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Infrared light is even used to heat food sometimes - special lamps that
emit thermal infrared waves are often used in fast food restaurants!
| Shorter, near infrared waves are not hot
at all - in fact you cannot even feel them. These shorter
wavelengths are the ones used by your TV's remote control.
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How can we "see" using the Infrared?
Since the primary source of infrared radiation is heat or thermal
radiation, any object which has a temperature radiates in the infrared.
Even objects that we think of as being very cold, such as an ice cube,
emit infrared. When an object is not quite hot enough to radiate visible
light, it will emit most of its energy in the infrared. For example, hot
charcoal may not give off light but it does emit infrared radiation which
we feel as heat. The warmer the object, the more infrared radiation it
emits.
| Humans, at normal body temperature,
radiate most strongly in the infrared at a wavelength of about 10
microns. (A micron is the term commonly used in astronomy for a
micrometer or one millionth of a meter.) This image ( which is
courtesy of the Infrared Processing and Analysis Center at CalTech),
shows a man holding up a lighted match! Which parts of this image do
you think have the warmest temperature? How does the temperature of
this man's glasses compare to the temperature of his hand?
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To make infrared pictures like the one
above, we can use special cameras and film that detect differences
in temperature, and then assign different brightnesses or false
colors to them. This provides a picture that our eyes can interpret.
The image at the left (courtesy of SE-IR Corporation, Goleta, CA)
shows a cat in the infrared. The orange areas are the warmest and
the white-blue areas are the coldest. This image gives us a
different view of a familiar animal as well as information that we
could not get from a visible light picture.
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Humans may not be able to see infrared light, but did you
know that snakes in the pit viper family, like rattlesnakes, have sensory
"pits", which are used to image infrared light? This allows the snake to
detect warm blooded animals, even in dark burrows! Snakes with 2 sensory
pits are even thought to have some depth perception in the infrared!
(Thanks to NASA's Infrared Processing and Analysis Center for help with
the text in this section.)
Many things besides people and animals emit infrared light - the Earth,
the Sun, and far away things like stars and galaxies do also! For a view
from Earth orbit, whether we are looking out into space or down at Earth,
we can use instruments on board satellites.
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Satellites like GOES 6 and Landsat 7
look at the Earth. Special sensors, like those aboard the Landsat 7
satellite, record data about the amount of infrared light reflected
or emitted from the Earth's surface.
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Landsat 7
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Other satellites, like the Infrared Astronomy Satellite (IRAS) look up
into space and measure the infrared light coming from things like large
clouds of dust and gas, stars, and galaxies!
What does the Infrared show us?
| This is an infrared image of the Earth
taken by the GOES 6 satellite in 1986. A scientist used temperatures
to determine which parts of the image were from clouds and which
were land and sea. Based on these temperature differences, he
colored each separately using 256 colors, giving the image a
realistic appearance.
Why use the infrared to image the Earth? While it is easier to
distinguish clouds from land in the visible range, there is more
detail in the clouds in the infrared. This is great for studying
cloud structure. For instance, note that darker clouds are warmer,
while lighter clouds are cooler. Southeast of the Galapagos, just
west of the coast of South America, there is a place where you can
distinctly see multiple layers of clouds, with the warmer clouds at
lower altitudes, closer to the ocean that's warming them.
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Space Science and Engineering Center,
University
of Wisconsin-Madison,
Richard Kohrs, designer
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We know, from looking at an infrared image of a cat, that many things
emit infrared light. But many things also reflect infrared light,
particularly near infrared light. Near infrared radiation is not related
to the temperature of the object being photographed - unless the object is
very, very hot.
Infrared film 'sees' the object because the Sun (or some other light
source) shines infrared light on it and it is reflected or absorbed by the
object. You could say that this reflecting or absorbing of infrared helps
to determine the object's 'color' - its color being a combination of red,
green, blue, and infrared!
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This image of a building with a tree and grass shows how
Chlorophyll in plants reflect near infrared waves along with visible
light waves. Even though we can't see the infrared waves, they are
always there. The visible light waves drawn on this picture are
green, and the infrared ones are pale red. |
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| This image was taken with special film
that can detect invisible infrared waves. This is a false-color
image, just like the one of the cat. False-color infrared images of
the Earth frequently use a color scheme like the one shown here,
where infrared light is mapped to the visible color of red. This
means that everything in this image that appears red is giving off
or reflecting infrared light. This makes vegetation like grasa and
trees appear to be red. The visible light waves drawn on this
picture are green, and the infrared ones are darker red. |
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This is an image of Phoenix, Arizona
showing the near infrared data collected by the Landsat 5 satellite.
The light areas are areas with high reflectance of near infrared
waves. The dark areas show little reflectance. What do you think the
black grid lines in the lower right of this image represent?
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This image shows the infrared data
(appearing as red) composited with visible light data at the blue
and green wavelengths. If near infrared is reflected off of healthy
vegetation, what do you think the red square shaped areas are in the
lower left of the image? |
Instruments on board satellites can also take pictures of things in
space. The image below of the center region of our galaxy was taken by
IRAS. The hazy, horizontal S-shaped feature that crosses the image is
faint heat emitted by dust in the plane of the Solar System.
Infrared Processing
and Analysis Center, Caltech/JPL
RETURN TO THE ELECTROMAGNETIC SPECTRUM
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