 ### What is a Vegetation Index ?

A Vegetation Index is a single value calculated by transforming the observations from multiple spectral bands. It is used to enhance the presence of green, vegetation features and thus help to distinguish them from the other objects present in the image. Depending on the transformation method and the spectral bands used, different aspects pertaining to the vegetation cover in the image could be evaluated say, the percentage of vegetation cover, amount of chlorophyll content, leaf area index and so on.
All the ratio indexes, in general, are independent of the illumination conditions at the time of acquisition and slope effects.

### Simple Ratio (SR)

This is the simplest VI which is a ratio between the reflectance recorded in the Near Infra-Red (NIR) and Red bands. This is a quick way to distinguish green leaves from other objects in the scene and estimate the relative biomass present in the image. Also, this value may be very useful in distinguishing stressed vegetation from non-stressed areas.

Simple Ratio = ρNIR / ρRed = ρ850 / ρ675

According to the spectral signature of green leaves, they exhibit very low reflectance in the Red and Blue regions (leaves reflect more in the green region and hence appear green). However, the reflectance is relatively higher in the NIR region. Thus, the SR value is close to 1 when the object has similar reflectance in both Red and NIR bands – for example, soil. Whereas, for a green object the value would be much greater than 1. ### Normalized Difference Vegetation Index (NDVI)

This is one of the most commonly used method for monitoring the percentage of green cover in an area. Since it is a ratio, this index is invariant to the difference in illumination conditions, slope, seasons, etc. and thus suitable for crop monitoring throughout the growth season. It is calculated by taking a ratio between the difference of reflectance from NIR and Red bands and the sum of reflectance from NIR and Red bands.

NDVI = (ρNIRRed) / (ρNIRRed) = (ρ850675) / (ρ850675) ### Photochemical Reflectance Index (PRI)

The Photochemical Reflectance Index is a measure of the light-use efficiency of foliage and thus is primarily used as an indicator of water stress and for the assessment of carbon-dioxide uptake by plants.

PRI = (ρ570530) / (ρ570530)

It is sensitive to the variations in the carotenoid pigments (for example, xanthophyll) in the leaves. These carotenoid pigments are involved in converting the absorbed photosynthetic radiation into fixed carbon. If you want to know more about vegetations indices, take a look at our article about Vegetation Indices on Chlorophyll Content !