Why Apply Near Infrared (NIR) Reflectance to Crops and Other Vegetation
Related page: NIR and Index Correlation to Soybean Yield
The advantages of using NIR reflectance to assess vegetation canopies are shown in the graphic below from Campbell (2007). Visible radiation (light) is predominantly absorbed by leaves, as is shown by the graphic below. For many plant-types, a healthy green vegetation canopy reflects about 2% percent blue light, 5% percent green light, and 3% red light (vegetation appears green because slightly more green light is reflected). Much more NIR light (listed as IR in the above graphic) is reflected for the same vegetation canopy (about 50% to 60% for many plant-types).
NIR light can reveal vegetation density differences because (unlike visible radiation) it can transmit through the top leaf layer and reflect off lower layers, then transmit back through the canopy to the sensor (revealing variability below the top of the canopy). NIR radiation can also reflect off soil (which has a lower NIR reflectance than vegetation) in less dense, but closed, canopy areas (worse crop condition areas), which will lower reflectance and reveal thinner areas of the canopy. "The near-IR channel can see through roughly eight leaf layers, while the red channel sees only one leaf layer or less" (Lillesaeter, 1982).
References
Campbell, J.B. 2007. Introduction to Remote Sensing: Fourth Edition. The Guilford Press: New York, NY.
Gao, B.C. 1996. NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space. Remote Sensing of Environment 58: pp. 257-266.
Lillesaeter, O. 1982. Spectral reflectance of partly transmitting leaves: laboratory measurements and mathematical modeling. Remote Sensing of Environment 12: pp. 247-254. Cited in Gao (1996).