Green Leaf Index
The Green Leaf Index is an algorithm developed around wheat that can distinguish living plants from soil and non-living matter in the field. Like the Visual NDVI it leans heavily on the green color of a healthy plant to calculate the index. Therefore, it can be used as an indicator of chlorophyll.
GLI = (2 x G - R - B) / (2 x G+R+B)
The difference between the GLI and the Visual NDVI is the use of the Blue band in the calculation. Results of the vegetation index will range between -1.0 and 1.0.
In most cases you can threshold the index near zero into two classes- Green leaves and non-living material. Negative values tend to be soil or non-living material. Positive values tend to be living plant material. However due to potential changes in light and environmental conditions in the field the threshold does not always lie directly at zero.
References- Louhaichi, M., Borman, M.M., Johnson, D.E., 2001. Spatially located platform and aerial photography for documentation of grazing impacts on wheat. Geocarto International 16, 65–70.