2. Diurnal Heating Effects
It has been implied earlier in this Section that thermal
images will vary considerably in appearance depending
on whether they are acquired during the warm part of the
day or after a night of absence of the Sun and resultant
cooling of the atmosphere as well as heat loss from the
surface and shallow depths beneath.
This is evident in these two images of central Atlanta,
GA. taken during the day (left, or top) and then just
before dawn (right, or bottom). The thermal sensor
was flown on an aircraft.
3. Contrasts between heated buildings
This is evident in these two images of central Atlanta, GA. taken during the day
4. Heated Areas In Shadow
In the day thermal image, contrasts between heated buildings
and streets and areas in shadow create a scene that resembles an
aerial photo. But in the dawn image, differences in temperature have
decreased sharply (no shadows), although a part of that image is
brighter, representing a local "heat island" effect. Also, in the dawn
image, many streets are evident because, being asphalt paved, they
absorb more heat and remain warmer through the night.
The "heat island" effect is commonplace over urban areas and towns.
Activities of people, such as heating or cooling of homes and operation of
autos, add thermal energy to the air. Temperatures can be several degrees (F
or C) hotter than rural areas. This holds for both day and night. A pair of
Landsat-7 images - the upper true color, the lower a color-coded map of
temperature variations as measured by the thermal band on the ETM+
sensor - illustrates this effect for Atlanta, for an observation made on
September 28, 2000.
6. Unlike remote sensing of reflected light from surfaces in
which only the topmost layers (a few molecular layers thick)
are involved,
Thermal remote sensing includes energy variations extending
to varying shallow depths below the ground surface.
This takes time and is the normal consequence of heating
during the day and cooling at night.
The most critical consideration in analyzing and interpreting
thermal data and imagery is that of knowing the physical and
temporal conditions that heat the near surface layers.
7. Over the seasons, minor shifts in the mean temperature in bedrock can
occur to depths of 10 m (33 ft) or more.
Solar radiation and heat transfer from the air significantly heat materials
at and immediately below the surface during the day.
Temperatures usually drop at night primarily by radiative cooling
(maximum radiative cooling occurs under cloudless conditions),
accompanied by some conduction and convection.
During a single daily (diurnal) cycle, the near surface layers (commonly,
unconsolidated soils) experience alternate heating and cooling to depths
typically between 50 and 100 cm (20-40 in). The daily mean surface
temperature is commonly near the mean air temperature.
Observed temperature changes are induced mainly by changes during the
diurnal heating cycle, but seasonal differences in temperature and local
meteorological conditions also affect the cycle response from day to day.
9. Changes In Radiant Temperature In Surface
The curves shown here summarize the qualitative changes in
radiant temperature during a 24-hr cycle, beginning and
ending at local midnight, for five general classes of materials
found at the surface.
From these curves we can estimate the relative gray levels
that a thermal sensor could record, as a function of the
material and the time of day.
Given two thermal images of the same locale, taken 12 hours
apart, about noon and about midnight, we might determine
the identities of co-registered pixels, based on their
temperatures and thermal inertias.
10. Differences in thermal inertia, reflectance, and emissivity of various materials and
variable atmospheric radiance are important factors that modify the measured
surface temperatures. Consider the four sets of curves plotted below:
11. materials and variable atmospheric radiance
Thetop curves (A) show temperature variations resulting solely from
differences in thermal inertias of materials, with other factors held
constant.
Note the distinct crossover points. Values of P much higher than 0.05
(commonly, metallic objects) produce diurnal curves that approach a
straight line passing through the crossover points.
This effect is consistent with the earlier statement that materials
with high thermal inertias undergo smaller radiant temperature
changes during a full heating/cooling cycle.
Curves in B show the effects of different reflectance's the ratio of
reflected solar radiation to incident radiation, in percent.
12. The maximum and minimum daily temperatures and their differences
increase with decreasing reflectance of solar insolation.
In C, the curves represent changes associated with different emissivities;
where most natural materials have values of ranging from 0.80 to 0.98.
Curves in D indicate the temperature effects introduced by the
atmospheric radiances, which are caused by re-emission from water
vapor, a common source of error. From these sets of curves, we learn that
natural surface materials show considerable modifications in radiant
temperatures because of these variables.
cont.…
13. Regardless of the time of day, temperature profiles in the
heating zone converge on a nearly steady value at some depth
below 30-50 cm (depending on whether it is rock or soil, and its
moisture content), but that convergent temperature slowly
increases with depth into the Earth as the geothermal gradient
takes over.
14. The least variation in temperature
The maximum and minimum temperatures at the observed
surface (topmost centimeter or so) depend mostly on the extent of buildup of the
heat reservoir and its storage capacity through the affected layers.
For materials that have the same density and albedo but different
conductivities and/or specific heats, the difference in predawn (lowest T values)
and midday (highest T values) increases with increasing conductivity and
decreasing specific heat.
The actual magnitude of the difference decreases with depth, down to the
stable (convergent) value at the base.
The effect of increasing the material density (holding other variables
constant in P = (ρ Kc)1/2) is to require more thermal energy to heat the additional
mass in a given volume, so that less heat transfers to lower layers.
With increasing density, the total added heat (derived from Sun and air)
distributes over a decreased thickness of surficial layers. So, in this instance, the
maximum T reached at midday is lower, and the minimum at night is greater.
15. the spread of daily temperature extremes tends to be lower
relative to less dense materials.
The corresponding rise in thermal inertia for denser materials
simply expresses their increased resistance (sometimes
referred to as thermal impedance) to temperature changes
(smaller as heating or cooling progresses).
For materials with higher thermal conductivity's, more heat
transfers to greater depths, less remains concentrated at the
surface, and, again, temperature extremes diminish, as
evidenced by lower daytime surface temperatures and higher
nighttime temperatures compared with materials having
lower Ks (e.g., soils).
cont…
16. Thermal properties of objects
Emissivity:
• Emissivity(e) of a material is defined as the ratio of radiant flux emitted from
the real body to radiant flux emitted by a reference black body at the same
temperature.
• The emissivity of a real body means the emitting ability of a real body as
compared to that of a black body.
• Emissivity is a measure of material’s ability to absorb and radiate energy.
• Emissivity describes how efficiently an object radiates energy compared to a
black body.
• Water is the only object whose emissivity is constant under different
conditions.
• Emissivity depends upon temperature, emission angel and wave length.
• Emissivity of true black body is e=1.emissivity of real object e<1.
• Emissivity for all selectively radiating bodies is from 0 to less than 1.It
fluctuates according to the wavelength of energy.
17. •A grey body is defined as a body with constant emissivity over all wave
-lengths and temperatures.
•Emissivity of a Non-black body depends on surface geometry and the
chemical composition. An object which does not absorb all incident light
will also emit less radiation than an ideal black body. s
•Surface emissivity is generally measured by assuming
emissivity =1-reflectivity
•A perfect specular surface may reflect 98% of the incident energy
absorbing the remaining 2%. This is reverse in black body.
emissivity of selective materials
Material Emissivity (mu m)
Granite 0.185
Quartz sand, large grains 0.914
Asphalt paving 0.959
Concrete walkway 0.966
Water with thin film of oil 0.972
Pure water 0.993
Polished metal surface 0.060
18. Kinetic Heat, Temperature, Radiant Energy
And Radiant Flux
•A body in thermal state can be expressed by two temperature
1.internal temperature or kinetic temperature T (kin)
2.External temperature or radiant temperature T (rad)
•Internal temperature is due to the motion of its atom and it is measured
by thermometer.
•External temperature is measured remotely by a radiometer.
•Kinetic heat refers to energy generated due to random motion of
Particles of matter. All objects having temperature above absolute 0
Exhibits this random motion.
Internal kinetic heat of an object is also converted to radiant energy
often called external or apparent energy.
Both T (kin) and T (rad) exhibits high correlation.
Radiant temperature always being slightly less than the true kinetic
temperature of the object due to thermal property called emissivity.
Radiant flux is the flow of radiant energy from a square surface area of
A black body per second. It is commonly measured as watts.
19. The emissivity of a object is influenced by a number of factors which
Includes
1.Color:
Darker colored objects are better absorbers and emitters than the
lighter colored ones.
2.Surface roughness
The greater the surface roughness of object relative to the size of the
Incident wavelength, the greater the surface area of the object and
Potential for absorption and re-emission of energy.
3.Moisture content:
The moisture content object has greater ability to absorb energy and
become a good emitter. Ex:: wet soil particles.
4.Compaction:
The degree of soil impaction can affect emissivity.
5.Field of View:
The emissivity of a single leaf measured with very high resolution
thermal radiometer will have a different emissivity than an entire tree
Crown viewed using a coarse spatial resolution radiometer.
20. Thermal conductivity
Thermal conductivity (k) is the intrinsic property of a material which
indicates its ability to conduct heat.
Conduction of heat refers to transfer of energy within a material
without any motion of the material as whole.
Conduction takes place when a temperature gradient exists in a solid
medium.
The rate of heat transfer depends upon the temperature gradient and
the thermal conductivity of the material.
It is measured as calories.
Heat flows in the material in the direction of decreasing temperature.
Energy is transferred from the more energetic to less energetic
Molecules when neighboring molecules collide.
K=Q/t*L/A*triangle T
Q=quantity of heat transmitted.
L=thickness of a material
A=surface area of the material
Tri T=temperature gradient
21. Thermal capacity
tThermal capacity c is the number of calories required to raise the
temperature of 1 gm material by 1degree centigrade.
It also represents the ability of a material to store heat.
Water has the highest thermal capacity of 1.01. The temperature of a
lake usually varies very little between night and day. water has a very
high thermal capacity compared to other materials.
C=rou c
Rou= density of material
c = product of the density
22. Thermal inertia
Thermal inertia p is a measure of the thermal response of a material
to temperature changes.
P=(kpc)1/2[J m-2 k-4 s -1/2]
K=thermal conductivity
Rou=density
C=thermal capacity
The higher the density, higher is the thermal inertia. Materials having
high thermal inertia are cooler in day time and warmer at night.
Materials having low thermal inertia are warmer in day time and
Cooler at night.
Thermal inertia represents the ability of a material to conduct and
store heat.
Thermal inertia predominantly depends upon the physical properties
of the near surface materials such as particle size, degree of
Indurations, rock abundance and exposure of bedrock.
Thermal inertia is a measure of the geological properties of the
surface on the same depth scale.
23. Lowest thermal inertia probably represents loose, fine surface dust
and very few rocks.
Medium thermal inertia probably represents a combination of coarse
loose particles, crusted fines, a fair number of scattered rocks, and
Perhaps a few scattered bedrock outcrops.
Higher thermal inertia is a combination of coarse sand, dune sand,
Strongly-crusted fines, abundant rocks, and scattered bedrock
Exposures.
24. 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….
25. thermal sensors….
Quantum and photons are used for this purpose.
These detectors are capable of very rapid (less than 1nano
seconds) response.
They operate on the principle of direct interaction between
photons of radiation incident on them and the energy level of
electrical charge carriers within the detectors.
For maximum sensitivity detectors must be cooled to
temperatures approaching absolute zero to minimize their
own thermal emission.
26. Thermal imagers are typically across-track scanners that
detect emitted radiation in only the thermal portion of the
spectrum.
Thermal sensors employ one or more internal
temperature references for comparison with the detected
radiation, so they can be related to absolute radiant
temperature.
The data are generally recorded on film and/or magnetic
tape and the temperature resolution of current sensors
can reach 0.1 °C.
Thermal sensors essentially measure the surface
temperature and thermal properties of targets.
27. Because energy decreases as the wavelength increases, thermal
sensors generally have large IFOVs to ensure that enough energy
reaches the detector in order to make a reliable measurement.
Therefore the spatial resolution of thermal sensors is usually fairly
coarse, relative to the spatial resolution possible in the visible and
reflected infrared.
28. Thermography
Thermography, thermal imaging, or thermal video, is a type
of infrared imaging. Thermographic cameras detect radiation in
the infra red range of the electromagnetic radiation (roughly
900–14,000 nanometres or 0.9–14 µm) and produce images of
that radiation.
Since infrared radiation is emitted by all objects based on their
temperatures, according to the black body radiation law,
thermography makes it possible to "see" one's environment with
or without visible illumination.
29. Image of a small dog taken in mid-infrared ("thermal")
light (false color)
30. Finally, let's take a quick peek at two aspects of military use -
here the appearance of a tank and a supply truck operating actively
at night during maneuvers.
31. Individuals in everyday clothing appear as distinct temperature variants. Their
faces usually are warmer (reds) than their outside clothes (in cooler greens and
blues):
32. Thermographic image of a traditional building in the background and a
‘passive house' in the foreground
33. Difference between IR film and thermography
IR film is sensitive to temperatures between 250 °C and 500 °C
while thermography is sensitive to approximately -50 °C to over
2,000 °C.
So for a IR film to show something it must be over 250 °C or be
reflecting infrared radiation from something that is at least that
hot.
Night vision goggles normally just amplify the small amount of
light that is available outside like starlight or moon light and can't
see heat or work in complete darkness
34. Advantages of Thermography
You get a visual picture so that you can compare temperatures
over large area
It is real time capable of catching moving targets
Measurement in areas inaccessible or hazardous for other
methods
Limitations & disadvantages of thermography
Quality cameras are expensive and are easily damaged
Images can be hard to interpret accurately even with experience
Accurate temperature measurements are very hard to make
because of emissivities
Training and staying proficient in IR scanning is time consuming
35. Condition monitoring
Medical imaging
Research
Process control
Non destructive testing
Chemical mapping
Applications….
36. References
lillisand t.m and r.w. kiefer remote sensing and image interpretation(1994)
anand p.h remote sensing &gis
www.rst.gsfc.nasa.gov
https://en.wikipedia.org/wiki/Thermography
Lillisand T.M And R.W. Kiefer Remote Sensing And Image
Interpretation(1994
Anand Ph. Remote Sensing &Gis
www.rst.gsfc.nasa.gov
Remote Sensing a Tool for Environmental Observations Steven M. de Jong
(Ed.) Raymond Sluiter, Maarten Zeijlmans ,Elisabeth Addink January 2005
