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What is a LED light?
A LED lamp is a light-emitting diode (LED)
product that is assembled into a lamp (or light bulb) for use in
lighting fixtures. LED lamps have a lifespan and electrical
 efficiency
that is several times better than incandescent lamps, and
significantly better than most fluorescent lamps, with some
chips able to emit more than 100 lumens per watt.
Like incandescent lamps and unlike most
fluorescent lamps (e.g. tubes and Compact Flourescent Lamps -
CFL), LED lights come to full brightness without need for a
warm-up time; the life of fluorescent lighting is also reduced
by frequent switching on and off. Initial cost of LED is usually
higher. Degradation of LED die and packaging materials reduces
light output to some extent over time.
With research into organic LEDs (OLED) and
polymer LEDs (PLED), cost per lumen and output per device have
been improving so rapidly according to what has been called
Haitz's law, analogous to Moore's law for semiconductor devices.
(Moore's law is the observation that the number of transistors
on integrated circuits doubles approximately every two years)
Some LED lamps are made to be a directly
compatible drop-in replacement for incandescent or fluorescent
lamps. An LED lamp packaging may show the lumen output, power
consumption in watts, color temperature or description ("warm
white") and sometimes the equivalent wattage of an incandescent
lamp of similar luminous output.
LEDs do not emit light in all directions, and
their directional characteristics affect the design of lamps.
The light output of single LEDs is less than that of
incandescent and compact fluorescent lamps; in most applications
multiple LEDs are used to form a lamp, although high-power
versions are becoming available.
LED chips need controlled direct current (DC)
electrical power; an appropriate
 power
supply is needed. LEDs are adversely affected by high
temperature, so LED lamps typically include heat dissipation
elements such as heat sinks and cooling fins.
General-purpose lighting needs white light. LEDs
emit light in a very narrow band of wavelengths, emitting light
of a color characteristic of the energy bandgap of the
semiconductor material used to make the LED. To emit white light
from LEDs requires either mixing light from red, green, and blue
LEDs, or using a phosphor to convert some of the light to other
colors.
Comparisons to current lamps
Incandescent lamps (light bulbs) generate light
by passing electric current through a resistive filament,
thereby heating the filament to a very high temperature so that
it glows and emits visible light over a broad range of
wavelengths. Incandescent sources yield a "warm" yellow or white
color quality depending on the filament operating temperature.
Incandescent lamps emit 98% of the energy input as heat. A 100 W
light bulb for 120 V operation emits about 1,180 lumens, about
11.8 lumens/W; for 230 V bulbs the figures are 1340 lm and 13.4
lm/W.
Incandescent lamps are relatively inexpensive to
make. The typical lifespan of an AC incandescent lamp is 750 to
1,000 hours. They work well with dimmers. Most older light
fixtures are designed for the size and shape of these
traditional bulbs. In the U.S. the regular sockets are E26 and
E11, like E27 and E14 in some European countries.
Compact fluorescent lamps' specified lifespan
typically ranges from 6,000 hours to 15,000 hours.
Fluorescent lamps work by passing electricity
through mercury vapor, which in turn emits ultraviolet light.
The ultraviolet light is then absorbed by a phosphor coating
inside the lamp, causing it to glow, or fluoresce. Conventional
linear fluorescent lamps have life spans around 20,000 and
30,000 hours based on 3 hours per cycle according to lamps NLPIP
reviewed in 2006. Induction fluorescent relies on
electromagnetism rather than the cathodes used to start
conventional linear fluorescent. The newer rare earth
triphosphor blend linear fluorescent lamps made by Osram,
Philips, Crompton and others have a life expectancy greater than
40,000 hours, if coupled with a warm-start electronic ballast.
The life expectancy depends on the number of on/off cycles, and
is lower if the light is cycled often. The ballast-lamp combined
system efficacy for then current linear fluorescent systems in
1998 as tested by NLPIP ranged from 80 to 90 lm/W. For
comparison, general household LED bulbs available in 2011 emit
64 lumens/W
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lifetimes.
Energy Star specifications require the bulbs to typically drop
less than 10% after 6,000 or more hours of operation, and in the
worst case not more than 15%. LED lamps are available with a
variety of color properties. The purchase price is higher than
most other, but the higher efficiency may make total cost of
ownership (purchase price plus cost of electricity and changing
bulbs) lower.