Our
eyes are detectors which are designed to detect visible
light waves (or visible radiation). Visible light is one
of the few types of radiation that can penetrate our atmosphere
and be detected on the Earth's surface. As we learned from the discovery
of infrared, there are forms of light (or radiation)
which we cannot see. Actually we can only see a very small
part of the entire range of radiation called the electromagnetic
spectrum.
The
electromagnetic spectrum includes gamma rays, X-rays,
ultraviolet, visible, infrared, microwaves, and radio
waves. The only difference between these different types
of radiation is their wavelength or frequency. Wavelength increases and frequency
(as well as energy and temperature) decreases from gamma
rays to radio waves. All of these forms of radiation travel
at the speed of light (186,000 miles or 300,000,000 meters
per second in a vacuum). In addition to visible light,
radio, some infrared and a very small amount of ultraviolet
radiation also reaches the Earth's surface from space.
Fortunately for us, our atmosphere blocks out the rest,
much of which is very hazardous, if not deadly, for life
on Earth.
Infrared radiation lies between the visible and microwave
portions of the electromagnetic spectrum. Infrared waves
have wavelengths longer than visible and shorter than
microwaves, and have frequencies which are lower than
visible and higher than microwaves. Infrared is broken
into three categories: near,
mid and far-infrared. Near-infrared refers to the
part of the infrared spectrum that is closest to visible
light and far-infrared refers to the part that is closer
to the microwave region. Mid-infrared is the region between
these two.
Far
Infrared (FIR), by its physical properties, has a deep penetrating
power. It can penetrate through the skin into the subcutaneous
tissues. Upon penetrating
the skin, Far Infrared (FIR) transforms from radiant energy like light, into
heat energy conducted in our cells. The thermal effect within the deep layers of
the tissues causes blood vessels and capillaries to dilate,
and has other physiological effects.
Courtesy of Inframetrics
The
primary source of infrared radiation is heat or thermal radiation. This is
the radiation produced by the motion of atoms
and molecules in an object. The higher the
temperature, the more the atoms and molecules
move and the more infrared radiation they
produce. Any object which has a temperature,
i.e., anything above absolute zero (459.67
degrees Fahrenheit or -273.15 degrees Celsius
or 0 degrees Kelvin), radiates in the infrared.
Absolute zero is the temperature at which
all atomic and molecular motion ceases. 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. The infrared image of a landing space
shuttle (left) shows how the tiles underneath
the shuttle have been heated during re-entry.
Courtesy of Meditherm
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). In the image to the
left,the red areas are the warmest,
followed by yellow, green and blue (coolest).
The image to the right shows a cat in the infrared.
The yellow-white areas are the warmest and the
purple 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. Notice the cold nose
and the heat from the cat's eyes, mouth and
ears.
Some
animals can "see" in the infrared. For example,
snakes in the pit viper family (e.g. rattlesnakes) have
sensory "pits," which are used to detect infrared
light. This allows the snake to find warm-blooded animals
(even in dark burrows), by detecting the infrared heat
that they radiate. Snakes with 2 sensory pits are thought
to have some depth perception in the infrared. We experience
infrared radiation every day. The heat that we feel from
sunlight, a fire, a radiator or a warm sidewalk is infrared.
Although our eyes cannot see it, the nerves in our skin
can feel it as heat. The temperature-sensitive nerve endings
in your skin can detect the difference between your inside
body temperature and your outside skin temperature. We
also commonly use infrared rays when we operate a television
remote.
