Thermal Infrared remote sensing

1. Thermal Infrared
Remote Sensing

2. Introduction
• Earths atmosphere system derives its energy from the sun which being at a very high temperature
radiates maximum energy in the shorter wavelengths (visible 0.20 to 0.80µm).
• Earths atmospheric system absorbs part of this energy which in turn heats it up and raises its
temperature. This temperature being in the range of 300 deg kelvin it will emit its own radiation in
the longer wavelengths called Thermal radiation.
• The observations in the thermal wavelength of the electromagnetic spectrum (3 -35µm) is generally
referred to as thermal remote sensing.
• In this region the radiations emitted by the earth due to its thermal state are far more intense than the
solar reflected radiations, therefore any sensor operating in this wavelength region would primarily
detect the thermal radiative properties of ground material.
• All materials having a temperature above absolute zero (-273 deg celcius or 0 deg Kelvin) both day
and night emit infra red energy.

3. IR REGION OF THE ELECTROMEGNETIC
SPECTRUM
• IR Region covers wavelengths from 0.7 to 300µm. The reflected IR
region ranges from wavelengths 0.7 to 3µm and includes the
photographic IR bands.(0.7 to 0.9µm) that may be detected from the IR
film.
• IR radiation at wavelengths 3 to 14µm is called the thermal IR region.
• Thermal infra red is absorbed by the glass lenses of the conventional
cameras and cannot be detected by photographic film. Hence special
optical mechanical scanners are used to detect and record images in the
thermal IR region.
• IR radiation at wavelengths larger than 14µm is not utilised for remote
sensing as these are absorbed by the earths atmosphere.

4. ATMOSPHERIC TRANSMISSION
• Not all wavelengths of thermal Infrared radiation are
transmitted uniformly through the atmosphere.
• CO2, Ozone and water vapour absorb energy at certain
wavelengths IR and radiation at wavelengths from 3-5µm and
from 8-14 µm is readily transmitted through the atmosphere
windows.
• A narrow absorption band occurs from 9-10 µm due to the
ozone layer present at the top of the earths atmosphere.
• To avoid this absorption band, satellite thermal IR systems
operate in 10.5 – 12.5 µm.

5. Thermal properties of the material
• Radiant energy striking the surface of the material is
partly reflected, partly absorbed and partly
transmitted through the material. Therefore
Reflectivity + absorptivity + transmissivity = 1
• For materials having negligible transmissivity it
reduces to
Reflectivity + Absorptivity = 1
• The absorbed energy causes an increase in kinetic
energy

6. Emissivity
• It is the measure of the ability of a material to radiate and to absorb
energy.
• Materials with high emissivity absorb large amounts of incident energy
and radiate large quantities of kinetic energy and vice versa.
Radiant Temperature
The final product of the thermal mapping system is a map showing
apparent surface temperature. The system is sensitive not only to changes
in surface temperature but also change in emissivity of the scanned
surfaces
Thermal Conductivity
Thermal Conductivity K of the material is the quantity of heat in calories
that will pass through a 1cm cube of the material when two opposite faces
are maintained at 1 deg difference in temperature.

7. • Thermal capacity
• Thermal capacity C is the number of calories required to
raise the temperature of 1 gm of the material by 1 deg and
is expressed in cal gm-1. Water has the highest thermal
capacity of 1.01.
Thermal Inertia
Thermal inertia is the measure of the thermal response of a
material to temperature changes. The higher the density the
higher the inertia. The materials having high thermal inertia
strongly resist temperature changes.

8. Thermal Diffusivity
Thermal Diffusivity is a measure of the rate at which heat is transferred
within the substance.
It governs the rate at which heat is conducted from the surface to depth
in the daytime and from depth to the surface in the night time. Water
possesses high specific heat and therefore minor changes in moisture
content have significant effect on thermal diffusivity of soils.
IR – Radiometers
Non-imaging IR radiometers measure the radiant temperature using the
detectors sensitive to 8-14 µm wavelengths.
Radiant temperature may be measured either from air or on the ground
using portable IR radiometers. These measurements are useful for
collecting ground truth.

9. Airborne and Satellite TIR Scanner system
• Thermal remote sensing data is collected by radiometers and scanners. The most basic form of radiant temperature is the
thermal radiometer. This non-imaging device measures the radiant temperature using detectors sensitive to 8-14 µm
wavelengths.
• Thermal scanners are used in aircrafts or space-crafts.
• Airborne scanner systems consists of three basic components.
 An optical mechanical scanning subsystems.
 A thermal infrared detector.
 Imaging recording subsystems.
• The use of broadband thermal scanners from orbital platforms commenced from meteorological missions.
Eg. TIROS, NOAA having a typical spatial resolution of 1- 5 km.
 Later a thermal channel was included in missions like Heat Capacity Mapping Mission (HCMM) and
Landsat TM.
 HCMM satellite carried Heat Capacity Mapping Radiometer as the sensor operating in spectral range of
10.5 – 12.5 µm, with a spatial resolution of 600m.
 Whereas for Landsat the sensor used is Thermal Mapper (TM) working in spectral range 10.4 – 12.5 µm
(band 6) with a spatial resolution of 120m.

10. Scanner Distortions in IR images
• Thermal Scanners also generate geometric errors as they gather data.
• Some errors are caused by aircraft instability. As the aircraft rolls and pitches the scan
lines lose their correct positional relationships. Some of the errors are
1. Some of the imageries produce relief displacement like aerial photographs.
2. They do not have a central perspective instead they have separate Nadir for each scan
line. Thus the relief displacement is projected from a line that follows the cneter of the
long axis of the image.
3. As the distance from the center line increases, sensor tends to view the side rather than
the top of the features.
4. The scanner mirror rotates at a constant angular rate but the imagery is recorded at a
constant linear rate.
5. The projection of IFOV onto the ground surface does no move at equal speed because of
varied distance from the aircraft to the ground. At Nadir the sensor is closer to the
ground than the edge of the image resulting in image compression towards the edges.
This is known as tangential scale distortion.
6. Images near the flight line are more circular, whereas the shape of those nearest to the
edges are compressed along the axis perpendicular to the flight line.

11. Effects of Weather on Images
• 1. Clouds: have patchy warm and cool pattern, where the dark
signatures are relatively cool and bright signatures are relatively
warm. Scattered rain showers produce a pattern of streaks parallel to
the scan lines on the images. A heavy overcast reduces the thermal
contrast between terrain and cloud layer.
• 2. Surface Winds: produces smears and streaks on images. Avoided by
acquiring images in calm nights.
• 3. Penetration of smoke plumes: Smoke plumes conceals the ground in
the visible image whereas the terrain features are clearly visible in TIR
image and the burning from the fire has a bright signature.

12. INTERPRETATION OF THERMAL
IMAGERY (Radiant Temperature)
• The image generated by a thermal scanner is a strip of black-and-white film depicting thermal
contrasts in the landscape as variations in gray tones (e.g., Figures 9.9 and 9.10).
• Usually, brighter tones (whites and light grays) represent warmer features; darker tones (dark
grays and blacks) represent cooler features.
• For any thermal infrared image, the interpreter must always determine
(1) whether the image at hand is a positive or a negative image and
(2) the time of day that the image was acquired.
Sometimes it may not be possible to determine the correct time of day from information within
the image itself; misinterpretation can alter the meaning of gray tones on the image and render
the resulting interpretation useless.

13. • Thermal scanners are generally uncalibrated, so they show relative radiances rather
than absolute measurements of radiances.
• However, some thermal scanners do include reference sources that are viewed by the
scanning system at the beginning and end of each scan.
• The reference sources can be set at specific temperatures that are related to those
expected to be encountered in the scene. Thus each scan line includes values of
known temperature that permit the analyst to estimate temperatures of objects
within the image.
• Even when accurate measures of radiances are available, it is difficult to derive data
for kinetic temperatures from the apparent temperature information within the
image.
• Derivation of kinetic temperatures requires knowledge of emissivities of the
materials. In some instances, such knowledge may be available, as the survey may
be focused on a known area that must be repeatedly imaged to monitor changes over
time (e.g., as moisture conditions change).
• But many other surveys examine areas not previously studied in detail, and
information regarding surface materials and their emissivities may not be known.

14. • Emissivity is a measure of the effectiveness of an object in translating temperature
into emitted radiation (and in converting absorbed radiation into a change in observed
temperature).
• As objects differ with respect to emissivity, observed differences in emitted infrared
energy do not translate directly into corresponding differences in temperature.
• As a result, it is necessary to apply knowledge of surface temperature or of emissivity
variations to accurately study surface temperature patterns from thermal imagery.
• As knowledge of these characteristics assumes a very detailed prior knowledge of the
landscape, such interpretations should be considered as appropriate for examination of
a distribution known already in some detail rather than for reconnaissance of an
unknown pattern (e.g., one might already know the patterns of soils and crops at an
agricultural experiment station but may wish to use the imagery to monitor
temperature patterns).
• Often estimated values for emissivity are used, or assumed values are applied to areas
of unknown emissivity.

15. • In many instances, a thermal image must be interpreted to yield qualitative rather
than quantitative information.
• Although some applications do require interpretations of quantitative information,
there are many others for which qualitative interpretation is completely
satisfactory.
• An interpreter who is well informed about the landscape represented on the image,
the imaging system, the thermal behavior of various materials, and the timing of
the flight is prepared to derive considerable information from an image, even
though it may not be possible to derive precise temperatures from the image.
• The thermal landscape is a composite of the familiar elements of surface material,
topography, vegetation cover, and moisture. Various rocks, soils, and other surface
materials respond differently to solar heating.
• However, the thermal behavior of surface materials is also influenced by other
factors. For example, slopes that face the sun will tend to receive more solar
radiation than slopes that are shadowed by topography. Such differences are, of
course, combined with those arising from different surface materials.

16. • Thermal sensors can be very effective in monitoring the presence and
movement of moisture in the environment.
• In any given image, the influences of surface materials, topography,
vegetation, and moisture can combine to cause very complex image
patterns.
• However, often it is possible to isolate the effect of some of these
variables and therefore to derive useful information concerning, for
example, movement of moisture or the patterns of differing surface
materials
• Timing of acquisition of thermal imagery is very important.
• The optimum times vary according to the purpose and subject of the
study, so it is not possible to specify universally applicable rules.
• As the greatest thermal contrast tends to occur during the daylight
hours, sometimes thermal images are acquired in the early afternoon to
capture the differences in thermal properties of landscape features.

17. Conducting IR Surveys
• 1. Time Of the Day: frequently at night when there is no interference from
reflected solar radiations. Flying time is just before dawn when the effect of
differential solar heating is low.
• 2. Spatial Resolution: Depends on flight altitude and IFOV of the detector. (
Altitude of 2000m and IFOV OF 3mrad, the ground resolution becomes 6m.
• 3. Wavelength Bands: 3 – 5 µm and 8-14 µm.
• 4. Orientation & Altitude of the flight line
• 5. Ground measurements

18. Applications of TIR remote sensing
• 1. Geology
• 2. Military
• 3. Hydrology
• 4. Agriculture
• 5. Botany
• 6. Forestry
• 7. Heat Loss Survey