is a form of absorption spectroscopy that identifies organic functional groups present in a used oil sample by measuring their infrared absorption at specific infrared wavelengths; absorbance is proportional to concentration. The test can indicate the presence of water, hydrocarbon contamination of a synthetic lubricant, oxidation, nitration, and glycol contamination from coolant. Fourier Transforms Infrared (FT-IR) permits the generation of complex curves from digitally represented data.

Also see chromatography, ferrography, mass spectrometer, particle count, spectrographic analysis.