Charts
Pixel Pitch Vs. Viewing Distance
It is safe to say that LED displays have dramatically changed the options available for stage and lighting designers worldwide. For both indoor and outdoor applications, no other display medium compares to the brightness LED displays can emit. Combined with dynamic color and resolution options, there is usually an LED display for every need.
A variety of elements, however, need to be regulated and secured to achieve maximum viewing quality when using LED screens and panels. From brightness to color to pixels, following are certain factors that need to be considered when working with and installing these screens for use in concerts, presentations, and productions.
Screen Resolution
The key to a quality image on a large-format video display is resolution. A screen's resolution is defined by the total number of vertical and horizontal pixels (dots that form the picture). The video signal the screen will be reproducing has a native resolution of 486 to 576 (NTSC/PAL) vertically, and about 240 to 720 horizontally, depending on the quality of the source. To reproduce these signals with no loss of image resolution requires a minimum screen resolution of about 648×486 (NTSC) or 768×576 (PAL).
If a screen has fewer pixels than the input source, the images will have less resolution than the source. If the display is designed properly, however, it can still give an acceptable appearance for video images. Screens of approximately one-third VGA resolution can provide a very acceptable video image, so around 200×150 is fine. For example, to achieve a 640×480 VGA resolution screen with a medium-sized 3m × 2.25m screen, the pixels would need to be spaced around 4.5mm apart. This distance between pixels is called pixel pitch, and it is usually measured in millimeters.
Typically, pixel pitches for indoor screens are 6mm, 10mm, 16mm, and 20mm, and for outdoor screens, they are 16mm, 20mm, 25mm, and 30mm. Outdoor screens tend to be larger than indoor screens because the viewing distance is often greater.
One consideration to remember is that the larger the pixel pitch, the more prone the image is to pixelization — when viewers can start to see the pixel structure, much like looking at a newspaper photograph with a magnifying glass. This is a function of the distance between the viewer and the screen, and it needs to form part of the design calculation.
The choice of pixel pitch and screen resolution is dictated by any physical size constraints, viewing distance and sight lines, and of course, budget — these displays are usually priced by area. A good rule of thumb is shown in the chart that accompanies this article (shown opposite page, above) and is described below.
When pixels are viewed at close range, the RGB light-emitting diodes appear as independent dots. The distance from the screen, where these LEDs mix to form a single color, is known as the color compound distance. Superior color compound ability allows images to appear clear and sharp at close range, which is a vital factor in indoor displays. For outdoor lamp-type individual LED screens, the color compound distance can be calculated by multiplying the pixel pitch in millimeters by 500mm. For indoor (SMD or discreet LED) surface-mounted devices that have three-in-one RGB in one packet, multiply pixel pitch by 250mm, as the LEDs are very close together.
The minimum viewing distance is calculated by multiplying the pixel pitch by 750 to 1,000. This value will produce a smooth image. Closer viewing will produce an image with individual LEDs appearing as dots. The maximum viewing distance is generally 20X to 30X the screen height.
Video Processing
Another factor to consider with LED screens is drive electronics. A standard video signal cannot be directly displayed on an LED screen without first being processed. The quality of this processing is most often overlooked by prospective users of this technology. The primary rule of all broadcasting should apply: garbage in equals garbage out.
Video images are made up of a number of horizontally scanned lines, but these don't all appear on a screen at the same time. In the first 1/60th of a second (1/50th for PAL), the odd lines are shown, and in the second 1/60th of a second, the even lines are shown. A television works this way — this is called interlaced display. As most displays don't use this broadcast signal directly, the video must first be de-interlaced.
The simplest way to do this is to take the first set of lines (field), double it, and show it, ignoring the second field. Some low-end video processors do this and throw away half of the original picture information. More sophisticated approaches involve storing the first line information and then combining it with the second-line information 1/60th of a second later. A complete frame can then be displayed.
If, however, an object were moving rapidly, it may be in a different position in the second line than in the first line, and this can lead to unacceptable video effects, such as flickering. Resolving this requires interpolation of the two sets of lines, done in realtime, and then scaling the image to fit the output screen (this is normally a resolution different from the source). The combination of these processes, especially the scaling, requires a lot of powerful processing to generate clean, artifact-free, and fast-moving flicker-free video. Generally, this is done by dedicated video processing equipment. Because the processing can make a dramatic difference in the quality of the displayed image, it is worthwhile to investigate how display manufacturers process the video signal for display.
When building a very large screen display, ensure the input signal type has enough pixels to fill the screen area. For example, for a PAL or NTSC signal going into a screen of 1280x1024, processing magnification is needed to fill the screen area, and this can soften the image. If the input signal is poor quality, then magnifying it will only make it look worse. Content production companies often look at the material on a standard TV monitor, which is not always a true representation of how it will look on an LED screen. When using text, it is important to know the minimum text size so that it can be clearly read on the screen.
DVI is quickly becoming the industry standard in the PC world, and it is starting to be used in the A/V industry as well. Now, 90 percent of permanent LED installations use DVI. This is a superior way to transport material between devices and greatly improve quality by refining the picture by 30 percent in some cases.
Brightness & Contrast
The unit of measurement for LED screen brightness is the nit (cd/m2), with higher numbers meaning a brighter display. As a general rule, accept no less than 1,000 nits for an indoor display and 5,000 for outdoor displays. The way to measure this is at a normal angle to the screen using a light meter. The color temperature of the screen should normally be set to 6,500K for outdoor screens, 5,000K for indoor screens. If set this way, a full white signal should be measured at several points (usually 12, being the center and then evenly spaced around the screen) from the normal minimum viewing distance.
The screen should be set to black and then re-measured for the ambient reflected light — one measurement at the center will suffice. The brightness is an average of the 12 points of white, minus the measured ambient when the screen is black.
The viewing angle is normally defined at the point when brightness is 50 percent of the maximum. Walk around the screen to see the brightness change, and review the three primary colors and white to see if the color remains uniform at all angles.
One challenge unique to LED display technology is shouldering. Here, a color shift is caused by one LED blocking the view of another LED at extreme angles. The viewing angles should include color shifts, and if a significant color shift occurs before the brightness falls to 50 percent, then this is the viewing angle.
If screen manufacturers drive the LEDs using high currents, they can quote brightness figures in excess of 8,000 nits. However, high drive currents lead to faster degradation in the LEDs, and the screen uniformity can shift dramatically in short periods of time. Quoted life figures for the LEDs range from 20,000 to 100,000 hours. These figures are clearly only meaningful if they are determined at the actual drive current that will be used under real display conditions and at the drive levels used to produce the brightness measurement.
Electrical Considerations and Maintenance
Other things to consider include the issue of in-rush current at time of screen switch-on. It is important to have correctly specified the type of circuit breaker. Limit the amount of panels on one circuit breaker to avoid overload. Here are the rules of thumb regarding circuit breakers: breakers with the B curve have a surge rating of 3X to 5X their trip current; breakers with the C curve have a surge rating of 5X to 10X their trip current; breakers with the D curve have a surge rating of 10X to 14X times their trip current.
Also, for the earth leakage circuit breaker, variable rated devices are the best options. Electrical switchboards are typically fitted with 30mA, 100mA, or 300mA earth leakage breakers. These are also known as RCDs (Residual Current Disconnects), RCCB (Residual Current Circuit Breakers), or ELCBs (Earth Leakage Circuit Breakers).
A/V rental screens should always be operated with earth leakage breakers. Earth leakage trip value should be slightly (10 percent to 20 percent) higher than maximum calculated value per breaker. Permanent installations do not require earth leakage breakers. When designing a mains distribution system for an outdoor display, consider the above- mentioned comments on in-rush and earth leakage, and rate the power cables/looms and Y-connections IP65.
Finally, when working with LED screens, ensure the right spare parts are available for the equipment. When removing faulty parts from a display, clearly label them with a description of the fault. There is nothing worse than replacing a part with a new one that is also faulty. Become familiar with the equipment. Spend time understanding how it works before going to a trade show or demonstration. And, as simple as it sounds, RTFM (read the factual manual) and carry important pages, such as dip switch settings, with you at the gig.
When installed and configured properly, LED screens can add color, bright- ness, and excitement to any stage display. There are, however, various pro- cedures to follow when maximizing results with this technology. By following these few guidelines mentioned above, users can avoid common pitfalls of working with LEDs and can secure the best results for both the rental operators and viewing audience.
Brent Watson is the international technical support manager for Lighthouse Technologies and has worked in the lighting and A/V field for more than 20 years. He currently conducts technical training sessions with rental-and-staging operators and installers around the world. For more technical information about LED screens and panels, visit www.lighthouse-tech.com/techstuff. Reach him at target="_blank">[email protected]