**3.2. Objects and spectrum**

When light encounters an object, they can interact in several different ways: transmission, reflection, and absorption. The interaction depends on the wavelength of the light and the nature of the material of the object.

Most materials exhibit all three properties when interacting with light: partly transmission, partly reflection, and partly absorption. According to the dominant optical property, we categorize objects into two typical types: transparent materials and opaque materials.

Transparent material allows light to pass through the material without being scattered or absorbed. Typical transparent objects include plate glass and clean water. **Figure 2** shows the transmission spectrum of soda-lime glass with a 2-mm thickness. Soda-lime glass is typically used in windows (also called flat glass) and glass containers. From **Figure 2**, it can be seen that soda-lime glass nearly blocks UV radiation. Nevertheless, it has high transmittance in the

**Figure 2.** Transmission spectrum of soda-lime glass with a 2-mm thickness. Obtained from Wikipedia [22].

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).

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

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

When light encounters an object, they can interact in several different ways: transmission, reflection, and absorption. The interaction depends on the wavelength of the light and the

Most materials exhibit all three properties when interacting with light: partly transmission, partly reflection, and partly absorption. According to the dominant optical property, we cat-

Transparent material allows light to pass through the material without being scattered or absorbed. Typical transparent objects include plate glass and clean water. **Figure 2** shows the transmission spectrum of soda-lime glass with a 2-mm thickness. Soda-lime glass is typically used in windows (also called flat glass) and glass containers. From **Figure 2**, it can be seen that soda-lime glass nearly blocks UV radiation. Nevertheless, it has high transmittance in the

egorize objects into two typical types: transparent materials and opaque materials.

**Figure 2.** Transmission spectrum of soda-lime glass with a 2-mm thickness. Obtained from Wikipedia [22].

historical documents without actually having to touch or open them and risk damage.

smoke [21]. Microwave sensors work in any weather condition and at any time.

**3.2. Objects and spectrum**

nature of the material of the object.

26 Multi-purposeful Application of Geospatial Data

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].

**4. Spaceborne sensors**

**4.1. Optical imaging sensors**

**Figure 6.** Spaceborne remote sensing sensors.

Spaceborne sensors have been developed for over 40 years. Currently, approximately 50 countries are operating remote sensing satellites [9]. There are more than 1000 remote sensing satellites available in space, and among these, approximately 593 are from the USA, over 135

A Review: Remote Sensing Sensors

29

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

Conventionally, remote sensors are divided into two groups: passive sensors and active sensors, as we described in the first section. However, as sensor technology has advanced, nothing has been absolute. For example, an imaging camera is usually regarded as a passive sensor. However, in 2013, a new approach that integrates active and passive infrared imaging capability into a single chip was developed. This sensor enables lighter, simpler dual-mode active/passive cameras with lower power dissipation [28]. Alternatively, remote sensing sensors can be classified into imaging sensors and non-imaging sensors. In terms of their spectral characteristics, the imaging sensors include optical imaging sensors, thermal imaging sensors, and radar imaging sensors. **Figure 6** illustrates the category in terms of imaging sensors and non-imaging sensors.

Optical imaging sensors operate in the visible and reflective IR ranges. Typical optical imaging systems on space platform include panchromatic systems, multispectral systems, and hyperspectral systems. In a panchromatic system, the sensor is a monospectral channel detector that is sensitive to radiation within a broad wavelength range. The image is black and white or gray scale. A multispectral sensor is a multichannel detector with a few spectral bands. Each channel is sensitive to radiation within a narrow wavelength band. The resulting image is a multilayer image that contains both the brightness and spectral (color) information of the targets being observed. A hyperspectral sensor collects and processes information from 10 to 100 of spectral bands. A hyperspectral image consists of a set of images. Each narrow spectral band forms an image. The resulting images can be utilized to recognize objects, identify materials, and detect elemental components. **Table 2** gives a more detailed description of

are from Russia, and approximately 192 are from China [27].

**Figure 4.** Reflective spectrum of metals: aluminum, gold, and silver.

wavelength of 310 nm, the reflectance of aluminum is zero [25]. The reflectance of gold significantly increases at a wavelength of approximately 500 nm, reaching a very high reflectance starting in the infrared. This figure indicates that regardless of the wavelength at which the sensor operates, it is inevitable to encounter high reflection from aluminum surfaces.

The physical characteristics of the material determine what type of electromagnetic waves will and will not pass through it. **Figure 5** shows examples of the reflection spectrums of dry bare soil, green vegetation, and clear water. The reflection of dry bare soil increase as the wavelength increases from 400 to 1800 nm. Green vegetation has a high reflectance in the red light and near-infrared regions. These characteristics have been applied for distinguishing green vegetation from other objects. In addition, the previous figure shows that water has a low absorbance in the visible light region. **Figure 5** shows that water reflects visible light at a low rate (<5%). Indirectly, the figure indicates that water has a high transmittance in the visible light range.

**Figure 5.** Examples of reflective materials. Image referenced from Wikimedia [26].
