Forest Health is calculated using three different vegetation indicies - greenness, leaf pigments, and canopy water or light use efficiency. These indicies are discussed in detail in the ENVI online help. I believe the help lists the following reference which you might find useful: Asner, G.P., 1998, Biophysical and Biochemical Sources of Variability in Canopy Reflectance, Remote Sensing of Environment, 64:234-253.