IR = (R - G)/(R + G)
It uses the following data: channel green (G) and channel red (R).
The soils with a very light gray outcropping gypsum crust (low IR), the soils with aeolian sail, composed of reddish quartz sand (high IR). When; under the combined effect of agricultural practices, the wind, and their deposits move, the composition of the soil surface is modified and their color as well. By following the variations in the redness index, it is then possible to follow these modifications (Escadafal and al., 1991). The soils and mineral surface materials are the dominant components of arid surfaces; the interpretation of images acquired by satellites over these regions requires knowing and taking into account the corresponding spectral properties. Information of a geological and ecological nature can be drawn from it, by detecting the presence of outcrops of clear materials (limestone, gypsum, salts, etc.), or sand. Only a knowledge of the environment and its different aspects according to the level of degradation makes it possible to use the index IR to diagnose a possible evolution of the state of the environment, positive or negative (Escadafal and Huete ,1991).