SWIR
Shortwave Infrared. A spectral region (approximately 1400-3000nm) sensitive to moisture content in vegetation and soil, mineral composition, and snow/ice properties. Used in indices like NDMI and MNDWI, and for geological mapping and burn severity assessment.
Overview
Shortwave Infrared (SWIR) refers to the electromagnetic spectrum region spanning approximately 1.0 to 2.5 μm. SWIR radiation is invisible to the human eye but rich in diagnostic information about surface materials. It is particularly sensitive to moisture content in vegetation and soil, mineral composition of rocks, and the presence of fire. Two atmospheric windows within SWIR — around 1.6 μm (SWIR-1) and 2.2 μm (SWIR-2) — allow satellite sensors to measure surface properties in this region.
How It Works
SWIR sensitivity to moisture arises because liquid water has strong absorption features around 1.4, 1.9, and 2.5 μm. While the 1.4 and 1.9 μm regions are blocked by atmospheric water vapor, the flanking regions (1.55-1.75 μm and 2.08-2.35 μm) capture moisture-related reflectance changes from the surface. Wet vegetation reflects less SWIR than dry vegetation because leaf water absorbs the energy. Minerals have diagnostic absorption features in SWIR: clay minerals absorb near 2.2 μm, carbonates near 2.35 μm, making SWIR essential for geological mapping.
SWIR also penetrates thin smoke, haze, and some cloud types better than visible wavelengths. Active fires emit strongly in SWIR, enabling fire detection even through smoke plumes.
Key Facts
- Two atmospheric windows: SWIR-1 (~1.55-1.75 μm) and SWIR-2 (~2.08-2.35 μm).
- Sentinel-2 Band 11 (1610 nm, 20 m) and Band 12 (2190 nm, 20 m) cover both SWIR windows.
- Landsat 8/9 Band 6 (1.6 μm) and Band 7 (2.2 μm) provide SWIR at 30 m resolution.
- SWIR is used in NDMI, MNDWI, NBR (Normalized Burn Ratio), and BSI (Bare Soil Index).
- Cloud/snow discrimination exploits the fact that snow absorbs SWIR while clouds reflect it.
Applications
Vegetation Moisture and Drought
SWIR bands are used in NDMI and NDWI to detect vegetation water stress and monitor drought conditions.
Geology and Mineral Exploration
Diagnostic mineral absorption features in SWIR enable lithological mapping and identification of clay, carbonate, and sulfate minerals.
Fire Detection and Burn Scar Mapping
Active fires emit strongly in SWIR, and burned areas show elevated SWIR reflectance from charred organic matter and exposed mineral soil.
Snow and Cloud Discrimination
Snow reflects strongly in visible but absorbs in SWIR, while clouds reflect in both — enabling accurate snow/cloud separation.
Limitations & Considerations
SWIR bands on free sensors (Sentinel-2, Landsat) have coarser spatial resolution (20-30 m) than visible/NIR bands (10 m on Sentinel-2). Atmospheric water vapor absorption near 1.4 and 1.9 μm limits usable SWIR regions. SWIR detectors require cooling and have lower signal-to-noise ratios than visible detectors. Solar energy decreases with wavelength, further reducing signal strength in SWIR-2.
History & Background
SWIR measurement from space began with Landsat 4's Thematic Mapper in 1982, which added SWIR bands 5 (1.6 μm) and 7 (2.2 μm). These bands were revolutionary for geological mapping, moisture detection, and land cover discrimination. Sentinel-2's SWIR bands maintain this capability at improved spatial resolution. The upcoming CHIME mission (Copernicus Hyperspectral Imaging Mission for the Environment) will provide continuous spectral coverage through the SWIR region.
Analyze SWIR data with LYRASENSE
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