# Rock Emissivity Measurement for Infrared Thermography Engineering Geological Applications
## 1 Introduction  
Infrared thermography (IRT) is a scientific application for the acquisition and analysis of thermal information, based on the detection of the thermal radiation emitted by an object. At temperatures above absolute zero, all bodies emit electromagnetic radiation, with wavelengths mainly falling within the infrared band of the electromagnetic spectrum, as the result of thermally excited electron oscillations or transitions within the matter [1, 2]. Infrared radiation is therefore invisible to the human eye and specific devices are required to process such signals. These are infrared cameras, which can detect and record the radiation emitted by a material to build a thermal image, from which the temperature pattern can be reconstructed. The feature controlling the relation between the emitted radiation and the surface temperature of the investigated material is known as emissivity (\(\epsilon\)) [3]. This surface property, characterizing all forms of matter capable of emitting thermal radiation, ranges between 0 (referred to as a perfect emitter--mirror) and 1 (referred to as a theoretical perfect absorber--blackbody). Based on this value, the surface temperature of an object can be quickly estimated in a non-destructive way. Thanks to this, IRT has experienced a growing diffusion across a wide range of scientific fields; a literature review [4] andreferences therein highlighted that IRT is employed in medicine or biology, chemistry, engineering and material science, life and sport sciences, meteorology and agriculture, although the most predominant fields involving IRT applications are applied science and engineering. In the latter cases, the use of IRT is also focused on building materials and structures characterization: [PERSON] et al. [5] studied the thermal behavior of concrete specimens aiming at assessing their density, while [PERSON] et al. [6] and [PERSON] et al. [7] employed IRT to detect deterioration signals in concrete structures. More recently, IRT has found its utility even in engineering geological sciences, with satisfactory results in the study of landslides, hydrogeology and rock mechanics, e.g., [8; 9; 10; 11; 12; 13; 14]. With specific reference to rock materials, [PERSON] and [PERSON] [15] used IRT for the indirect estimation of rock porosity in laboratory, while [PERSON] at al. [16] and [PERSON] et al. [17] suggest that rock spectral emissivity is subject to variation during mechanical loading and with respect to the rock composition, respectively. Estimating the emissivity of a rock is possible according to specific procedures, although often, it is useful to have a reference value due to time issues or the impossibility of measuring. NASA's ECOSTRESS (ECosystem Spaceborne Thermal Radiometer Experiment on Space Station) Spectral library [18; 19] is a good reference in terms of the reflectance spectra of rocks, minerals, lunar/terrestrial soils covering the visible through thermal infrared wavelength region (0.4-15.4 \(\upmu\)m). FLIR Systems [20] collected several experimental data on emissivity values belonging to different materials. The provided table, compiled by using a shortwave camera (i.e., a camera sensitive to shorter IR wavelengths between visible light and 3 \(\upmu\)m), lists numerous kinds of material, but only a few examples of rocks. Some articles provide a mention on the emissivity of marble, granite, sandstone, limestone and gypsum, due to their employment in constructions, clay and soil [21; 22]. On this trend, this paper aims at increasing the scientific casuistry providing representative emissivity values for different rock types, according to their laboratory assessment through the method suggested by FLIR [20]. Specimens of igneous, sedimentary and metamorphic rocks were chosen among the most common lithologies considered for engineering geological and cultural heritage purposes.