Understanding LEDs in Lighting & Displays

Dec. 20, 2004
Increasingly, architects find themselves needing to understand light-emitting diode (LED) technologies in order to ensure that their design teams intelligently specify two very different lines of products: public video displays and highly efficient lighting systems.Industry interest in LEDs for both kinds of applications has accelerated in the past year. In the lighting industry alone, 80 companies exhibited LED products at Lightfair 2004, and LED products won four out of the six top new product awards, including Best of Show.The reason for this growing popularity is that LEDs satisfy an ever-increasing demand for innovative equipment that is smaller, smarter and produces brighter colors. LEDs, in particular colored LED systems, offer a number of advantages over traditional display and lighting technologies.In this AIA accredited continuing education course, readers will be introduced to both technologies. On the lighting side, Craig DiLouie, communications director for the Lighting Controls Association, details critical aspects of the LED driver, the device that controls LED fixtures. Our review of LED displays is derived from an AIA accredited course offered both live and online by Al Jensen, marketing vice president of BARCOMedia. LED Drivers: The Power of New LightingBy Craig DiLouieResearchers have been advancing LED lighting technologies for years. At the forefront of this group has been Dr. Nadarajah Narendran, Director of Research for the Lighting Research Center at Rensselaer Polytechnic Institute, who has witnessed a surge of interest in recent years.“LEDs are already an important technology,” Dr. Narendran says, “because of their advantages – life, controllability, both spatial and temporal, and the ability to create rugged, unique fixtures.”While the basic principles of the LED are well understood, as this technology becomes more popular, architects and designers will need to understand a crucial component of the LED lighting system – the LED driver. In the Driver’s SeatThe LED driver functions much like a fluorescent ballast in that it supplies the proper starting voltage and then regulates the current flowing through the lamp after startup. “An LED driver is the power supply for an LED system, much like a ballast is to a fluorescent or HID lighting system,” says Al Marble, Manager – Sales & Market Development for Philips/Advance Transformer. As low-voltage lamps, LEDs require a constant direct current (DC) voltage to operate optimally. Operating on low-voltage DC power enables LEDs to be easily adapted to different power supplies, permits longer standby power and increases both safety and the life of the device. An individual LED needs just 2 to 4 volts of DC power  and several hundred milliamps (mA) of current – the equivalent of two to three penlight batteries. But when LEDs are connected in series to form an array, higher voltages are required. The device that supplies this power is the LED driver, which converts standard line voltage – typically 120 volts alternating current (AC) cycling at 60Hz in the U.S. – to the low DC voltages required by LEDs.While they are operating, it is crucial that LEDs be protected from line-voltage fluctuations, because changes in voltage can have several consequences. If current exceeds the manufacturer’s recommendations, the LEDs will become brighter but their light output can degrade at a faster rate due to heat, which shortens their useful life. (One definition of useful life is the time prior to the age at which an LED’s light output declines by 30 percent.) The LED driver regulates the current flowing through the LED during operation and protects it from voltage fluctuations.Once we understand the function of the LED driver, we can differentiate various drivers by type. They may be constant voltage types (usually 10V, 12V or 24V) or constant current types (350mA, 700mA or 1A). Some drivers are manufactured to operate specific LED arrays, while others can operate most commonly available LEDs. LED drivers are usually compact enough to fit inside a junction box, include isolated Class 2 output for safe handling of the load, operate at high system efficiency, and offer remote operation of the power supply.Advanced Lighting ControlOther drive functions that can be specified include dimming, color-changing or sequencing of LEDs. In fact, a major advantage of LEDs is that they are easily integrated with advanced electronics that enable not just dimming and color-changing but also preset automated commands, the ability to sense the presence of occupants or other specific environmental conditions. Dimming. Drivers with dimming capability can control light output over the full range of an LED’s lumen capacity, from zero to 100 percent.
Dimming drivers can dim LEDs in one of several ways – by reducing the current, using a traditional potentiometer; by controlling the rate at which the LED flickers, which is a digital control scheme known as pulse width modulation (PWM); or by using even more sophisticated methods. Most dimming drivers operate using the PWM method. With this method, the frequency could range from a hundred modulations per second to as high as hundreds of thousands of modulations per second, so that the LED appears to be continuously lighted without flicker. A benefit of the PWM method is that it enables dimming with minimal color shift in the LED output. According to the Lighting Research Center, dimming causes LEDs to experience a shift in spectral power distribution similar to what you get when you dim an incandescent lamp. If colored LEDs in an array are clustered to produce white light, the amount of shift, particularly with red and yellow LEDs in the cluster, may produce an undesirable color shift.
Dimming does not result in a loss of efficiency. During dimming, the LEDs are still operated at the same voltage and current as during full light output. In addition, lamp life is not affected by dimming, as is sometimes the case with frequently dimmed fluorescent lighting. Rather, dimming LEDs may lengthen the useful life of LEDs by reducing operating temperatures inside the light source.Color Control. Drivers can be specified to control color-changing or sequencing by dimming a mix of colored LEDs in an array.
Alternatively, the driver can work with a color sequencer, a device that receives the 10V or 24V output from the LED driver and converts it into three-channel output – usually red, blue and green – that can be mixed to create a wide dynamic range of colors. When a sequencer is used, it generates a preset sequence, with color changes occurring at a speed that can be programmed dynamically.
A third option is for each LED to be individually controlled and programmed by interfacing with a DMX digital controller, enabling thousands of LEDs to dynamically dim up or down to create a seemingly infinite spectrum of colors.Interoperability with other control devices. Most LED drivers are compatible with commercially available 0-10V control devices as well as with occupancy sensors, photocells, wallbox dimmers, remote controls, architectural and theatrical controls, and building and lighting automation systems. LEDs can also work with devices governed by the DMX and digital addressable lighting interface (DALI) protocols. In the future, they may include wireless (RF) as a control option. Craig DiLouie serves as communications director for the Lighting Controls Association (www.aboutlightingcontrols.org) and principal of ZING Communications, Inc. (www.zinginc.com), a marketing communications, consulting and research firm specializing in the lighting and electrical industries.LED Displays: From Pixels to PicturesBy Al JensenLEDs are plastic capsules that contain a specific chemical compound, mounted on a microscopic wafer, that emits light when an electric current is passed through it. Known as Surface Mount Diodes (SMD), these kinds of LEDs are used on indoor products. A surface mount diode may be a discrete single color or a 3-in-1 color package.Individual LEDs are available in many grades of quality and shades of color, including high-grade red, green, blue and amber LEDs. When red, green and blue LEDs are clustered together and shine at their full brightness, their combined light emission appears white to the human eye. Varying the intensity of the three colors independently creates all the other shades. This is known as RGB color, for red-green-blue.Single-color amber LEDs are used for monochrome displays. A cluster of three LEDs creates a pixel, the basic picture element. Each dot on a video display monitor is a single pixel. On an LED display, each pixel is an RGB cluster of three LEDs and has its own color and brightness attributes. Pitch A display’s pitch is its measurement of distance from the center of one pixel to the center of another pixel. The pitch determines the viewing distance. The smaller the pitch, the shorter the viewing distance. Viewing DistanceMinimum Distance is the point at which the fully illuminated red, green, and blue components appear to the eye to blend into white. Typically, minimum distance is calculated as 2’ times the pitch of the display. Thus a display with a 12 mm pitch would require a minimum distance from the viewer of 24 feet:  2’ x 12 mm = 24’Maximum Distance is the point at which the smallest characters the display can generate begin to be illegible. This point varies greatly with the content of the display, but in general it can be calculated as seven times the minimum distance. Thus, in the above example, the maximum distance would be 168 feet:24’ x 7 = 168’ Maximum distance can also be calculated as roughly 30 to 40 times the height of the display. So, for example, the maximum distance for a nine-foot high display would be between 270 and 360 feet. MatrixAn array of pixels forms a matrix, which is usually described as the product of the total number of pixels spanning the width times the total pixels spanning the height. So, for example, a 600 pixel by 800 pixel screen would be described as a 480K screen (800 x 600 = 480,000). Scoreboards and similar data signage may be constructed as either a matrix or segmented digits. Fill Factor refers to how densely pixels are packed into a space.  A high fill factor means more LEDs per square foot and generally produces a smoother looking picture. Resolution is the total number of pixels in a display. The higher the number of pixels, the greater the possible detail. A standard video signal (NTSC in the United States) has a native resolution of 645 x 485 pixels. Aspect Ratio is the relationship in a video image between the width of the image and the height of the image. It is expressed as a ratio of width to height (4:3, 16:9). The aspect ratio for NTSC video and most computer monitors is 4:3. The standard aspect ratio for widescreen movies and digital HDTV is 16:9. Viewing Angle is the number of degrees left or right of center that a viewer can be positioned before a display’s output drops to half brightness. It is expressed in degrees horizontal and vertical. For example, Horizontal 140°(±70°), and vertical 60º(±30°). Color Shift: Brightness and color do not necessarily drop off at the same rate as the viewer moves to one side or the other of a display, because the red, green and blue LEDs may not change consistently. As the viewing angle increases LED’s begin to shadow one another. The color shift should happen after the viewing angle drop off.Vertical color shift may also be caused by the shaders at extreme angles. Brightness is measured in candelas per square meter (cd/m2), also known as “nits,” which are roughly 0.3 foot lamberts. Note that there is no correlation between nits and measures of illuminated brightness such as LUX (Lumens per square meter). The higher the  number of nits, the brighter the display.For example, 1,500 nits provides readable text in outdoor daylight, while outdoor video requires up to 5,000 nits for good color depth. Indoor video requires 1,500 to 2,500 nits. Color TemperatureMeasured in degrees Kelvin, color temperature is an indication of the amount of “warmth”(yellow, red tones) or “coolness”(blue tones) in a white light. Standard video color temperature is 6,500ºK. Color temperature is adjustable on higher-quality LEDs. Contrast RatioContrast ratio is a measure of the dynamic range of a displayed image – the distance between the “whitest” white and the “blackest” black in a video wave form. There is no industry standard for measuring contrast ratio. LED has high numbers because it has no residual glow when turned off in a dark room. A more important number for video is shades of gray. Brightness is the amount of overall brightness of the image. CalibrationFor even picture quality, the brightness of each LED is calibrated (adjusted) to match the brightness of the surrounding LEDs. A combination of hardware and software is used to automatically calibrate pixel-to-pixel and tile-to-tile. HOW to choose a Daylight DisplayFirst check the International Protection rating, a unique reference base for all types of industrial applications.Choose tiles that are resistant to water, dust, heat, salt, UV, animal intrusions, and vibration.Check the life expectancy, which is determined by how much operating time you will have before the LED fades to half brightness. LEDs fade significantly faster after they dim to half brightness. Quality pixels range from 50,000 to 100,000 hours.Barco is a recognized world leader in state-of-the-art large-screen visualization solutions. At Barco Media & Entertainment, we specialize in developing advanced projection and LED (light emitting diode) display solutions targeted at the specific needs of the markets in the Media & Entertainment industry. From the professional digital cinema (www.barco.com/digitalCinema/) market to large-scale entertainment, or corporate events (www.events.barco.com/); from installations at entertainment (www.barco.com/entertainment/) sites, or in sports (www.barco.com/sports/) venues to branding (www.barco.com/branding/) your company’s new head-quarters, Barco has a solution tailor-made to answer your specific needs. Call today for more information 800-429-3711 (www.media.barco.com).

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