A Blackbody: the Perfect Emitter
Imagine looking into a small opening of a deep cave. In the visible
wavelengths, the opening looks black because the light that enters
the cave is not easily reflected back out. However, the cave glows with
emitted thermal IR energy. This energy emerges as a complete spectrum
of all wavelengths of IR light. The radiance at each infrared wavelength
is the maximum amount possible for a given temperature. If we used
an infrared spectrometer to measure this emitted energy and plotted the
result, it would follow a Planck distribution.Anything that emits energy
with a Planck distribution can be called a blackbody. A blackbody emitter
is useful for comparison with materials that do not emit perfectly at all
wavelengths, which is the case for most of the matter in the universe.
A Universe of Selective Emitters
The molecules that form the stuff of the universe (gases, liquids, and
solids) result from atoms bonding together. They behave like microscopic
balls on the ends of molecular springs, vibrating when agitated.
This agitation arises when light of just the right wavelength hits a
particular molecule. Once it starts vibrating, the molecule re-radiates
the same wavelength of light. This is the process of absorption and
emission. The wavelengths of light that cause molecular vibrations occur
in the infrared region. Every unique molecule has its own characteristic
frequency of vibration. So, unlike a blackbody emitter,
molecules emit energy that
departs from a Planck distribution. This means that
the infrared light emitted by vibrating molecules can be used to
identify them.
Emissivity: the Temperature Equalizer
One of the ways to describe the infrared energy emitted by molecules
is in terms of radiance: watts of energy per unit of area. With changes
in temperature, come changes in radiance. For example,
the radiance from a
mineral at one temperature will be different from that at another
temperature. In order to make comparisons of emission from materials at
different temperatures, we need to remove the temperature effect. This
is done mathematically by dividing the radiance spectrum of a selective
emitter by that of a blackbody (perfect emitter) at the same temperature.
The result is called an emissivity spectrum. Because it results from
dividing one radiance spectrum by another, the units of watts/area
cancel. Emissivity then, is a fractional representation of the amount of
energy from some material vs. the energy that would come from a blackbody
at the same temperature. The places in an emissivity spectrum that have
a value less than one are the wavelength regions that molecules are
absorbing energy. In the case of quartz (SiO2), the silicon-oxygen
molecules are responsible for the absorptions.
