Introduction
The Earth’s atmosphere plays a crucial role in remote sensing applications, affecting the radiation that is reflected or emitted by the Earth’s surface and atmosphere. Atmospheric gases, such as water vapor, carbon dioxide, and ozone, absorb and scatter radiation within specific ranges of wavelengths, which can make it difficult to measure radiation accurately for remote sensing applications. Atmospheric window and spectral reflectance curve are related because the atmospheric window determines the wavelengths of radiation.
To overcome this challenge, scientists have identified a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without being absorbed or scattered by atmospheric gases. This range of wavelengths is known as the atmospheric window.
Remote sensing applications use the atmospheric window to measure the radiation reflected or emitted by the Earth’s surface or atmosphere in specific wavelengths that are not affected by atmospheric absorption. By measuring radiation in these specific wavelengths, scientists can create spectral reflectance curves, which show how much radiation is reflected by different surfaces at different wavelengths.
what is atmospheric window in remote sensing?
In remote sensing, the atmospheric window refers to a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without being absorbed or scattered by atmospheric gases, such as water vapor, carbon dioxide, and ozone. The atmospheric window is important in remote sensing because it allows scientists to measure the reflected or emitted radiation from the Earth’s surface or atmosphere in specific wavelengths that are not affected by atmospheric absorption.
The atmospheric window is typically divided into several sub-windows, including the visible window (0.4 to 0.7 micrometers), the near-infrared window (0.7 to 1.3 micrometers), and the thermal infrared window (8 to 14 micrometers). These sub-windows are important for different types of remote sensing applications.
For example, the visible window is used for measuring radiation reflected by the Earth’s surface in visible wavelengths, which can provide information about surface features, such as vegetation, water bodies, and urban areas. The near-infrared window is used for measuring radiation reflected by vegetation and other types of vegetation cover, which can provide information about vegetation health and biomass. The thermal infrared window is used for measuring radiation emitted by the Earth’s surface in the form of heat, which can provide information about surface temperature and thermal properties.
Atmospheric Window and Spectral Reflectance Curve
An atmospheric window is a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without being absorbed or scattered by atmospheric gases, such as water vapor, carbon dioxide, and ozone. The atmospheric window is important for remote sensing applications because it allows scientists to measure the reflected or emitted radiation from the Earth’s surface or atmosphere in specific wavelengths that are not affected by atmospheric absorption.
On the other hand, a spectral reflectance curve is a graph that shows the percentage of radiation that is reflected by an object or surface at different wavelengths within the electromagnetic spectrum. Spectral reflectance curves are unique for different materials and surfaces and are used in remote sensing applications to identify and classify objects and features on the Earth’s surface.
The atmospheric window and spectral reflectance curve are related because the atmospheric window determines the wavelengths of radiation that can be measured without atmospheric interference, while the spectral reflectance curve provides information about how much radiation is reflected by different surfaces at those wavelengths. By using the atmospheric window to measure the radiation reflected by different surfaces in specific wavelengths, scientists can create spectral reflectance curves that help them identify and classify objects and features on the Earth’s surface, such as vegetation, water bodies, and urban areas.
Atmospheric Window
In remote sensing, an atmospheric window refers to a range of wavelengths in the electromagnetic spectrum where the Earth’s atmosphere is relatively transparent to incoming and outgoing radiation. These atmospheric windows allow remote sensing instruments to detect and measure the radiation emitted or reflected by the Earth’s surface, without interference from the atmosphere.
The Earth’s atmosphere contains various gases and particles that can absorb, scatter, and reflect electromagnetic radiation at different wavelengths. This can make it difficult for remote sensing instruments to accurately detect and measure the radiation emitted or reflected by the Earth’s surface, particularly in the presence of clouds or other atmospheric conditions.
Atmospheric windows occur at specific wavelength ranges where the Earth’s atmosphere is relatively transparent to radiation. These windows allow remote sensing instruments to penetrate the atmosphere and detect radiation emitted or reflected by the Earth’s surface. The major atmospheric windows used in remote sensing include the visible, near-infrared, shortwave infrared, mid-infrared, and thermal infrared windows. Each of these windows has a specific wavelength range and is used for different types of remote sensing applications.
Atmospheric windows are an important consideration in remote sensing, as they can affect the accuracy and reliability of remote sensing data. Remote sensing scientists use atmospheric correction algorithms to account for the effects of the atmosphere on remote sensing data and to improve the accuracy and reliability of the data.
Atmospheric Window Definition in Remote Sensing
Here are two definitions of atmospheric window in remote sensing with their respective authors:
“An atmospheric window is a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without being absorbed or scattered by atmospheric gases, such as water vapor, carbon dioxide, and ozone. The atmospheric window is important for remote sensing applications because it allows scientists to measure the reflected or emitted radiation from the Earth’s surface or atmosphere in specific wavelengths that are not affected by atmospheric absorption.”
“The atmospheric window is a spectral range where the atmosphere is essentially transparent to incoming and outgoing radiation. This spectral region is used extensively in remote sensing to study the Earth’s surface and atmosphere.” – Author: Dr. Prasad S. Thenkabail in the book “Remote Sensing of Global Croplands for Food Security” (2019).
Both definitions convey the same idea that the atmospheric window is a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without interference from atmospheric gases, which is important for remote sensing applications. The second definition emphasizes the use of the atmospheric window in remote sensing to study the Earth’s surface and atmosphere.
Atmospheric Window Range
The atmospheric window is a range of wavelengths within the electromagnetic spectrum that can pass through the Earth’s atmosphere without being absorbed or scattered by atmospheric gases, such as water vapor, carbon dioxide, and ozone. The atmospheric window is typically divided into several sub-windows, each with its own range of wavelengths.
Here are the ranges of the main sub-windows of the atmospheric window:
Visible window: 0.4 to 0.7 micrometers. This sub-window is important for measuring radiation reflected by the Earth’s surface in visible wavelengths, which can provide information about surface features, such as vegetation, water bodies, and urban areas.
Near-infrared window: 0.7 to 1.3 micrometers. This sub-window is important for measuring radiation reflected by vegetation and other types of vegetation cover, which can provide information about vegetation health and biomass.
Shortwave infrared window: 1.3 to 3.0 micrometers. This sub-window is important for measuring radiation reflected by the Earth’s surface in the shortwave infrared portion of the spectrum, which can provide information about mineralogy and rock type.
Mid-infrared window: 3.0 to 8.0 micrometers. This sub-window is important for measuring radiation emitted by the Earth’s surface and atmosphere in the mid-infrared portion of the spectrum, which can provide information about surface temperature and thermal properties.
Thermal infrared window: 8.0 to 14.0 micrometers. This sub-window is important for measuring radiation emitted by the Earth’s surface and atmosphere in the form of heat, which can provide information about surface temperature and thermal properties.
It’s important to note that the exact ranges of the atmospheric window sub-windows can vary slightly depending on the source and context.
| Atmospheric Window | Wavelength Range | Typical Applications in Remote Sensing |
|---|---|---|
| Visible Window | 0.4 – 0.7 μm | Vegetation, urban areas, water bodies, and land cover classification |
| Near-Infrared Window | 0.7 – 1.3 μm | Vegetation health, biomass, and leaf area index |
| Shortwave Infrared Window | 1.3 – 3.0 μm | Rock and mineral identification, soil moisture, and vegetation water content |
| Mid-Infrared Window | 3.0 – 8.0 μm | Surface temperature, thermal properties, and mineral identification |
| Thermal Infrared Window | 8.0 – 14.0 μm | Surface temperature, thermal properties, and cloud properties |
Spectral Reflectance Curve
A spectral reflectance curve, also known as a reflectance spectrum or spectral signature, is a graphical representation of the reflectance of a material or surface at different wavelengths of the electromagnetic spectrum.
The spectral reflectance curve is generated by measuring the amount of electromagnetic radiation reflected by a material or surface at different wavelengths using a spectrometer or other remote sensing instrument. The resulting data is then plotted on a graph, with the wavelengths on the x-axis and the reflectance values on the y-axis.
The shape and features of the spectral reflectance curve are determined by the physical and chemical properties of the material or surface being measured. Different materials and surfaces have unique spectral reflectance curves, which can be used to identify and distinguish them from each other. For example, vegetation typically has a high reflectance in the near-infrared portion of the spectrum, while water has a low reflectance in the visible portion of the spectrum but a high reflectance in the thermal infrared portion of the spectrum.
Spectral reflectance curves are important tools in remote sensing for identifying and classifying objects on the Earth’s surface based on their spectral properties. By comparing the spectral reflectance curve of an unknown object to a library of known spectral curves, remote sensing scientists can determine the type of material or surface that the object is composed of, and can use this information to make inferences about the object’s properties and characteristics.
Spectral Reflectance Curve Definition
According to Jensen (2016), spectral reflectance is defined as “the ratio of the energy reflected from a surface to the energy incident upon it as a function of wavelength or frequency”. The spectral reflectance curve is the graphical representation of this ratio as a function of wavelength or frequency. The curve can show variations in reflectance across the electromagnetic spectrum, ranging from the ultraviolet to the thermal infrared regions.
Spectral reflectance is affected by a variety of factors, including the composition, texture, and geometry of the surface, as well as the angle and polarization of the incident light. Different materials have distinctive spectral reflectance curves, which can be used to identify and distinguish them from one another. For example, vegetation typically has a high reflectance in the near-infrared region of the spectrum and a low reflectance in the visible and shortwave infrared regions, whereas water has a low reflectance in the visible and near-infrared regions and a high reflectance in the thermal infrared region.
Vegetation
Vegetation is a general term used to describe plant life, including trees, shrubs, grasses, and other types of plants. Vegetation plays a critical role in the Earth’s ecosystem, providing habitat and food for many species of animals, regulating the water cycle, and absorbing carbon dioxide from the atmosphere through photosynthesis.
(Image source: https://ltb.itc.utwente.nl/)
In remote sensing, vegetation is a common target for study and analysis. Vegetation has unique spectral properties that can be detected and measured using remote sensing instruments. For example, vegetation tends to have high reflectance values in the near-infrared portion of the spectrum, due to the high reflectance of plant cell walls and chlorophyll, and low reflectance values in the visible portion of the spectrum. This makes it possible to distinguish vegetation from other types of land cover, such as water, urban areas, and bare soil, using spectral analysis.
Remote sensing data can be used to monitor and map vegetation cover, health, and productivity over large areas and over time. For example, vegetation indices such as the Normalized Difference Vegetation Index (NDVI) can be calculated from remote sensing data to provide an estimate of vegetation biomass, productivity, and health. This information can be used for a variety of applications, including crop yield prediction, land use planning, and ecosystem monitoring.
Soil
The spectral reflectance curve of soil is a graphical representation of the reflectance of soil at different wavelengths of the electromagnetic spectrum. Soil reflectance is affected by factors such as soil composition, moisture content, texture, and organic matter content, among others. Here is a general description of the spectral reflectance curve of soil:
In the visible portion of the spectrum (0.4 to 0.7 µm), soil reflectance is relatively low, typically ranging from 5-10%. This is due to the absorption of visible radiation by soil particles and organic matter.
In the near-infrared portion of the spectrum (0.7 to 1.3 µm), soil reflectance increases dramatically to values of 30-50% or higher. This increase in reflectance is due to the strong scattering of near-infrared radiation by soil particles, particularly those that are relatively large and non-spherical, such as sand and clay particles.
In the shortwave infrared portion of the spectrum (1.3 to 3 µm), soil reflectance decreases gradually with increasing wavelength, typically ranging from 20-30%. This decrease in reflectance is due to the increasing absorption of shortwave infrared radiation by soil minerals such as quartz, feldspar, and mica.
In the mid-infrared portion of the spectrum (3 to 5 µm), soil reflectance increases slightly, typically ranging from 25-35%. This increase in reflectance is due to the absorption of mid-infrared radiation by water and organic matter in the soil.
In the thermal infrared portion of the spectrum (8 to 14 µm), soil reflectance is low, typically ranging from 5-15%. This is due to the strong absorption of thermal infrared radiation by soil particles and water.
Soil can be used to identify and distinguish different types of soil based on their spectral properties. For example, soils with high clay content tend to have higher reflectance in the near-infrared portion of the spectrum, while soils with high organic matter content tend to have higher reflectance in the mid-infrared portion of the spectrum. Remote sensing data can be used to map and monitor soil properties over large areas and over time, providing valuable information for agricultural, environmental, and geological applications.
Water
The spectral reflectance curve of water is an important concept in remote sensing, as it is used to distinguish water bodies from other types of land cover and to monitor water quality and quantity. Here is a general description of the spectral reflectance curve of water with references to relevant literature:
Water has a low reflectance in the visible and near-infrared regions of the electromagnetic spectrum, as these wavelengths are strongly absorbed by the water molecules. The spectral reflectance curve of water typically shows a deep minimum in the visible region and a steep increase in the near-infrared region (Jensen, 2016). This spectral signature is different from that of most other land cover types, such as vegetation and soil, which have higher reflectance in the visible and near-infrared regions.
The spectral reflectance properties of water are affected by several factors, including the angle of incidence and the presence of suspended particles, dissolved organic matter, and chlorophyll. These factors can alter the spectral signature of water and make it more difficult to distinguish from other types of land cover. Remote sensing techniques can be used to account for these factors and to retrieve accurate information on water properties, such as water depth, turbidity, and chlorophyll concentration.
The spectral reflectance curve of water has important applications in remote sensing, including the mapping of water bodies, the monitoring of water quality and quantity, and the assessment of hydrological processes. For example, satellite sensors such as Landsat and Sentinel-2 can be used to detect changes in the spectral reflectance of water bodies over time, which can indicate changes in water quality or quantity. Understanding the spectral properties of water is therefore essential for effective water resource management and environmental monitoring.
(Reference: Jensen, J. R. (2016). Remote sensing of the environment: An earth resource perspective. Pearson Education.)
Conclusion
In conclusion, the atmospheric window and spectral reflectance curve are both fundamental concepts in remote sensing. The atmospheric window is a range of wavelengths in the electromagnetic spectrum where atmospheric gases are transparent and allow electromagnetic radiation to pass through with minimal absorption and scattering. The spectral reflectance curve describes the way different materials reflect electromagnetic radiation across the spectrum, and is influenced by factors such as composition, structure, and physical and chemical characteristics.
The atmospheric window is important in remote sensing because it allows sensors to detect and measure radiation emitted or reflected by the Earth’s surface without interference from atmospheric gases. This is essential for accurate mapping of land cover types and monitoring of environmental processes such as vegetation health, water quality and quantity, and snow cover.