The reflectance spectra of water, vegetation, and typical soil vary significantly across different wavelengths, especially in the visible, near-infrared (NIR), and shortwave infrared (SWIR) regions. Here's a brief comparison of their reflectance properties:
- Water:
- Visible Spectrum: In the visible range (approximately 400 to 700 nanometers), water has relatively low reflectance, typically below 10%. It absorbs most of the visible light, appearing dark in color.
- Near-Infrared: In the NIR region (approximately 700 to 1400 nanometers), water's reflectance increases gradually, reaching about 50% or more. This rise in reflectance is primarily due to the scattering of light by water molecules.
- Shortwave Infrared: In the SWIR region (approximately 1400 to 3000 nanometers), water shows a sharp increase in reflectance, reaching nearly 100%. This high reflectance is caused by strong absorption bands related to water molecular vibrations.
- Vegetation:
- Visible Spectrum: In the visible range, vegetation exhibits relatively high reflectance, typically above 30% to 50%. The green color of plants is a result of strong reflection in the green part of the spectrum (around 550 nanometers).
- Near-Infrared: Vegetation shows even higher reflectance in the NIR region, often exceeding 50% to 70%. This high reflectance is primarily due to the presence of chlorophyll, which absorbs light in the visible spectrum and reflects strongly in the NIR.
- Shortwave Infrared: In the SWIR region, vegetation's reflectance starts to decline significantly, with values dropping below 10% as wavelengths increase beyond 2000 nanometers. This decline is due to strong water absorption features and other cellular components' absorption.
- Typical Soil:
- Visible Spectrum: Soil reflectance in the visible range varies widely depending on factors like moisture content, organic matter, and mineral composition. Generally, soil has moderate reflectance in the visible spectrum, around 10% to 40%.
- Near-Infrared: In the NIR region, soil reflectance typically drops to around 5% to 20% due to the presence of iron oxide and other mineral absorptions.
- Shortwave Infrared: In the SWIR region, soil reflects relatively weakly, often below 5%. This low reflectance is attributed to several factors, including absorption by various minerals and water.
In summary, water has low reflectance in the visible spectrum but becomes highly reflective in the NIR and SWIR regions due to water's molecular vibrations. Vegetation has high reflectance in both the visible and NIR regions due to chlorophyll content, but its reflectance drops significantly in the SWIR region. Soil shows moderate reflectance in the visible spectrum, lower reflectance in the NIR region, and even lower reflectance in the SWIR region, mostly due to mineral and water absorption features. These distinct reflectance characteristics make remote sensing techniques based on these wavelengths invaluable for environmental monitoring, agriculture, and geological applications.