Near infrared
Also called: NIR · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"
Meaning
Near infrared (NIR) is the part of the electromagnetic spectrum just beyond visible red, at about 0.7 to 1.1 micrometre. Our eyes cannot see it, but satellite sensors can. Healthy green leaves reflect NIR very strongly because their leaves have many layers. Clear water absorbs it almost completely, so water looks dark in an NIR band. This big contrast makes NIR the key band for mapping vegetation and water. It is also used in NDVI, a vegetation index.
Example
In Resourcesat-1 images of Najafgarh, Delhi (2005), fresh water absorbs energy in band 4, the infrared band, so it appears dark. In standard false colour composites, NIR is shown as red. That is why healthy crops and forests look bright red.
Don't confuse with
- Thermal infrared: this lies much further along the spectrum, at about 8 to 14 micrometres. It records heat given off by surfaces, whereas NIR records reflected sunlight.
Related concepts
- Electromagnetic radiation
- Electromagnetic spectrum
- Spectral band
- Atmospheric absorption and scattering
- Thermal infrared remote sensing
- Reflectance
- Absorptance
- Spectral signature
- NDVI