# Rock Emissivity Measurement for Infrared Thermography Engineering Geological Applications
## 2 IRT Background  
Infrared radiation, or infrared light, is part of the electromagnetic spectrum (Figure 1). Due to its wavelength (700-1 mm) the human eye cannot see it, but humans can sense some of this energy as heat [23]. According to the temperature of an object, most of its infrared radiation can be \"seen\" by specific devices known as thermal cameras, which convert infrared radiation into visible images [24]. According to the Wien's displacement law, the peak wavelength of the emitted radiation is inversely proportional to the temperature. For a body at ambient temperature, the peak occurs in the infrared band (Figure 1). For this reason, thermal imaging is considered a very powerful remote sensing technique for numerous field studies, thanks to the fact that thermal radiation can penetrate smokes, aerosols, dust, and mists more effectively than visible radiation [24]. The Stefan-Boltzmann law (Equation (1)), explains the relationship between the energy emitted by a body (\(J\)), across all wavelengths, and the Stefan-Boltzmann constant (\(\sigma\)), the surface temperature of the body (\(T\)) to the fourth power and its emissivity (\(\varepsilon\)) [25]:  
\[J=\varepsilon\sigma T^{4} \tag{1}\]Mathematically, emissivity is the ratio between the infrared radiation emitted by a body and the radiation emitted by a blackbody at the same temperature. It is self-evident that it plays an essential role in infrared thermographic surveys and it is dependent on temperature, wavelength and surface condition [3]. For example, a surface with a low emissivity value (i.e., aluminum, steel, etc.) acts as a mirror (high reflectance). With reference to the engineering geology, IRT use represents a new frontier for both the remote surveys of landslides/rock masses and rock mechanics laboratory applications. It has proved a useful tool in detecting crevasses, cave openings and rock fracturing [26; 27; 28], or the integrity of rock mass behind a shotcreted slope [14]. Some authors [29] tested the application of IRT to rock masses under different daily and seasonal conditions, finding out a statistical relation between the rock mass cooling and its grade of fracturing. Based on such innovative results, [PERSON] et al. [30] used IRT to characterize unstable rock wedges, while [PERSON] et al. [31] studied the thermal response of jointed rock masses. IRT was further employed to characterize landslides, even in combination with other remote survey or geophysical techniques, e.g., [32; 33; 34], and for the detection of rock bridges within rock mass cracks [35]. Further applications refer to the study of rock behavior under stress, where IRT can highlight the unstable crack propagation and/or flaw coalescence [36], to the evolution monitoring of excavation damaged zones [37], and to the detection of microstructural changes of reservoir rocks [38]. A parallel pioneering scientific trend is the indirect estimation of rock porosity in laboratory by using IRT to monitor the cooling rate of oven-heated rock specimens. More specifically, the strong relationship existing between the rock porosity and its cooling speed has recently been highlighted, demonstrating that a porous rock cools faster than a massive one [15; 39]. Such evidence leads the way to a potentially increasing number of IRT applications on rocks, which must be supported by reliable data. In this light, the emissivity knowledge of a framed target is essential to achieve a correct estimation of its surface temperature by IRT. For this reason, emissivity values belonging to different natural and human-made matters have been experimentally estimated and are available in the literature, although there are few data on rocks [20]. Among the low-emissivity materials, polished bronze, copper, chromium, iron, steel, gold and platinum can be found, while masonry bricks, asphalt pavement, concrete, carbon and human skin are counted among the high-emissivity materials.