Light-emitting diodes (LEDs) are solid-state devices that convert electric energy directly into light of a single color. Because they employ “cold” light generation technology, in which most of the energy is delivered in the visible spectrum, energy is not wasted in production of heat at non-visible wavelengths. In comparison, most of the energy in an incandescent
lamp
is in the
infrared
(or non-visible) portion of the spectrum. Both fluorescent and HID lamps produce a great deal of heat as well. Additional characteristics of LEDs:
Can be powered from a portable battery pack
Are small in size and resistant to vibration and shock.
Have a very fast “on-time” (60 nsec vs 10 msec for an incandescent lamp).
Have good color resolution and present low, or no, shock hazard.
The centerpiece of a typical LED is a diode that is chip-mounted in a
reflector
cup and held in place by a mild steel lead frame connected to a pair of electrical wires. The entire arrangement is then encapsulated in epoxy. The diode chip is generally about 0.25 mm square. When current flows across the junction of two different materials, light is produced from within the solid crystal chip. The shape, or width, of the emitted light
beam
is determined by a variety of factors: the shape of the
reflector
cup, the size of the LED chip, the shape of the epoxy
lens
and the distance between the LED chip and the epoxy lens. The composition of the materials determines the
wavelength
and color of light. In addition to visible wavelengths, LEDs are also available in
infrared
wavelengths, from 830 nm to 940 nm.
The definition of “lifetime” varies from industry to industry. The useful life for a
semiconductor
is defined as the calculated time for the light
level
to decline to 50% of its original value. For the lighting industry, the average life of a particular
lamp
type is the point where 50% of the lamps in a representative group have burned out. The life of an LED depends on its packaging configuration, drive current, and operating environment. A high ambient
temperature
greatly shortens an LED's life.
Additionally, LEDs now cover the entire light spectrum, including red, orange, yellow, green, blue, and white. Although colored light is useful for more creative installations,
white light
remains the holy grail of LED technology. Until a true white is possible, researchers have developed three alternatives:
Blend the beams. This technique involves mixing the light from multiple single-color devices. (Typically red, blue, and green.) Adjusting the beams' relative intensity yields the desired color.
Provide a phosphor coating. When energized photons from a blue LED strike a phosphor coating, it will emit light as a mixture of wavelengths to produce a white color.
Create a light sandwich. Blue light from one LED device elicits orange light from an adjacent layer of a different material. The
complementary colors
mix to produce white.
Of the three methods, the phosphor approach appears to be the most promising technology.
Another shortcoming of early LED designs was light output, so researchers have been working on several methods for increasing lumens per watt. A new “doping” technique increases light output several times over compared to earlier generations of LEDs. Other methods under development include:
Producing larger semiconductors.
Passing larger currents with better heat extraction.
Designing a different shape for the device.
Improving light conversion efficiency.
Packaging several LEDs within a single epoxy dome.
One family of LEDs may already be closer to improved light output. Devices with enlarged chips produce more light while maintaining proper heat and current management. These advances allow the units to generate 10 times to 20 times more light than standard indicator lights, making them a practical illumination source for lighting fixtures.
Before LEDs can enter the general illumination market, designers and advocates of the technology must overcome several problems, including the usual obstacles to mainstream market adoption: industry-accepted standards must be developed and costs must be reduced. But more specific issues remain. Things like lumen-per-watt efficacy and color consistency must be improved, and reliability and lumen maintenance should be addressed. Nevertheless, LEDs are well on their way to becoming a viable lighting alternative.
LED applications in entertainment:
Advantages of using LEDs:
LEDs emitting light of an intended color without the use of color filters.
The shape of the LED package allows light to be focused. Incandescent and fluorescent sources often require an external
reflector
to collect light and direct it in a useable manner.
LEDs are insensitive to vibration and shocks, unlike incandescent and discharge sources.
LEDs are built inside solid cases that protect them.
LEDs have an extremely long life span: typically ten years, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs.
LEDs give off less heat than incandescent light bulbs with similar light output.
LEDs light up very quickly. An illumination LED will achieve full brightness in approximately 0.01 seconds, 10 times faster than an incandescent light bulb (0.1 second), and many times faster than a compact fluorescent lamp, which starts to come on after 0.5 seconds or 1 second, but does not achieve full brightness for 30 seconds or more. A typical red indicator LED will achieve full brightness in microseconds, or possibly less if it is used for communication devices.
Changing the full bar mood from red to blue with LEDs. Photo courtesy of D-LED Israel
