What is Infrared?

By Steve Hill, CO

In this months Technicians Corner, we’ll be examining a technology that has been greatly misunderstood. Infrared. It’s as different from convection ovens as a microwave and as easy to use. But, for some reason, there seems to be a shroud of mystery surrounding infrared. It is really a very simple thing and is something that we encounter every day. To better understand infrared we should start from the beginning and establish a little background.

Radiation

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.

(insert electromagnetic spectrum graph)

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.

(insert infrared spectrum graph)

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

(insert reflected, absorbed or transmitted drawing)

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 and controller that does this.

Raise Your Productivity

Does it matter what wavelength of infrared radiation will best be absorbed by your poly-olefins? Or is it enough that the plastic just gets hot? Understanding infrared’s effects can enable us to harness it and help us to better process our plastics, making us more efficient and raising our productivity.

* Absolute zero (-460^o F & -273^o C) is the temperature when all atomic vibration stops.