## 4 Results and Conclusions  
The results have shown that long infrared ranges present limitations. The signals ratio in the LW spectral range was not very sensitive and too noisy to be properly detected by the FLIR A655 sc thermal camera (both for 0.01 \(\upmu\)m and for 0.1 \(\upmu\)m central wavelengths distances). The investigation continued in the mid-near infrared spectral range. The experimental analysis, obtained with the FLIR SC5500 thermal camera, showed that this novel measurement technology guarantees the surface temperature with a measurement accuracy lower than 5% when proper narrow filters with limited bandwidths were used. Finally, in order to increase the sensitivity of the signal ratio between two near wavelengths, a camera with a working range starting from the NW spectral range (i.e., Orion SC 7600  
Figure 12: Dual-color versus standard thermographic techniques performance at high temperature for different emissivity classes of materials in the near-infrared spectral range. working in 1.5-5 \(\upmu\)m) was chosen. The curves, in Figure 11, were obtained interpolating the data from the dual-color simulation using the Orion SC 7600 as reference instrumentation. The goodness of the results was assessed on the basis of how the signals ratio curve, obtained analytically for the material, approximates the signals ratio curve of the black body. Figure 12 showed a comparison of the two approaches. The results were obtained using DC (circular points) and the single color (triangular points) with \(-40\%\) of uncertainty on the effective emissivity values. An error band of \(5\%\) was also reported to graphically quantify the goodness of the different approaches. The choice of the best thermographic approach depends on the degree of knowledge of emissivity curves. Therefore, the dual-color technique was preferable in applications at high temperatures, for which a relative temperature accuracy lower than \(5\%\) was obtained for medium-high emissivity (\(\epsilon>0.4\)) materials. It has been found that when the emissivity was higher than \(15\%\) from the real value, the DC technique was preferable with respect to the single-color (SC) technique. For low emissivity (\(\epsilon<0.4\)) materials, instead, the emissivity had to be higher than \(40\%\) of the real value to guarantee that the DC technique was preferable with respect to the SC technique with a DC temperature accuracy of the order of \(30\)-\(40\%\). Furthermore, the DC technique has to be used in applications where the emissivity of the materials is not well known, such as in the case of the test performed in the PWT facilities to verify the TPS withstanding the hypersonic plasma interactions [12; 13; 14; 15]. Before testing the methodology in the PWT aerospace field, the Center for Isotopic Research on the Cultural and Environmental heritage (CIRCE) accelerator [16; 17; 18; 19], upgraded both to perform accelerator mass spectrometry (AMS) up to actinides [20; 21; 22; 23; 24] and for experimental nuclear astrophysics [25; 26; 27; 28; 29; 30; 31; 32] as other AMS systems [33], can be used for the preliminary test at temperature values reaching \(1500\)\({}^{\circ}\)C, with materials that, at the beginning of the process, are characterized by a low emissivity, and increase their emissivity value with a carbon ions implantation. Conceptualization, M.D.C. and L.S. ; methodology, M.D.C. and L.S. ; validation, M.D.C. and L.S. ; formal analysis, M.D.C. and L.S. ; investigation, M.D.C. and L.S. ; data curation, M.D.C. and L.S. ; writing--original draft preparation, M.D.C. and L.S. ; writing--review and editing, M.D.C., L.S., A.D.V., F.D.C., M.M., U.G. and G.R. ; supervision, M.D.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.