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Design Possibilities with LEDs

Photonics Spectra
Jan 2006
Brighter and more versatile than ever, LEDs are used in applications ranging from small indoor displays to huge, interactive, outdoor backdrops and designs.

Mats Karlsson, Barco Media & Entertainment

Full-color LED displays evolved out of a need for much brighter and more rugged displays than were possible using other technologies. First introduced in the mid-1990s, they now are a mainstream product for large events as well as for large, fixed installations. LEDs now have efficiencies that exceed those of low-voltage halogen lamps and are becoming more common in display backlighting applications.

Barco_U2_Vertigo_MiSPHERE.jpg

Building blocks enable displays to be created like curtains, as shown here in the stage design used in the band U2’s Vertigo tour. A transparent effect and multiple-angle viewing ability were achieved.

Efficiency does not equal brightness, however. Brightness involves absolute numbers of lumens. LEDs are still low-power devices, and achieving high brightness levels requires a lot of them. Fortunately, they are low in cost, so arrays for achieving a certain level of brightness are not prohibitive from a component cost perspective.

But there are other issues to consider. LEDs are mass-produced semiconductors, and the exact performance of any given unit cannot be predetermined. Following manufacture, they are tested and sorted according to their individual performance, a process that is known as binning.

Binning divides them into groups according to color and/or brightness level within specific limits. Of course, the tighter the limits, the fewer the LEDs that will comply and the higher the price per LED. But even the closest binning will not provide the level of consistency required for high-performance displays. To achieve this, the displays must be calibrated.

Tiling and calibration

LED displays are available in many types and resolutions. The biggest differentiator is the pixel density, which is the number of image pixels — either a group of discrete RGB LEDs or a single RGB emitter — per surface unit. This is measured from the center of one pixel to the center of the next, so the smaller the number, the greater the density. The smallest distance today is 3 mm, which probably is the limit for current LED display technology. A smaller distance is achievable only with plasma displays, typically starting at 2.5 mm. Mainstream LED displays are in the range of 10 to 20 mm, but the trend is to decrease this number.

A high pixel density provides a higher image quality at a given viewing distance, but there are practical limits. There is little point in having a display in a sports arena with a 3-mm pixel distance because the high resolution that results would not be seen from a great distance. For those applications, density of 15 to 25 mm is common.

LED displays are constructed of building blocks known as tiles, which can be calibrated at the time of manufacture. In the tiles produced by certain manufacturers, the performance of a particular tile is stored in its memory.

This is especially important for the events industry, for which very large displays may be assembled, some covering entire walls. For such applications, few audiovisual equipment suppliers own enough equipment themselves to set up the displays, so they subcontract to other companies to supply a customer with the required number of tiles, sometimes more than 1000.

Consider such an event — an auto show, for example. To produce a display for the show, 500 tiles come from a supplier who purchased them in a couple of batches over the past three years. Another 200 come from a second audiovisual company that, again, acquired them over some period of time. Finally, another 100 come from a third supplier and are brand new.

Even with strict manufacturing tolerances, no supplier can guarantee that a LED tile out of the box will perform exactly the same as one supplied three months ago, let alone three years ago. A large display stitched from these 800 tiles thus would be expected to exhibit highly inhomogeneous image quality.

The performance information stored in the tiles’ onboard memory overcomes this. When the tiles are assembled into one large display, they “talk” among themselves, using the previously stored information to autocalibrate, thereby achieving a consistent performance. The result is uniform brightness, contrast and color, regardless of the screen size.

A new trend in the rental and staging market is turnkey solutions. Customers want their equipment to be ready to go from Day One. To enable this, LED manufacturers have developed supporting products such as sophisticated control systems, mechanisms to hang or stack the screens, and practical solutions for quick setup, teardown and transportation. Huge displays also are becoming portable, with the screens being mounted on mobile trailers for sporting events such as downhill skiing and auto racing.

New packaging opens the door to innovative LED display solutions. An example is Barco NV’s MiPix technology, square blocks with four pixels that can be linked to create structures of any size and shape. To expand the creative possibilities, diffusers can be added to individual blocks or to groups of blocks. The system allows for customer-designed displays without custom manufacturing and opens up the possibility to redesign, change or reuse the components for other projects.

Bridging the gap

LED manufacturers also have incorporated image-processing technology with the displays, so it is possible to integrate full video and data. These new displays bridge the gap between existing video and lighting products. The modules can be viewed from all angles and can be daisy-chained into a string, with several strings forming a 3-D visualization curtain.

Heat management has been an issue for designs that employ LEDs. The wall-plug efficiency of an LED package typically is on the order of 5 to 40 percent, so somewhere between 60 and 95 percent of the input power is lost as heat. There have been advances in LED chip efficiency as well as in thermal transfer from chip to package, but heat management will remain a challenge into the foreseeable future.

With the convergence of video and lighting, LEDs have become a means of artistic expression. They may be used to create moods and ambience, rather than just to deliver information. With increasing demands from lighting designers, coupled with investments in the technology, the future of these displays will be brighter and more creative than ever.

Meet the author

Mats Karlsson is product manager of the creative light imaging group at Barco Media & Entertainment in Kuurne, Belgium; e-mail: mats.karlsson@barco.com.


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