The inhabitants of big cities are used to bright LED screens which usually broadcast live dynamic video clips and are installed in the most populous places. But only few know that such screens are closely linked to developments in light-emitting diode (LED - Light Emitting Diodes). Today only this technology permits to obtain acceptable image quality under any light condition - even at the direct sunlight. Known for a long time, LED technology within the last decade moved fast forward. On streets of Las Vegas we can trace practically the entire history of LED screens. Couple of years ago, in Las Vegas it was even possible to see the predecessors of LED - the incandescent lamp screens.
Initially, the discrete diodes of red, green and blue colors were grouped into square pixels, each pixel representing one imaging point. The needed color in each point was achieved by the appropriate selection of luminous intensity from multi-colored diodes inside the pixel. Actually this three-dimensional synthesis is also the basis of practically all contemporary display technologies (one can look on the computer or plasma display through the tenfold magnifying glass to understand the principle).
Light-emitting diode is one of the most effective luminous sources. In terms of luminous efficiency it five times exceeds the usual incandescent lamp, but developers continue to work hard in order to increase efficiency coefficient. Apparently, 60-70 lumens per watt will soon be achieved and then the efficiently of light-emitting diodes will be equal to fluorescent lamps.
Furthermore, light-emitting diodes are produced with the assigned spectral characteristics of emission, and therefore, assembling the LED display does not require additional colored light filters (these filters decrease brightness and are essential for all other technologies).
Of course, the image on a big outdoor LED screen is visible even from shorter distances, but the picture becomes more unpleasant and uncomfortable to watch. The closer we approach the more screen divides into smaller separate segments and becomes unattractive. The situation worsens due to the reason that each pixel contains 3-5 LEDs and free space between these LEDs is made of black plastic. This plastic surface minimizes sun reflection from the screen, which increases contrast, permitting the screen to operate at sunlight.One of the most important characteristics of LED screen, which basically determines the area of its application, is the distance between centers of nearest pixels – the pixel pitch. In the beginning, this parameter was quite significant that was the reason for outdoor application only. Today, the average pixel pitch for outdoor screens is 20-30 mm. Human eye’s resolution is roughly equal to 2 angular minutes, meaning that the image is perceived as merged only from 30-50 meter distance. The screen with 30 mm pixel pitch and width of 10 meters will only have 330 horizontal points, which is almost equal to regular home video player.
Everything for LED screen resolution
The LED screens are built from separate modules, the number of which can be very high, making it possible to broadcast high quality signals. However, even 10 mm pixel pitch screen will need to be 8 meters in width in order to transmit the SVGA input signal. That is why developers always wanted to decrease the pixel pitch, but due to the technical reasons it was problematic on discrete screens. At the same time screen contrast went down, the problems with overheating arose and switching to less powerful LEDs caused the brightness decrease.
But the progress does not stand still. The latest developments in LED technology allowed using screens in those areas, where a few years ago it was impossible. With the appearance of surface mounting technology (also called SMD - surface mounting device) it became possible to create light-emitting diode construction that decreased heating, by placing the light-emitting crystals into one small housing and to obtain instead of several discrete light-emitting diodes one full color (in specifications it is usually called RGB 3-in-1).To boost the image quality producers came up with some interesting solutions, for instance, the technology of “virtual pixel”. In specifications the virtual resolution was indicated as double of physical resolution. Simply saying, in such models the additional (virtual) pixel can be formed from separate LEDs, which are the part of neighboring “real” pixels. This method does not improve the imaging of small details on the screen, but permits to smooth out the inclined and curved lines. “Virtual pixel” actually somewhat improves the quality of dynamic video picture, but if it comes to a clear static image, such as text symbols it proves to be useless.
It made possible to decrease the size of luminous point in each pixel without its brightness reduction and simultaneously to reduce the pixel pitch. The author of this article along with associates had to compare two models of LED screens with identical pixel pitch (10 mm), one of which was built on discrete light-emitting diodes and another on integrated. The choice was unanimous in favor of the latter. The screens with “3-in-1” diodes shorten by one forth the viewing distance in comparison with traditional.
The integration of three light-emitting diodes into one housing, ensuring the smaller pixel pitch, made the creation of high-resolution LED screens (can be used indoors) possible. The systems with 3 (3.5) mm pixel pitch have already been developed.
The most recognized companies present their high-resolution LED displays on international exhibitions: BARCO (has serial production of 3.5, 6, 8, 10 mm pixel pitch), Daktronics (3, 4, 6 and 10 mm), Lighthouse (6 mm and above), Toshiba (6 mm and above). Some Russian firms also offer screens with 6 mm pitch (“Gelitron Video Systems”, “Inkoteks – Display Systems”, “New Light Technologies”).
LED technology indoor
Optical characteristics of LED screens
However, what does the light-emitting diode technology give in this new area of application? First of all, image contrast with significant external illumination is the best in comparison with all other technologies. More noticeable pixels are compensated by increased brightness and extended color palette and also by more saturated and clean colors. Thus, front projection even with the use of high-end projectors requires intricate control of external illumination and there are not many examples of successful solutions. Rear projection is much more effective than front in terms of contrast, but requires significant amount of free space behind the screen and color saturation cannot even compete with LED displays.
LED screen Video walls (see PC Week/RE, N 36/2005, p. 22) also occupy considerable space (usually 60 cm + in depth) and have too obvious seams on the screen. Furthermore, video cubes, from which video walls are assembled, preserve their characteristics at the acceptable level only under stable working conditions, for example in the dispatcher halls, where they never work. Video walls also experience some problems with lamps used in projection modules. The problems arise from differences in spectral characteristics, which cause the imbalance of color reproduction on separate cubes and the need for their periodic tuning by qualified engineer.
The same problems of LED-displays are a lot easier to solve. LED screen reliability is higher while operating costs are lower. LED screen with size of 6x4.5 meters and 6 mm pixel pitch contains approximately 750 000 light-emitting diodes “3-in-1”, i.e. actually 2.25 million separate light-emitting diodes. How is the uniformity and color cleanliness achieved in this case? LEDs are selected during manufacture according to brightness and color characteristics. For screen production the diodes of one category are thoroughly selected (with close characteristics). For more precise balancing, each diode characteristic is measured by robotized equipment; therefore, characteristics are practically identical within one module. So when adding the module into the screen the characteristics of module are transferred to the general controlling processor, which equalizes brightness and color reproduction of all modules.
The viewing angle of indoor LED screens is quite significant and such disadvantage of outdoor models as disguising sunlight visors (or levers), which limit vertical viewing angle is not needed indoor.
Brightness of indoor LED screen equals 1000-3000 cd/m2. For comparison: cinema screen’s brightness – 20-60 cd/m2, operating brightness of computer display – 200 cd/m2. Therefore indoor LED screens are losing in terms of brightness only to their outdoor modifications, the brightness range 5000-7000 cd/m2. One of the major advantages of LED screens – brightness adjustment (up to 100 percentage units), which is almost unreachable to any other technology (except CRT). There are no competitors in terms of operating lifetime as well; LED manufacturers claim that LED will work for 50 000 hours before their brightness decreases by half.
LED screen construction
The advantages of LED screens are not only in their electrical characteristics, but also in technical structure, based on module principle. The typical module size for 6-10 mm pixel pitch screens is about 50x50 cm, sometimes bigger. The size of BARCO ILite 3 with 3.5 mm is 258x193 mm. The module depth is from 10 to 20 cm (up to 30 cm with cables and joints). Some manufacturers have extremely tough module frames, which permit them to assemble big screens (2.5-3 meters high) without additional support structure. However it is possible to hang up to 15 modules (6-7 meters in height).
LED screens are supplied in two variations: as separate modules with extra accessories or as complete sets, which include the external framing (frame) for specific screen size and for example, a standing support pole. In the first case the buyer can assemble the screen himself and have it with any shape and any aspect ratio. The second version is preferable when it is necessary to obtain a standard screen (4:3 or 16:9). Usually complete sets cost cheaper. Producers attempt to propose buyers greater possibilities of solutions by decreasing the module size. Thus, in new BARCO OLite products with 10-mm pixel pitch, the module dimensions are only 116.6x83.6 mm. From such modules it is possible to assemble screens of almost any form or to lay out different symbols and logos from modules, on surface of which high-quality video image can be demonstrated.
Price of LED screens
Like any equipment for professional purposes with the prolonged service period, LED screens are expensive. Modules with dense arrangement of light-emitting diodes (with smaller pixel pitch) will cost more. If the cost of specific outdoor display with 23 mm pixel pitch is around 8300 dollars per square meter (display with the sizes of 8x6 with the real resolution 380x256 will cost approximately 400 000 dollars), then the price of 3 mm pitch screen reaches 100 000-150 000 dollars per square meter. Nevertheless the popularity of similar solutions for high-quality indoor image will definitely grow. To a big extend this trend will develop due to renting business that is going to demand more screens for special events and shows. LED displays are completely suitable for that, since they allow repeated assembling/disassembling and are relatively simple in tuning.