## 3 Innovative SHM Techniques
### Image and Computer-Vision Based Approach  
Computer vision (CV) enables computers to analyse, understand, and interpret visual information from static images and video sequences. These methods embody several advantages, such as non-contact and long-distance application, automated inspection, and low interference with the daily operation of the structures. Therefore, their adoption has emerged as a suitable non-destructive method for preventive conservation in CHS. [PERSON] and [PERSON] [144] presented a comprehensive review of CV-based monitoring approaches with applications at both global (e.g., modal identification, displacement measurement, vibration serviceability) and local (e.g., crack, spalling, delamination) structural levels. They also highlighted some limitations of CV algorithms that can affect the accuracy of the results, including the adverse effect of the hardware used (e.g., electric noise, camera  
Figure 10: Concept of smart brick technology (a) and an example of its application for strain monitoring within a damaged masonry building using smart brick ((b), from [142]).  
self-heating, lens distortion), as well as the environmental conditions (e.g., illumination change, ground vibration, rain, etc.). Therefore, in applying CV approaches in CHS, the aim should not be to completely replace the conventional SHMs, but to complement them.  
Among the most common digital technologies (based on CV methods) that are used for documenting, monitoring, and inspecting historical constructions are photogrammetry and laser scanning [145,146]. A comparison between the two techniques in terms of their main features, advantages, and limitations is summarized in Table 11. As a general rule, laser scanning is more appropriate if a high level of accuracy over ample space is needed. At the same time, photogrammetry is an optimal solution for smaller spaces and for obtaining a more visual photo realism. Due to each method's intrinsic pros and cons, they are usually integrated to ensure complete and accurate documentation [147].  
Another technology based on many standard CV techniques is infrared thermography, or thermal imaging, the science of analysing images captured from thermal infrared cameras. The use of infrared thermography can help identify potential structural issues before they become major problems. In addition, this technique can be used to assess the effectiveness of any restoration work that has been performed on the building. However, it is important to note that this technique should be used in conjunction with other methods of structural analysis, as it has limitations, and it may not detect all types of damage.  
Some researchers have also explored the use of 2D image-based methods for the monitoring of cracks. [PERSON] et al. [148] investigated the use of this technique for the digital indoor monitoring of cracks in heritage listed buildings of lower social projection, for whose financial resources dedicated to preventive conservation are scarce. This technique proved to be a valid alternative to a higher-cost SHM image-based approach (such as 3D photogrammetry), guaranteeing effectiveness, non-invasiveness, and affordability. The use of thermography and a moisture meter, together with the 2D image-based method, was recommended by the authors to check for the impact of moisture on cracks and to predict any potential future damage to the building's stability.