This video lecture addresses image acquisition
in remote sensing, with a focus on thermal
imagery
There are three important properties to consider
for electromagnetic energy.
First, the wavelength is represented as the
distance from one wave crest to the next.
Second, the frequency is measured as number
of crests passing a fixed point in given time
period.
Third, the amplitude is the height of each
peak.
While most students of remote sensing are
familiar with visible spectrum ranging from
roughly 0.4-0.7 micrometers (um), it is important
to consider the other portions of the electromagnetic
spectrum beyond what the human eye can detect.
Our specific focus in this lecture is on the
range beyond the visible spectrum in the near
and mid infrared and the thermal infrared.
Some other portions of the spectrum used in
remote sensing include the ultraviolet and
microwave regions of the electromagnetic spectrum.
In general, the infrared radiation is sometimes
called "below the red" because frequencies
are lower than the red portion of visible
spectrum.
The infrared portion of the electromagnetic
spectrum was discovered in 1800 by British
astronomer Sir William Herschel (1738-1822).
Then, in 1847, two Frenchmen AHL Fizeau (1819-1896)
and JBL Foucault (1819-1868) demonstrated
how the optical properties of infrared are
similar to those of visible light.
The use of infrared in remote sensing focused
on emissive infrared or thermal infrared with
radiation from approximately 7-18 um.
In practice, what we call far infrared has
overlap with passive remote sensing of microwave
radiation.
In some applications, absorption by atmospheric
particles restricts use.
Thermal infrared remote sensing collects information
on surface temperatures and thermal properties
of soils, rocks, vegetation, and man-made
structures.
Detectors are devices formed of substances
known to respond to energy over a defined
wavelength interval.
The detector generates weak electric signal,
and the strength of the signal is related
to radiances in the field of view of the sensor.
An example of uncommon materials that are
used in thermal detectors are indium antimonide
(InSb) and mercury-doped geranium (Ge:Hg)
with peak sensitivity near 10 um in the far
infrared portion of the spectrum.
Another example of a material used in a detector
is mercury cadmium telluride (MCT), which
is sensitive over the range of 8-14 um.
Detectors are cooled to very low temperatures
(~ -200C) for maximum sensitivity.
As Natural Resources Canada (2014, para.
1) explains [quote] "Many multispectral (MSS)
systems sense radiation in the thermal infrared
as well as the visible and reflected infrared
portions of the spectrum.
However, remote sensing of energy emitted
from the Earth's surface in the thermal infrared
(3 ?m to 15 ?m) is different than the sensing
of reflected energy.
Thermal sensors use photo detectors sensitive
to the direct contact of photons on their
surface, to detect emitted thermal radiation.
The detectors are cooled to temperatures close
to absolute zero in order to limit their own
thermal emissions.
Thermal sensors essentially measure the surface
temperature and thermal properties of targets"
[end quote].
Thermal detectors with coarse radiometric
resolution have low sensitivity so that only
large differences in brightness are recorded
and most finer detail in a scene is lost.
In contrast, thermal detectors with fine radiometric
resolution have high sensitivity and finer
differences in scene brightness are recorded.
You may have heard of the concept of signal-to-noise-ratio,
often written as SNR or S/N ratio, as the
measure of signal strength relative to the
background noise.
In sound communications, the SNR is generally
measured in decibels (dB).
In remote sensing, we talk about the signal
as the differences in image brightness caused
by actual variation in scene brightness, and
the noise as variations in brightness unrelated
to scene brightness, for example, due to system
performance or as a contribution from a sensor.
Thermal scanners are the most widely used
imaging sensors for remote sensing.
These scanners sense radiance of features
beneath an aircraft flight path and in turn
produce images of terrain.
There are two main types of thermal scanners.
First, the object-plane scanners, which view
the landscape by means of a moving mirror.
Second, the image-plane scanners, which have
a wider field of view for a more comprehensive
image of the landscape.
Thermal scanners are used across many fields
and for many applications.
Thermal radiation is emitted by all objects
at temperatures greater than absolute zero,
but the intensity and peak wavelength vary
with object temperature.
The immediate source of emitted thermal radiation
is shortwave solar energy, with exceptions
in geothermal energy, man-made thermal sources,
and range and forest fires.
Short (n.d.) notes [quote] "remote sensors
that cover two thermal intervals - the 3-5
um and 8-14 um broad bands (corresponding
to two atmospheric windows) allowing sensing
of thermal emissions from the land, water,
ice and the atmosphere - have been flown on
airplanes for several decades.
Here are temperature variations in Mt. Hope
Bay and part of Narragansett Bay, Rhode Island,
made from an aircraft survey" [end quote].In
this image, the warmer colors, for example
oranges and reds, represent warmer temperatures,
and the cooler colors, for example greens
and blues, represent cooler temperatures.
8 - Emissivity equals the radiant emittance
of an object in relation to emittance of a
blackbody at the same temperature.
This slide offers a few emissivity coefficients
of common materials.
In applications like remote sensing, keep
in mind that a thermal scanner may detect
radiation that is reflected from the surface
of the object, that is emitted by the object,
and that may be transmitted through the object.
Thermal remote sensing may be captured from
plane or space based sensors.
As Short (n.d.) notes, [quote] "thermal imaging
has been done from various space systems.
Many of the meteorological satellites ... include
at least one thermal channel.
These include most geostationary satellites.
This is a map of thermal variations off the
east coast of the U.S. made from meteorological
data" [end quote].
The following references were used in developing
this video lecture.
