# Imaging Diagnostics of Inside of a Building Wall Using Millimeter-Wave Reflectometer
## 1 Introduction  
In Japan, tile materials are attached to make the outer wall of a building look attractive, and to protect the concrete wall. The detachment of tile materials may occur due to adhesion defects and aging, which may cause dangerous accidents. Therefore, the inspection of the adhesive condition, as well as the peeling state, is of importance. Conventionally, a hammering sound inspection is utilized, which detects invisible defects under the attached tile by striking the surface with a hammer and listening to the sound. An inspector judges the state by the sound. Usually, veteran engineers are engaged in this work. This method does not need a sophisticated system and is a non-destructive measurement. However, it has serious problems: for example, it requires skilled experts and is time-consuming. There is no convenient way to quantitatively evaluate the results. As a further note, organic adhesive exterior wall tiles have become more widespread in recent years. It is difficult to detect the adhesion state of these tiles by using a hammering inspection.  
Recently, studies of hammering robots have been progressing [1; 2; 3]. The robot imitates the hammering sounds of skilled inspectors. The hammering sound is evaluated by inspectors remotely or analyzed by using a computer. Hammering robots belong to the remote measurement systems, since one can control the system and analyze the resulting sound data remotely. However, the above-mentioned difficulties with the manual hammering inspection still exist. Additionally, considering dangerous activities, such as building walls and other constructions, the application of non-contact, as well as non-destructive, diagnostic systems should be expected.  
As an alternative to the hammering method, electromagnetic wave diagnostics using infrared waves has been studied [4; 5; 6; 7; 8; 9; 10]. Infrared thermography has advantages, since it is a perfectively non-intrusive and non-contact measurement method. However, it still has anissue in the quantitative diagnosis of the tile adhesion measurement since it can be difficult to understand the results due to ambient thermal effects, as well as the lack of spatial resolution. Although not commonly used for exterior wall diagnosis, ultrasonic methods are also available as a non-destructive testing technique [11; 12]. The ultrasonic C-scan obtains the depth-directed damage distribution. However, it is difficult to apply to exterior wall diagnostics because the sample must be immersed in liquid. Air-coupled ultrasonic has also been studied, but there is still no standardized method for detecting internal defects in exterior walls with pulse echoes. Microwave/millimeter-wave reflectometric measurement has also gained importance in the present measurement due to its non-invasive nature. Two characteristics of microwave/millimeter-wave diagnostic systems combine to act effectively. The first is the ability to penetrate dielectric materials. The second is the use of sensitive phase fluctuation measurements [13]. In this work, a type of radar reflectometer was first applied for the measurement, which utilizes the phase interference effect of reflected waves due to the multiple reflection between the two layers (Fabry-Perot effect). The results show the imaging inside the surface related to the peering condition between the tile and concrete wall, and the quantitative evaluation of the condition.  
In Section 2, the diagnostic principle using Fabry-Perot interferometry is briefly described. An initial measurement is compared with a theoretical calculation. The practical hardware system together with the signal processing of the results is described in Section 3. In Section 4, the experimental results using various simulated samples and quantitative evaluation of the tile adhesion condition are provided, followed by the conclusion.