Light Emitting Diodes (LED) Information
Image Credit: Digi-Key Corporation, Kingbright Corporation, Newark
What are LEDs?
Light emitting diodes (LEDs) are PN junction devices that give off light radiation through electroluminescence when forward biased. They are used as various indicators in aviation, automotive and traffic lighting, as well as lighting for some lamps and torches. Most light emitting diodes function in the near infrared and visible ranges, though there are now UV LEDs as well.
LEDs Compared to Other Light Sources
LEDs sport a number of advantages over other lighting sources such as incandescent and fluorescent bulbs. The efficiency of LEDs is greater than incandescent bulbs and unlike fluorescent bulbs, it is independent of size and shape. LEDs turn on and off quickly, can be easily dimmed by modulation or current reduction, and can easily be cycled on and off. They emit little heat in the form of IR and heat waste is channeled through the base rather than the bulb. They are also resistant to shock, fail slowly over time, and have a much longer lifetime (approximately 40,000 hours) than fluorescents (approximately 12,000 hours) or incandescents (approximately 1,500 hours).
There are certain disadvantages to LEDs however. LEDs tend to cost more than conventional lights. The voltage sensitivity of LEDs requires series resistors or current-regulated power supplies to prevent current overload. Performance of LEDs is also largely dependent on temperature. Excessive heat from the environment or heat generated internally can damage or degrade the diode if not removed via heat sinking, shortening its life. The light quality of LEDs is also considered inferior to incandescent and natural light, since it can affect the color rendering properties of the environment it illuminates.
When selecting a diode, industrial buyer's should consider the wavelength or type of light, the forward current and voltage, the optical power and luminous intensity, the type of construction, and various other specifications.
Wavelength or Color
The wavelength or color is usually one of the most important criteria when selecting an LED. The wavelength emitted by the diode can be within the visible, infrared, or ultraviolet spectrum. Multicolored LEDs can emit a range or number of different wavelengths. It is important that the color of an LED is appropriate for its intended use, since different applications involve different colors or types of light.
Forward Current and Voltage
LEDs operate at a rated current and voltage, usually within a certain range. For LEDs, voltage is normally measured in volts (V) and current is measured in milliamps (mA). If a system supplies a predetermined voltage to the system, it is important to select an LED designed with those specifications. Since current overload is a main cause of LED failure, it is important to correctly match this specification and provide any necessary current regulators.
Optical Power and Luminous Intensity
Optical power and luminous intensity are important performance specifications for LEDs. Optical power is the energy output of the LED, usually measured in milliwatts (mW) because LEDs require less power than other light sources. Luminous intensity is the amount of light an LED can put out, usually measured in milliCandelas. Luminosity and power become more important in the selection of larger LEDs in applications where more light is required.
LEDs can function in different ways depending on the type of construction.
- Edge emitting LEDs have outputs that emanate from between the heterogeneous layers. They have relatively small beam divergence, allowing for more efficient fiber coupling than surface-emitting LEDs. They allow for higher data rates in optical fiber communications.
- Surface emitting LEDs have a thin active layer parallel to the surface from which the light is extracted. They emit light perpendicular to the active region.
- Super luminescent LEDs (SLDs) are a variation of edge emitting LEDs. They are based on stimulated emission with amplification but insufficient feedback for oscillation to build up. SLDs are applied in situations where a smooth and broadband optical spectrum (i.e. low temporal coherence), combined with high spatial coherence and relatively high intensity, is required.
Other specifications that may be important when selecting LEDs include lens type, viewing angle, and features such as duality and bipolarity. Lens types shapes can change the way emitted light is projected, and typically are either flat or domed. Viewing angle is dependent upon diffusion from the lens. Usually the larger the viewing angle, the less bright the LED. Diffused types generally have larger viewing angles and non-diffused types have smaller viewing angles. Dual devices include LEDs with two diode lamps in the same housing. Bipolar featured LEDs are those that, unlike most LEDs, can operate even if the voltage is reversed. Some models offer different color outputs when the voltage is reversed.
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