Thermal Infrared Remote Sensing

1. Thermal Infrared Remote Sensing

2. Introduction • All objects that have a temperature > absolute zero (0 K; -273.16 ºC; -459.69 ºC) emit thermal infrared EM radiation (3.0 – 14 mm)  Our eyes can’t see it but certain sensors can • Landscape components (soil, rock, vegetation, etc.) have predictable thermal characteristics • Reason: selective absorption of solar short-wavelength energy and radiation of thermal infrared energy • TIR images can be used to: • Determine types of materials • Evaluate changes in thermal characteristics of various phenomena (e.g., pollution, heat loss from buildings, etc.

3. Some Uses of Thermal Infrared RS Assessment of Building Efficiency … Heat Loss Medical Applications Moisture around Cold Vent A “HotDog” Breaker Box

4. 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 www.geocarto.com

5. 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 • 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

6. Think Thermally • 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

7. TIR Radiation Properties • All objects that have a temperature above absolute zero exhibit random motion • Kinetic Heat (measured in calories): • Energy of particles of molecular matter in random motion • Collision of these particles due to random motion causes their energy state to change and to emit EM radiation • Radiant Energy: • External or apparent energy • Results from conversion of internal kinetic heat (see above) • Facilitates use of RS technology for thermal assessments • Kinetic Temperature (Tkin; measured with thermometer): • Concentration of kinetic heat • Internal temperature of an object

8. TIR Radiation Properties • All objects that have a temperature above absolute zero exhibit random motion • Radiant Flux (Ф; measured in Watts): • EM radiation exiting an object • Radiant Temperature (Trad; measured with radiometer): • Concentration of the amount of radiant flux emitted from an object; radiating temperature of an object • Highly correlated with true kinetic temperature (Tkin) • Measure Tradwith radiometer from a distance to derive Tkin • BASIS OF THERMAL INFRARED RS • Problem:Trad always slightly higher than Tkin(due to emissitivity – see below)

9. Methods of Energy Transfer

10. Interactions of EM Energy Space Sensor Sun Longwave Shortwave Atmosphere Earth’s Surface

11. TIR Atmospheric Windows • Infrared region of EM spectrum quite broad (0.7-103mm) • Reflective IR (0.7-3mm) • Near-IR: 0.7-1.3 mm • Mid-IR: 1.3-3 mm • Thermal IR (3-14mm) • (includes part of Far-IR – 103mm) • Problem: • Atmosphere absorbs most of the IR energy present(e.g., O2, H2O, CO2, O3 absorb a lot) • Bands where this happens = absorption bands • Atmosphere passes only some of the IR energy present • Bands where this happens = atmospheric windows

12. TIR Atmospheric 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

13. Thermal Radiation Laws • Blackbody: • Theoretical construct • 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) • 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

14. Thermal Radiation Laws • Stefan-Boltzmann Law • Total spectral radiant exitance leaving a blackbody is proportional to the fourth power of its temperature • Mb – total spectral radiant exitance (Watts m-2) • T – absolute temperature (º Kelvin) • σ – Stefan-Boltzmann constant (5.6697 x 10-8 W m-2 K-4) • Example: • T = 6000  Mb = 7.3 x 10-8 Watts m-2 • T = 300  Mb = 4.6 x 10-2 Watts m-2

15. Thermal Radiation Laws • Wien’s Displacement Law • Describes the relationship between the true temperature of a blackbody and its peak spectral exitance or dominant wavelength • T – absolute temperature (º Kelvin) • lmax – dominant wavelength • k – constant (2898 mm  ºK) • Example: • T = 6000   lmax = 0.48 mm • T = 300   lmax = 9.66 mm

16. Thermal Radiation Laws • The higher the temperature, the higher the amount of radiant energy emitted • The higher the temperature, the shorter wavelength of the radiant energy peak • Why do we need to know the dominant wavelength? • Provides info regarding the part of the EM spectrum in which we want to sense the object • Forest fires at 800 K  lmax = 3.62 μm Optimum TIR detector: 3-5 μm • Soil, etc. at 300 K  lmax = 9.67 μm Optimum TIR detector: 8-14 μm

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

18. Thermal Radiation Laws • Emissivity (ε) • Graybody • ε ranges from 0 to ≤ 1 (depends on wavelengths considered) • Outputs a constant emissivity that is less than 1 at all wavelengths

19. 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 • 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

20. Thermal Radiation Laws • Emissivity of an object is influenced by: • Moisture content • The higher an object’s moisture content, the greater its ability to absorb energy and become a good emitter • Compaction • Affects emissivity • Field of view • Emissivity varies with spatial resolution of sensor • Wavelength • Affects emissivity • Viewing angle • Emissivity varies with viewing angle of sensor

21. Thermal Radiation Laws • Kirchoff’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? • Fil = incident radiant flux in specific wavelength (l) • rl = spectral hemispherical reflectance • al = spectral hemispherical absorptance • tl = spectral hemispherical transmittance • If el= al and tl = 0 (most real-world materials are opaque, i.e., little radiant flux exits from the other side, then

22. Thermal Radiation Laws • Kirchoff’s Radiation Law • Describes why objects appear as they do on TIR imagery • If reflectivity increases, then emissitivity must decrease. • If reflectivity decreases, then emissitivity must increase. • (Because real-world materials theoretically don’t lose any incident energy to transmittance) • Dark objects on TIR Objects that reflect most of the incident energy (e.g., metal roofs)  Cold objects • Light objects on TIR Objects that reflect little of the incident energy (e.g., water)  Warm objects

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

24. Thermal Radiation Laws • Goal of TIR RS: • 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 to the effects of emissivity) • Knowing emissivity of an object allows modification of the Stefan-Boltzmann law originally applicable to blackbodies (Mb = s x T4) so that it pertains to the total spectral radiant flux of real world materials (Mr):

25. 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 (See textbook, p. 252), then: • Trad = radiant temperature of an object recorded by TIR sensor • e can be estimated if an object’s Trad and Tkin are measured in the field:

26. Thermal Properties of Terrain • Thermal conductivity • Ability to conduct heat • Thermal capacity • Ability to store heat • Materials respond differently (more rapidly or slowly) to changes temperatures • These different thermal properties affect interpretation of TIR imagery

27. Thermal Properties of Terrain • Heat/Thermal capacity (c – cal g-1ºC-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 ºC

28. Thermal Properties of Terrain • Thermal conductivity (K – cal cm-1 sec -1ºC-1) • Rate at which a substance transfers heat through it • Number of calories that will pass through 1 cm3of material in 1 second when two opposite faces are maintained at 1 ºC difference in temperature • Varies with moisture content

29. Thermal Properties of Terrain • Thermal inertia (P – cal cm-2 sec -1/2ºC-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

30. 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 (DT) 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

31. Thermal Properties of Terrain • Thermal inertia: • High DT value is usually associated with terrain materials that have a low thermal inertia value • Low DT 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 • Etc. • Useful sensors (collection during the day and night): • ASTER, TIMS, ATLAS, etc.

32. Thermal Properties of Terrain • Use of TIR sensor remote for practical purposes (e.g., temperature mapping) requires calibration of brightness values (BVs) in image  Radiometric calibration: • Internal Source Referencing (using modeling techniques) • External Referencing (in situ) … requires: • Thermometer (measure true kinetic temperature) • Radiometer (measure radiant exiting temperature) • Radiosonde (obtain atmospheric profiles of temperature, barometric pressure, and water vapor) BVij – brightness value in uncalibrated image a – slope of linear equation b – intercept of linear equation

33. TIR Data Collection • TIR RS data may be collected by • Across-track thermal scanner • Daedalus DS-1260 and DS-1268 • Airborne Multispectral Scanner (AMS) • NASA TIMS • NASA ATLAS • Pushbroom linear- and area-array charge-coupled-devices • Forward-Looking Infrared (FLIR) systems • Thermal Airborne Broadband Imager (TABI) • Thermal Airborne Spectrographic Imager (TASI) • Geometric Correction ... more next time • Radiometric Calibration ... more next time

34. TIR Environmental Considerations • Diurnal Temperature Cycle of Typical Materials • Diurnal cycle – 24 hours • Sunrise: • Earth intercepts mainly solar shortwave energy (0.4-0.7 μm) • Dawn to dusk: • Earth intercepts incoming solar shortwave energy • Much of it is reflected back into the atmosphere (0.4-0.7 μm) • Some of it is absorbed and then reradiated back into the atmosphere as TIR longwave radiation (3-14 μm)  Lag of 2 to 4 hours after midday peak of incoming solar shortwave radiation (takes time to heat terrestrial materials) • Reflected shortwave plus reradiated longwave energy  Creates energy surplus during the day

35. TIR Environmental Considerations • Diurnal Temperature Cycle of Typical Materials • After sunset: • Incoming and outgoing shortwave radiation = 0 • Outgoing longwave radiation exiting terrain  Continues all night long … Diurnal cycle of reflected shortwave and emitted longwave energy Note the peak period of daily outgoing longwave radiation and the general daily maximum temperature.

36. TIR Environmental Considerations • Diurnal radiant temperatures of various surface materials Good/bad times for acquiring imagery?! Why are the curves the way they are?

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

38. TIR Environmental Considerations • Pre-Dawn TIR image of effluent entering the Savannah River Swamp System; obtained at 4:28 AM on March 31, 1981

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

40. ? Any questions ?