This blog post delves into how spectral reflectance, which varies by wavelength, serves as a key clue for identifying the composition and state of the Earth’s surface. It focuses on the principles and applications of remote sensing.
Numerous satellites orbiting Earth carry various imaging sensors capable of detailed observation of the surface. Developed primarily for military purposes since the early 1960s, satellite imaging sensors are now widely used for scientific purposes to understand the Earth’s environment. Remote sensing is the science of acquiring and analyzing information about objects non-contact via these sensor systems. To properly understand it, we must closely examine the complex interactions between the energy used in remote sensing and the objects being studied.
Radiant energy emitted from the Sun travels through space at the speed of light as electromagnetic waves. It passes through Earth’s atmosphere, is reflected at the surface, and then travels back through the atmosphere to reach satellite sensors, where it is measured. The ratio of incident energy to reflected energy is called reflectance. Remote sensing utilizes spectral reflectance—reflectance at different wavelengths—to determine an object’s properties.
Objects emit radiant energy across various wavelengths, with the wavelength at which the energy is maximized termed the ‘maximum energy wavelength’. The Sun, with a surface temperature of approximately 6,000K, has a maximum energy wavelength of 0.48μm. Initial satellite imagery utilized only visible light (0.4–0.7μm) to match this peak. However, with recent technological advancements, it has become possible to utilize various wavelength bands invisible to the human eye, such as near-infrared, mid-infrared, and thermal infrared. Consequently, the usefulness of remote sensing has greatly expanded.
For example, while natural grass and artificial turf both appear green to the human eye, they are clearly distinguishable using near-infrared (0.7–1.2μm). This is because green leaves strongly reflect about 50% of light in this band, appearing bright in satellite imagery, whereas artificial turf reflects only about 5%, appearing dark.
Mid-infrared (1.2–3.0 μm) offers far greater sensitivity to leaf moisture content than visible light, making it valuable for obtaining critical information related to crop growth status. Mid-infrared is also effective for resource exploration, utilizing the unique spectral reflectance characteristics of water or rock. For example, kaolinite, the raw material for ceramics, absorbs mid-infrared radiation at 2.17, 2.21, 2.32, and 2.58μm. If an object’s spectral reflectance exhibits these characteristics, it can be identified as kaolinite.
Thermal infrared radiation (3–14μm), which concentrates the Earth’s emitted thermal radiation energy, provides information about surface temperature distribution. Since the maximum wavelength of radiant energy emitted by an object is inversely proportional to its temperature, thermal infrared sensors are particularly useful for monitoring wildfires (temperature ~800K, maximum wavelength 3.62μm) or detecting temperatures of surface features like soil, water, and rock (temperature ~300K, maximum wavelength 9.67μm).
A key point to note here is that electromagnetic waves undergo scattering and absorption by atmospheric particles both before reaching the surface and after being reflected. Even on clear days with no dust, fog, or clouds in the atmosphere, scattering occurs due to atmospheric particles like oxygen or nitrogen molecules, which have effective diameters much smaller than the wavelength of the incident wave. This is called Rayleigh scattering, and its intensity is inversely proportional to the fourth power of the wavelength. For example, ultraviolet light with a wavelength of 0.32μm exhibits approximately 16 times stronger scattering than red light with a wavelength of 0.64μm. Rayleigh scattering serves as an important indicator of atmospheric composition and density, but it must be accounted for as it attenuates the brightness and contrast of satellite images capturing the Earth’s surface. Some remote sensing systems boldly abandon acquiring natural-color imagery by excluding the blue wavelength band, where Rayleigh scattering effects are significant, and instead use only green, red, and near-infrared sensors.
Absorption of electromagnetic waves in the atmosphere occurs at specific wavelength bands corresponding to the intrinsic resonance frequencies of the constituent materials. The overlapping absorption effects of various atmospheric substances—water vapor, carbon, oxygen, ozone, nitrogen oxides, etc.—mean that electromagnetic waves in certain wavelength bands barely penetrate the Earth’s atmosphere, even on clear days. Fortunately, several electromagnetic wave bands, including visible light, belong to the ‘atmospheric window’ where energy passes through very efficiently. Therefore, satellite sensors must be designed to operate within these atmospheric window wavelength bands. Consequently, mid-infrared sensors are designed to exclude the strong absorption wavelengths of 1.4, 1.9, and 2.7μm caused by atmospheric water vapor, while thermal infrared sensors primarily utilize only the 3–5μm and 8–14μm bands.
Understanding these characteristics of electromagnetic waves, their interaction with the atmosphere, and the utility of various wavelength bands is essential for accurately interpreting and applying remote sensing systems. Based on this understanding, satellite imagery serves as a crucial tool for more sophisticated analysis of the Earth’s surface and environment.
