A Normalized Difference Vegetation Index (NDVI) is an equation that takes into account the amount of infrared reflected by plants. Live green plants absorb solar radiation, which they use as a source of energy in the process of photosynthesis. The reason NDVI is related to vegetation is that healthy vegetation reflects very well in the near-infrared part of the electromagnetic spectrum. Green leaves have a reflectance of 20% or less in the 0.5 to 0.7 micron range (green to red) and about 60% in the 0.7 to 1.3 micron range (near-infrared). These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually; hence, they take on values between 0.0 and 1.0. Thus, the NDVI itself varies between -1.0 and +1.0. Negative values of NDVI (values approaching -1) correspond to deep water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while high values indicate temperate and tropical rainforests (values approaching 1). The typical range is between about -0.1 (for a not very green area) to 0.6 (for a very green area). Overall, NDVI provides a crude estimate of vegetation health and a means of monitoring changes in vegetation over time, and it remains the most well-known and used index to detect live green plant canopies in multispectral remote sensing data. The NDVI ratio is calculated by dividing the difference in the near-infrared (NIR) and red color bands by the sum of the NIR and red colors bands for each pixel in the image as follows:
In this particular NDVI image, the shades of blue and faint greens represent manmade surfaces (concrete, macadam, roofs), yellows/faint oranges represent grassy fields of varying health, and bright reds represent healthy, vigorous tree crowns (circular features in the campus fields). The pixel values of this image range from around zero (and even slightly negative) for non-vegetated surfaces to values of (positive) 0.78-0.82 for very bright vegetation.