Selection of normalized spectra between 6400 and 8000 Å. The horizontally dotted lines indicate the regions across which the vegetation index ΔPVI, associated with the VRE: blueward (between 6750 and 6850 Å; green dotted line) and redward (between 7480 and 7780 Å, but avoiding the region affected by the O₂ -A between 7580 and 7680 Å; red dotted line). The difference between the green and red dotted lines defines the amount of ΔPVI. The derivation of EW(O₂-A) is explained in the text.