Baseline-free retrieval algorithm for broadband absorption spectra of H₂O at high temperatures and pressures

Absorption spectroscopy is a widely used non-contact combustion diagnostic technique. H₂O is frequently used as the target molecule for combustion temperature measurement. However, correcting the baseline in broadened absorption spectroscopy under high pressure remains challenging. In this Letter, we propose a new, to the best of our knowledge, processing algorithm, the Interpolated Envelope Correction Method (IECM), for baseline-free correction in broadband absorption spectra, enabling simultaneous retrieving of pressure, temperature, and H₂O concentration in high-temperature and high-pressure scenarios. The IECM first calculates the lower envelope from the measured spectrum and subtracts it from the spectrum and then processes least-squares fit based on the theory of absorption spectroscopy, to retrieve measured pressure, temperature, and concentration. Experimental results indicate that the IECM outperforms the background polynomial method at temperatures below 1273 K and pressures below 70 bar. An RMSE (root mean squared error) of 1.49 bar and 17 K was achieved using the IECM to retrieve pressure and temperature, which are smaller than other commonly used methods. The proposed IECM avoids the problem of the baseline, making it more convenient in the application of high-pressure combustion diagnostics.