Fig. 1. Schematic diagram of direct absorption spectroscopy (DAS) technology

Fig. 2. Schematic diagram of wavelength modulation spectroscopy (WMS) technology

Fig. 3. Schematic diagram of linear TDLAT technology

Fig. 4. Beam arrangements and corresponding sinogram plots^[47]. Beam arrangements are suitable for (a) 32 beams with regular geometry; (b) 32 beams with unoptimized irregular geometry; (c) 27 beams with optimized irregular geometry

Fig. 5. Schematic of pentagon LAS sensor with five fan beams^[53]

Fig. 6. Schematic diagram of nonlinear TDLAT technology

Fig. 7. Application of hyperspectral tomography (HT) sensor in exhaust flow of J85 engine^[65]. (a) Experimental device containing 30 beams of HT sensors; (b) optical testing hardware; (c) schematic diagram of position of measurement plane in exhaust gas and sample measurement of 2D distribution of temperature measured at this position

Fig. 8. Comparison of three imaging methods. (a) Moving line-of-sight (LOS) scanning; (b) multi-projection TDLAT technology; (c) LAI method

Fig. 9. Optical setup used to break laser light’s coherence and provide diffraction-free LAI^[76]

Fig. 10. Interband cascade laser (ICL) alignment and transmission image and projected absorbance images of CH₄ shown with respective angle of Bunsen-style flames^[79]

Fig. 11. Comparison of reconstruction effects based on deep learning and tomography^[79]