## 2 Standardization of the Method-Emissivity  
Surface emissivity is an important factor in IRT surveys, especially when dealing with passive thermography. Accurate temperature profiles can be obtained from such surveys as can be seen in numerous applications presented in the paper (a parameter which is taken into account in all the relevant applications, i.e., cleaning interventions, moisture detection, and consolidation treatments). In this situation, the obtained emmisivity values from the ASTM standard approach were used, but nonetheless the obtained values from the empirical approach were also taken into consideration in order to see if there is an effect on the actual outcome of the in situ thermographic surveys. Furthermore, as for Cultural Heritage materials, it is difficult to find extended literature reports with emissivity values. The two described approaches for determining emissivity on materials from historic structures give the scientist-conservator an advantage in correctly obtaining a precise temperature pattern/image. Values for emissivity (\(\epsilon\)) can be between 0 (perfect reflector--mirror) and 1 (perfect emmiter--blackbody). As a result, emissivity plays an essential role in infrared thermographic surveys and is dependent on temperature, wavelength and surface condition. A surface with a low emissivity value (i.e., aluminium, steel, etc.) acts as a mirror (high reflectance). However, such a problem is usually overcome using high emmittance flat paints (i.e., black colour water based paints) for painting the investigated surface(s). Two different approaches were used for the calculation of the emissivity values on various materials. The aim was to present different ways of determining emissivity values, even for materials used in cultural heritage applications, where the literature of emissivity values is rather limited. Both approaches are presented here in order to show how this was achieved (i.e., either by using a standardized approach or even an empirical developed one). In both instances the obtained result could be used in an IRT survey. Nonetheless, the standardized procedure gives more flexibility  
Figure 1: General graph for infrared thermography (IRT) application by National Technical University of Athens. to the thermographer-scientist as it attains values in various temperatures (this is the limitation of the empirical presented approach). The first approach was in accordance with the ASTM standard E1933-97 [17], whereas the second one was completed according to an empirical procedure [18]. In the case of the empirical approach, a piece of electrical tape (3M Scotch Super 88 Vinyl) with a known emissivity of 0.95, at 48.88 \({}^{\circ}\)C was attached as a reference emitter to the investigated sample(s). Batches of three specimens (Table 1) were measured using mid and long wave IRT (AVIO TVS-2000 Mk II LW, Nippon Avionics Co, Tokyo, Japan) (3-5.4 \(\upmu\)m and 8-12 \(\upmu\)m), at numerous temperatures. The importance of emissivity in IRT building surveys was seemingly acknowledged. A non-contact thermometer (Gann IR40, Gann, Gerlingen, Germany) was also utilised to confirm that the examined specimens were in a thermal equilibrium state while inside the chamber. The IRT systems (AVIO TVS-2000 Mk II LW, Nippon Avionics Co, Tokyo, Japan) were then adjusted in order to determine the materials' emissivity values. To overcome the problem of moisture (porous materials), the samples were first heated at a temperature of 105 \({}^{\circ}\)C for a period of 24 h and then were stored in a desiccator for at least two hours. Figure 2 shows the emissivities of the investigated materials using both approaches. The standardized approach has more advanced analysis as it can obtain emissivity values at various temperatures when compared to the empirical developed approach. Furthermore, the results presented in the MWIR (i.e., 3-5.4 microns) show greater variability than in the LWIR (i.e., 8-12 microns).