Light and Heat
Thermal IR energy is more commonly known as "heat". Everyone is familiar with heat because of our sense of touch. But what exactly is heat? Heat is a form of light invisible to our eyes, but detectable with our skin. Visible light is part of a large spectrum of energy that includes other familiar electromagnetic energy regions: microwaves, radio waves, ultraviolet, and X-rays all are forms of light that we can not see. The colors of a rainbow form a continuous spectrum of light in the visible wavelength region as does the "light" in the other regions. Infrared light occurs at wavelengths just below red light, hence the name, infra- (below) red. Near-infrared is the "color" of the heating coil on an electric stove just before it glows red. The thermal (or mid-) infrared colors are found at even longer wavelengths.
Glowing vs. Reflecting
The light that we see with our eyes originates from a glowing source, a light bulb or the sun, for example. But that light also can reflect off of surfaces and reach our eyes. This allows us to see things that don't emit their own light. Infrared light also is emitted and reflected. Unlike visible light though, infrared light is emitted by any object that has a temperature above absolute zero. With hotter temperatures comes brighter infrared light until the object emits visible light.
Temperature and Color
We can determine the temperature of a glowing object from the color of the emitted light. This is true of the sun, a light bulb, or a rock heated by the sun. The color that we see results from the wavelength region where the maximum emitted energy occurs. In the first two cases, this radiance maximum occurs in the visible wavelengths that we call yellow. In the case of the rock, its peak radiance is in the thermal IR wavelengths. Determining temperature in this manner is a useful feature of infrared measurements. As described in the next section, much more information is available if we can see a full spectrum of infrared colors.
