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Crop Agriculture

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):

The advantages of using NIR reflectance to assess vegetation canopies are shown in the graphic below from Campbell (2007).

All visible band radiation is predominantly absorbed by leaves, slightly more green is reflected which is why vegetation appears green; much more NIR (listed as IR in the graphic) radiation is reflected to the sensor, in large part, because it can transmit though the canopy. NIR radiation can reveal vegetation density differences because (unlike visible radiation) it can transmit through the top leaf layer and reflect off lower areas (sensing variability below the top of the canopy) and transmit back through the canopy to the sensor. NIR radiation will more likely reflect off soil (which has a lower NIR reflectance than vegetation) in less dense, lower crop condition areas, which will lower reflectance. "The near-IR channel can see through roughly eight leaf layers, while the red channel sees only one leaf layer or less" (Lillesaeter, 1982). Download the free Tutorial imagery and open the data in GIS to view visible and NIR surface reflectance values.

 

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).