# Remote Spectral Identification in the THz Band with Reflection Spectroscopy in an Open Atmosphere
## 5 Discussion  
This paper brings up the subject of the practical application of THz radiation. The discussion about the best selection of the equipment is one without an absolute solution. Preferences will change depending on the field, application and specialist expertise. In our considerations, besides strictly technical parameters, user-friendliness, ease of implementation, and a low to moderate level of expertise needed by the designated operator to use the whole setup in an industrial situation were taken into account. Our selection, as any other  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Correlation @1 m** & **Correlation @3 m** & **Correlation @5 m** \\ \hline RDX & 0.84 & 0.86 & 0.85 \\ Lactose & 0.94 & 0.87 & – \\ PABA 0.67–1 THz & 0.83 & 0.62 & 0.58 \\ PABA 1.2–1.6 THz & 0.58 & 0.24 & – \\ \hline \hline \end{tabular}
\end{table}
Table 4: Results of identification of three materials at 1, 3, and 5 m.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Correlation @1 m** \\ \hline RDX & 0.87 \\ Lactose & 0.36 \\ PABA & 0.27 \\ Sugar & 0.68 \\ Salicylic Acid & 0.23 \\ PETN & 0.34 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Results of identification of six materials at 1 m distance.  
among various possibilities, will often generate discussion in scientific world. Hopefully, it will lead to a better or more cost-effective solution.  
The experiment was conducted in an atmosphere with 38-42% RH, as these were the dominant conditions indoors. Only low variability was observed during the one day period, and minimal variations not exceeding about 1% during the time of one measurement were observed. According to the accepted methodology, when the reference and sample measurements are conducted in similar conditions, the background influence should be eliminated. In our study, the water vapor had observable minimal to minor influence on the measured spectra. The observed spectral features occurred in close proximity to the material absorption peak. From our observations, the water vapor absorption features appeared on the measured spectra when the sum of the material absorption, water vapor absorption, and the atmosphere absorption exceeded the value of the detector dynamic range. The water vapor influence could be limited by the software. Furthermore, with engineering and calibration, the THz source output power could be controlled in an automatic manner by appropriately attenuating the signal. This could provide overall improved performance and limit the water vapor influence only to the strongest lines. Although the number of the samples was relatively small, they were selected to have absorption peaks spread across the range of the radiation generated by the OPO. The authors see significant room for improvement in terms of the methods used for analytical comparison for identification purposes. The novelty in using AI, neural networks, and other advanced computing methods could provide a higher certainty of positive identification. This is a necessary direction for progress toward practical application. The results showed that it is possible to use THz reflection spectroscopy for remote identification of some materials at a distance of a few meters. Although the methods used do not provide adequate results to prove clear positive identification they provide sufficient results for future, more sophisticated analysis and methods to be implemented.