Live green vegetation absorbs visible light (solar radiation) as part of photosynthesis. At the same time plants reflect solar energy in the near infrared frequencies of the electromagnetic spectrum. This difference in absorption and reflection is unique to plants and to plant species.
By measuring the difference between the light absorbed and reflected by crops, the presence of weeds can be detected. When crops are stressed, photosynthesis is affected therefore changing the amount of light absorbed and reflected, so this can be seen with drones too.
NDVI stands for Normalised Difference Vegetation Index and is an algorithm that can take into account conditional variations such as bright sunlight and cloud cover.
SAVI stands for Soil Adjusted Vegetation Index. It’s an algorithm used where vegetation is low and the soil surface is exposed.
Once processed, the NDVI shaped files can be used for a variety of purposes including detection and location of weeds, counting plants and assessing crop health and biomass.