# Terahertz Inspection of Buildings and Architectural Art
## Structural Integrity Inspection Of Buildings  
As mentioned earlier, the inspection of the structural integrity of buildings is also a very important aspect. [38] Terahertz imaging has been proven useful in finding voids in concrete samples, as depicted in Figure 4[39]. Other studies have reported the inspection of defects in materials as varied as wood, cement, and tile adhesives, which have been imaged with terahertz [40] (see Figure 5b,c). The use of water percolation process in cracks has been used to enhance the contrast in order to find them, as shown in Figure 5a,b. We finally want to mention that the THz-optical properties of a number of common building materials have been reported [41; 42]. These properties are fundamental for the appropriate analysis and understanding of many of the studies that we have reviewed here, and some that may come in the future.  
Figure 3: (**a**) Facade of the townhouse in Lünerstrasse 3, Lüneburg, Lower Saxony where the medallion (shown in **b**) was removed from for analysis and restoration [photo by [PERSON]]. (**b**) Polychrome glazed Terracotta medallion-portrait of a man-from the 16 th century. The red lines indicate the examined area. (**c**) A tomographic cut of the air gap obtained from the terahertz time-of-flight measurements. Adapted with permission from Journal of Infrared Millimeter and Terahertz Waves 38, 495 (2017) [36] Adapted with permission from [Journal of Infrared, Millimeter, and Terahertz Waves], Springer, 2017.  
It is worth pointing out that there is still a broad number of construction materials that have not been properly characterized in the terahertz band, and this is a pending task that will be of importance not only in building measurements, but in other areas, like terahertz telecommunications. Terahertz waves have also been used as non-contact inspection tool for measuring the water content of concrete, which is an important parameter in terms durability of structures made from it [43; 44]. Furthermore, terahertz has been used to characterize metal objects. It has been demonstrated to provide quantitative information on the roughness of archeological metal surfaces [45] and for distinguishing corroded/mineralized layers in metal artifacts [46].  
Figure 4: (**a**) Photograph and (**b**) terahertz image of a defect-free concrete sample. (**c**) Photograph and (**d**) terahertz image of a concrete sample with defects. Adapted with permission from 38 th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) [39]. Adapted with permission from [38 th International Conference on Infrared, Millimeter, and Terahertz Waves], IEEE, 2013.  
Figure 5: (**a**) Terahertz imaging of cracks in concrete, in which water diffuses along the cracks which enhances the detection sensitivity of the defect. (**b**) Is an illustration that depicts the process of water percolation which increases the contrast around the crack. (**c**) Is a terahertz image of the voids of the adhesive cement under a tile. (**d**) illustrates the geometry of the measurement shown in (**c**) [40]. Adapted with permission from [NDT & E International], Elsevier, 2009.