Thermal Sensors in Remote Sensing

Thermal sensors are remote sensing instruments that detect naturally emitted thermal infrared (TIR) radiation from the Earth's surface.
Unlike optical sensors (which detect reflected sunlight), thermal sensors measure heat energy emitted by objects because of their temperature.

They work mainly in the Thermal Infrared region (8–14 µm) of the electromagnetic spectrum.

1. Thermal Infrared Radiation

All objects above 0 Kelvin (absolute zero) emit electromagnetic radiation.
This is explained by Planck's Radiation Law.
For Earth's surface temperature range (about 250–330 K), the peak emitted radiation occurs in the 8–14 µm thermal window.

Thus, thermal sensors detect emitted energy, not reflected sunlight.

2. Emissivity

Emissivity is the efficiency with which a material emits thermal radiation.

Values range from 0 to 1:

Water, vegetation → high emissivity (0.95–0.99)
Bare soil → medium (0.85–0.95)
Metals → low (0.1–0.3)

Emissivity affects brightness temperature recorded by sensors.

3. Brightness Temperature

Thermal sensors measure radiance and convert it to brightness temperature, which is the "apparent temperature" of a surface.

Brightness temperature ≠ actual temperature
(because emissivity and atmosphere affect the measurement)

4. Thermal Window (Atmospheric Window)

Thermal remote sensing works best in the 8–14 µm wavelength range because:

the atmosphere absorbs less radiation
energy can travel from the surface to the sensor

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

Types

1. Passive Thermal Sensors

Use naturally emitted heat.

Landsat TIRS
Landsat ETM+ (Band 6)
ASTER TIR
MODIS thermal bands
VIIRS TIR channels

These are the most common.

2. Active Thermal Sensors

Rare.
Active sensors supply their own heat source (e.g., thermal LiDAR), but these are not widely used in Earth observation.

How Thermal Sensors Work

1. Objects emit thermal radiation based on temperature and emissivity.

Hotter objects → emit more radiation.

2. Thermal sensor detects this emitted energy.

The detector measures radiance in the thermal infrared bands.

3. Radiance is converted to brightness temperature.

Using physical laws (Planck's Law, Stefan–Boltzmann Law).

4. Atmospheric correction is applied.

Because the atmosphere absorbs some thermal radiation (mostly by water vapor and CO₂).

5. Surface temperature maps are generated.

These maps show spatial variation in land surface temperature.

Applications

🌡 1. Land Surface Temperature (LST) Mapping

Used in:

heatwave monitoring
microclimate studies
urban heat island analysis

🔥 2. Forest Fire Detection and Monitoring

Hot objects emit strong thermal radiation
Thermal sensors detect fire hotspots even through smoke

🌋 3. Volcano Monitoring

Detection of lava flows and thermal anomalies.

💧 4. Soil Moisture and Evapotranspiration

Cooler surfaces often indicate high moisture levels.

🏙 5. Urban Studies

Mapping heat islands
Thermal comfort analysis

🌾 6. Agriculture

Plant water stress detection
Crop health monitoring

🛰 7. Oceanography

Sea Surface Temperature (SST) mapping

Satellite	Sensor	Thermal Bands
Landsat-8/9	TIRS	10 & 11 (TIR)
Landsat-7	ETM+	Band 6
ASTER	TIR	Bands 10–14
MODIS (Terra & Aqua)	MODIS	Multiple TIR bands
VIIRS	VIIRS	Longwave IR
NOAA AVHRR	AVHRR	Thermal channels

Thermal sensors are remote sensing instruments that detect naturally emitted thermal infrared radiation (8–14 µm) from the Earth's surface. They measure radiance and convert it into brightness temperature, which is used to map land surface temperature, fire hotspots, soil moisture, volcano activity, and thermal characteristics of objects.

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