# Frequency Range Optimization for Continuous Wave Terahertz Imaging
## 7 Conclusions  
This paper proposes an FRO algorithm for CW-THz imaging based on a CNN and an occlusion sensitivity algorithm. THz imaging is based mainly on THz-TDS systems. However, THz-FDS systems could be more suited for certain applications. THz-FDS systems suffer from two main disadvantages. The first disadvantage is low acquisition speed, and the second disadvantage is high tuning error and low repeatability. A novel imaging principle for THz-FDS imaging is proposed in this paper. The imaging principle is a combination of raster imaging and full-field imaging. A relatively wide collimated THz beam passes through the sample and is detected in transmission mode. With a linear \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(\
u_{0}\) (GHz) & \(\
u_{1}\) (GHz) & Bandwidth (GHz) & \
\begin{tabular}{c} **Pixel Acquisition** \\ **Time (s)** \\ \end{tabular} &
\begin{tabular}{c} **Image Acquisition** \\ **tion Time (s)** \\ \end{tabular} \\ \hline
50 & 1210 & 1160 & 348 & 125,280 \\
710 & 810 & 100 & 30 & 10,800 \\
1170 & 1200 & 30 & 9 & 3240 \\
770 & 810 & 40 & 12 & 4320 \\
416 & 430 & 14 & 4.2 & 1512 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Estimated image acquisition times for frequency ranges from Table 2 and for the reference measurement. Figure 15: Spectrograms and images for acquisition in the frequency range 1170-1200 GHz: (**a**) Peak envelope spectrogram, (**b**) Unwrapped phase spectrogram. (**c**) Peak envelope spectrogram processed using GSF. (**d**) Unwrapped phase spectrogram processed using GSF. (**e**) Compressed 1D intensity image with marked plastic plates’ positions. (**f**) Compressed 1D phase image with marked plastic plates’ positions. rail, the position of the PCAs change in a way thatoverlaps other measurements. If the linear rail's movement step is much smaller than the THz beam's aperture, the proposed GSF can be used for the THz-EDS's frequency tuning error mitigation. The proposed imaging principle solved the problem of long acquisition times. In this paper, a CNN and occlusion sensitivity are used for frequency range optimization, which, in principle, would find an ideal frequency range. The deal frequency range should have a relatively short bandwidth, and would have to retain the spectral characteristics of the observed samples or materials. The proposed frequency range algorithm was evaluated experimentally, and compared to an expert's opinion. The expert determined the ideal frequency range from 710 GHz to 810 GHz, while the proposed algorithm determined three ranges: 1170 GHz to 1200 GHz, 770 GHz to 810 GHz, and 416-430 GHz. One-dimensional images were reconstructed for all the optimized frequency ranges. Each of the images seemed to retain all the spectral information, but the most visually distinguishable was the frequency range between 770 GHz and 810 GHz. In addition, measurements in all frequency ranges were faster than the reference measurement. The proposed frequency range optimization was used successfully for frequency range optimization. The proposed THz imaging principle and frequency range optimization algorithm could mitigate the two shortcomings of THz-EDS systems in imaging applications: low acquisition speed and frequency tuning error. In further research, the proposed imaging principle and frequency range optimization could be used for 2D image acquisition and reconstruction. Conceptualization, B.P., A.S. and D.G. ; methodology, B.P., A.S. and D.G. ; software, B.P. and A.S.; validation, B.P. and D.G. ; formal analysis, B.P. and D.G. ; investigation, B.P. ; resources, B.P. ; data curation, B.P. ; writing--original draft preparation, B.P. and A.S.; writing--review and editing, D.G. ; visualization, B.P. ; supervision, D.G. ; project administration, D.G. ; funding acquisition, D.G. All authors have read and agreed to the published version of the manuscript. This research was funded by the Slovenian Research Agency (ARRS) Grant number J7-9408 and Research program number P2-0065. Not applicable. The authors declare no conflict of interest.