LEDs: What are they?
Mike Bourget - Orbital Technologies
Corp.
email: bourgetm@orbitec.com
Light Emitting Diodes (LEDs) are
semiconductor devices that produce non-coherent, narrow-spectrum
light when a forward voltage is applied. LEDs range in wavelength
from the UVC band to infrared and are available in packages from
milliwatts to more then 10 watts. The first LED was an infrared
emitting device and was patented in 1961_ In 1962, the first
practical, visible spectrum LED was developed. The first high
power (1 watt) LEDs were developed in the late 1990's. LEDs create
light via a semiconductor process called electron-hole
recombination; rather then with a superheated element, ionized
gas, or an arc-discharge as in traditional light sources. The
wavelength of the light emitted is determined by the materials
used to form the semiconductor junction. LEDs produce more light
per electrical watt then incandescent lamps, with the latest
devices rivaling fluorescent tubes in energy efficiency. They are
solid-state devices, which are much more robust then any
glass-envelope lamp, and contain no hazardous materials as
fluorescent lamps do. LEDs also have a much longer lifetime then
incandescent, fluorescent, and HID lamps. While LEDs possess many
advantages over traditional light sources, a total system approach
must be considered when designing an LED-based lighting system.
LEDs do not radiate heat directly, but do produce heat that must
be removed to ensure maximum performance and lifetime. LEDs
require a constant-current, DC power source rather then a standard
AC line voltage. Finally, since LEDs are directional light
sources, external optics may be necessary to produce the desired
light distribution. A properly designed LED light system is
capable of providing performance and lifetime well beyond any
other traditional lighting source.
HortScience, Vol. 42(4), July 2007.
AMERICAN SOCIETY
FOR HORTICULTURAL SCIENCE
