over the rainbow!
The agitation of the charged particles that are present in all
matter causes all objects to emit electromagnetic radiation. The
objects emit this energy, but can also transmit, absorb, and reflect
it. The sun is one of the main natural sources of the electromagnetic
energy on Earth, but there are also many artificial sources, such
as electric lamps, microwave ovens, and cell phones.
The suns electromagnetic spectrum includes radiation with
very short wavelengths, such as gamma rays and X-rays (of the order
of 1/100 of a micron) and very long wavelengths, such as radio waves
(that can be several kilometres long). A relatively small part of
the electromagnetic spectrum is especially important for us.
This is the part that covers wavelengths from
0.4 to 0.7 µm, or visible light. Within this frequency range
each of the colours of the rainbow corresponds to a specific wavelength.
Thus, blue = approx. 0.45µm, green = approx. 0.55µm,
and red = approx. 0.65µm.
Electromagnetic radiations characteristics
can undergo changes during transmission. For example, the Earths
atmosphere (notably the ozone layer) luckily blocks some of the
solar radiation that is extremely harmful to human beings. The radiations
properties are sometimes changed partially during the transfer.
Thus, the oceans upper layers absorb the parts of the solar
radiation that correspond to red and green light, which explains
the blue colour of the deeper layers. At a certain depth the water
absorbs all of the visible light rays and the ocean becomes pitch
When radiation reaches an object, the object may
absorb one part and reflect another part of the radiation. In all
cases, the electromagnetic radiations characteristics will
be modified. Plants use mainly the red part of the solar spectrum
to carry out photosynthesis. The reflected light spectrum is thus
devoid of this red band and the light reflected by the leaves appears
signatures can be represented graphically as shown here, with
the reflected electromagnetic radiations frequencies (or wavelengths)
plotted on the abscissa (x axis) and intensities on the ordinate
As we can see from the graph,
different objects do not all absorb the same parts of solar radiation.
Consequently, their reflected ray spectra are different. The pattern
of the electromagnetic spectrum reflected by an object is called
its spectral signature.
Remote sensing makes use
of this property, for analysing the characteristics of the electromagnetic
spectra reflected by objects (their spectral signatures) allows
one to determine some of the objects properties, within limits.
Human vision basically uses the same principle: it uses colours
to identify objects, for example, to select the ripest apple. The
sensors used in remote sensing, however, make it possible to broaden
the field of analysis to include parts of the electromagnetic spectrum
that are well beyond visible light.
A spectro-radiometer is usually used to analyse all the details
of an electromagnetic spectrum. This instrument can analyse all
of the frequencies of a given electromagnetic radiation. Other,
simpler, instruments measure only certain parts of the frequency
spectrum, known as spectral bands. Although the data provided by
such multispectral radiometers are discrete, that is, non-continuous,
they can also be used to distinguish different types of materials.
The sensors used in remote
sensing cover the ultraviolet (<0.3 µm), visible (0.4-0.7
µm), near-infrared (0.7-1.5 µm) and thermal infrared
(up to 1000 µm or 1 mm) ranges. As a rule, they merely measure
and analyse the radiation reflected by the objects that are lit
by the sun; they are thus passive systems. Other remote
sensing systems send out signals that strike the Earths surface
and then analyse their echoes; these are active systems.
The latter usually operate in the microwave or radar wave range,
working with wavelengths of from 1cm to 1m.