A type of absorption spectroscopy used to identify organic functional groups present in a used oil sample by measuring their light absorption at specific infrared wavelengths This is helpful because absorbance is proportional to concentration. The test can show additive depletion, the presence of water, hydrocarbon contamination of a synthetic lubricant, oxidation, nitration, and glycol contamination from coolant. Fourier Transform Infrared (FTIR) allows for the generation of complex curves from digital data.