Application of Band-Target Entropy Minimization to Infrared Emission Spectroscopy and the Reconstruction of Pure Component Emissivities from Thin Films and Liquid Samples

Abstract

Thermal emission spectral data sets were collected for a thin solid film (parafilm) and a thin liquid film (isopropanol) on the interval of 298–348 K. The measurements were performed using a conventional Fourier transform infrared (FT-IR) spectrometer with external optical bench and in-house-designed emission cell. Both DTGS and MCT detectors were used. The data sets were analyzed with band-target entropy minimization (BTEM), which is a pure component spectral reconstruction program. Pure component emissivities of the parafilm, isopropanol, and thermal background were all recovered without any a priori information. Furthermore, the emissivities were obtained with increased signal-to-noise ratios, and the signals due to absorbance of thermal radiation by gas-phase moisture and CO₂ were significantly reduced. As expected, the MCT results displayed better signal-to-noise ratios than the DTGS results, but the latter results were still rather impressive given the low temperatures used in this study. Comparison is made with spectral reconstruction using the orthogonal projection approach–alternating least squares (OPA-ALS) technique. This contribution introduces the primary equation for emission spectral reconstruction using BTEM and discusses some of the unusual characteristics of thermal emission and their impact on the analysis.