Different surfaces reflect electromagnetic waves such as the visible light in different intensities. Black surfaces reflect less sunlight than white ones.
But there is more to that: Surfaces reflect some parts of the electromagnetic spectrum more intensely than others. For example, green vegetation absorbs blue and red light quite intensely whereas it reflects green and especially infrared light even more intensely.
Objects differ in their specific reflection and absorption characteristics. Just like every human being has their individual fingerprint, each object has its characteristic spectral behaviour. That is why this specific kind of behaviour is called the spectral fingerprint.
Take a look at the animation below in order to see what the spectral fingerprints of dry ground, water and different plant vitalities look like. Can you see how the surfaces differ in their reflections of the blue, green, red and infrared light?
Click on the buttons below in order to find out about the spectral fingerprints of water, soil and different plant vitalities in the diagram.
As you can see, healthy and green vegetation has a very specific spectral fingerprint. This is due to the special characteristics of the leafs. The leaves contain pigments such as chlorophyll, which absorb the main part of the blue and the red range of the impinging light but reflect the green range more intensely. That is why we perceive leaves as green.
In the infrared range, which is invisible to the human eye, the intensity of the reflection increases even further. This is due to the fact that infrared light is being reflected multiple times at the cell walls of the leaf cells. Due to the high reflectances of the infrared range, healthy plants (a lot of chlorophyll and stable cell walls) stick out in the infrared band of satellite images.
Due to its structure and its chemical set-up, a leaf reflects infrared light twice as intensely as green light.
How to measure vegetation
The spectral fingerprint of a leaf in the course of time
This animation shows you how the reflection characteristics and thus the spectral fingerprint of a leaf change in the course of time. You can clearly see that the difference between reflected light in the visible range and reflected light in the invisible range of healthy plants is quite striking. If a plant withers, this difference diminishes rapidly.
In remote sensing, this phenomenon led to the development of a measurement indicating the vitality (= health) of vegetation. This measurement is called NDVI, which is short for Normalised Difference Vegetation Index.
NDVI-Bild des Mittelmeeraumes. Deutlich treten die vegetationsarmen Regionen hervor (bearb. nach USGS/NASA Landsat Program).
In the coloured NDVI image of the Mediterranean above the vegetation-free areas of the Sahara stand out. You can also easily spot the mountainous regions of Mount Etna (volcano in Sicily) and of Mount Parnassus (Greek mainland) containing only very little vegetation. Can you detect any other differences in the spatial NDVI pattern? Can you, for example, locate Athens at the southern tip of Greece?
Satellite images help capture and analyse plant vitalities worldwide.