# Frequency Range Optimization for Continuous Wave Terahertz Imaging
## 6 Experimental Results  
This paper proposes a frequency range optimization based on a CNN's occlusion sensitivity. Proposed frequency range optimization was experimentally evaluated using the THz-PDS imaging principle proposed in preliminary work [17] and described in Section 3. In this paper, a CNN and occlusion sensitivity were used for frequency range optimization, as shown in Section 5. The optimal frequency range should have a relatively short bandwidth, and would have to retain the spectral characteristics of the observed sample or materials. The expert empirically determined the optimal frequency range from 710 GHz to 810 GHz, while the proposed algorithm in Section 5 determined three ranges: 1170 GHz \begin{table}
\begin{tabular}{c c c} \hline \hline \(v_{0}\) **(GHz)** & \(v_{peak}\) **(GHz)** & \(v_{1}\) **(GHz)** \\ \hline
1170 & 1185 & 1200 \\
760 & 790 & 820 \\
416 & 420 & 430 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Extracted frequency ranges based on the proposed algorithm. Figure 11: OSF function with marked peaks. to 1200 GHz, 770 GHz to 810 GHz, and 416-430 GHz. New datasets were acquired in frequency ranges stated in Table 3 and the 1D images were reconstructed for all the acquired datasets. Each of the images seemed to retain 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 measurements. In the second part of the experiment, measurements were performed in the frequency ranges from Table 2. Other settings remained the same as in the reference measurement. Figure 12 shows the resulting images for measurements in the frequency range between 416 GHz and 430 GHz. Figure 12e,\(f\) show a compressed 1D intensity image and compressed 1D phase image, respectively. In both images the plastic plates are distinguishable. However, the spectral features are not as clearly visible as in the reference measurement. Figure 13 shows the resulting images for measurements in the frequency range between 770 GHz and 810 GHz. Figure 13e,f show a compressed 1D intensity image and compressed 1D phase image, respectively. The plastic plates are distinguishable among them and are clearly distinguishable from the environment. The results in Figure 13 are similar to the results from the reference measurements shown in Figure 14. However, because of the shorter frequency range, some uncertainties are present. Figure 15 shows the resulting images for measurements in the frequency range between 1170 GHz and 1200 GHz. Figure 15e,f show a compressed 1D intensity image and compressed 1D phase image, respectively. However, their spectral characteristics are not distinguishable, and this image could be deemed unusable for further processing. Figure 12: Spectrograms and images for acquisition in the frequency range 416–430 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. The visual results show that the selected optimal frequency ranges could be adequate for further processing. The proposed FRO algorithm was intended to help mitigate the long acquisition times in CW THz imaging. The acquisition time of a single pixel is  
Figure 14: Spectrograms and images for acquisition in the frequency range 710–810 GHz: (**a**) Peak envelope spectrogram. (**b**) Unwrapped phase spectrogram. (**c**) Peak envelope spectrogram processed using GSF. (**d**) Compressed 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. Figure 13: Spectrograms and images for acquisitions in the frequency range 770–810 GHz: (**a**) Peak envelope spectrogram. (**b**) Unwrapped phase spectrogram.