## 1 Introduction  
Infrared radiation, an invisible electromagnetic radiation, was accidentally discovered by Sir [PERSON] in 1800. At this moment, a long journey began and this invisible 'light' was exploited for many different applications. The first infrared thermography (IRT) systems were used for military purposes, during the First World War. In the building sector, the first applications started in the early 1980s, mostly for energy saving and insulation issues in building diagnostics [1]. With the passage of time, there was an evolution of the various thermographic systems used, in terms of sensitivity, applicability, image processing, etc. Until now, there have been two main modes of IRT application; the passive and the active infrared thermography [2]. Commonly, all active IRT techniques can be used in the NDT assessment of materials and also provide image processing results. Nonetheless, selection of the most suitable energy sources depends on the application. In cultural heritage applications, the most commonly used ones are optical pulsed thermography (PT) and optical lock-in thermography (LT). The first one is easy to use providing prompt results, whereas the second one permits enhanced control of the energy deposited on the material, which could be attractive if a low power source is to be employed or if distinctive attention has to be given to the investigated material (i.e., inspection of cultural heritage objects) [3]. Through the different image processing tools that one can use (i.e., the Pulsed Phase Thermography (PPT) and Thermographic Signal Reconstruction (TSR) techniques are great examples) it allows detection of defects to certain depths (i.e., of 2.5 mm), for defects with a size-to-depth ratio of approximately 2 and higher and thus provides the user with quantitative results. In particular, image processing approaches like background subtraction [4] or Machine Learning algorithms like expectation-maximization [5], can reveal additional features that are not able to be detected from the original thermal image. IRT can be used for the identification of the conservation state of a monument in real time. From its inception to the present day, the National Technical University of Athens research team have investigated many monuments belonging to different historic periods. IRT was used as a tool in the diagnostic level, for the detection of invisible superficial cracks or/and disparities, as well as the revelation of moisture presence within structures. Additionally, IRT used for the assessment of materials interventions and the determination of incompatible materials might have been utilized. Last but not least, at the Holy Aedicule rehabilitation works when it is applied for continuous monitoring, not only in real time for the detection of emergence issues that arise, but also for tracking and documenting all the interventions in every work phase [6; 7; 8]. This scientific monitoring supported decision-making concerning the quality assessment of the integrated project of Holy Aedicule rehabilitation, contributing to the quality control of restoration and conservation interventions. This use of IRT is currently adopted by small SMEs providing consultancy services to the stakeholders of cultural heritage. In combination with other non-destructive techniques, further information can be obtained. In this framework a new field of application in civil infrastructure has emerged for the quality control of structures like airport pavements, aircrafts, metro railways and photovoltaic panels [9; 10]. Today, IRT is widely utilized in the sector of cultural heritage. In recent years, many researchers have used this technique all over the world, in Korea for example, at the temple of [PERSON] [11], or at the Church of the Nativity in Bethlehem [12], or in Italy at the Longobard Temple [13].