4. Non-invasive detection of internal structures of objects

Wave attenuation, reflections from the material surface, and internal scattering from embedded objects or cavities in the material causes "shadows" on the opposite side of the material from the electromagnetic source. An X-ray image (Figure 7) is a good example of how shadows associated with propagation delay and wave attenuation can be used to interpret the internal structures of objects. Effectively the combination of attenuation and phase delay, which are directly linked to the dielectric properties of the material, provide information about the material through which the electromagnetic wave travels.

Wave penetration into any material can be defined by the penetration depth (d), when the ratio of the wave amplitude to initial amplitude is <sup>1</sup> <sup>¼</sup> <sup>0</sup>:3679: <sup>e</sup>

$$d = \frac{1}{k\_o a} \tag{17}$$

The penetration depth of an electromagnetic wave is inversely proportional to the wave frequency and the attenuation factor of the material. If the penetration

Figure 7. Example of an X-ray image.

Figure 8. Schematic of a 'look through' microwave system for assessing the properties of materials.

depth of a material is far greater than the thickness of the material at a given frequency, the material appears to be 'transparent' to the electromagnetic wave. If the penetration depth of a material is far smaller than the thickness of the material at a given frequency, the material appears to be 'opaque'. If the penetration depth of the material is similar to the thickness of the material at a given frequency, the material may be regarded as 'translucent'.

Although X-rays have been used to investigate the internal features of objects, other frequencies of the electromagnetic spectrum can also be used [24]. For example, microwaves can be used to assess the internal structure of materials, by 'looking through' the objects, as illustrated in Figure 8. Such microwave systems have been used to assess moisture content of materials [25–28], detection of decay is timber [29–31], detection of insects in bulk materials such as grains and wood [32–34], assessment of wooden structures of cultural significance [24], and 'see through walls' Wi-Fi imaging [35, 36].

## 5. Energy transfer

Electromagnetic waves can transfer energy from one object to another through open space. Generally, the amount of energy transferred depends on: the intensity of the electromagnetic fields; the frequency of the fields' oscillations; and the dielectric properties of the material. The power dissipated per unit volume in a non-magnetic, uniform materials, exposed to electromagnetic fields can be expressed as [37]:

$$P(\mathbf{x}) = \mathbf{55.63} \times \mathbf{10^{-12}} \bullet f \bullet (\boldsymbol{\tau} \bullet \boldsymbol{E})^2 \bullet \boldsymbol{\kappa''} \bullet e^{-2k\_o \alpha \mathbf{x}} \tag{18}$$

where f is the frequency, κ" is the dielectric loss factor of the heated material, ko is the wavenumber of free space, α is the attenuation factor, and x is the distance

below the surface of the material. Therefore, more power is dissipated in a material at higher frequencies; however, the wave attenuation factor is higher at higher frequencies and therefore the penetration of the heating into the surface of the material is lower at higher frequencies.
