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Understanding Infrared
 First,
it is important to understand that hot objects do not radiate heat.
They radiate electromagnetic waves, that when absorbed by a surface,
result in it heating up. Example: On a cold day you step out and face
the sun. Your face heats up, but your back stays cool. If the sun
were radiating heat, it would have heated up the air before you stepped
into it. Electromagnetic radiation can pass through space without
heating it. That is why outer space is so cold even though it is actually
closer to the sun. It is only when electromagnetic waves hit an object
and are absorbed that they transform into the heat that we can feel.
We can't feel the electromagnetic waves, but we can feel the heat
that they produce.
 What
are electromagnetic waves?
All materials are made up of atoms that are constantly in motion.
As energy is absorbed by an object the motion of it's atoms is increased.
The temperature of an object is directly proportional to the vibration
of it's atoms*. The more they vibrate, the hotter they get. Atoms
contain protons and electrons which are electrically charged particles.
These particles create an electric field around themselves, and when
they move it creates a magnetic field. The atoms of a hot object vibrate
frantically. As these atoms move, the electric and magnetic fields
that are created by the charged particles are disrupted. This disruptance
is called an electromagnetic wave (not unlike the waves in a pond).
Objects that are hot are radiating electromagnetic waves. When these
waves reach a cool object, the electric and magnetic fields of the
electromagnetic wave will pull on the charged particles of the cool
objects atoms and cause them to vibrate. The more they vibrate, the
hotter they will get. The atoms of the cool object absorb the energy
of the electromagnetic wave created by a hot object some distance
away.
The Electromagnetic Spectrum
 Electromagnetic
waves are like other waves in that they are described by their velocity,
frequency and wavelength but are different in that they don't need
a medium in which to travel. Ocean waves need water & sound waves
need atmosphere, but electromagnetic waves can travel through the
vacuum of space. Visible light, microwaves, radio waves, X-rays and
infrared are all different types of electromagnetic waves. They all
travel at the same velocity commonly referred to the "speed of
light". The only way in which they differ from each other is
in the length and frequency of their waves. The electromagnetic spectrum
is divided up into many different frequencies with many different
wavelengths. The infrared band is bordered on one side by visible
light and on the other by microwaves and is defined as the area between
.72 and 1000 microns. There are three parts to the infrared band.
Near IR is defined as the area between .72 and 1.5 microns, middle
IR is between 1.5 and 5.6 microns and far IR is 5.6 to 1000 microns.
A micron is the measurement used gauge wave lengths and is equivalent
to 1/1,000,000 of a meter or about 0.00004 inches (the average human
hair is about 50 microns in diameter).
 Absorption
Curves
All energy is either absorbed, transmitted or radiated when it hits
an object and all materials have absorption curves which show what
wavelengths the material will best absorb. To find out what frequency
and wavelength of infrared radiation we want our heater to output,
we need to establish the absorption curve of the material we want
to heat. In orthotics and prosthetics we require our heater to cook
Poly-olefin type plastics. The absorption curve indicates that the
carbon/hydrogen bond of most plastics breaks in the middle IR range
(at about 3.5 microns). Ideally, we would like our heater to output
most of it's energy in this area so it's important to select an infrared
emitter that does this. The wavelength can also be fine tuned with
the proper controller. A digital controller can regulate the wavelength
so well that it's accurate within +/- 2° of the set temperature.
* Absolute zero (-460° F & -273° C) is the temperature when
all atomic vibration stops. |
