Absorptance
Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"
Meaning
Absorptance is the share of the energy falling on a surface that the surface takes in (absorbs), instead of reflecting it or letting it pass through. It is written as a fraction or a percentage of the energy received. A surface with high absorptance in one band (a chosen range of wavelengths) sends little energy back to the sensor, so it looks dark in that band. Each object absorbs differently in different wavelengths. This helps build its spectral signature, its unique "fingerprint" of reflected energy, which is how objects are identified in satellite images.
Example
In NCERT's Resourcesat-1 images of Najafgarh, Delhi (2005), fresh water absorbs strongly in band 4, which is infrared. So the water looks dark in that band. Dry surfaces reflect strongly in band 2 (green), so they look light.
Don't confuse with
- Reflectance: the share of the energy received that a surface sends back. Absorptance is the share it keeps. Healthy vegetation absorbs red light but has high reflectance in near infrared (NIR).
- Turbid water vs clear water: both are water, but they absorb differently. Clear fresh water absorbs red and NIR and looks dark. Turbid (muddy) water reflects more blue and green, so it looks lighter.
Related concepts
- Electromagnetic radiation
- Electromagnetic spectrum
- Spectral band
- Near infrared
- Atmospheric absorption and scattering
- Thermal infrared remote sensing
- Reflectance
- Spectral signature
- NDVI