## 2 Materials and Methods  
### Modified Raman Spectroscopy Setup  
Raman spectroscopy is a method for measuring the Raman scattering spectrum, which involves the inelastic scattering occurring in a material. Raman scattering is a phenomenon in which an incident electromagnetic wave loses or gains energy while generating or absorbing phonons within a material [49]. Scattering that loses energy while generating phonons is called Stokes scattering, whereas scattering that gains energy while absorbing phonons is called anti-Stokes scattering. Since the energy of phonons is influenced by factors such as the type of material, bonding structure, stress, and temperature, the analysis of the Raman scattering spectrum provides valuable information about these characteristics [41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55]. Because Raman scattering requires interaction between electromagnetic waves and phonons, its probability of occurrence is very low--approximately 1 in 100,000 photons. This probability is significantly smaller compared with other elastic interactions such as Rayleigh scattering or reflection. As a result, elastically scattered light must be filtered out, and a highly sensitive measuring system is required. Stokes scattering occurs when phonons are generated, making it largely independent of the number of phonons already present in the material. In contrast, anti-Stokes scattering occurs when existing phonons are absorbed, meaning that higher temperatures--where more phonons are already present--lead to an increased probability of occurrence. Consequently, the temperature of a material can be determined by measuring the intensity of anti-Stokes scattered light. However, a challenge arises in that the intensity fluctuates during the Raman measurement process. To address this issue, a widely known method involves calculating the temperature using the ratio of Stokes scattering intensity to anti Stokes scattering intensity [42; 43; 46; 47; 48]. The relationship between this intensity ratio and temperature can be expressed as Equation (1) [42; 43]. \[\frac{I_{s}}{I_{a}}=\left(\frac{\mu+\
u}{\mu-\
u}\right)^{4}\left[exp\left( \frac{\hbar\omega}{k_{B}T}\right)-1\right] \tag{1}\]  
where \(I_{s}\) and \(I_{a}\) represent the intensities of Stokes scattering and anti-Stokes scattering, respectively; \(\mu\) and \(\
u\) represent the wavenumbers of the excitation light and Raman scattering, respectively. \(\hbar\), \(\omega\), and \(k_{B}\) are [PERSON]'s constant, Raman scattering wavenumber, and Boltzmann's constant, respectively. Generally, the calculated temperature is hardly affected by peak shift, because the difference in \(\mu\) and \(\
u\) is very small. Therefore, the spectroscopic system for anti-Stokes Raman thermography does not require a high spectral resolution. However, in order to measure Stokes scattering and anti-Stokes scattering simultaneously, a wide spectral range must be measured, and a very sensitive measuring system is required to measure anti-Stokes scattering, which is about eight times smaller than Stokes scattering at room temperature. This has a significant impact on the mapping time for Raman thermography. A typical Raman spectrometer consists of optical fibers, a slit, a reflective grating, and spherical mirrors. While using a reflective grating and slit provides good resolution, it also results in significant signal loss. Additionally, commercial spectrometers receive signals through binning, meaning the vertical focus of the Raman signal on the CCD or CMOS detector is not a primary consideration. In fact, the vertical focus size of a commercial Raman spectrometer has been observed to exceed 300 \(\upmu\)m. However, to enhance the speed of Raman mapping, it is crucial to focus the Raman signal onto the narrowest possible area. Doing so reduces the number of pixels required by the detector, improving the measurement speed and increasing the sensitivity, as the signal threshold of each pixel is more easily exceeded.