Visual Atmospheric Resistance Index
The Visual Atmospheric Resistance Index is a vegetation index that was originally designed for satellite imagery. It is found to be minimally sensitive to atmospheric effects, allowing the estimation of vegetation fraction in a wide range of atmospheric conditions.
VARI = (Green-Red)/(Green+Red-Blue)
As sunlight reaches the earth’s atmosphere it is scattered in all directions by the gasses and particles in the air. But blue light tends to scatter more than all the other colors because it travels in smaller wavelengths than the rest of the visual spectrum. Therefore, we see the sky as blue most of the time. By including the Blue band in the denominator of the VARI equation we are accounting for the effects of the atmosphere on this Vegetation Index calculation.
For each pixel VARI is calculating a ratio of green vegetation cover. Values can be anywhere from -1.0 and 1.0 in the final output.
References- Gitelson, A.A., Kaufman, Y.J., Stark, R., Rundquist, D., 2002. Novel algorithms for remote estimation of vegetation fraction. Remote Sensing of Environment 80, 76–87.