2. A light-emitting diode (LED) is a two-lead semiconductor light source.
It is a p–n junction diode, which emits light when activated. When a
suitable voltage is applied to the leads, electrons are able to recombine
with electron holes within the device, releasing energy in the form
of photons. This effect is called electroluminescence, and the color of
the light (corresponding to the energy of the photon) is determined by
the energy band gap of the semiconductor.
4. Blue LEDs were first developed by RCA in 1972. However, these initial blue LEDs
were not very bright.
The first high-brightness blue LED was demonstrated by Shuji
Nakamura of Nichia Corporation in 1994 and was based on InGaN. In
parallel, Isamu Akasaki and Hiroshi Amano in Nagoya were working on
developing the important GaN nucleation on sapphire substrates and the
demonstration of p-type doping of GaN. Nakamura, Akasaki and Amano were
awarded the Nobel prize in physics for their work.
5. White LED
The existence of blue LEDs and high-efficiency LEDs quickly led to
the development of the first white LED phosphor coating to
mix down-converted yellow light with blue to produce light that
In January 2012, Osram demonstrated high-power InGaN LEDs grown
on silicon substrates commercially. It has been speculated that the
use of six-inch silicon wafers instead of two-inch sapphire wafers
and epitaxy manufacturing processes could reduce production
costs by up to 90%.
6. Illumination breakthrough
The invention of the blue LED made possible a simple and
effective way to generate white light. By coating a blue LED
with a phosphor material, a portion of the blue light can be
converted to green, yellow and red light. This mixture of
colored light will be perceived by humans as white light and
can therefore be used for general illumination. The first white
LEDs were expensive and inefficient.
RGB LEDs consist of three LEDs. Each
LED actually has one red, one green and
one blue light. These three colored LEDs
are capable of producing over 16 million
7. White light
There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that
generate high-intensity white light. One is to use individual LEDs that emit three primary
colors—red, green, and blue—and then mix all the colors to form white light.
There are three main methods of mixing colors to produce white light from an LED:
•blue LED + green LED + red LED (color mixing; can be used as backlighting for displays)
•near-UV or UV LED + RGB phosphor (an LED producing light with a wavelength shorter
than blue's is used to excite an RGB phosphor)
•blue LED + yellow phosphor (two complementary colors combine to form white light;
more efficient than first two methods and more commonly used)
Typical efficacy (lm/
Red 620 < λ < 645 72 0.39
Red-orange 610 < λ < 620 98 0.29
Green 520 < λ < 550 93 0.15
Cyan 490 < λ < 520 75 0.26
Blue 460 < λ < 490 37 0.35
Efficiency and operational parameters
Efficiency: LEDs emit more lumens per watt than incandescent light
Color: LEDs can emit light of an intended color without using any
color filters as traditional lighting methods need.
Size: LEDs can be very small and are easily attached to printed
Warmup time: LEDs light up very quickly.
Cycling: LEDs are ideal for uses subject to frequent on-off cycling,
unlike incandescent and fluorescent lamps that fail faster when
often, or high-intensity discharge lamps (HID lamps) that require a
Dimming: LEDs can very easily be dimmed either by pulse-width
modulation or lowering the forward current.
Cool light: In contrast to most light sources, LEDs radiate very little
the form of IR that can cause damage to sensitive objects or fabrics
10. Slow failure: LEDs mostly fail by dimming over time, rather than the
abrupt failure of incandescent bulbs.
Lifetime: LEDs can have a relatively long useful life. One report
estimates 35,000 to 50,000 hours of useful life, though time to
failure may be
Shock resistance: LEDs, being solid-state components, are difficult to
damage with external shock, unlike fluorescent and incandescent
which are fragile.
Focus: The solid package of the LED can be designed to focus its light.
Incandescent and fluorescent sources often require an external
to collect light and direct it in a usable manner.
•High initial price: LEDs are currently more expensive (price per
lumen) on an initial capital cost basis, than most conventional
•Temperature dependence: LED performance largely depends on
the ambient temperature of the operating environment – or
"thermal management" properties. Over-driving an LED in high
ambient temperatures may result in overheating the LED package,
eventually leading to device failure.
•Voltage sensitivity: LEDs must be supplied with the voltage above
the threshold and a current below the rating.
•Electrical polarity: Unlike incandescent light bulbs, which
illuminate regardless of the electrical polarity, LEDs will only light
with correct electrical polarity. To automatically match source
polarity to LED devices, rectifiers can be used.
12. •Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now
capable of exceeding safe limits of the so-called blue-light hazard as defined
•Efficiency droop: The efficiency of LEDs decreases as the electric
current increases. Heating also increases with higher currents which
lifetime of the LED. These effects put practical limits on the current through an
•Impact on insects: LEDs are much more attractive to insects than sodium-
vapor lights, so much so that there has been speculative concern about the
disruption to food webs.
•Use in winter conditions: Since they do not give off much heat in comparison
to traditional electrical lights, LED lights used for traffic control can have snow
them, leading to accidents.