LED Screen Displays in Manifold Applications

Illumination-industry leaders see broad-use, LED-lamp applications as a real possibility, but on a 10-year horizon, if not sooner. Already, LEDs have overtaken the incumbent lamps in traffic lights; some automotive head, tail and interior lights; signage channel letters and specialized, architectural-lighting applications. In most instances, lighting specifiers utilize inorganic, III-V compound-semiconductor LEDs – they see organic LEDs (OLEDs) as less-mature counterparts, at least for the present.
LEDs aren’t yet the overwhelming choice in various lighting and illumination markets, although they’re commonly applied in electronic billboards and other large-format indoor and outdoor displays. Their considerable viewing distances allow for coarse resolution (pixel pitch), which is achieved with individual, RGB-packaged LEDs (the more mature, inorganic III-V semiconductor type) placed in proximity.
However, smaller, consumer-electronic devices that require higher-resolution screens, such as handheld cellphones, cameras and MP-3 players, as well as prototype laptops and television screens, OLEDs are chosen above the III-V LEDs. Why?
Because a planar array of uniform, high-quality LEDs arranged in the tight proximity needed to form a smaller, but high-resolution viewing screen isn’t possible with III-V semiconductors. The reasons are multifold.
First, high-quality, RGB LEDs come from different III-V compound materials; therefore, monolithic integration of three-color pixels in a single, flat plane isn’t feasible. A hybrid integration of inorganic RGB LEDs with a 16-micron pixel pitch, with adequate color isolation, is a daunting task (16-micron pitch is needed to achieve the 160 pixel/in. required in 1920 x 1200 WUXGA laptop screen; 1 micron = 1/1,000 cm).
Even if such RGB pixels could be constructed in a III-V plane, they would be limited to the 2-in. standard wafer size used in this industry. Currently, only a few high-end, LED manufacturers have 3- or 4-in., wafer-processing capacities, which is still rather small for close-viewing or consumer-device display technologies.

LED manufacturing challenges
Furthermore, single-color LEDs processed into a wafer are subject to various challenges: non-uniformity, impurities, dislocation and lithographic variations, all of which lead to the well-known “binning” issue. These challenges are particularly pronounced at the wafer’s edge, which renders a good portion unusable. Finally, even if III-V RGB LEDs could be produced over a 1-sq.-centimeter area by using a selected, flip-chip, wafer bonding technique — tiling them to create, say, a cellphone screen, is also unlikely because of the inevitable (and unacceptable) 1/100 in. tiling gap that would remain between the wafer plates.
OLEDs, on the other hand, are fabricated via a method similar to inkjet printing. This system can precisely deposit, with sufficient isolation, organic, RGB-molecule dye chemicals, or predetermined emissive polymers, into miniature “wells” fabricated into organic layers. The RGB-well combination, then, forms a passive- or active-matrix pixel group, known as passive-matrix OLED (PMOLED) or active-matrix OLED (AMOLED) displays. These screens can provide minute detail and beautiful colors. Further, the repeatable, chemical-fabrication process and the monolithic integration of organic-layer transistors (TFTs) allow for superior color display and video processing.
The best AMOLEDs can display nearly four times as many colors as equivalent-size LCDs can produce, with much wider viewing angles.
Despite the OLED promises, laptops, most cellphones and other handheld display devices predominantly use LCDs, a technology invented in 1963 with a goal to replace CRTs. However, LCDs are difficult to scale up to large surfaces and incur high production and other related expenses. Therefore, this technology is vulnerable to new innovations. OLEDs, however, need further “lifespan” improvement.
By 2010, iSupply Corp. believes, factories will churn out 289 million AMOLED displays annually. Approximately 88% of them will end up on cellphones. Computer-screen and television applications may take another five years, as companies develop longer-lasting chemicals that will increase the display’s effective lifetime.

Designing for pixels
Large-format displays, viewed from much longer distances than consumer electronic devices, can use bigger pixels and lower resolutions. Inorganic LEDs are best suited for such applications because these high-quality, robust and long-lasting LEDs can be packaged as surface-mounts and integrated into a motherboard with drive electronics, with a resulting pixel pitch as little as 4mm.
Such pitch-size (or greater) is appropriate for large-screen indoor and outdoor displays found in billboards or instore, mall, theatre and other, on-premise signage. A display designer must, however, regulate many elements to maximize viewing quality when specifying LED display panels: screen resolution, brightness and contrast, video processing, electrical usage and maintenance.

Screen Resolution
Resolution, the first key to a quality image on a large-format video display, isdefined by the total number of vertical and horizontal pixels that form the image. The on-screen image is typically reproduced from a video signal that carries a native NTSC resolution of 486 (vertical) x 240 (horizontal) or 576 x 20 (PAL). For best results, a minimum resolution of about 648 x 486 (NTSC) or 768 x 576 (PAL) is recommended. However, a properly designed display with lower resolution can still give an acceptable appearance for video images, like those seen in a standard VGA screen. Taking this as a benchmark, to achieve the same 640 x 480 VGA (15-in. diagonal) resolution with a 3m x 2.25m screen, the pixels should be spaced approximately 4.5mm apart.
Larger pixel pitch leads to higher pixelization, meaning the viewer sees the pixel structure from an extended viewing distance. Hence, screen designers need to calculate the pixel size based on the distance between the viewer and the screen. The choice is also dictated by any physical-size constraints, sight lines and, of course, budget.
When pixels are viewed up close, the RGB LEDs appear as separate dots. These LEDs mix to form a single color at a distance from the screen known as the color-compound distance (CCD). For indoor displays a more advanced color-compound scheme is required to make images appear clear and sharp from close range. The best indoor displays use 3-in-1 RGB in one packet with surface-mounted device (SMD) LEDs, whereas outdoor displays may use individual, RGB LEDs.
Typically, CCD is calculated by multiplying the pixel pitch in millimeters by 250mm for indoors and by 500mm for outdoors. The minimum viewing distance is determined by multiplying the pixel pitch by 750 to 1,000. The maximum viewing distance is generally 20 to 30 times the screen height.

Brightness and Contrast
LED screens B=brightness, or more accurately, luminance, is measured in nits (cd/m2) using a light meter. The general rule is to require at least 1,000 nits for an indoor display and 5,000 for outdoor displays. (In use, these settings can be automatically regulated, to respond to ambient light.) The designer first measures a full white signal at a normal, screen-viewing angle (VA), and sets color temperature at 6,500K (D65) for outdoor screens and 5,000K (D50) for indoor screens. He then repeats this measurement several times at various, equally spaced screen points. Setting the screen to black, the designer then re-measures for the ambient reflected light. The brightness is an average of the various points of white, minus the measured ambient when the screen is black.
The VA is defined at the point when brightness falls to 50% of the maximum value. A challenge unique to LED display technology is known as shouldering, a color shift caused by one LED that blocks the view of another at extreme angles. The VA check should include color shifts as well as brightness; if a significant color shift occurs at an angle before the brightness falls by 50%, then the VA rating is reduced to this angle.

Video processing
A standard video signal cannot be directly displayed on an LED screen without being first electronically processed, and many sophisticated electronics now accomplish such work. Video images comprise many horizontally scanned lines, but not all appear simultaneously on screen. In the first 1/60th sec. (1/50th for PAL), the odd lines appear, and in the next 1/60th second the even lines follow, to complete an “interlaced” frame.
Resolving video that involves rapidly moving objects requires sophisticated, real-time interpolation, along with appropriate scaling to fit the output screen that, normally, has a different resolution than the source. These manipulations, especially scaling, require powerful processing to generate vivid, fast-moving, artifact- and flicker-free video.
A standard step is to view the content first via a CRT TV monitor, although the monitor isn’t always a true representation of how the picture will look on an LED screen. The PC world extensively uses Digital Visual Interface (DVI), which transports material between devices through a refining process. It has now become overwhelmingly popular in LED installations.

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Electrical usage and maintenance
LED-screen design and practices also include proper electrical adoptions and maintenance. It’s important to have the appropriate circuit breakers to handle switch-on overloads and earth leakage. Finally, when an LED screen needs a repair, the sign’s owner must ensure the availability of vendor service with the correct replacement parts.
LED-type displays’ market ranges from cellphones, cameras and car-dashboard displays to large-format, electronic displays, and we can expect more gigantic growth in worldwide use. And, as manufacturers work to increase longevity, expect to see OLED-based television and laptop screens in the not-too-distant future.

As president of LED Lighting Technologies, Dr. M. Nisa Khan provides education and awareness to the lighting industry and consumers about LED lighting. She has a bachelor’s degree in physics and mathematics, and master’s and Ph.D. in electrical engineering. She is also a money manager in her firm, IEM Asset Management LLC. Email her at nisa.khan@iem-asset.com