Visual Normalized Difference Vegetation Index
The Visual NDVI algorithm is based from a vegetation index called NGRDI- Normalized Green Red Difference Index. In this algorithm the Red and Green bands of a visual image are utilized to calculate a Vegetation Index value. This vegetation index value is designed to detect differences in green canopy area. It leans heavily on the green color of a healthy plant.
The algorithm has been a tested indicator of chlorophyll content in several different crop types including Corn, Alfalfa, Soybean, and Wheat.
Visual NDVI = (Green-Red) / (Green+Red)
The difference between the green and red values of the image differentiates between plants and soil, and the sum normalizes for variations in light intensity between different images. The possible range of the Visual NDVI output is from -1.0 to 1.0
References- Tucker, C.J., 1979. Red and photographic infrared linear combinations for monitoring vegetation. Remote Sensing of Environment 8, 127–150.