Simple Ratio (SR) = NIR / Red. First “true” vegetation index (Cohen 1991, building on Jordan 1969). Image ratioing highlights the spectral contrast between vegetation and non-vegetation.

• On Landsat TM/ETM+, SR = Band 4 / Band 3 (NIR ÷ Red).
• Weakness: unbounded; values can become very large over dense canopies, making comparisons across scenes tricky. NDVI fixes this.