1. Fourier Transform Infrared Spectroscopy
Gas sample is temperature controlled to allow comparison to previously measured spectra
Interferometer path length is measured with a He-Ne laser, one intensity peak every 632.8nm of path length

2. Signal Processing and Analysis
Measured transmission spectra is subtracted from a background measurement of IR-inactive gas to account for attenuation and non-linearity in the instrument.
Computer algorithm compares a set of reference spectra to find the combination of species and concentrations that best matches the measurement.
H2O Reference Spectra
Engine Exhaust Measurement