What is Emissivity?

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.

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