## 4 Emerging Microwave NDT Techniques
### Active Microwave Thermography  
Microwave thermography combines the features of both microwave technology and thermographic techniques [114]. It is an NDT technique that detects microwave radiation from objects and can be divided into two types: passive and active [115]. In the passive method shown in Figure 12a [115], the natural microwave radiation from objects is detected by recording the specimen's temperature using an infrared (IR) camera without exposing the object to external microwave sources. On the other hand, in active microwave thermography (AMT) shown in Figure 12 [115], electromagnetic energy is radiated onto the object, which heats it. Further, the thermal excitation or response is measured using IR cameras. The thermal excitation can be varied by controlling the microwave source's frequency, polarization, and power intensity. In passive and active microwave thermography, the thermal response allows for depth measurement and precise localization of defects, enhancing the ability to assess the severity and extent of subsurface anomalies with greater accuracy [116]. Moreover, passive thermography facilitates qualitative defect assessments, while active thermography offers qualitative and quantitative analyses [117]. Figure 11: The microwave microscope probe’s design: (**a**) a schematic representation; (**b**) a simulation showing the fringe electric field at the open tip of the probe in free space; (**c**) a simulation of the electric field distribution on a metal surface under the probe with a 0.5 mm standoff; and (**d**) the normalized reflection coefficient magnitude (\(|S_{11}|\)) as the distance between the probe and the surface increases [112]. The more common thermography technique is the active method. Thus, various active thermography techniques have emerged based on the nature of the external thermal excitation. Such methods include pulsed thermography (PT) [118], step thermography (ST) [119], and modulated thermography (MT), also known as lock-in thermography (LT) [120]. Furthermore, a hybrid advancement in this field is pulsed phase thermography (PPT), which merges the features of both PT and MT to offer enhanced detection capabilities [121]. [PERSON] et al. [122] presented the use of AMT for the NDT of external defects in metallic materials. The findings indicate that, when a metal with a dielectric-filled crack is exposed to an electric field polarized perpendicular to the length of the crack, a propagating mode is initiated within the crack, resulting in dielectric heating. The study revealed that cracks are identifiable through the AMT detection technique at angles ranging from 0\({}^{\circ}\) to around 65\({}^{\circ}\), with the ideal heating duration for effective detection being approximately 5-30 s [122]. Furthermore, in another investigation, [PERSON] et al. [123] utilized AMT to inspect and detect water ingress within structures. This paper established a mathematical analysis relating water location and level to the temperature distribution on the inspection surface. The results from AMT measurements on a rubber sample with a small volume of water demonstrated the technique's potential for water detection and accurate depth estimation of water ingress. The experiments showed that even a small amount of water could be detected after a short duration of microwave excitation, with an average in-depth estimation error of approximately 5% [123]. Another significant study related to AMT is that of [PERSON] et al. [124], which significantly contributes to using the NDT method for composite materials. This paper investigated the application of AMT in assessing microwave-absorbing structures, notably carbon-fiber-reinforced polymers equipped with RF-absorbing materials. The findings highlighted the enhanced detection capabilities of AMT, as evidenced by a pronounced  
Figure 12: Microwave thermography setups: (**a**) passive thermography and (**b**) active thermography [115].