2. • Electromagnetic Waves
• Electromagnetic waves are energy transported
through space in the form of periodic
disturbances of electric and magnetic fields.
• All electromagnetic waves travel through space
at the same speed, c = 2.99792458 x 108 m/s,
commonly known as the speed of light.
• An electromagnetic wave is characterized by:
1. frequency
2. wavelength.
3. These two quantities are related to the speed of
light .
3. • So speed of light = frequency x wavelength
The frequency ( and the wavelength) of an
electromagnetic wave depends on its
source.
• There is a wide range of frequency in our
physical world, ranging from the low
frequency of the electric waves generated
by the power transmission lines, to the very
high frequency of the gamma rays. This
wide frequency range of electromagnetic
waves constitute the Electromagnetic
Spectrum.
8. The Electromagnetic Spectrum
• The electromagnetic spectrum can be divided
into several wavelength (frequency) regions also
called bands, only a narrow band from about
400 to 700 nm (0.7-0.4 ) µm is visible to the
human eyes. Note that there is no sharp
boundary between these regions. The
boundaries shown in the above figures are
approximate and there are overlaps between two
adjacent regions.
• Wavelength units: 1 mm = 1000 µm; 1 µm =
1000 nm.
10. • The wavelength of electromagnetic energy has
such a wide range that no instrument can
measure it completely. Different devices,
however, can measure most of the major
spectral regions.
• The division of the spectral wavelength is based
on the devices which can be used to observe
particular types of energy, such as thermal,
shortwave infrared and microwave energy. In
reality, there are no real abrupt changes on the
magnitude of the spectral energy. The
spectrum are conventionally divided into
various parts as shown below:
11. • Radio Waves: 10 cm to 10 km wavelength.
• Microwaves: 1 mm to 1 m wavelength. The
microwaves are further divided into different
frequency (wavelength) bands: (1 GHz = 109 Hz)
– P band: 0.3 - 1 GHz (30 - 100 cm)
– L band: 1 - 2 GHz (15 - 30 cm)
– S band: 2 - 4 GHz (7.5 - 15 cm)
– C band: 4 - 8 GHz (3.8 - 7.5 cm)
– X band: 8 - 12.5 GHz (2.4 - 3.8 cm)
– Ku band: 12.5 - 18 GHz (1.7 - 2.4 cm)
– K band: 18 - 26.5 GHz (1.1 - 1.7 cm)
– Ka band: 26.5 - 40 GHz (0.75 - 1.1 cm)
12. • The portion of the spectrum of more recent
interest to remote sensing is the microwave
region from about 1 mm to 1 m. This covers the
longest wavelengths used for remote sensing.
• The shorter wavelengths have properties similar
to the thermal infrared region while the longer
wavelengths approach the wavelengths used for
radio broadcasts. Because of the special nature
of this region and its importance to remote
sensing in Canada
13. Infrared: ( 0.7 to 300) µm wavelength. This region is
further divided into the following bands:
1. Near Infrared (NIR): 0.7 to 1.5 µm.
2. Short Wavelength Infrared (SWIR): 1.5 to 3 µm.
3. Mid Wavelength Infrared (MWIR): 3 to 8 µm.
4. Long Wavelength Infrared (LWIR): 8 to 15 µm.
5. Far Infrared (FIR): longer than 15 µm.
The NIR and SWIR are also known as the Reflected
Infrared, referring to the main infrared component
of the solar radiation reflected from the earth's
surface. The MWIR and LWIR are the Thermal
Infrared.
14. Visible Light: This narrow band of electromagnetic
radiation extends from about 400 nm (violet) to
about 700 nm (red). The various color
components of the visible spectrum fall roughly
within the following wavelength regions:
1. Red: 610 - 700 nm
2. Orange: 590 - 610 nm
3. Yellow: 570 - 590 nm
4. Green: 500 - 570 nm
5. Blue: 450 - 500 nm
6. Indigo: 430 - 450 nm
7. Violet: 400 - 430 nm
15. The light which our eyes - our "remote sensors" - can
detect is part of the visible spectrum. It is important to
recognize how small the visible portion is relative to the
rest of the spectrum. There is a lot of radiation around us
which is "invisible" to our eyes, but can be detected by
other remote sensing instruments and used to our
advantage. The visible wavelengths cover a range from
approximately 0.4 to 0.7 µm. The longest visible
wavelength is red and the shortest is violet. It is
important to note that this is the only portion of the
spectrum we can associate with the concept of colours
16. Ultraviolet: 3 to 400 nm ,X-Rays and Gamma Rays
• For most purposes, the ultraviolet or UV
portion of the spectrum has the shortest
wavelengths which are practical for remote
sensing. This radiation is just beyond the violet
portion of the visible wavelengths, hence its
name. Some Earth surface materials, primarily
rocks and minerals, fluoresce or emit visible
light when illuminated by UV
17. • Blue, green, and red are the primary colors or
wavelengths of the visible spectrum. They are
defined as such because no single primary color can
be created from the other two, but all other colors
can be formed by combining blue, green, and red in
various proportions. Although we see sunlight as a
uniform or homogeneous color, it is actually
composed of various wavelengths of radiation in
primarily the ultraviolet, visible and infrared
portions of the spectrum. The visible portion of this
radiation can be shown in its component colors
when sunlight is passed through a prism, which
bends the light in differing amounts according to
wavelength.
18. • The optical region covers 0.3 - 15 mm where
energy can be collected through lenses. The
reflective region, 0.4 - 3.0 mm, is a subdivision of
the optical region. In this spectral region, we
collect solar energy reflected by the earth
surface. Another subdivision of the optical
spectral region is the thermal spectral range
which is between 3 mm to 15 mm, where energy
comes primarily from surface emittance. Table
lists major uses of some spectral wavelength
regions.
20. Region Name Wavelength Comments
Gamma Ray < 0.03 nanometers
Entirely absorbed by the
Earth's atmosphere and not
available for remote
sensing.
X-ray 0.03 to 30 nanometers
Entirely absorbed by the
Earth's atmosphere and not
available for remote
sensing.
Ultraviolet 0.03 to 0.4 micrometers
Wavelengths from 0.03 to
0.3 micrometers absorbed
by ozone in the Earth's
atmosphere.
Photographic Ultraviolet 0.3 to 0.4 micrometers
Available for remote
sensing the Earth. Can be
imaged with photographic
film.
Visible 0.4 to 0.7 micrometers
Available for remote
sensing the Earth. Can be
imaged with photographic
film.
21. Region Name Wavelength Comments
Infrared 0.7 to 100 micrometers
Available for remote
sensing the Earth. Can be
imaged with photographic
film.
Reflected Infrared 0.7 to 3.0 micrometers
Available for remote
sensing the Earth. Near
Infrared 0.7 to 0.9
micrometers. Can be
imaged with photographic
film.
Thermal Infrared 3.0 to 14 micrometers
Available for remote
sensing the Earth. This
wavelength cannot be
captured with photographic
film. Instead, mechanical
sensors are used to image
this wavelength band.
22. Region Name Wavelength Comments
Microwave or
Radar
0.1 to 100
centimeters
Longer wavelengths
of this band can
pass through
clouds, fog, and
rain. Images using
this band can be
made with sensors
that actively emit
microwaves.
Radio > 100 centimeters
Not normally used
for remote sensing
the Earth.
24. • Most earth observation satellites record in several
spectral bands, in other words; the satellite records a
number of small wavelength intervals within the
electromagnetic spectrum (visible light, near and short
wave infrared). By means of the basic colors red, green
and blue (RGB) it is possible to construct several band
combinations in which the colors tell something about
the parts of the spectrum that are represented in RGB.
Demonstrated hereunder is how various band
combinations are shown by the Landsat satellite. Landsat
records in 7 spectral bands, see the Landsat TM
wavelength bands in the figure above, and RGB
combination of certain bands lead to images with
different information content. Demonstrated is how a
band combination will show in flat agricultural area (The
Netherlands) and how this will be for a mountainous area
(Bosnia
25. 432: combination of VNIR (Visible Near Infra Red)
(4) - red (3) - green (2)
VNIR: 0.76 - 0.90 µm
red: 0.61 - 0.69 µm
green: 0.51 - 0.60 µm
These three bands are typically combined to
make a 'traditional' false colour composite as
one also knows from aerial photography. In
band 4, especially the high reflectance peak
from vegetation is detected, also enabling
discrimination of numerous vegetation types.
Also detecting land-water is well possible with
band 4. This false colour combination makes
vegetation appear as redtones, brighter reds
indicating more the growing vegetation. Soils
with no or sparse vegetation range from white
(sand, salt) to greens or browns depending on
moisture and organic matter content. Water
appears blue; clear water will be dark blue to
black while shallow waters or waters with high
sediment concentrations are lighter blue.
Urban areas will appear blue towards gray.
26. SWIR: 2.08 - 2.35 µm
VNIR: 0.76 - 0.90 µm
green: 0.51 - 0.60 µm
In this band combination the vegetation shows in
various green shades because band 4 (high
reflectance of vegetation) is presented in the colour
green. Like Landsat band 5 (also SWIR), band 7 is
sensitive to variations in moisture content and
especially detects this in hydrous minerals in
geologic settings (such as clays). This band can
discriminate in various rock and mineral types.
Differences originating from these various types are
presented in shades of red to orange in this band
combination but also the brighter shades in the
blue can give information about soils. In
comparison to the other IR channels and apart from
recording the normal reflective radiation, band 7 is
increasingly sensitive to the emissive radiation so
that it's possible to detect heat sources with this
band. Bright green spots indicate vegetation and
the waters appear dark blue or black. Urban areas
will be also dark blue or pink
742: combination of SWIR (7) -
VNIR (4) - green (2)
27. VNIR: 0.76 - 0.90 µm
SWIR: 1.55 - 1.75 µm
red: 0.61 - 0.69 µm
The short wave infrared band (band 5 for
Landsat) is sensitive to variations in water
content, for leafy vegetation as well as soil
moisture. This band features a very high
water absorption, thus enabling detection
of very thin water layers (less than 1 cm).
Also variations in ferric iron (Fe2O3)
content in rocks and soils can be detected;
higher reflections with higher contents. In
this combination vegetation appears in
shades of red. When a crop has a relative
lower moisture content, the reflection
from band 5 will be relatively higher,
meaning more contribution of green and
thus resulting in a more orange colour.
The colour green will begin to dominate in
this combination when the vegetation
reflects lower in the VNIR and higher in
the SWIR. Non vegetated soils and urban
areas will appear in blue towards gray
colours
–
453: combination of VNIR (4) -
SWIR (Short Wave Infra Red)
(5) - red (3
28. 321: combination of red (3) - green (2) -
blue (1)
red: 0.61 - 0.69 µm
green: 0.51 - 0.60 µm
blue: 0.45 - 0.51 µm
This band combination is used to
represent an image in natural colour
and therefore best approaches the
appearance of the landscape in reality.
Band 3 detects chlorophyll absorption
in vegetation (thus low reflection).
Band 2 detects the green reflectance
from vegetation. Band 1 is more suited
for penetration in water, in clear water
this can be some 25 meters. On the
other hand one can also derive
information about sediment
transportation in water from this band.
Band 1 also differentiates between soil
and vegetation and distinguishes forest
types
