A white surface reflects equal amounts of radiation of all wavelengths of visible light, whereas a green leaf reflects less radiation in the red and blue parts of the visible spectrum than in the green.

This gives an excess of green light (compared to red and blue) and the leaf looks green. So the composition of the electromagnetic reflection, the spectral signature, tells us about the surface emitting or reflecting the radiation.

The ability of satellites to distinguish between various spectral signatures is critical for their use in mapping, where the distinction between different surface and area types is essential.

The human eye can only perceive radiation within a limited range of the electromagnetic spectrum. So instruments for remote sensing outside the visible wavelengths actually represent an extension of our visual field and they give access to additional information about the physical world surrounding us.

Electromagnetic radiation from a surface is either a reflection (reflected light) or an emission (radiation emitted from the surface itself). Reflected sunlight is for obvious reasons only measurable in daylight, whereas emission can be measured at all times.

Thermodynamics indicates that a body at temperature different from 0 Kelvin, i.e. an object on Earth, emits radiation proportionally to its temperature.

Surface temperature is a key factor for emission. The sun has a surface temperature of 6000 degrees Kelvin (K) and maximum emission in the visible light range. A surface with a temperature of about 1000K, e.g. a fire in the Amazon, has its maximum emission in the middle infrared spectrum. The surface temperature of the Earth is about 290K and a emission maximum around 14 micrometres, also called the thermal infrared range, see illustration. There is a direct correlation between surface temperature and degree of emission at a given wavelength. The surface temperature can be calculated on the basis of remote sensing of the thermal infrared emission.

The maximum emission of the sun is in the wavelength 0.483 micrometres, whereas that of the Earth is at 14 micrometres.

Emission from a surface is a function of its surface temperature, which means that surface temperature can be calculated on the basis of remote sensing of surface emission.

As the Earth only radiate small amounts of energy in visible light, it can only be seen because it reflects visible light from the sun. Sun-rays hitting the Earth can either be absorbed and thus contribute to the heating of the Earth or be reflected and seen by the human eye or sensed by a satellite. The albedo value of a surface indicates how large a percentage of the sunlight is reflected.

The spectral signature for green plants is very characteristic. The chlorophyll in a growing plant absorbs visible and especially red light to be used in photosynthesis, whereas near infrared light is reflected very effectively as it is of no use to the plant, see illustration. In this way the plants avoid unnecessary heating and loss of juice through evaporation. Therefore the reflection from vegetation in the near infrared and in the visual ranges of the spectrum varies considerably. The degree of difference reveals how large a part of the area is covered with growing green leaves (leaf area index).

The Normalized Difference Vegetation Index (NDVI) is usually calculated as follows,

NDVI =	near infrared - red
	near infrared + red

On the basis of this simple formula the global distribution of vegetation is currently mapped.  

Due to recurrent drought problems in the Sahel area south of the Sahara special attempts have been made to map the vegetation in great detail here. A series of vegetation maps covering the whole season will give an impression of the total biomass production in the growth period. Satellite data can be transformed into kilogramme (Kg) biomass per hectare (ha) with great accuracy by measuring selected control areas and adjusting the remote sensing results. In this way large geographical areas can be mapped at short intervals and drought problems be detected at an early stage. Visit for example: 

During image classification it is possible to identify a specific area type on the screen (and use it as a training area), determine the spectral signature and then let the computer identify all the pixels having the same spectral signature. In that way large regions can be mapped very quickly and easily by means of satellite data.

However, there are still several unresolved problems. It is especially difficult to distinguish between different types of vegetation as their spectral signatures can be so much alike. Furthermore, the same type of vegetation has different signatures dependent on stage in growth season and on other factors, such as soil humidity or atmospheric conditions.

So research is concentrated on the potential refinement of area classification based on satellite data. One way is to try to optimize the spread of sensors covering specific channels in the visible and near infrared ranges of the spectrum.

A satellite with many narrow channels is said to have a high degree of spectral resolution. In the future satellites with high spectral resolution may make it possible to map the changes in the vegetation provoked by the stress of pollution or drought. Remote sensing is expected to become an increasingly important tool in connection with environmental mapping.

Even though remote sensing takes place in the atmospheric windows it is still to some extent interfered with by diffusion and absorption in the atmosphere.

So remote sensing may often be slightly distorted and has to be adjusted with subsequent digital image processing.