Under certain conditions, radiant energy incident on an internal surface of a high refractive index transparent material is totally reflected. When an infrared absorbing material is in optical contact with a totally internally reflecting surface, the intensity of the internally reflected radiation is diminished for those wavelengths or energies where the material absorbs energy. Since an internal reflecting surface is essentially a perfect mirror, the attenuation of this reflected intensity by a material on its surface provides a means of producing an absorption spectrum of the material. Such spectra are called internal reflection spectra or attenuated total reflection (ATR) spectra. ATR is the most commonly accepted descriptor for this spectral collection technique. It is convent to regard the process as internal reflection and the data as ATR spectra. The high index of refraction material used to create internal reflection is called an internal reflection element (IRE) or an ATR crystal. Both terms are common. Figure 1 illustrates the general features of an internal reflection configuration and labeling of various components.