NDVI = (NIR − Red) / (NIR + Red) — developed by Rouse et al. (1974), ranges −1 to 1.

• Well-vegetated pixels have high values (NIR ≫ Red).
• Water / shadow have negative values (Red > NIR).
• Rock, dry soil, senesced veg. are near zero (Red ≈ NIR).
• On Landsat TM/ETM+: NDVI = (B4 − B3) / (B4 + B3). On Landsat 8/9 OLI: (B5 − B4) / (B5 + B4).