**3.1. Electromagnetic spectrum**

**Figure 1** contains the EMS range from gamma rays to radio waves. In remote sensing, typical applications include the visible light (380–780 nm), infrared (780 nm–0.1 mm), and microwave (0.1 mm–1 m) ranges. This paper treats the terahertz (0.1–1 mm) range as an independent spectral band separate from microwaves. Remote sensing sensors interact with objects remotely. Between sensors and the earth surface, there is atmosphere. It is estimated that only 67% of sunlight directly heats the Earth [11]. The remainder of the light is absorbed and reflected by the atmosphere. The Earth's atmosphere strongly absorbs infrared and UV radiation. In visible light, typical remote sensing applications include the blue (450–495 nm), green (495–570 nm), and red (620–750 nm) spectral bands for panchromatic or multispectral or hyperspectral imaging. Current bathymetric and ice LIDAR generally uses green light (e.g., NASA's HSRL-1 LIDAR, with a spectrum of 532 nm). However, new experiments have shown that in the blue spectrum, such as at 440 nm, the absorption coefficient for water is approximately an order of

**Figure 1.** The electromagnetic spectrum. Image from UC Davis ChemWiki, CC-BY-NC-SA 3.0.

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 shows high absorption, and biological systems are highly transmissive.

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

and opaque (partly or fully absorbed).

24 Multi-purposeful Application of Geospatial Data

**3.1. Electromagnetic spectrum**

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

**Figure 1** contains the EMS range from gamma rays to radio waves. In remote sensing, typical applications include the visible light (380–780 nm), infrared (780 nm–0.1 mm), and microwave (0.1 mm–1 m) ranges. This paper treats the terahertz (0.1–1 mm) range as an independent spectral band separate from microwaves. Remote sensing sensors interact with objects remotely. Between sensors and the earth surface, there is atmosphere. It is estimated that only 67% of sunlight directly heats the Earth [11]. The remainder of the light is absorbed and reflected by the atmosphere. The Earth's atmosphere strongly absorbs infrared and UV radiation. In visible light, typical remote sensing applications include the blue (450–495 nm), green (495–570 nm), and red (620–750 nm) spectral bands for panchromatic or multispectral or hyperspectral imaging. Current bathymetric and ice LIDAR generally uses green light (e.g., NASA's HSRL-1 LIDAR, with a spectrum of 532 nm). However, new experiments have shown that in the blue spectrum, such as at 440 nm, the absorption coefficient for water is approximately an order of

**Figure 1.** The electromagnetic spectrum. Image from UC Davis ChemWiki, CC-BY-NC-SA 3.0.

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

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 terahertz applications was that MIT invented a terahertz camera that can read a closed book. This camera can distinguish ink from a blank region on paper. The article indicates that 'In its current form the terahertz camera can accurately calculate distance to a depth of about 20 pages' [19]. It is expected that in the future, this technology can be used to explore and catalog historical documents without actually having to touch or open them and risk damage.

visible light and NIR wavelengths. It is easy to understand that when a laser scanner with a wavelength of 905, 1064, or 1550 nm hits a flat glass window or a glassy balcony, over 80% of the laser energy passes through the glass and hits the objects behind the window. Another typical example of transmissive material is clear water. Water transmittance is very high in the blue-green part of the spectrum but diminishes rapidly in the near-infrared wavelengths (see **Figure 3**). Absorption, on the other hand, is notably low in the shorter visible wavelengths (less than 418 nm) but increases abruptly in the range of 418–742 nm. A laser beam with a wavelength of 532 nm (green laser) is typically applied in bathymetric measurements as this wavelength has a high water transmittance. According to the Beer-Lambert law, the relation between absorbance and transmittance is as follows: Absorbance = −log (Transmittance).

A Review: Remote Sensing Sensors

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http://dx.doi.org/10.5772/intechopen.71049

Opacity occurs because of the reflection and absorption of light waves off the surface of an object. The reflectance of light depends on the material of the surface that the light encounters. There are two types of reflection: one is specular reflection and another is diffuse reflection. Specular reflection is when light from a single incoming direction is reflected in a single outgoing direction. Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle, as in the case of specular reflection. Most objects have mixed reflective properties [24]. Representative reflective materials include metals, such as aluminum, gold, and silver. From **Figure 4**, it can be seen that aluminum has a high reflectivity over various wavelengths. In the visible light and NIR wavelengths, the reflectance of aluminum reaches up to 92%, while this value increases to 98% in MIR and FIR. Silver has a higher reflectance than aluminum when the wavelength is longer than 450 nm. At a

**Figure 3.** Liquid water absorption spectrum. Obtained from Wikipedia [23].

Regarding microwaves, shorter microwaves are typically used in remote sensing. For example, this region is used for radar, and the wavelength is just a few inches long. Microwaves are typically used for obtaining information on the atmosphere, land, and ocean, such as Doppler radar, which is used in weather forecasts, and for gathering unique information on sea wind and wave direction, which are derived from frequency characteristics, including the Doppler effect, polarization, back scattering, that cannot be observed by visible and infrared sensors [20]. In addition, microwave energy can penetrate haze, light rain and snow, clouds, and smoke [21]. Microwave sensors work in any weather condition and at any time.
