Spatial filtering based terahertz imaging of low absorbing objects

•A phase contrast and dark field methods are proposed in THz imaging that have been not yet investigated comprehensively in the context of THz imaging recording schemes. In diversity to the dark-field method, both positive (PPC) and negative phase contrast (NPC) images allow to visualize linear mapping of the phase variations induced by a low absorbing objects into corresponding changes in a registered intensity. Here, we apply monochromatic THz emission to realize both dark-field and phase-contrast methods at 0.3 THz.•Employment of two different THz imaging setups of distinctive geometries – using focused and collimated beams – facilitated to demonstrate that the proposed fertile approach enables to use optical setup with relatively small aperture sizes (50 mm diameter) and still allowing to obtain image of the large surface sample (object size was 82 mm x 50 mm) with good quality without indicative influence of diffraction effects. Relying on similarity to 4f imaging setup configuration we noticed that we can record images using small aperture size setup and single detector by shifting the sample in the object plane.•Proposed optical setup can be used to apply amplitude spatial filtering methods. Therefore, taking advantage of shifting property of Fourier transform, we moved the sample in object plane introducing changes in phase distribution in the Fourier plane without variation in amplitude distribution. We implemented these findings via exploitation of two alternative spatial filtering methods – dark field and phase contrast – to create a contrast-enhanced THz images of weakly-absorbing objects under focused and collimated radiation at 0.3 THz and recording it by sensitive antenna-coupled titanium-based microbolometers.