## 3 Measurement Results and Discussion
### Imaging with Spatial Filtering Methods  
To illustrate the sensitive operation of the developed Ti-mBs and their versatile operation using weak power sources we have proceeded in an opposite way that is usually applied in a conventional THz imaging. Here, as a rule, THz images come from scanning the object with the focused beam and registering the focused radiation on the detector making thus induced signal much larger--we have oriented our research for future applications when images without raster scanning, e.g., employing detectors arrays will be required. In such cases, the apertures of imaging optical elements become of a particular importance as the THz wavelength is large in comparison with diameters of optical elements and propagation distances. Therefore, a novel optical solutions [15] are needed allowing to design and manufacture compact optical elements with large aperture sizes and reasonable focal lengths. A special role can be attributed to the case of low absorbing or transparent samples, when the object observation and registration of its internal structure grow into a tremendous challenge [29]. It is related to the fact that power detectors record intensity of the radiation which suffers almost no observable change in the given circumstances, and the shift of the phase of incident radiation becomes a single quantity to be recorded. In what follows, we employ both single pixel and arrays of the sensitive Ti-mB to record THz images of low absorbing objects under unfocused THz illumination. Both techniques--direct imaging and spatial filtering methods--are used to create a THz image of object inducing phase changes  
Figure 6: THz beam profiles of different polarizations obtained using optically-pumped molecular THz laser operating in a continuous wave mode at frequencies: 0.76 THz, 1.33 THz and 2.52 THz. Reference measurements using pyroelectric detector (**a**); results using microbolometer when the dipole antenna is in parallel to the electric field of the incident light (**b**); results using microbolometer when the dipole antenna is perpendicular to the electric field of the incident light (**c**). White lines indicate intensity cross sections along the relevant axes. (uniform amplitude) and that can be registered by intensity sensitive Ti-mB. Conventionally, such filter must be inserted in the Fourier plane in the used imaging system and it will affect only some spatial frequencies of the Fourier spectrum of the object. One can distinguish different methods like bright and dark field [30], Schlieren [31] or phase contrast [32; 33] depending on the filter type. They differ in principles and type of the filter; however, all of them permit to create an intensity pattern corresponding to phase changes in the object. The phase contrast method enables linear mapping between phase shift introduced by the object and recorded intensity pattern. In a given study we preferred a phase contrast and dark field methods to register not focused radiation with sensitive Ti-mB. Results of the investigation together with experimental set-up are given in Figure 7. To elucidate sensing features of the microbolometers, a sample consisting of various low absorbing objects--rubber, paper towel, napkin, sponge, sponge with metal wire inside, plastic (from 1 to 4 layers), aperture of 4 mm in diameter, dots made of silicone, acrylic and wax as well as metal as the reference--was composed and prepared. Measurement set-up for this kind of research is depicted in Figure 7a. As one can see, it is slightly different from that displayed in Figure 2 because of the filter inserted.