LEDs: Rugged, versatile light sources
Once used only as simple status indicators, LEDs, or light emitting diodes, now play prominent roles in many applications, including back lighting, panel indication, decorative illumination, emergency lighting, and signage. Greater use of LEDs can be attributed to new manufacturing technologies, packaging innovations, and availability of an increasing number of colors, along with a growing awar...
Once used only as simple status indicators, LEDs, or light emitting diodes, now play prominent roles in many applications, including back lighting, panel indication, decorative illumination, emergency lighting, and signage. Greater use of LEDs can be attributed to new manufacturing technologies, packaging innovations, and availability of an increasing number of colors, along with a growing awareness of LED advantages, which include long life spans, vivid sunlight-visible colors, and low power requirements.
A basic LED (see illustration) consists of a semiconductor diode chip mounted in a reflector cup of a lead frame connected to electrical (wire bond) wires, and then encased in a solid epoxy lens. LEDs emit light when energy levels change in the semiconductor diode. This energy shift generates photons, some of which are emitted as light. Specific wavelength of the light depends on the difference in energy levels as well as type of semiconductor material used to form the LED chip.
Molding different LED chips within a common housing creates multicolor LEDs (for example, red, green, blue chips for RGB LEDs). Applying positive and negative voltages turns on each color. Beam angle (narrow or wide) depends on how the LED chip is packaged and is affected by reflector cup shape, LED chip size, epoxy lens shape, and distance between the LED chip and the top of the epoxy lens.
LEDs are available in visible and infrared wavelengths. Infrared LEDs reach wavelengths of 830 to 940 nm. Visible colors include red, yellow, orange, amber, green, blue/green, blue, and white, which fall into the spectral wavelength region of 400 to 700 nm. Colored light of an LED is determined exclusively by the semiconductor compound used to make the LED chip and is independent of the epoxy lens color.
Advances in technology in the past few years have dramatically broadened LED application. At one time, red was the only 'daylight-visible' colored LED. Developments have led to an increase in LED light output as much as 20 times over earlier generations and to the production of daylight-visible devices in virtually any color. Unlike incandescent bulbs that give off the full spectrum of light in a spherical pattern, LEDs emit a focused beam of a single wavelength (color) in one direction in a variety of angles, which is sufficient for many applications. However, it was with the introduction of multi-chip arrays and high-flux LED chips (see illustration) that LEDs began to achieve the effect of an incandescent filament. Even white light, long thought to be impossible to obtain, is now available (see more on 'White-light LEDs' in this article online at www.controlen.com/archive ).
LEDs are efficient light sources, delivering 100% of their energy as colored light. They come in a variety of standard bases, in sizes ranging from T1 (3 mm) to medium-screw 25-mm G30-sized bulbs and larger. Lens configurations range from clear to tinted, diffused, and non-diffused, depending on the application. Clear lenses produce the greatest light output while non-diffused types are often used for backlighting. Overall, LEDs are rugged, durable, daylight visible and have a lifespan measured in years not hours.
Jordon Papanier, LEDtronics Inc., Torrance, CA, www.ledtronics.com , firstname.lastname@example.org . Download a pdf on LED technology at www.ledtronics.com/pages/utilizing_LEDs/util_led.pdf
Two methods are used to produce white light from LEDs:
Combining red, green, and blue LED chips in one discrete package or cluster LED lamp. The technique of mixing red, green, and blue (RGB) LED chips is typically used in signage because RGB LEDs can be combined to create 256 colors cost-effectively and efficiently.
Coating blue InGaN (indium-gallium-nitride) LED chips with phosphorus. Blue light from an InGaN LED chip filters through the phosphorus and generates a cool-white or fluorescent-light appearance. InGaN technology offers high reliability and color integrity. Brightness and color purity depend on the amount of phosphorus coating. Three shades of InGaN-white LEDs exist: cool white, pale white, and incandescent white. Cool white has the least amount of phosphorus and is the brightest.
At one time, lack of white-light LEDs limited integration of LEDs into some applications. However, it is a misconception that InGaN-white LEDs can illuminate a lens of any color, and simplify lighting requirements and designs. Because red is not represented in white LEDs, they can only be used behind a clear or milky white lens or panel. Placing a white LED behind a red lens produces pink, a green lens shifts to aqua, and so on. To maintain accurate and brilliant colors, LED color must match lens color. Therefore, white LEDs made from a blue chip cannot be used as a general backlighting light source for different-colored lenses and panels.
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