Chlorophyll in healthy leaves absorbs red light for photosynthesis, while the leaf's internal cell structure strongly scatters near-infrared (NIR) light. Bare soil, water and concrete do neither — they reflect red and NIR more evenly (water absorbs both, but NIR most of all). The satellite measures reflected Red and NIR for every ground cell and combines them into one number:
NDVI = (NIR − Red) / (NIR + Red)
- Dense forest ≈ 0.7–0.9 — huge NIR reflectance, tiny red reflectance.
- Cropland / grassland ≈ 0.3–0.7 depending on health and moisture.
- Bare soil / built-up ≈ 0.0–0.15 — red and NIR reflect almost equally.
- Water is negative — it absorbs NIR strongly.
Real-world relevance: comparing NDVI maps of the same ground cells at two dates reveals deforestation, drought stress or land conversion from orbit, without ever setting foot on site — the basis of real satellite biodiversity monitoring (e.g. Sentinel-2, Landsat).