1. Thermal Remote Sensing

2. Student Learning Outcomes
Differentiate between kinetic and radiant temperature.
Differentiate between absorption bands and atmospheric windows.
Define blackbody.
Describe the Stefan-Boltzman Law, Wien’s Displacement Law, and Kirchhoff’s Radiation Law.
Discuss the relevance of these laws to TIR remote sensing.
Define emissivity.
Explain the fundamental premise of TIR remote sensing.
Think: the thermal capacity, conductivity, and inertia of materials differ and, as a result, objects respond differently in the TIR; the differential response can be used to discriminate features, assess heat loss, plant stress, and many other things.

3. Student Learning Outcomes
Explain why TIR data must be radiometrically calibrated.
Discuss the major steps required in the radiometric calibration of TIR data using atmospheric radiative transfer models.
Summarize the diurnal cycle of reflected shortwave and emitted longwave energy.
Given TIR imagery, describe and explain, in general terms, temperature differences (of objects / materials) across space and through time.

4. Introduction
All objects with temperature > absolute zero (0° K; -273° C; ° F) emit TIR EMR (3-14 μm)
We may feel it but not see it
Some sensors can detect it
Features have predictable thermal characteristics
TIR images can be used to:
Determine types of materials
Evaluate changes in thermal characteristics of phenomena (e.g., plant stress, pollution, heat loss from buildings)
Landscape features have predictable thermal characteristics due to selective absorption of solar short-wavelength energy and radiation of thermal infrared energy

5. Some Uses of Thermal Infrared RS
Assessment of building efficiency: heat loss, etc.
Medical Applications
Moisture around cold vent
A “HotDog”
Breaker box

6. Some Uses of Thermal Infrared RS
MIR of Baghdad:
Locations of fire (white spots)
and rising smoke plume from ablazing oil ditches
TIR of Baghdad:
Locations of fire (white spots) against black smoke background
Oil surrounded by seawater (CASI)
Natural color of Baghdad:
City obscured by smoke
Night-time surveillance

7. History of Thermal Infrared RS
1800: Discovery of the TIR portion of the EM spectrum
WW I: Ability to detect men at 120 m and later aircraft
WW II: Major developments in IR technology, incl. the invention of detector element; IR surveillance
1960s: Use of TIR images for few select civilian clients
1968: On-demand supply of TIR data that did not exceed a certain spatial resolution and temperature sensitivity
Since 1970s:
Declassification of some TIR RS data (e.g., TIROS – U.S. Television Operational Satellite, launched in 1960)
Launch of satellites with scientifically oriented TIR systems (e.g., TIMS, ATLAS, GOES, Landsat TM/ETM+, ASTER)
TIROS: cloud patterns and frontal movement
CZCS – Coastal Zone Color Scanner, on NASA’s Nimbus 7, for SST, 1978
TIMS – Thermal Infrared Multispectral Scanner, NASA and JPL, 1980
ATLAS – Airborne Terrestrial Applications Sensor
GOES – weather prediction
ASTER – Advanced Spaceborne Thermal Emission and Reflection Radiometer

8. Think Thermally Analyst must understand:
How solar shortwave radiation interacts with atmosphere
How solar shortwave radiation interacts with Earth materials
How emitted terrestrial radiation interacts with atmosphere
How a RS detector records emitted TIR EM radiation
How sensor system and terrain introduce noise into TIR image

9. TIR Radiation Properties
All objects with T > 0° K exhibit random motion
Kinetic (true, real, internal) heat (measured in calories):
Energy of particles of molecular matter in random motion
Collision of particles due to random motion causes their energy state to change and to emit radiant energy
Kinetic temperature (Tkin; measured with thermometer):
Concentration of kinetic heat
Internal temperature of an object
Radiant (apparent, external) energy (measured in joules):
Energy exiting an object (see kinetic heat above)
Facilitates use of RS technology for thermal assessments

10. TIR Radiation Properties
All objects with T > 0° K exhibit random motion
Radiant Flux (Ф; measured in Watts):
Radiant energy per unit time
Radiant Temperature (Trad; measured with radiometer):
Concentration of amount of radiant flux emitted from an object
Highly correlated with kinetic temperature (Tkin), but tends to overestimate it (due to emissivity, see below)
Key idea behind TIR RS:
Measure Trad with radiometer from a distance to derive Tkin

11. EMR-Matter Interactions
RECALL:
EMR emitted from Sun and Earth …
Absorption
Reflection
Scattering, Refraction
Transmission
Sensible heat transfer (e.g., conduction)
Latent heat transfer (e.g., evaporation)
Sensor
Sun
Atmosphere
Earth’s Surface

12. The Infrared Portion IR of EM spectrum quite broad (0.7-103 μm)
Reflective IR (0.7-3 μm)
Includes NIR ( μm) and SWIR (1.3-3 μm)
Thermal IR (3-14 μm)
Includes MIR (3-5.5 μm) and “TIR” ( μm)
Reflective
Thermal

13. “Windows & Shutters” Absorption bands Atmospheric windows
Where atmosphere absorbs most of the IR energy present (e.g., due to O2, H2O, CO2, O3)
Atmospheric windows
Where atmosphere passes IR energy
Primary TIR windows: 3-5 and 8-14 micrometers

14. RS Instruments and Windows
RS instruments engineered to only be sensitive to the IR energy present in atmospheric windows
In:Sb (indium antimonide): peak sensitivity near 5 μm
Ge:Hg (mercury-doped germanium): peak sensitivity near 10 μm
Hg:Cd:Te (mercury-cadmium-telluride): sensitive from 8-14 μm

15. Thermal Radiation Laws
Blackbody
Theoretical construct
Absorbs all the radiant energy incident on it (i.e., no reflected or transmitted energy)
Radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature
No objects in nature are true blackbodies, but think of
Sun as a 6,000° K blackbody
Earth as a 300° K blackbody
Two important physical laws
Stefan-Boltzmann Law
Wien’s Displacement Law
The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition.
In terms of frequency () or wavelength (λ), Planck's law is written: …
where B is the spectral radiance, T is the absolute temperature of the black body, kB is the Boltzmann constant, h is the Planck constant, and c is the speed of light.

16. Temperature, Energy, and Waves
The hotter the object,
the more energy it radiates (Stefan-Boltzmann Law)
the shorter its dominant wavelength (Wien’s Displacement Law)
Sun-to-Earth incoming shortwave radiation from 6,000° K (ca. 6,000º C or 11,000º F) Sun with a dominant wavelength of 0.48μm
Blackbody
Radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature
Absorbs all the radiant energy incident on it (i.e., no reflected or transmitted energy)
Earth-to-space outgoing longwave radiation from 300° K (ca. 15º C or 60º F) Earth with a dominant wavelength of 9.66 μm
All objects above absolute zero (-273º C or 0º K) emit EM energy!

17. Thermal Radiation Laws
Stefan-Boltzman Law
Mλ = total emitted radiation from a blackbody (W m-2)
σ = Stefan-Boltzmann constant ( × 10-8 W m-2 K-4)
T = absolute temperature (degrees Kelvin)
What does this mean?
The hotter the object, the greater the amount of energy emitted by that object
Actual amount of energy emitted corresponds to sum/integral of the area under its curve
Total spectral radiant exitance leaving a blackbody is proportional to the fourth power of its temperature

18. Thermal Radiation Laws
Wien’s Displacement Law
λmax = dominant wavelength
k = constant (2898 μm K)
T = absolute temperature (degrees Kelvin)
What does this mean?
The hotter the object, the shorter its dominant wavelength or the higher its dominant frequency
Describes the relationship between the true temperature of a blackbody and its peak spectral exitance or dominant wavelength

19. Thermal Radiation Laws
Why do we need to know the dominant wavelength for TIR RS?
Provides info regarding the part of the EM spectrum in which we want to sense the object
Forest fires at 800º K
λmax = 3.62 μm
Optimum TIR detector: 3-5 μm
Soil, etc. at 300º K
λmax = 9.67 μm
Optimum TIR detector: 8-14 μm

20. Thermal Radiation Laws
Emissivity (ε)
All objects are selectively radiating bodies (NOT blackbodies)
Emit only a certain proportion of the energy emitted from a blackbody at the same kinetic temperature
Ratio between the radiant flux exiting a real-world selectively radiating body (Mr) and the radiant flux exiting a blackbody at the same kinetic temperature (Mb)
ε ranges from 0 to ≤ 1 (depending on wavelengths considered)

21. Thermal Radiation Laws
Emissivity (ε)
Graybody
Constant ε that’s < 1 at all wavelengths
Radiates energy having a blackbody distribution reduced by a constant factor
Distilled water has emissivity close to 1

22. Thermal Radiation Laws
Emissivity (ε)

23. Thermal Radiation Laws
Why do we need to know about emissivity?
Two objects lying next to one another (e.g., two rocks) could have the same kinetic temperature but different apparent/radiant temperatures b/c their emissivities vary
Emissivity of an object is influenced by:
Color
Surface roughness
Moisture content
Compaction
Wavelength
Sensor characteristics

24. Thermal Radiation Laws
Emissivity of an object is influenced by:
Color: darker objects are better absorbers, poorer reflectors, and better emitters than lighter objects
Surface roughness: the greater the surface roughness of an object relative to the size of the incident wavelength, the greater the surface area of the object and potential for absorption and reemission of energy
Moisture content: the higher an object’s moisture content, the greater its ability to absorb energy and become a good emitter
Compaction
Wavelength
Sensor characteristics: e.g., field of view viewing angle

25. Thermal Radiation Laws
Kirchhoff’s Radiation Law
States that the spectral emissivity of an object equals its spectral absorptance at a given wavelength
Why? Remember the Radiation Budget Equation?
Φiλ = incident radiant flux at specific wavelength (λ)
rλ = spectral hemispherical reflectance
αλ = spectral hemispherical absorptance
τ λ = spectral hemispherical transmittance
If αλ = ελ (as observed by Kirchhoff in IR portion of EMS) and τ λ = 0 (most real-world materials are opaque), then
ε=α
Good absorbers = good emitters
Good reflectors = poor emitters
Radiation budget equation divided by Φiλ yields 1 = rλ +αλ +τ λ
Φiλ = incident radiant flux at wavelength λ (radiant energy per unit time)
Φrλ = radiant flux reflected from the surface
Φaλ = radiant flux absorbed by the surface
Φtλ = radiant flux transmitted through the surface
Hemispherical reflectance, absorptance, transmittance = dimensionless

26. Thermal Radiation Laws
Kirchhoff’s Radiation Law
Describes why objects appear as they do on TIR imagery
Good absorbers are good emitters and good reflectors are poor emitters.
If reflectivity increases, then emissivity must decrease.
If reflectivity decreases, then emissivity must increase.
Dark objects on TIR
Cold objects: Objects that reflect most of the incident energy (e.g., metal roofs) --- poor emitters
Light objects on TIR
Warm objects: Objects that reflect little of the incident energy (e.g., water) --- good emitters

27. Thermal Radiation Laws
Metal
hangar
Aircraft
(recently
turned off)
Aircraft
Concrete
tarmac
Aircraft
(turned on)

28. Thermal Radiation Laws
Goal of TIR RS when measuring LST:
Point radiometer at an object and have the recorded apparent temperature (Trad) equal the true kinetic temperature of the object (Tkin)
Problem: radiant flux from a real-world object is NOT the same as the radiant flux from a blackbody at the same temperature (largely due emissivity effects)
Knowing emissivity of an object allows modification of the Stefan-Boltzmann law originally applicable to blackbodies (Mb = σ × T4) so that it pertains to the total spectral radiant flux of real world materials (Mr):

29. Thermal Radiation Laws
Takes into account an object’s temperature and emissivity
More accurate estimate of radiant flux exiting an object and recorded by the TIR sensor
If we assume several things (book pp. 258/259), then:
ε can be estimated if an object’s Trad and Tkin are measured in the field:
ε (not T) is of interest to geologists
ε = (Trad/Tkin)1/4 ~ Measure RS and field simultaneously

30. Thermal Properties of Terrain
Earth materials are characterized by:
Thermal capacity: ability to store heat
Thermal conductivity: ability to conduct heat
Thermal inertia: ability to conduct and store heat in response to temperature changes
Differential abilities
Differential responses to temperature changes
Different thermal properties affect TIR image interpretation!

31. Thermal Properties of Terrain
Thermal capacity (c – cal g-1 °C-1)
Ability of a material to absorb heat energy
Quantity of heat required to raise the temperature of one gram of that material by 1 °C
Material
Thermal capacity
Glass
0.16
Water
1.0
Wood
0.327
Granite
Sandy soil
0.24

32. Thermal Properties of Terrain
Thermal conductivity (K – cal cm-1 sec -1 °C-1)
Rate at which a substance transfers heat through it
Number of calories that will pass through 1 cm3 of material in 1 second when two opposite faces are maintained at 1 °C difference in temperature
Varies with moisture content
Material
Thermal conductivity
Glass
0.0021
Water
0.0013
Wood
0.0050
Granite
0.0075
Sandy soil
0.0014

33. Thermal Properties of Terrain
Thermal inertia (P – cal cm-2 sec-1/2 °C-1)
Measure of the thermal response of material to temperature changes
Computed using:
K – thermal conductivity
p – density (g cm-3)
c – thermal capacity
Important biophysical variable, because thermal inertia generally increases linearly with increasing material density
Material
Thermal inertia
Thermaldensity
Glass
0.029
2.6
Water
0.036
1.0
Wood
0.009
0.5
Granite
0.056
Sandy soil
0.024
1.8
Measure of how quickly/slowly T changes

34. Thermal Properties of Terrain
Problem:
Thermal conductivity, capacity, inertia, and density can not be directly remotely sensed – must be measured in situ
Per-pixel RS of apparent thermal inertia is possible:
Acquire night- and early day-time TIR images of study area
Geometrically and radiometrically correct images to one another
Determine change in temperature (ΔT) for each pixel
Daytime apparent temp. – Nighttime apparent temp.
Apparent thermal inertia (ATI) per pixel:
A = albedo measured in the visible spectrum during the daytime

35. Thermal Properties of Terrain
Thermal inertia:
Roughly inversely related with measured temperature change
High ΔT value is usually associated with terrain materials that have a low thermal inertia value
Low ΔT value is usually associated with terrain materials that have a high thermal inertia value
Some uses:
Distinguish boundaries between bedrock and alluvium
Discriminate rock units with similar spectral properties
Identify zones of hydrothermal alteration
Useful sensors (collection during the day and night):
ASTER, TIMS, ATLAS, etc.

36. Radiometric Calibration …
… of TIR RS data is required to:
compensate for atmospheric absorption and emission in the radiance arriving at the sensor
correct for surface emissivity effects, which are coupled with temperature in the radiance emitted by the surface
… of TIR RS data may be performed using:
internal blackbody source referencing
external empirical referencing based on in situ data
external empirical referencing based on atmospheric profile characteristics and atmospheric radiative transfer models

37. Internal Source Referencing
Calibration of all collected BVs to apparent true temperature values relative to known “cold” and “hot” reference targets
Problem: does not account for intervening atmosphere
Emits spurious radiant energy into IFOV
Absorbs energy emitted from ground before it reaches detector optics

38. External Source Referencing
Calibration of all collected BVs to kinetic temperature values using empirical reference data collected in situ
Thermometer (measure kinetic temperature)
Radiometer (measure radiant temperature)
Radiosonde (obtain atmospheric profiles of temperature, barometric pressure, and water vapor)
BVij – BV in uncalibrated image
a – slope of linear equation
b – intercept of linear equation

39. External Source Referencing
Calibration of all collected BVs to kinetic temperature values using atmospheric profile characteristics and atmospheric radiative transfer models
Obtain atmospheric profiles of air temperature, pressure and humidity (e.g., Atmospheric Correction Parameter Calculator:
Incorporate atmospheric data in atmospheric transmission model (e.g., MODTRAN) to generate atmospheric correction parameters (transmissivity, upwelling radiance, downwelling radiance)
Combine atmospheric correction parameters with BVs in empirical formulas (radiative transfer equation and Planck function)

40. External Source Referencing
NAMCORR

41. TIR Environmental Considerations
Diurnal Cycle of Reflected Shortwave and Emitted Longwave Energy
Note the pattern of daily temperatures
Max. temperature: lag of 2 to 4 hours after midday peak of incoming solar shortwave radiation (takes time to heat terrestrial materials)
Note the peak period of daily outgoing longwave radiation

42. TIR Environmental Considerations
Diurnal Cycle … Energy
Sunrise:
Earth intercepts mainly solar shortwave energy ( μm)
Dawn to dusk:
Earth intercepts incoming solar shortwave energy
Much of it is reflected back into atmosphere ( μ m)
Some of it is absorbed and then reradiated back into the atmosphere as TIR longwave radiation (3-14 μ m)
Energy surplus: reflected shortwave + reradiated longwave
After sunset:
Incoming and outgoing shortwave radiation = 0
Outgoing longwave radiation exits terrain all night long

43. TIR Environmental Considerations
Diurnal Radiant Temperature of Various Materials
Good/bad times for acquiring imagery?!
Why are the curves the way they are?

44. TIR Environmental Considerations
Patterns?
ASTER TIR imagery of a sandbar in the Mississippi River
obtained at 5 AM and 10:30 AM on September 10, 1999 (2.5 × 2.5 m spatial resolution; Predawn: 5 AM; Daytime: 10:30 AM)

45. TIR Environmental Considerations
Pre-dawn: best unless trying to compute thermal inertia > no shadows from sun in daytime
4 pm: equilibrium temperatures > convective wind currents settle down (no wind smear, no wind streaks, more accurate flight lines)
Pre-Dawn TIR image of effluent entering the Savannah River Swamp System;
obtained at 4:28 AM on March 31, 1981

46. TIR RS Examples Etc. Etc. Water Pollution Monitoring
Leaking of Septic Tanks
Residential Insulation Surveys
Commercial / Industrial Roof Moisture Surveys
Urban Heat Island Effect
Etc.
Etc.

47. Urban Heat Island Effect
TIR RS Examples
Urban Heat Island Effect

48. Urban Heat Island Effect
TIR RS Examples
Urban Heat Island Effect
Atlanta, GA

49. Urban Heat Island Effect
TIR RS Examples
Urban Heat Island Effect
Atlanta, GA

50. Urban Heat Island Effect
TIR RS Examples
Urban Heat Island Effect

51. TIR RS Examples LCC, 1985-2009 × 1.7 1985 (13,585 ha) 1990 1995 2001
2005
2009 (22,816 ha)
Las Cruces, NM

52. TIR RS Examples
Change in Min LST (°K)
+ 193
- 148
Las Cruces, NM

53. TIR RS Examples
Change in Max LST (°K)
+ 194
- 223
Las Cruces, NM

54. TIR RS Examples
Change in Mean LST (°K)
+ 140
- 119
Las Cruces, NM

55. TIR RS Examples St Dev from Mean LST (°K) 2009 - 3 - 2 - 1 + 1 + 2 + 3
Las Cruces, NM