## 4 Experiments and Results Discussion  
### Target Gas Detection Experiment  
According to the principle of the three-layer radiance transfer model, a target gas detection system was built in the laboratory, as shown in Figure 7. The infrared radiation emitted by the MTB-6D surface array blackbody was used as the light source, and different background radiation conditions were simulated by controlling the temperature of the blackbody. The MH4021 multi-channel gas sample compounder was used to mix gases into a CaF\({}_{2}\) gas cell with an effective axial length of 30 cm and a bottom diameter of 10 cm. The target gas was standard CO\({}_{2}\) with a purity of 99.99%, and the dilution gas was standard N\({}_{2}\) with a purity of 99.99%. The flat-field radiation light emitted by the surface array blackbody passed through the gas cell filled with the target gas before entering the SMWIR-IFTS. After completing one full cycle of scanning by controlling the scanning mirror, the interferogram data cube of the target gas was obtained. Gas detection experiments were conducted using the two filter channels of the SMWIR-IFTS, with the experimental parameters listed in Table 2.  
The radiance spectrum reconstruction of the interferogram data cube collected by SMWIR-IFTS was performed. First, the interferogram units with different interference dimensions in the same spatial dimensions were extracted and stitched to obtain the target gas's interference image sequence. Then, the interference image sequence underwent baseline correction, apodization, phase correction, and Fourier transform to obtain the DNspectrum. Finally, spectral and radiometric calibration were applied to the DN spectrum to obtain the target radiance spectrum. The collected radiance spectrum of the gas cell filled with N\({}_{2}\) at 296 K is used as the background radiance spectrum \(R_{bg}\), while the collected radiance spectrum of the gas cell with a specified concentration of CO\({}_{2}\) at 296 K is used as \(R_{in}\), and the radiance spectrum of a blackbody at 296 K is used as \(B_{gas}\). The CO\({}_{2}\) absorbance spectrum is then reconstructed using the three-layer radiance transfer model. Figure 8a shows the background radiance spectrum \(R_{bg}\) and 90% CO\({}_{2}\) radiance spectrum \(R_{in}\) measured at 383 K through the 1 st filter channel, with the reconstructed CO\({}_{2}\) absorbance spectrum shown in Figure 8b. Correspondingly, Figure 9a presents the radiance spectra acquired at 408 K through the 2 nd filter channel, and Figure 9b shows its reconstructed CO\({}_{2}\) absorbance spectrum. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Filter Channel** & **Gas Type** & **Temperature/K** & **Pressure/atm** & **Concentration Range** & **Concentration Sampling Interval** \\ \hline
1 st & CO\({}_{2}\) & 353, 373, 383 & 1 & 10–90\% & 10\% \\
2 nd & CO\({}_{2}\) & 353, 373, 408 & 1 & 10–90\% & 10\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental parameters of two filter channels. Figure 8: **(a)** Gas radiance spectra; **(b)** 90% CO\({}_{2}\) absorbance spectrum collected by the 1 st filter channel at a blackbody temperature of 383 K.  
Figure 7: Target gas detection system **(a)** schematic diagram; **(b)** experimental scene. As shown in Figures 8 and 9, there are obvious CO\({}_{2}\) absorption phenomena at the spectral bands of 2037-2061 cm\({}^{-1}\), 2061-2077 cm\({}^{-1}\), and 2251-2268 cm\({}^{-1}\) compared with other bands, which is consistent with the results of the Hitran standard gas database [34]. A total of 27 sets of gas detection experiments were conducted in the two filter channels of the SMWIR-IFTS, with each experiment involving 1000 consecutive data acquisitions. Each filter channel collected 27,000 absorbance spectra, which were then randomly split into training, validation, and test sets in a 6:2:2 ratio. The mean CO\({}_{2}\) absorbance spectra at different concentrations measured by the 1 st and 2 nd filter channels at various blackbody temperatures are shown in Figures 10 and 11, respectively.