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Table 2 Calculation methods and reflectance of spectral indices

From: Estimation of nitrate nitrogen content in cotton petioles under drip irrigation based on wavelet neural network approach using spectral indices

Spectral index Abbreviation Formula References
Trilateral parameters Red edge amplitude Dr Maximum first derivative within 680–760 [38]
Red edge area SDr Sum of first derivative values in red edge [38]
Yellow edge amplitude Dy Maximum first derivative within 560–640 [39]
Yellow edge area SDy Sum of first derivative values in yellow edge [39]
Blue edge amplitude Db Maximum first derivative within 490–530 [39]
Blue edge area SDb Sum of first derivative values in blue edge [39]
Vegetation index Red edge ratio spectral index RD R740/R720 [40]
Red edge model index CIred-edge (R780/R710) − 1 [41]
Normalized difference red edge index NDRE (R790R720)/(R790 + R720) [42]
Normalized difference spectral index ND705 (R750 − R705)/(R750 + R705) [43]
Near infrared ratio spectral index NIR R780/R740 [44]
Red edge ratio spectral index RI-1 dB R735/R720 [45]