Plant leaves absorb or capture several colors of visible light to drive the food-making process, photosynthesis. We can tell if a plant is healthy or stressed by monitoring the proportions of these colors reflected by the leaves, which is a good indicator of what is being absorbed. Several additional wavelengths of light, such as the near infrared, are also useful in monitoring plants because of the difference in reflectance between healthy and stressed plant leaves.

The technologies that measure the intensity of reflected light help us monitor environmental health in a number of ways:

Determine plant health, 
Identify the types of land cover (plants, roads, bare soil, lakes, ocean, snow, ice, etc.),

Measure the amount of land cover types,

Monitor the growth and timing of plant cycles (greening up, flowering, greening down, senescence).


Image showing how light reflects off objects and is perceived by different types of light sensors (i.e. eyes, cameras, satellites)

Illustration by Ling Hsiao, Museum of Science, Boston.