A vegetation index (VI) is a spectral imaging transformation of two or more image bands designed to enhance the contribution of vegetation properties and allow reliable spatial and temporal inter-comparisons of terrestrial photosynthetic activity and canopy structural variations.

There are many VIs, with many being functionally equivalent. Many of the indices make use of the inverse relationship between red and near-infrared reflectance associated with healthy green vegetation. Since the 1960s scientists have used satellite remote sensing to monitor fluctuation in vegetation at the Earth's surface. Measurements of vegetation attributes include leaf area index (LAI), percent green cover, chlorophyll content, green biomass and absorbed photosynthetically active radiation (APAR).

VIs have been historically classified based on a range of attributes, including the number of spectral bands (2 or greater than 2); the method of calculations (ratio or orthogonal), depending on the required objective; or by their historical development (classified as first generation VIs or second generation VIs). For the sake of comparison of the effectiveness of different VIs, Lyon, Yuan et al. (1998) classified 7 VIs based on their computation methods (Subtraction, Division or Rational Transform). Due to advances in hyperspectral remote sensing technology, high-resolution reflectance spectrums are now available, which can be used with traditional multispectral VIs. In addition, VIs have been developed to be used specifically with hyperspectral data, such as the use of Narrow Band Vegetation Indices.

Uses
Vegetation indices have been used to:
 examine climate trends;
 estimate water content of soils remotely;
 monitor drought;
 schedule crop irrigation, crop management;
 monitor evaporation and plant transpiration.
 assess changes in biodiversity
 classify vegetation
 detection and quantification of crop diseases

Types of vegetation index

Multispectral Vegetation Index

Ratio Vegetation Index (RVI): Defined as the ratio between the Red and Near Infrared lights of multispectral images 
Normalised Difference Vegetation Index (NDVI): The most commonly used remote sensing index  that calculates the ratio of the difference and sum between the Near Infrared and Red bands of multispectral images. It normally takes values between -1 and +1. It is mostly used in vegetation dynamics monitoring, including biomass quantification.
Kauth-Thomas Tasseled Cap Transformation: A spectral enhancement index that transforms the spectral information of a satellite data into spectral features 
Infrared Index
Perpendicular Vegetation Index
Greenness Above Bare Soil
Moisture Stress Index: A spectral index that measures the level of moisture stress in leaves  
Leaf Water Content Index (LWCI) 
MidIR Index
Soil-Adjusted Vegetation Index (SAVI): An adjusted form of NDVI developed to minimize the effects of soil brightness on spectral vegetation indices, particularly in areas of high soil composition 
Modified SAVI: Mostly applied in to areas with low NDVI measures.
Atmospherically Resistant Vegetation Index
Soil and Atmospherically Resistant Vegetation Index
Enhanced Vegetation Index (EVI): Very similar to NDVI. The only difference is that it corrects atmospheric and canopy background noise, particularly in regions with high biomass
New Vegetation Index
Aerosol Free Vegetation Index
Triangular Vegetation Index
Reduced Simple Ratio 
Visible Atmospherically Resistant Index
Normalised Difference Built-Up Index
Weighted Difference Vegetation Index (WDVI)
Fraction of absorbed photosynthetically active radiation (FAPAR)
Normalised Difference Greenness index (NDGI)
Temperature Vegetation Water Stress Index (TVWSI)

Hyperspectral Vegetation Index
With the advent of hyperspectral data, vegetation index have been developed specifically for hyperspectral data.
Discrete-Band Normalised Difference Vegetation Index
Yellowness Index
Photochemical Reflectance Index
Descrete-Band Normalised Difference Water Index
Red Edge Position Determination
Crop Chlorophyll Content Prediction
Moment distance index (MDI)

Advanced Vegetation Indices 
With the emergence of machine learning, certain algorithms can be used to determine vegetation indices from data. This allows to take into account all spectral bands and to discover hidden parameters that can be useful to strengthen these vegetation indices. Thus, they can be more robust against light variations, shadows or even uncalibrated images if these artifacts exist in the training data.

 Synthesis of Vegetation Indices Using Genetic Programming
 A soft computing approach for selecting and combining spectral bands 
 DeepIndices: Remote Sensing Indices Based on Approximation of Functions through Deep Learning

See also
Crop coefficient

References

Remote sensing
Biogeography