Diamond has become the standard optical material for ATR (attenuated total reflection) spectroscopy in the infrared. This technique uses the evanescent wave at the surface of single or multi-reflection ATR elements to evaluate the infrared absorption properties of liquids and solids with high sensitivity. This application takes advantage of the broad-band spectral transparency, the high refractive index and the robust surface of diamond.

The broadband transparency and the high refractive index make diamond an ideal material for infrared beam splitters as required by Fourier transformed infrared (FTIR) spectroscopy. Diamond beam splitters can be used for high resolution spectroscopy over a wide spectral range ranging from infrared to terahertz radiation.