# Rock Emissivity Measurement for Infrared Thermography Engineering Geological Applications
## 4 Methodology  
The experimental methodology for the emissivity estimation was carried out herein according to [20; 42] by employing a high-sensitivity thermal camera FLIR T420 (Table 1). It is a non-contact measurement method requiring the use of an emissivity-known surface material. In this case, a piece of electrical tape, with known high emissivity (\(\varepsilon=0.97\)), was placed on the specimen face, adjacent to the area where the emissivity was to be measured (Figure 3). This method is based on the rock emissivity estimation by comparing its optic features with the ones of a nearby blackbody at the same temperature, simulated by the black tape [43]. The rock was then oven-heated to reach, at least, 20 K above room temperature, according to the specifications, and the temperature difference between the specimen and the ambient was stable during the quick test, thus minimizing potential measurement errors. \begin{table}
\begin{tabular}{l c} \hline
**IR resolution** & 320 \(\times\) 240 pixels \\  & \(-20\,^{\circ}\)C to +120 \({}^{\circ}\)C (\(-4\,^{\circ}\)F to +248 \({}^{\circ}\)F) \\  & 0 \({}^{\circ}\)C to +650 \({}^{\circ}\)C (+32 \({}^{\circ}\)F to +1202 \({}^{\circ}\)F) \\
**Accuracy** & \(\pm 2\,^{\circ}\)C (\(\pm 3.6\,^{\circ}\)F) \\
**Spectral range** & 7.5–13 \(\upmu\)m \\
**Focal Plane Array** & Uncooled microbolometer \\ \end{tabular}
\end{table} 
Table 1: Main technical specification of the employed thermal camera. The second step (Figure 3) was the determination of reflected apparent temperature, i.e., a parameter used to compensate the radiation reflected in the object [44]. This step is crucial, because it allows the avoidance of potential error sources. The reflected apparent temperature was estimated herein according to the \"reflector method\", which consists in crumbling up a large piece of aluminum foil and then unscrambling it on a piece of cardboard of the same size. This was placed in front of the target to frame and pointed by the thermal camera with the emissivity set to 1.0. The apparent temperature of the aluminum foil was measured. Since laboratory measurements were carried out from an acquisition distance of 0.4 m, atmosphere effects were negligible. Moreover, the experimental set was a laboratory room with constant ambient temperature and humidity and no artificial sources of light; heating conditions were constant for all the specimens, so this ensured a strong reduction in potential measuring errors. For the emissivity determination, the rock specimen with the attached electrical tape was placed in front of the camera, after having verified that the tape was in good contact with the specimen, and the IRT image was taken. Such an image was processed by the software Flir Tools, which allowed one to adjust the level and span for best image brightness and contrast. Emissivity was set according to the tape value and the reflected temperature was set according to the previous measure. The temperature of the tape (Tt) was measured according to the \"area function\" (indicated for surfaces with varying emissivity such as rocks composed of different minerals or affected by voids). Then, the measurement function was moved to the sample surface and the emissivity setting was adjusted until the same temperature Tt was recorded. The emissivity corresponding at the Tt detected on the rock surface was the emissivity of the tested rock. Furthermore, measures were repeated a minimum of three times and at different portions of the specimen surface. This was carried out because the rock surface may be affected by specific macroscopic features, such as a preponderant concentration of pores or minerals, which could play a role on the final emissivity value. Therefore, in order to find out their influence on emissivity, along with a reliable range of values to be considered  
Figure 3: Phases of the methodological approach carried out to determine the emissivity of rock specimens. Sketch in inset (2) is modified from [20].