# Pulsed Thermography Applied to the Study of Cultural Heritage
## 1 Introduction  
In recent years, the number of non-destructive investigations applied to the study of cultural heritage artefacts has increased. In fact, the characterization of structures and materials, as well as of the artistic features of cultural heritage, enables us to consistently improve our understanding and helps in determining their preservation state. When investigating cultural heritage, two of the main purposes consist of the characterization of the artefact constituent materials and the study of the manufacturing process. They are often achieved by the application of specific diagnostics, namely non-destructive testing (NDT), which enables the investigation of artwork materials without causing damage [1,2]. Among them, infrared thermography (IRT) has been successfully applied to the investigation of cultural heritage structures and materials [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. In particular, in the active IRT approach, an external stimulus is required to generate temperature variations whose time evolution may witness the presence of subsurface anomalies [16]. Active IRT allows researchers to obtain both qualitative and quantitative evaluations of the sample structure as a function of depth by monitoring the transient behavior of the induced temperature change. Depending on the external excitation mode, different configurations of active IRT can be identified, the main ones being pulsed and lock-in IRT. They are based on the photothermal techniques which in general have been proven to be useful in determining some thermophysical parameters in liquids and solids [17, 18, 19, 20, 21, 22]. Pulsed IRT involves recording the time dependence of the distribution of the infrared (IR) radiation locally emitted across the sample surface following the heating of the sample produced by a pulsed illumination [16]. On the other hand, in the lock-in IRT configuration, an oscillating temperature field is produced by a periodicheating of the sample surface, then detected by a synchronized IR camera according to the lock-in processing technique [16; 23]. With respect to pulsed IRT, a larger signal to noise ratio can be obtained in the thermographic images, but the gathering of that information originating at different depths in the sample requires several measurements performed at different frequencies instead of a single one as a function of time. In the last years, pulsed IRT has been the most applied configuration in the study of different kinds of small-size artefacts, such as library and archival materials, archaeological findings [24; 25; 26], and works of art [27]. In particular, it has been proposed for the investigation of the mosaic preservation state [28; 29; 30; 31] and for the study of sculptures [27; 32]. In 2015, IRT was used by [PERSON] et al., to analyze the cracks of the \"Ratto delle Sabine\", a model mainly composed of raw clay in the outer part and of several different materials in the inner part, to evaluate the level of weathering of the statue, in particular that related to the previously mentioned cracks [32]. Concerning the study of books and documents, IRT has been used to study historical bindings, providing information on both the assembled structure and the various component materials. In the bookbinding analysis, the thermographic imaging allowed researchers to detect the presence of damage, to investigate the adhesion state of the different parts of the bookbinding, and consequently to evaluate their preservation state [33]. In the study of paintings, IRT has been applied to reveal the adhesion of the paint layer to the support, to observe the canvas or wood texture, and to detect defects [34; 35; 36] also investigated by other imaging techniques [37; 38; 39; 40; 41].