Thermal Infrared Remote Sensing

1. Principles

Thermal Infrared Remote Sensing is based on the detection of naturally emitted electromagnetic radiation from objects, rather than reflected solar energy.
According to Planck's Radiation Law, all objects with a temperature above absolute zero (0 K) emit electromagnetic radiation.
For Earth surface features, the peak emission lies in the Thermal Infrared (TIR) region of 3–14 μm of the electromagnetic spectrum.
The amount of radiation emitted is primarily a function of surface temperature and emissivity.
Sensors measure the radiant energy flux density (W/m²), which is later converted to surface temperature using Stefan-Boltzmann's Law.

2. Radiation Properties in TIR

Emissivity (ε): Ratio of radiation emitted by a surface to that emitted by a perfect blackbody at the same temperature. Natural surfaces like water (ε ≈ 0.98) have high emissivity, while bare soils and metals have lower values.
Blackbody: An idealized object that absorbs and emits all incident radiation perfectly.
Graybody: Real-world objects that emit less than a blackbody but with emissivity less than 1.
Kirchhoff's Law: At thermal equilibrium, absorptivity = emissivity.
Stefan-Boltzmann Law: Total energy emitted (E) = σT⁴ (σ = Stefan-Boltzmann constant, T = temperature in Kelvin).
Wien's Displacement Law: The wavelength of maximum emission (λmax) shifts inversely with temperature (λmax = 2897/T).

3. Thermal Infrared Atmospheric Windows

The Earth's atmosphere selectively absorbs and transmits thermal radiation.
Absorption bands are caused mainly by water vapor (H₂O), carbon dioxide (CO₂), and ozone (O₃).
The atmospheric windows in the TIR region allow maximum transmission of radiation to space and to sensors.
- 3–5 μm window: Useful for high-temperature targets like volcanoes, forest fires, and engines.
- 8–14 μm window: Used for Earth surface temperature monitoring, land cover studies, and meteorology.
These windows are crucial because radiation outside them is strongly absorbed by the atmosphere and cannot be sensed effectively.

4. Satellites and Sensors for TIR

Landsat series:
- Landsat 5 TM (Band 6: 10.4–12.5 μm, 120 m resolution).
- Landsat 7 ETM+ (Band 6, 60 m resolution).
- Landsat 8 & 9 TIRS (Bands 10: 10.6–11.2 μm, Band 11: 11.5–12.5 μm, 100 m resolution).
ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer): 5 TIR bands (8.125–11.65 μm) with 90 m resolution.
MODIS (Moderate Resolution Imaging Spectroradiometer): Multiple TIR bands with 1 km resolution, useful for global monitoring.
ECOSTRESS (ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station): Measures plant water stress via surface temperature.
NOAA AVHRR (Advanced Very High Resolution Radiometer): Long-term monitoring of sea surface and land surface temperature.

5. Applications

Land Surface Temperature (LST): Monitoring urban heat islands, agricultural drought, and land use/land cover changes.
Geological Studies: Mapping geothermal activity, volcano monitoring, mineral exploration.
Hydrology: Detecting soil moisture, evaporation rates, wetland mapping.
Atmospheric Studies: Estimating cloud top temperature, greenhouse gas distribution.
Oceanography: Measuring Sea Surface Temperature (SST), monitoring El Niño and La Niña events.
Disaster Management: Detecting and monitoring forest fires, volcanic eruptions, thermal pollution.
Military & Surveillance: Night vision imaging, target detection using heat signatures.
Agriculture: Crop stress detection, irrigation management, evapotranspiration estimation.

Thermal Infrared Remote Sensing utilizes the natural emission of radiation in the 3–14 μm range, governed by physical radiation laws (Planck, Stefan-Boltzmann, Wien). With atmospheric windows (3–5 μm and 8–14 μm) providing clear observation, satellites like Landsat TIRS, ASTER, and MODIS make it possible to study environmental processes such as surface temperature, vegetation stress, urban heat islands, volcanic activity, and global climate dynamics.

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