Beyond ratios, two linear transformations to know:

• PCA (Principal Component Analysis) — decorrelates bands into components ordered by variance. PC1 usually captures brightness; later PCs isolate features (e.g., vegetation).
• K-T (Kauth-Thomas) “Tasseled Cap” — a fixed linear transform tuned for Landsat that produces interpretable axes: Brightness, Greenness, Wetness (and sometimes a 4th).