Issue 46

A. Maione et alii, Frattura ed Integrità Strutturale, 46 (2018) 240-251; DOI: 10.3221/IGF-ESIS.46.22 244 (a) (b) Figure 3 : "Salottino di Porcellana" (room 52): (a) planimetric position and (b) timber vault realized under the earlier masonry vault, visible from the empty space behind the room. T HE INSTRUMENTAL INVESTIGATIONS Basic theoretical principles of the infrared thermography he thermographic surveys were carried out using an infrared camera (thermal camera) that allows detecting infrared energy emitted from an object, converting it into apparent temperature, and displaying the result as an infrared image. In fact, all elements with a temperature above absolute zero emit heat and therefore radiation in the infrared field. The thermal or infrared energy is characterized by a wavelength too large to be visible; it is, in fact, the portion of the electromagnetic spectrum that is perceived as heat. Thereafter the infrared cameras operate in wavelengths as long as 14000 nm (14 µm) while common cameras, forming images using visible light, operate in the range of 400–700 nanometres. The existence of the infrared radiation was discovered in the 19 th century by William Herschel [26]. He used a prism to produce a spectrum of colours and noticed that the temperatures of the colours increased from the violet to the red part of the spectrum. He also found that the highest temperatures are beyond the red portion of the spectrum in a region where no sunlight is visible. The functional principle of the infrared thermography is governed by three essential physical laws [27]: – Kirchhoff’s law defines the relation between emission and absorption of energy, which states that the absorption of a body has to equal its emissivity at every wavelength; – Planck’s law of radiation describes the distribution of the amplitudes of the energy that a black-body emits as radiation vs. different frequencies or wavelengths (i.e. spectral radiance); – Stefan–Boltzmann law is applied to the emission of a surface over all wavelengths and states that the radiant power, I, grows with the fourth power of its absolute temperature. The results of the infrared thermography are influenced by three essential factors: the surface configuration, the surface roughness and the environmental conditions. The surface configuration influences the thermal conductivity that can be reduced if air voids and low density areas are present. The surface roughness has the effect of increasing the surface emissivity with respect to that related to a smooth surface. This parameter is strictly related to the material characterizing the surface. The environmental conditions such as solar exposition, temperature, wind speed, surface moisture that surrounds the surface affects the validity of the image interpretation. The application of the infrared thermography to the case study For the thermographic investigations into the Capodimonte Museum the ThermaCAM B640 model of the FLIR Company was used. The system is provided with a Focal Plane Array detector operating in the band 7.5-13 lm and detects a temperature range from -40°C to +120°C with a measurement accuracy of  2° C. A passive approach of investigation was adopted by using the natural heat sources of the building as the solar radiation or the slow microclimate temperature. T