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Posted 01-SEP-98
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From The Sept/Oct 1998 issue of Electrical Line Magazine:
"LEDs
THE FUTURE OF LIGHTING"
Light emitting diodes have come a long way in the twenty-some years since they debuted as those novel little red indicator lights on electronic gadgetry. But advances in LED technology and new production techniques in the intervening years have dramatically broadened the applications for these rugged little lights. Today they are just about everywhere, satisfying the needs of business, industry and even medical science for lightweight, energy-saving, and cost efficient lighting.
When LEDs were first introduced, colour was not an option. Red was only "daylight visible" colour available. While it didn’t present a problem for applications such as indicators or switch illuminators, it did hinder their acceptance for other uses. Another drawback was that unlike the incandescent bulbs’ full spectrum, spherical pattern of light, LEDs had only a focused beam of a single wavelength (colour), in a single direction, in a variety of angles. But all that has changed. The development of multi-chip and multi-LED arrays, new doping technologies to increase LED output by 20 times over earlier generations, and the production of daylight visible LEDs in virtually every color of the spectrum, including white light (long thought to be an impossibility for light emitting diodes) are just a few of the many advances that have taken LEDs from the fringes into mainstream lighting.
A leading manufacturer and pioneer of LED technology, which has put its expertise to work in product development for a broad cross-section of industry, is LEDtronics Inc. of Torrance, California. The company, founded in 1983 by Mr. Pervaiz Lodhie as a small two-person manufacturing organization, has grown into one of the world’s leading suppliers of LED products with 170 employees and sales representatives world-wide. From his company we received an overview of LED technology.
The basic LED comprises a diode chip mounted in the coined reflector cup of a lead frame, connected to electrical wires, and encased in a solid epoxy lens. It operates as a PN junction semiconductor diode – ‘P’ representing the material that conducts by virtue of an electron deficiency; and ‘N’ that conducts by virtue of free electrons. The LEDs emit a monochromatic light when energy levels change in the semiconductor diode, the shift in energy generating photons emitted as light energy. The specific colour wavelength of the light depends on the difference in energy levels and the type of semiconductor material used to form the LED chip – primarily compounds, formed by elements from groups III & V of the periodic table, which emit light when current is passed through them. LED luminosity increases as current increases depending on the semiconductor material used, and up to some maximum rating beyond which the LED will break down. To guard against breakdown, current limiting devices such as resistors, constant current ICs, or current limiting power supplies, are used in all applications.
The wavelength for visible colours such as blue, green, yellow and red fall into the spectral range from 400 to 700 nanometers (nm). Infrared LEDs reach wavelengths from 830 to 940 nm and higher. The colour of the LEDs is determined exclusively by the semiconductor compound used and not by the lens colour.
It is in colour applications where LEDs really shine. White light from a typical incandescent bulb must be filtered so that only light from a particular part of the spectrum is made visible. That takes its toll in wasted energy and colour purity. Incandescent bulbs can waste 90% and more of their energy in light blocked by the lens of the filter. By contrast, because it is monochromatic, a light emitting diode is able to deliver 100% of its energy as brilliant, pure unfiltered light in the full spectrum of colour, and without any colour shift.
Multicolour LEDs are created by combining different LED chips within a common LED housing and applying positive and negative voltages to turn on the individual colours.
Lenses are available in several different configurations:
1. Clear types having no tint of diffusion and producing the greatest light output and narrowest viewing angle. They are designed for applications requiring very high intensity, or colourless LEDs in the "off" state.
2. Tinted types to indicate in the "off" state of what the lamp’s colour will be when it is in the "on" state.
3. Diffused types having tiny glass particles embedded in the epoxy lens. This spreads the light to a viewing angle of approximately ± 35 degrees from centre. These types of LEDs are often used for applications in which the LED protrudes through a hole in the front panel of the equipment.
4. Non-diffused types with no glass particles in the epoxy lens and producing a narrow viewing angle of ± 12 degrees from centre. They are often used in backlighting applications in which the LED is focused on a translucent window of a panel.
The technical data for just a few of the LED colours available, along with descriptions of the semiconductor compounds are shown in the table.
Long life, low power requirements and low operating costs are some important reasons for LEDs growing popularity. LEDs use a fraction of the power required by conventional filament bulbs and their long life means a significant decrease in maintenance costs and downtime losses. Unlike incandescent lights, which generate a high intensity light for a short operating life, typically, LEDs with no filaments to wear out, have a life span of 100,000 hours or more, about 10 times that of the average incandescent lamp. Their solid state design enables them to withstand shock, vibration, frequent switching, temperature and environmental extremes without compromising their performance. For industrial and motor control application and plant automation there is now a wide range of product available: stacklights, bargraph displays, chip-on-board LEDs, discrete LED replacements for based applications, based replacements for incandescents from 3mm "grain of wheat" to 25mm screw base sizes, as well as a full array of LED pilot and indicator lamps. Also, ultra-bright, multi-chip lights packaged into based lamps are now available as replacements for incandescent-lighted pushbuttons, annuciators or relampable panel indicators.
Colour Wavelength
nm Fwd
Voltage
VF@20ma Intensity
mcd Viewing
Angle Material
Infrared 940 1.5 14 (100mA) 20° GaAIAs
Infrared 880 1.7 20 (100mA) 40° GaAIAs
Ultra red 626 2.0 900 30° InGaAIP
Ultra orange 612 2.0 6500 15° InGaAIP
Ultra p. yellow 583 2.1 1300 30° InGaAIP
Ultra green 570 2.1 1200 8° InGaAIP
Aqua green 525 3.5 6000 15° Sic/GaN
Blue green 505 3.5 2000 30° SiC/GaN
Super blue 470 3.6 1200 15° GaN
Ultra blue 430 4.9 400 15° SiC/GaN
GaAIAs Gallium Aluminum Arsenide
InGaAIP Indium Gallium Aluminum Phosphide
SiC/GaN Silicon Carbide/Gallium Nitride
nm nanometer
mcd millicandela
Colour purity is one of the unique qualities of LEDs that is making it popular with the photographic industry. In an industry where even a slight shift in colour temperature can spell disaster for sensitive film emulsions, LEDs’ ability to maintain their precise Kelvin colour temperature over their 100,000 hour lifetime has been a boon for manufacturers as well as colour labs and darkrooms. LED colour-correct safelights protect photosensitive materials and LED striplights and pathlights are able to define perimeters and specific areas for personnel.
LEDs are also adding a new flair and dimension in creative effects for lighting professionals, architects and interior designers. While LEDs have not yet reached the stage where they are replacing general lighting, they have emerged as an effective tool to enhance the overall lighting scene. The availability of a wide range of LED options from direct screw-in replacements for incandescent bulbs to multi-LED arrays attached to conventional electrical bases, and directional lighting for stairways and walkways, is allowing their use in a variety of innovative ways, both indoors and outdoors. Also, with LEDs’ long life, architects and designers are able to make use of accent lighting for those hard-to-reach locations which, before, would have been unthinkable and impractical with conventional lighting. For outdoor situations, the minimal energy requirements of LEDs make possible dramatic solarpowered lighting effects.
While there are literally thousands of ‘off the shelf’ LED products available, LED manufacturers are quick to point out, "if you don’t see what you want ask for it and we’ll make it for you."
Mr. Lodhie of LEDtronics, said, "Developing new products for customers is what we do best. We enjoy working with companies in the development of product to fit their specific needs." This philosophy resulted in the company working with Sreco-Flexible™, a California based manufacturer of plumbing inspection equipment, in developing an improved light source for the "Flexicam", one of the smallest cameras in the world. The camera is connected to a flexible and rugged "snake" that can negotiate 90° bends and go deep into the piping. The light source developed for the camera by LEDtronics was built into the ring surrounding the camera lens. It contained eight multi-chip LEDs, with each chip containing six LEDs per chip. The result was an extraordinarily powerful light output for its size, and an ideal match for the sensitivity of the camera. The light provides a well-lit view of the cracks and problem areas of pipes throughout the system for videotaping and analysis.
Another product of this philosophy is a photovoltaic/battery illumination system for emergency call boxes on university campuses, hospital grounds, parks and parking lots, developed by the RCS Communications Group. These boxes, which are placed in remote and unprotected areas, are identified by a bright blue LED on a pole above the box. Because of the LEDs low power consumption, the units are entirely wireless, self-contained units, powered by batteries charged from solar cells, providing a beacon of light from dusk to dawn and transmitting calls via two-way radio.
As a technology born of the space age, LEDs are no strangers to space travel either, having made many journeys into that hostile environment. The same critical demands for reliability and long life demanded by space exploration are also making LEDs popular in the aviation and avionics industries.
Jim Veitengruber, unit chief flight deck engineer with Boeing in Everett, Washington, commenting on a retrofit of incandescent pushbutton switches and lightplates previously used in Boeing 777 cockpits, said, "We changed over to LEDs because of a high failure rate with the incandescents. With LEDs we save on maintenance time for relamping and avoid the potential for introducing mechanical problems in relamping, too. With LEDs’ life expectancy, they’ll be good for the life of the plane." Mr. Veitengruber added that a common complaint from pilots that incandescent pushbutton switches were often hot to the touch, has been eliminated, too.
Moving from the cockpit into the main passenger cabin, based LED strip-lights, pathlights and overhead cabin light replacements of regular incandescents, have been developed for new aircraft installations and older aircraft retrofits. Along with LEDs’ long life and energy-saving attributes, their non-heat generating properties also have a significant potential for savings. Mr. Jordon Papanier, marketing coordinator for LEDtronics, said, "Incandescent light generates a lot of heat in the cabin area and that puts an extra load on the air conditioning system. An airline engineer I was dealing with recently, estimated that by using LEDs throughout that they would be able to save about 5,000 to 7,000 gallons per plane annually."
One of the unique safety products developed by LEDtronics for the aviation industry is its Cockpit FlashLED™. The unit’s pure unfiltered colour preserves the night vision of pilots and ground control personnel and provides an ideal light for reading air charts. It operates on a set of AA batteries and its solid state LED technology gives it an operating life 16 times greater than would be possible with conventional design.
LEDs’ lightweight, long life, durability, and intense monochromatic red light has also caught the eye of the transportation industry. If you haven’t already noticed it, many automobile manufacturers have already installed LEDs as the third brake light on their cars, and there is an industry commitment to replace incandescents on new cars with LEDs, wherever possible, by the year 2000. An interesting safety feature of LEDs is that they ‘come on’ 1/5th of a second sooner than incandescents. This split second difference was one of the reasons that the Washington D.C. Area Transit Authority replaced incandescent brake lights on their buses with LEDs. They had been experiencing a lot of rear end collisions with cars running into the buses, and felt that the brighter quality of the LEDs and quicker response time might alleviate the problem. After two and a half years, was the changeover worth it? Mr. William Campbell, supervisor of the Washington Metro bus overhaul program, said, "The program was cost effective just from the reduced maintenance alone. With the previous lens and socket design, we had problems with insulation breakdowns, along with dirt and moisture getting behind the lens and obscuring the light. With the sealed unit of the LEDs we’re getting a brighter, and much higher visibility light." But has it reduced accidents? In answer to that question, Mr. Campbell said, "I haven’t the statistics at hand, but we’re more than satisfied."
Another area of growing usage for LEDs is for traffic signals on city streets. A big incentive for their use has
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