Identifying Subalpine Fir (Abies lasiocarpa) Attacked by the Balsam Woolly Adelgid (Adelges piceae) Using Spectral Measurements of the Foliage

Table 3

Narrow waveband spectral indices available for selection in the stepwise discriminant analysis.

Spectral index	Formula
Red/Green Ratio (R/G)	𝜌659𝜌549
Simple Ratio (SR)	𝜌659𝜌859
Normalized Difference	𝜌859−𝜌659𝜌859+𝜌659
Vegetation Index (NDVI)
Water Index (WI)	𝜌894𝜌972
Normalized Difference	𝜌856−𝜌1243𝜌856+𝜌1243
Water Index (NDWI)
Infrared Index (II)	𝜌856−𝜌1641𝜌856+𝜌1641