## 1 Introduction  
A composite is a structure made of materials with different physical properties. When combined, a material with characteristics different from the individual components is obtained. The properties of the composite structure depend on the matrix and the reinforcement materials and their setups. Polymer matrix composites (PMC) play important roles in modern industry because of their high stiffness, corrosion resistance, and strength to weight ratio [1]. Increasing the number of such structures in aerospace, construction, and automotive applications, as well as in lightweight structures in the wind energy industry, enforces continuous monitoring of their condition. Due to high anisotropy, nondestructive inspection of layered composite materials is a much more complicated process than the evaluation of homogenous, (mostly metallic) structures [2; 3; 4; 5; 6; 7; 8]. Several nondestructive methods are utilized in this case (ultrasonics, shearography, tap testing, acoustic emission, digital radiography, infrared thermography) but none of them gives a full description of the evaluated structures. Thus, further development of NDT techniques should be studied. The use of terahertz inspection of dielectric or semiconductive structures is attracting increasing attention in the research community and industry. Recently, various applications have been demonstrated by many research groups. Continuous-wave imaging (CW), frequency domain measurement using a vector network analyzer (VNA), and pulsed time domain spectroscopy (TDS) are utilized in this frequency range [9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. The first group of applications refers to spectroscopic evaluation of materials or potential inclusions compositions, i.e., polymer evaluation [16], drugs identification [19; 20], explosives detection [19], biological tissue identification in the case of biotechnological and medical applications [21; 22], hydration/moisture level determination [23; 24], polymerization state monitoring [25], chemical mixtures evaluation [26], and determination of tea geographical origin [27]. The second group of applications refers to observations of the inner structure of an object under test (OUT), similar to ultrasonic testing or the pulsed ground penetrating radar (GPR) technique. In this case homogenous or layered dielectric materials are examined in order to detect irregularities or defects [14; 15; 28; 29; 30]. Non-conductive coatings of dielectric or conductive materials, as well, as layers thicknesses can be also evaluated [28]. In the case of integrated circuits the inner structure is monitored [31]. TDS is also utilized in order to evaluate the integrity of layered tablet structures. Another group of applications refers to terahertz tomographic imaging [32; 33; 34; 35; 36]. Tomography refers to the cross-sectional imaging of an object by measuring transmitted or/and reflected waves (ray). There are various terahertz imaging options: time of flight reflection tomography (THz ToFRT), diffraction tomography (THz DT), computed tomography (THz CT), tomography with a binary lens, and digital holography. Continuous wave terahertz imaging [37; 38; 39] is a generally faster and less complicated technique compared to TDS. In this case the source (typically horn antenna, photoconductive antenna or far infrared laser) illuminates object under test and detector (photoconductive, pyroelectric, or bolometric) or a matrix of detectors receive the electromagnetic wave. In most cases this technique provides power or intensity information and phase data is not available. Higher power of sources and utilization of detector matrices enables inspection of thicker and bigger structures in comparison to TDS. Main advantages associated with terahertz imaging technique are:  
* non-contact measurement in reflection and transmission arrangement;
* non-ionizing nature;
* inner structure and spectral information is obtainable; and
* fraction of a millimeter resolution.