The normalized difference red edge index (NDRE) is a that can be used to analyse whether images obtained from multi-spectral image sensors contain healthy vegetation or not. It does this by measuring the amount of chlorophyll in a plant.
It is similar to Normalized Difference Vegetation Index (NDVI) but uses the ratio of Near-Infrared and the edge of Red as follows:
The red edge is the part of the spectrum centered around 715 nm.
The Index will give a value between -1.0 to +1.0, with a higher value showing a healthy plant environment.