Thermal Remote Sensing

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.

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.

Introduction All objects with temperature > absolute zero (0° K; -273° C; -459.4° 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

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

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

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

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

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

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

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

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

“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

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

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.

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!

Thermal Radiation Laws Stefan-Boltzman Law Mλ = total emitted radiation from a blackbody (W m-2) σ = Stefan-Boltzmann constant (5.6697 × 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

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

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

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)

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

Thermal Radiation Laws Emissivity (ε)

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

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

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

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

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

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):

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

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!

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

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

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

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

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.

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

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

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

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: http://atmcorr.gsfc.nasa.gov/) 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)

External Source Referencing NAMCORR

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

TIR Environmental Considerations Diurnal Cycle … Energy 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 atmosphere (0.4-0.7 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

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

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)

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

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.

Urban Heat Island Effect TIR RS Examples Urban Heat Island Effect

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

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

Urban Heat Island Effect TIR RS Examples Urban Heat Island Effect

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

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

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

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

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