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.Vegetation appears very different at visible and near-infrared wavelengths. In visible light , vegetated areas are very dark, almost black, while desert regions (like the Sahara) are light. At near-infrared wavelengths, the vegetation is brighter and deserts are about the same. By comparing visible and infrared light, scientists measure the relative amount of vegetation. Nearly all satellite Vegetation Indices employ this difference formula to quantify the density of plant growth on the Earth The result of this formula is called the Normalized Difference Vegetation Index (NDVI).
