# Remote Spectral Identification in the THz Band with Reflection Spectroscopy in an Open Atmosphere
## 1 Introduction  
Spectroscopy in the terahertz (THz) band has developed significantly since the introduction of pulsed techniques. Basic research [1; 2] and practical applications have benefited from THz technology development [3; 4]. Since the attacks on 11 September 2001, law regulations have enforced their highest ever security measures. While implementing new safety measures is a priority, the health and safety of people being screened are equally important. A perfect solution to a security checkpoint would be a system that informs the staff if someone is hiding something under their clothes and also provide information about what that object is made of. When considering the achievements of modern material technology such as ceramic knives or 3D-printed guns, in terms of security, none of the standard security solutions is sufficient by itself. Metal detectors are becoming a technology with a limited capability for threat detection. Due to the ionizing nature of X-ray radiation, it is not suitable for passenger screening. Chemical substance detectors are slow and need dedicated personnel and constant maintenance, reducing the throughput of the people checked. Only complex systems using many different techniques can be effective with a high probability of success. THz waves meet health requirements due to their low photon energy and lack of ionizing properties. For over a decade, the industry has offered THz and sub-THz imaging solutions, which are evolving towards multispectral imaging [5; 6]. Combining several safety precautions gives a high probability of finding suspicious objects, but remote identification of substances can be achieved only by spectroscopic methods. The presence of the spectral fingerprints of some explosive and dangerous materials in the THz range along with the ability of THz radiation to penetrate lighter clothing [7] makes it an ideal candidate as a sensing tool.  
The subject of using THz radiation in the spectroscopy-based identification of materials was mentioned in [8; 9; 10]. Although years of research have been dedicated to makingimprovements in THz techniques, technology puts limitations on the parameters of the available sources and detectors of THz radiation. Some work describing the research dedicated to THz sources, detectors, or optics has mentioned remote identification as a possible application for THz technology. The working environment of the existing systems able to perform measurements necessary for the identification of the aforementioned substances has been limited to laboratory conditions. In this paper, we present an experiment for which remote identification is the main purpose; hence, the experiment design and equipment selection were tailored to the subject and not the other way around. The first step was to confirm the correct operation of the setup and to acquire knowledge of the measurements in high relative humidity conditions. For this, a transmission setup was built. Transmission spectra are much easier to interpret than those measured in reflection mode. Thus, the water vapor impact or other experiment-related issues were easier to uncover.  
We obtained the identification results of six selected materials, three of them not previously measured, in the experimental setup for remote reflection THz spectroscopy with samples distanced at a 1, 3, and 5 m range. A key issue in the experiment was basing the identification on measurements made in a normal atmosphere with the presence of water vapor. Due to the many challenges water vapor creates for handling THz measurements, this is a significant step towards real-life applications. This approach was carefully planned to test the hypothesis of the practical remote identification of concealed material by use of THz reflective spectroscopy. This paper provides proof of concept that THz reflection spectroscopy can be used in the process of remote identification of some materials with a general focus on the setup configuration.