## 3 Measurement Results and Discussion
### Imaging with Spatial Filtering Methods
Two types of filters were used in the experiments: the filter 1, consisting of 2 mm circle shaped four paper sheets, was a phase filter introducing a phase retardation of \(\pi/2\) only in the central part of the Fourier spectrum and enabling thus to create an image using phase contrast method; second one, filter 2, circle shaped aluminum foil of 2 mm diameter, was an amplitude filter, blocking low spatial frequencies, which resulted in creating an intensity image of phase object having dark background (which is related to removal of low spatial frequencies). Sample photo and its direct image as well as two corresponding intensity images of phase objects are shown in the corresponding panels (b-d) of Figure 7. It can be easily seen that even direct imaging nicely resolves all the objects illustrating suitability of Ti-mB for imaging of low absorbing objects. As for deeper insight, transparent objects like sponge (5 and 6) and paper tissue (4) are much better visible in case of phase contrast method (filter 1), whilst dark field method (filter 2) gives much better results for imaging of thin rubber glove (2), where we can clearly see edges of the rubber shape. Figure 7: Principal scheme of phase contrast imaging set up for 0.3 THz, where L1—HDPE lens with \(f\) = 12 cm, M1, M2—gold plated flat mirrors, sample, L2—TPX lens with \(f\) = 6 mm, Filter—(1): \(Q\) = 2 mm circle shaped four paper sheets, (2)—\(Q\) = 2 mm circle shaped aluminum foil, PM1—off axis parabolic mirror with \(f\) = 5 cm (**a**). Photo of the sample with plastic substrate film, it consists of: (1)—metal, (2)—rubber, (3)—paper towel, (4)—napkin, (5)— sponge, (6)—sponge with metal wire inside, (7)—plastic (from 1 to 4 layers), (8)—aperture (diameter: 4 mm), (9)—dots, made of silicone, acrylic and wax (**b**). Direct raster scan image of the sample at 0.3 THz (**c**). Phase contrast images obtained using filter 1 (**d**) and filter 2 (**e**). THz image pixel size: 0.3 mm \(\times\) 0.3 mm; images consist of 292 \(\times\) 190 pixels. Finally, it is worth to notice the operation speed of the device. Earlier it was determined that the Ti-mB coupled with a 0.3 THz frequency dipole antenna exhibited the response time of 1 \(\upmu\)s range and maintained a sensitivity of 300 V/W and NEP as low as 14 \(\text{pW}/\sqrt{Hz}\)[26]. If compared with other similar devices based on thermoelectricity-related phenomena, for instance, with uncooled antenna-coupled terahertz detectors based on BiSb/Sb thermocouples with 22 \(\upmu\)s response time and NEP of 170 \(\text{pW}/\sqrt{Hz}\)[34] and bow-tie diodes operating on hot-electrons effects with sensitivity of 5 V/W, NEP of 5.8 \(\text{nW}/\sqrt{Hz}\) and response time less than 7 ns [35], the described titanium-based microbolometers with readout electronics exhibit high sensitivity exceeding 200 kV/W and relatively short response time (5 microseconds including electronics circuit). Therefore, these parameters together with electronics that can work up to 200 ksps (kilo samples per second) simultaneous sampling speed, makes the Ti-mB promising for real-time fine optical adjustment and mode profile control aims in THz spectroscopic and imaging systems as well as imaging applications and inspection of low-absorbing objects.