|
Infrared
Infrared (IR) radiation is electromagnetic
radiation of a wavelength
longer than visible light,
but shorter than microwave
radiation. The name means "below red"
(from the Latin
infra, "below"), red
being the color
of visible light of longest wavelength. Infrared radiation has
wavelengths between 700
nm and 1 mm.
IR is often subdivided into near-IR (NIR, 0.7-5 µm
in wavelength), mid-IR (MIR (also intermediate-IR (IIR)), 5 - 30
µm) and far-IR (FIR, 30 - 1000 µm). However, these terms are
not precise, and are used differently in the various study.
Infrared radiation is often linked to heat,
since objects at room temperature or above will emit
radiation mostly concentrated in the mid-infrared band (see black
body).
Uses
Infrared is used in night-vision
equipment, when there is insufficient visible
light to see an object. The radiation is detected and turned
into an image on a screen, hotter objects showing up brighter,
enabling the police and military to chase targets.
Smoke is more transparent to infrared than to visible light, so fire
fighters apply infrared imaging equipment when working in
smoke-filled areas.
A more common use of IR is in television
remote
controls. In this case it is used in preference to radio
waves because it does not interfere with the television
signal. IR data transmission is also employed in short-range
communication among computer peripherals and personal
digital assistants. These devices usually conform to
standards published by IrDA,
the Infrared Data Association. Remote controls and IrDA devices
use infrared light-emitting
diodes (LEDs) to emit infrared radiation which is focused by
a plastic lens
into a narrow beam. The beam is modulated,
i.e. switched on and off, to encode the data.
The receiver uses a silicon
photodiode
to convert the infrared radiation to an electric current.
It responds only to the rapidly pulsing signal created by the
transmitter, and filters out slowly changing infrared radiation
from sunlight, people and other warm objects.
The light used in fiber
optic communication is typically infrared.
History
In 1800
the physicist William
Herschel held a mercury thermometer in the spectrum
produced by a glass prism to measure the heat content of
different coloured lights. He found that the thermometer
registered an increase in temperature even when held beyond the red
end of the spectrum, where there was no visible light. This was
the first experiment to show that heat could be transmitted by
an invisible form of light.
This
content from Wikipedia
is licensed under the GNU
Free Documentation License.
|
