single (or monolithic) panels that can economically be made, so it is reasonable to assume that tiling will continue to occupy the top end of the market and that the largest displays available for any signage application will be tiled.
Several technologies now being sold into the market meet some or all of the requirements outlined above, but they have major differences in their characteristics. Many tiled displays have visible boundaries or mullions between the tiles; in others, the mullions are less visible or almost invisible. This feature will be used as the primary differentiator to group technologies in the following discussion. Brightness, contrast, the ability to be operated in ambients from dimly lit to full sunlight, and the viewing-angle dependence of the image will also be discussed.
© Clarity Visual Systems
Shown in the center of the photo are two 67-in.-diagonal Lion UXP (UXGA – 1600 x 1200) displays. The displays use Clarity's AP/LCD (Advanced Performance LCD) patented display technology. The installation is in a broadcast control room.
Highly Visible Mullions
Monolithic LCDs and plasma-display panels (PDPs) are commonly used to make video walls. Because space is required around the edges of displays for sealing the substrates together and making electrical connections to them, the active display area of each individual display is smaller than its total area. The "dead" space outside the active area is covered with black bands or mullions. The effect is one of looking at the displayed image through a window, with the difference that when one looks through an actual window, image content is lost behind the opaque frames. Instead, with a mullioned video wall, the image is displaced each time a mullion is crossed. While this may be acceptable for images, it is not an ideal solution for graphics or text, where the mullions produce dislocations in the displayed image. When text is displayed, this means that a character, or preferably a word, should not be displayed across a tile boundary. One company uses specially designed PDPs with inter-tile mullions less than 6 mm in width, which makes them much less visible. There are other aspects of scalability and resolution particular to tiled displays that, because of their modularity, often depart from standard display resolutions. These are beyond the scope of this article, but it is planned to cover them in a separate article later in the year.
Whether tiled displays have mullions or not, they must meet a basic set of requirements: They must have sufficient luminance and contrast to be viewable in the highest ambient illuminance for a particular application, and they must be viewable with insignificant color shift over the entire range of angles required for an application.
Luminance is one requirement for operating in high-ambient-lighting conditions; the other is ambient washout.
PDPs produce light by means of a UV discharge, which activates a phosphor. The brightness of PDPs is therefore limited by the maximum discharge current that can be maintained. Effectively, this limits the maximum large-area brightness to about 600 cd/m2, with peak brightness to accentuate highlights in video images of about 1200 cd/m2. On the other hand, LCDs modulate light from a backlight placed behind the display, so their brightness is limited by the backlight. There is nothing in principle to prevent luminance from exceeding 3000 cd/m2, but panels manufactured for the signage market are usually offered at about 600 cd/m2 and are claimed to be sunlight readable at this luminance.
The phosphor in a PDP, which is deposited on the inside of the front glass substrate, scatters ambient light. Placing the appropriate color filters between the phosphors and the glass substrate reduces this scattering, but PDPs suffer significant image washout in brightly lit environments. Manufacturers may claim a 10,000:1 contrast ratio, but this is measured in a dark room. PDPs have to be shielded from ambient light even in bright indoor areas such as glass-roofed buildings. LCDs are superior in this respect because the front polarizer used in all active-matrix LCDs (AMLCDs) absorbs about 50% of ambient light that has to pass through the color filters and a second polarizer before it reaches a scattering surface in the backlight and is scattered back toward the observer. However, a high-performance antireflection coating still must be applied to the front polarizer to prevent washout in the brightest environments. Darkroom contrast is in the range 600:1–2000:1, which is lower than that of PDPs, but provides superior contrast in high ambients.
The color displayed by PDPs does not change with viewing angle. This used to be a problem for LCDs, but today's technologies show no significant viewing-angle dependency of color.
If a PDP is used to display a fixed image or a changing image that always occupies a fixed position on the display, the efficiency of the phosphors in the driven area decreases and their reflectivity changes. This is called image burn-in and it limits the useful lifetime of these displays. LCDs do not suffer to any significant extent from image burn-in, making them the more suitable technology for the display of fixed images.
Because PDPs are cheaper than similarly sized LCDs, and the available size of LCDs has lagged behind that of PDPs, PDPs presently control about 70% of the market, a share that is expected to decline to about 33% by 2009. This prediction includes both monolithic and tiled displays.
Rear-projection systems can be packaged so that the tiles (usually known as projection cubes) can be stacked so that the gap between screens is about 1–2 mm. When viewed from a distance, these gaps, although visible, are much less obtrusive than for the tiled PDPs and LCDs discussed above.
9X Media, Inc.
This 9X Media X-Wall video wall and server was designed and built for a North American power company to view and monitor system status in real time. The X-Wall consists of 8 ´ 46-in. 9X SlimLine LCDs and an 9X Media VC-3208 server (expandable to 32 screens) to handle the information and enable processing.
The size of a projection cube is limited, for a given light output, by the relative sizes of the light source (an arc lamp), the light valve that modulates the projected beam, and the projection optics. The first two are real performance limiters in terms of the amount of light it is possible to direct onto the light valve with the correct angular control so that it can be transmitted by the projection optics. Light valves use either LCD (liquid crystal on sili-con or LCOS) or DLP™ technology. Of course, the light valve and optics can be made larger, but light-valve cost increases approximately linearly with area and that of the optics as the cube of the lens diameter. All this combines to limit the optical throughput of the display and the luminance or brightness at the screen.
Screens can be designed to throw more of the light within a restricted range of angles – this increases the brightness of the display within this angular range, but diminishes it outside the range. This is known as increasing screen "gain." Because screens produce an image by scattering light projected upon them, they also scatter ambient light, which reduces the contrast of the image in bright-ambient conditions.
By using so-called "black" screens, in which light from the projector is focused through small holes in a black matrix, ambient light rejection can be improved, but this is always at the expense of optical throughput. The main disadvantage of tiled projection displays is that to be sufficiently bright for well-lit ambients, they must use moderately high-gain screens. The maximum intensity of such screens tends to be along a line from the lens to the image point on the screen, so when viewed from the right, the right-hand edge of a screen will appear brighter than the adjacent left-hand edge of the next tile, making it impossible to achieve uniform display brightness at most viewing angles. This problem can be overcome by using unity gain (or Lambertian) screens, which scatter light uniformly in all directions, but because of their low brightness, these screens are suitable only for low-ambient-lighting applications such as control rooms. By using screens that concentrate light into a narrow vertical cone instead of a wide horizontal one, some improvement can be obtained, achieving a screen brightness of 300 cd/m2, but this is still not enough to be usable in bright environments.
Pixel size is easily changed by varying the focal length of the projection lens, but usually falls in the range 0.7–1.7 mm. These displays do not suffer from viewing-angle dependence of color.
For a display to be considered mullion-free, the pixel pitch must be constant across tile boundaries. Mullion-free does not mean that the tile edges will be invisible. The properties of the human-visual system, which is tuned to identify small linear features down to about one-tenth of the normal resolution limit of the eye, conspire to make this very difficult to achieve. However, mullion-free offers the very best chance of making tiled displays with invisible tile boundaries.
LED technology is used to make the largest displays now available. For the largest-sized displays, designed to be viewed from great distances in stadiums or at live events, the LED pitch is 10–35 mm. The LEDs are small and surrounded by a black matrix, which reduces ambient-light reflection. Moreover, the LEDs are extremely bright – delivering up to 6500 cd/m2 – so they work well in bright environments and can be used in direct sunlight. Because the image is formed from an array of small, very bright points of light that are well separated from one another, the image quality of these displays degrades as the viewing distance decreases. The LED industry has sought to overcome this dis-advantage by integrating a red/green/blue set of LEDs into a single package covered by a diffuser plate. Even for a 3-mm pixel pitch, the highest resolution available, the LED package occupies only about 40% of the display area. These high-resolution screens produce better image quality, but at the expense of brightness, which is reduced to 1500 cd/m2and is limited by the ability to extract heat from the LEDs and increased ambient-light reflection from the diffuser. Because of the low fill factor, images still pixelate when viewed up close.
Because a wide black matrix surrounds the LEDs, the width of the matrix on the outer sides of the peripheral pixels in a tile can be reduced so that pixel pitch is maintained across the tile boundary. Because the join line between tiles is inactive and black, the tile boundaries are effectively hidden.
LEDs do not suffer from viewing-angle dependence of color, and their viewing-angle properties can be adjusted by modifying the lenses and/or diffusers on the LEDs.
Barco's new high-definition LED Strip, the MiSTRIP, made its debut on the Bon Jovi 2006 tour. The 42-m-wide 3-D scenic stage consists of 384 m2 of LEDs made up of over 1490 MiSTRIPs which, if laid end to end, would cover a distance of more than 2.2 km.
To produce arrays with uniform color and brightness, LEDs must be selected ("binned") or selectively driven, or a combination of both these methods can be used. This problem is well known in the backlight industry, where intensive development of LED backlights is taking place because of international legislation to eliminate mercury (used in fluorescent tubes) from products. Large, tiled LED display systems are capable of driving the LEDs at up to 15 bits per color. The increased dynamic range offered by the extra bits is used to provide electronic correction so that 8–10 bits per color can still be displayed within this larger range. With this sophisticated correction, the color uniformity of LED displays can be made very good. However, after a few months of use, non-uniformity appears because of differential aging of the LEDs, so recalibration becomes necessary.
Despite these drawbacks, LEDs have captured a major portion of the market. LEDs are presently the dominant technology for the largest sunlight-readable displays,
The most recent development in mullion-free tiled displays is ITrans™ technology. It uses an expanding array of molded poly-carbonate light guides placed in front of and aligned to an AMLCD to expand the image to a size greater than that of the LCD panel. The output, or viewed faces, of the tiles have no border or frame around them, so they can be stacked into arrays in which the pixel pitch is maintained across tile boundaries and the interpixel gap between tiles and between pixels within a tile is the same. The light guides are designed so that the luminance maximum is always normal to the tile surface and the luminance profile is the same at all points on the tile. This condition must be met to achieve uniform viewing at all angles of view. At present, using 15-in. XGA LCDs, 25 LCD pixels feed each tile pixel with an output pitch of 1.7 mm, yielding a 17-in. tile of 204 x 152 full-color pixel resolution. The output fill factor is about 95% and, because there is no spatial subdivision of color, these displays remain legible at the closest viewing distance. Because of the high fill factor and minor optical differences in the interface between pixels within and between tiles, it is more difficult to make the tile boundaries invisible than for LEDs, where the tile boundaries are not lit. However, under most conditions, the boundaries are either invisible or do not intrude upon the viewed image.
ITrans™ uses standard normally white TN AMLCD panels. Because the angle at which a light ray is transmitted through the LCD is not conserved on transmission through the light guide, the maximum contrast of the panel (about 320:1) is reduced to about 120:1 at the tile output. The upside of this is that, at viewing angles greater than 40°, the contrast ratio of the tile actually exceeds that of the LCD and is >25:1 at 70°. Moreover, there is no viewing-angle dependence of color, and there are no regions of reverse contrast. Standard display luminance is more than 2000 cd/m2. In common with that of LCDs, ITrans™ displays do not suffer from image burn-in.
It would be reasonable to question whether a maximum contrast ratio of 120:1 is adequate. Remember that except for low-luminance displays for control rooms, all the displays discussed here will be used in bright ambients. The optical property of the ITrans™ tile is such that ambient light is transmitted back towards the LCD by the output face, and very little light is scattered back toward the viewer – less even than is scattered by an LCD in the absence of an ITrans™ tile. Consequently, these displays provide superior contrast in bright ambients to PDPs, LCDs, projection screens, or LEDs designed for indoor use. With their high luminance and excellent ambient-light rejection, they perform well in 100,000-lux conditions.
The market for tiled displays is diverse and the number of technologies capable of meeting the performance requirements is small, so all the technologies discussed here will surely find a place in the marketplace. Table 1 provides a useful comparison of the different technologies. •
Table 1: A comparison of different display technologies used in tiled displays.
aThe number of companies producing tiled or tileable display products is large. Therefore, none are mentioned by name for fear of displeasing those omitted. The companies active in the market can easily be found on the Internet by searching on the relevant technology.
bNo data are included for maximum display size. In general, this is limited by system mechanical and cost considerations rather than by limitations of the tile technology. LED displays currently hold the record for size, with installed displays exceeding 33 m on the diagonal.