## 2 Materials and Methods
### Spectral Analysis of Energy-Containing Materials  
#### 2.2.1 Spectral Analysis Using Laser-Induced Breakdown Spectroscopy  
In this paper, we utilized the Chem Reveal benchtop laser-induced breakdown spectrometer (TSI, Shoreview, MN, USA), which is equipped with an Nd:YAG laser from Quantel, France, operating at a wavelength of 1064 nm and capable of delivering energy ranging from 20 to 200 mJ. The optical system includes a seven-channel wide-range spectrometer from Avantes, Netherlands, with a spectral range of 190 nm to 950 nm and a spectral resolution of 0.1 nm. The appearance and schematic diagram of the instrument are presented in Figure 1, which primarily consists of a laser, a spectrometer, an optical system, a three-dimensional carrier stage, and a computer. To obtain spectral information about the energy-containing materials, we employed the Laser-Induced Breakdown Spectroscopy (LIBS) technique [35; 36] to analyze the elements present in nitrocellulose and their characteristic spectral lines, in conjunction with the NIST spectral database. The two most suitable bands were selected as the detection wavelengths for two-color temperature measurements.  
LIBS (Laser-Induced Breakdown Spectroscopy) technology employs a high-energy laser to interact with the surface of the object being measured, generating plasma that emits spectral information. This spectral signal is transmitted to a spectrometer through an optical system, and the collected spectral lines are processed and analyzed using computer software.  
Due to the substantial laser output energy of the LIBS (Laser-Induced Breakdown Spectroscopy) system, which can reach up to 200 mJ, the nitrocellulose sample was burned  
Figure 1: Chem Reveal laser-induced breakdown spectrometer and principle. (**a**) LIBS schematic; (**b**) LIBS Device Diagram.  
on a 310 stainless steel substrate to protect the spectrometer's detection lens from contamination. The nitrocellulose combustion product was then placed on the stage for analysis to obtain the spectral data of the nitrocellulose sample. Laser measurement did not trigger the combustion of nitrocellulose.  
Figure 2 illustrates the appearance of nitrocellulose before and after combustion. The LIBS (Laser-Induced Breakdown Spectroscopy) emission spectra of nitrocellulose at energy levels of 80, 90, 100, 110, and 120 mJ were recorded by directing a high-energy laser at the burned nitrocellulose samples, as depicted in Figure 3. The overall consistency of the spectra was satisfactory. According to data from the National Institute of Standards and Technology (NIST) spectral database, potassium (K) exhibits a notably strong excitation spectrum in the visible range, with no nearby interference bands. This suggests that potassium may be an intrinsic element of nitrocellulose, and the characteristic wavelength associated with this peak can be utilized for subsequent temperature measurements.  
Figure 4 illustrates the average energy spectrum of nitrocellulose residues. The potassium (K) element exhibits prominent spectral line peaks at 404.4 nm, 766.3 nm, and 769.8 nm. Notably, the spectral lines near 404.4 nm are more convoluted due to interference from other lines, while the peaks at 766.3 nm and 769.8 nm are relatively distinct and unaffected by surrounding spectral lines.  
Figure 3: Spectrogram of nitrocellulose by LIBS.  
Figure 2: Picture of nitrocellulose samples and combustion products. (**a**) Nitrocellulose samples; (**b**) Nitrocellulose combustion products diagram.