**3. Characteristics of materials in electromagnetic spectrum (EMS)**

Remote sensors remotely interact with objects on the surface of the Earth. Objects on the surface of the Earth generally include terrain, buildings, road, vegetation, and water. The typical materials of these objects that interact with the EMS are categorized into groups: transparent and opaque (partly or fully absorbed).

magnitude smaller than at 532 nm, and 420–460 nm light can penetrate relatively clear water and ice much deeper, offering substantial improvements in sensing through water for the same optical power output, thus reducing power requirements [11]. The red spectrum together with near-infrared (NIR) is typically used for vegetation applications. For example, the Normalized Difference Vegetation Index (NDVI) is used to evaluate targets that may or may not contain live green vegetation. Infrared is invisible radiant energy. Usually, infrared is divided into different regions: near IR (NIR, 0.75–1.4 μm), shortwave IR (SWIR, 1.4–3 μm), mid-IR (MIR, 3–8 μm), longwave IR (LWIR, 8–15 μm), and far IR (FIR, 15–1000 μm). Alternatively, according to the ISO 20473 scheme, another division is proposed as NIR (0.78–3 μm), MIR (3–50 μm), and FIR (50–1000 μm). Most of the infrared radiation in sunlight is in the NIR range. Most of the thermal radiation emitted by objects near room temperature is infrared [14]. In nature, on the surface of the Earth, almost all thermal radiation consists of infrared in the mid-infrared region, which is a much longer wavelength than that in sunlight. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible light energy. NIR is mainly used in medical imaging and physiological diagnostics. One typical application of MIR and FIR is thermal imaging, for example, night vision devices. In the MIR and FIR spectrum bands, water

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With regard to the terahertz spectrum band, terahertz frequencies are useful for investigating biological molecules. Unlike more commonly used forms of radiated energy, this range has rarely been studied, partly because no one knew how to make these frequencies bright enough [12] and because practical applications have been impeded by the fact that ambient moisture interferes with wave transmission [13]. Nevertheless, terahertz light (also called T-rays) has remarkable properties. T-rays are safe, non-ionizing electromagnetic radiation. This light poses little or no health threat and can pass through clothing, paper, cardboard, wood, masonry, plastic, and ceramics. This light can also penetrate fog and clouds. THz radiation transmits through almost anything except for not metal and liquid (e.g., water). T-rays can be used to reveal explosives or other dangerous substances in packaging, corrugated cardboard, clothing, shoes, backpacks, and book bags. However, the technique cannot detect

The terahertz region is technically the boundary between electronics and opt-photonics [15]. The wavelengths of T-rays—shorter than microwaves, longer than infrared—correspond with biomolecular vibrations. This light can provide imaging and sensing technologies not available through conventional technologies, such as microwaves [16]. For example, T-rays can penetrate fabrics. Many common materials and living tissues are semi-transparent and have 'terahertz fingerprints', permitting them to be imaged, identified, and analyzed [17]. In addition, terahertz radiation has the unique ability to non-destructively image physical structures and perform spectroscopic analysis without any contact with valuable and delicate paintings, manuscripts, and artifacts. In addition, terahertz radiation can be utilized to measure objects that are opaque in the visible and near-infrared regions. Terahertz pulsed imaging techniques operate in much the same way as ultrasound and radar to accurately locate embedded or distant objects [18]. Current commercial terahertz instruments include Terahertz 3D medical imaging, security scanning systems, and terahertz spectroscopy. The latest breakthrough research (9.2016) on

shows high absorption, and biological systems are highly transmissive.

materials that might be concealed in body cavities [14].
