"Spectral indices are combinations of surface reflectance at two or more wavelengths." (Mustafa et al., 2017) "The images emphasize a specific phenomenon that is present, while mitigating other factors that degrade the effects in the image. [...] Indices are often built by combinations of adding and subtracting bands, thereby making various band ratios. They are tied to specific bands that are in specific parts of the electromagnetic spectrum. As a result, they may only be valid for certain sensors or classes of sensors and it is critical that the proper bands are used in the calculation." (ESRI, 2018)
Mustafa Mustafa T, Hassoon Khalid I., Hassan Modher and Abd Modher H. "Using water indices (NDWI, MNDWI, NDMI, WRI and AWEI) to detect physical and chemical parameters by apply remote sensing and GIS techniques". International Journal of Research - Granthaalayah 5, no. 10 (2017): 117-128.
"Indices gallery". ArcGIS Pro, ESRI. 2018
Accessed February 1, 2019.