Measuring Vegetation  Normalized Difference Vegetation Index
(NDVI)  To determine the density of green on a patch of land,
researchers must observe the distinct colors (wavelengths) of
visible and near-infrared sunlight reflected by the plants. As
can be seen through a prism, many different wavelengths make up
the spectrum of sunlight. When sunlight strikes objects, certain
wavelengths of this spectrum are absorbed and other wavelengths
are reflected. The pigment in plant leaves, chlorophyll,
strongly absorbs visible light (from 0.4 to 0.7 µm) for use in
photosynthesis. The cell structure of the leaves, on the other
hand, strongly reflects near-infrared light (from 0.7 to 1.1
µm). The more leaves a plant has, the more these wavelengths of
light are affected, respectively.  Visible and Near-Infrared
Vegetation appears very different at visible and near-infrared
wavelengths. In visible light (top), vegetated areas are very
dark, almost black, while desert regions (like the Sahara) are
light. At near-infrared wa