THE PROGRESS IN INDOOR LED DISPLAYS-PART2

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.

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. 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. 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.