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Optical Sensors in Remote Sensing


1. What Are Optical Sensors?

Optical sensors are remote sensing instruments that detect solar radiation reflected or emitted from the Earth's surface in specific portions of the electromagnetic spectrum (EMS).

They mainly work in:

  • Visible region (0.4–0.7 µm)

  • Near-Infrared – NIR (0.7–1.3 µm)

  • Shortwave Infrared – SWIR (1.3–3.0 µm)

  • Thermal Infrared – TIR (8–14 µm) — emitted energy, not reflected

Optical sensors capture spectral signatures of surface features. Each object reflects/absorbs energy differently, creating a unique spectral response pattern.


a) Electromagnetic Spectrum (EMS)

The continuous range of wavelengths. Optical sensing uses solar reflective bands and sometimes thermal bands.

b) Spectral Signature

The unique pattern of reflectance or absorbance of an object across wavelengths.
Example:

  • Vegetation reflects strongly in NIR

  • Water absorbs strongly in NIR and SWIR (appears dark)

c) Radiance and Reflectance

  • Radiance: Energy measured by the sensor

  • Reflectance: The ratio of reflected energy to incoming solar energy
    Reflectance is used for analysis because it removes the effect of sunlight intensity.

 Types 

A. Passive Optical Sensors

These depend on sunlight as the energy source.

Examples:

  • Landsat OLI

  • Sentinel-2 MSI

  • MODIS

  • ASTER

  • Cartosat

Advantages:

  • Rich spectral information

  • Good for land cover, vegetation, water, soil studies

Limitations:

  • Cannot see through clouds

  • No data at night

B. Active Optical Sensors (less common)

They supply their own energy source in the optical range.

Examples:

  • LiDAR (Light Detection and Ranging) — uses laser pulses

  • Laser altimeters

Uses:

  • 3D mapping

  • Forest canopy height

  • DEM generation

Optical Sensor Technologies

a) Multispectral Sensors

Capture data in few broad bands.
Examples:

  • Sentinel-2 (13 bands)

  • Landsat-8/9 (11 bands)

Applications:
LULC, NDVI, water quality, soil mapping

b) Hyperspectral Sensors

Capture hundreds of very narrow bands (5–10 nm).
Examples:

  • Hyperion

  • PRISMA

Advantages:
Detect subtle material differences (minerals, crop stress)

c) Panchromatic Sensors

Capture a single broad band with high spatial resolution.
Example:

  • PAN band in WorldView, Cartosat-1
    Used for pansharpening.

How Optical Sensors Work

Step 1: Sunlight hits the Earth

Incoming solar radiation (insolation) interacts with objects.

Step 2: Interaction with surface

Radiation is reflected, absorbed, or transmitted.

Step 3: Sensor records reflected/emitted energy

The detector converts energy into digital numbers (DN).

Step 4: Processing

DN → Radiance → Reflectance → Thematic maps

Important Physical Principles

a) Reflectance Properties

Different materials have different albedo (reflectivity).

Example:

  • Snow: high reflectance

  • Water: low reflectance

  • Vegetation: high NIR reflectance (due to cell structure)

b) Atmospheric Effects

Scattering and absorption modify the signal.
Therefore, we apply:

  • Atmospheric correction

  • Haze removal

  • Cloud masking

Advantages of Optical Sensors

  • High-quality images

  • Rich spectral information

  • Suitable for vegetation and water studies

  • Many freely available missions (Landsat, Sentinel-2)

Limitations

  • Cannot penetrate clouds

  • Daytime-only (for reflective bands)

  • Affected by atmosphere

  • Not suitable for mapping beneath canopy or soil moisture (SAR needed)

Examples of Optical Sensor Applications

ApplicationSensor ExampleBands Used
Vegetation healthSentinel-2, LandsatRed, NIR → NDVI
Water bodiesMODIS, Sentinel-2Green, NIR, SWIR
Urban mappingLandsat, WorldViewSWIR, NIR
Mineral mappingASTER, HyperionSWIR, TIR
AgricultureSentinel-2Red-edge bands
....

 Important Optical Sensors in Remote Sensing

1. NASA / USGS Optical Sensors

Landsat Series

  • MSS – Multispectral Scanner

  • TM – Thematic Mapper

  • ETM+ – Enhanced Thematic Mapper Plus

  • OLI – Operational Land Imager

  • TIRS – Thermal Infrared Sensor

MODIS (Terra & Aqua)

  • Moderate Resolution Imaging Spectroradiometer

VIIRS

  • Visible Infrared Imaging Radiometer Suite

ASTER

  • Advanced Spaceborne Thermal Emission and Reflection Radiometer

AVHRR (NOAA)

  • Advanced Very High Resolution Radiometer

2. ESA (European Space Agency) Sensors

Sentinel Series

  • Sentinel-2 MSI – Multispectral Instrument (13 bands)

  • Sentinel-3 OLCI – Ocean and Land Colour Instrument

  • Sentinel-3 SLSTR – Sea and Land Surface Temperature Radiometer

3. ISRO (India) Optical Sensors

Indian Remote Sensing (IRS) Series

  • LISS-I, II, III, IV – Linear Imaging Self-Scanner

  • PAN & Super-PAN sensors

  • AWiFS – Advanced Wide Field Sensor

  • Cartosat-1 PAN

  • Resourcesat-2 LISS-III, AWiFS

Ocean Colour Sensors

  • OCM / OCM-2 – Ocean Colour Monitor (Oceansat missions)

4. Commercial High-Resolution Optical Sensors

DigitalGlobe / Maxar

  • WorldView-1, 2, 3, 4

  • GeoEye-1

  • QuickBird

Planet Labs

  • PlanetScope

  • RapidEye (5-band multispectral)

SPOT Series (France)

  • SPOT-1 to SPOT-7 HRV / HRG Sensors

5. Hyperspectral Sensors

Spaceborne Hyperspectral Sensors

  • EO-1 Hyperion

  • PRISMA (Italy)

  • EnMAP (Germany)

  • HySIS (India – Hyperspectral Imaging Satellite)

Airborne Hyperspectral Sensors

  • AVIRIS – Airborne Visible/Infrared Imaging Spectrometer

  • HyMap

6. Oceanographic Optical Sensors

  • SeaWiFS – Sea-viewing Wide Field-of-view Sensor

  • MODIS Ocean Bands

  • OLCI (Sentinel-3)

  • OCM (Oceansat)


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