Why does vegetation reflect ir

Water absorbs most incoming radiation across the entire range of wavelengths. Knowing their typical spectral response characteristics, it is possible to identify forests, crops, soils, and geological formations in remotely sensed imagery, and to evaluate their condition. The next graph demonstrates one of the advantages of being able to see beyond the visible spectrum. The two lines represent the spectral response patterns of conifer and deciduous trees. Notice that the reflectances within the visual band are nearly identical.

At longer, near- and mid-infrared wavelengths, however, the two types are much easier to differentiate. In general, if there is much more reflected radiation in near-infrared wavelengths than in visible wavelengths, then the vegetation in that pixel is likely to be dense and may contain some type of forest.

If there is very little difference in the intensity of visible and near-infrared wavelengths reflected, then the vegetation is probably sparse and may consist of grassland, tundra, or desert. NDVI is calculated from the visible and near-infrared light reflected by vegetation. Healthy vegetation left absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation right reflects more visible light and less near-infrared light.

The numbers on the figure above are representative of actual values, but real vegetation is much more varied. Hardwood trees will appear light red than Coniferous. Clearwater appears dark-bluish, while turbid water appears cyan. Being one of the popular band combinations is useful for studying vegetation, drainage monitoring various stages of crop growth and soil patterns.

Densely populated urban areas are indicated in light blue. This band combination offers similar results to the traditional color infrared aerial photography.

The spectral reflectance is based on water and chlorophyll absorption in the leaf. Needles have a dark response comparing to the leaves. Chlorophyll is a little better at absorbing blue and red wavelengths than it is at absorbing green wavelengths, so relatively more of the green radiation is reflected, giving vegetation its green appearance. Incoming photons at wavelengths in the near-infrared region individually contain less energy recall that the energy in a photon is proportional to its frequency, and hence inversely proportional to its wavelength.

These photons are therefore unable to cause an electronic transition so pigments used for photosynthesis cannot use them, and in general do not absorb them.

Other parts of plants, especially liquid water found in the leaves, do absorb these photons, which rotate and stretch chemical bonds in the water or in plant cells, effectively heating up the absorbing material. In general, plants have no need for heating up, and except for wavelengths around 1. The combination of low reflectance in the visible and high reflectance in the near-infrared wavelengths is the most characteristic signature of vegetation used in remote sensing, and is used for quick-and-dirty mapping of vegetation on Earth.

Consider that most land areas contain a mix of vegetated and unvegetated areas. Using the reflectance in the visible wavelengths alone does not tell us how much vegetation is in an area, because water also has low reflectance in this wavelength region. And using the reflectance in the near-infrared region alone also does not tell us much about vegetation, because other surface types, such as the bright sand and bright snow depicted in Figure 68 also have higher near-infrared reflectance.

However, the ratio of the visible and near-infrared reflectances is a useful indicator for the amount of vegetation present in an area because no other surface type has both as high near-infrared and as low visible reflectance as vegetation. This observation led initially to the development of what is called the Simple Ratio SR :.

Figure Four areas with rather different vegetation characteristics. Top left: The African savanna contain a mix of trees and grass cover. Top right: The Sahara desert is in most places void of vegetation.

Bottom left: The Amazon rainforest is covered in dense vegetation. Bottom right: The Canadian Arctic has large areas with no vegetation. Patterned ground in Canadian tundra by Raymond M. While surface reflectance values should in principle always be used to calculate this and any other vegetation index, in practice these calculations are often done instead based on TOA reflectance or even TOA radiance or even DN values! The Simple Ratio has one drawback that means it is rarely used — its values approach infinity when red reflectance approaches zero, which makes differences between large values difficult to interpret.