Thermal IR February 23, 2005

1. Thermal Properties Thermal IR Atmospheric Windows Thermal IR Environmental Considerations Thermal Radiation Laws Emissivity Reminder: Read rest of Chapter 8 for next class Midterm Exam on Monday. Review sheet is posted! Thermal IR Thermal IRFebruary 23, 2005

2. Selected Applications of Thermal Infrared Remote Sensing

3. Nighttime Thermal Infrared Imagery of an Airport

4. Kinetic Temperature (Tkin) – true kinetic temperature Radiant Temperature (Trad) – temperature calculated from radiant exitance (radiant flux) Usually a pretty darn good correlation, but not always!! Depends on the thermal emissivity of an object (and discussed later in this chapter). Thermal Properties Thermal Properties

5. Atmospheric Windows in the Electromagnetic Spectrum Where are the Thermal IR atmospheric windows?

6. Thermal IR region of the EM Spectrum is from 3 to 14 µm Three primary windows: 3 - 5 µm 8 - 9.2 µm 10.5 – 12.5 µm Thermal IR Atmospheric Windows Thermal IR Atmospheric Windows

7. Peak Period of Daily Outgoing Longwave Radiation and the Diurnal Radiant Temperature of Soils and Rocks, Vegetation, Water, Moist Soil and Metal Objects When is/are the best time(s) of day to acquire thermal imagery? Why? When is/are the worst time(s) of day to acquire thermal imagery? Why?

8. Thermal Infrared Radiation Principles • An analyst cannot interpret a thermal infrared image as if it were an aerial photograph or a normal image produced by a multispectral scanner or charge-coupled device. • Rather, the image analyst must thinkthermally. • The analyst must understand how energy from the Sun or from the Earth interacts with the various terrain components and how the detectors function as they record the terrain’s emitted thermal infrared electromagnetic radiation.

9. Pre-dawn Thermal Infrared Image of Effluent Entering the Savannah River Swamp System Savannah River Savannah River 2x reduction March 31, 1981 4:28 am; 3 x 3 m

10. Pre-dawn Thermal Infrared Image of a Residential Subdivision in Forth Worth, Texas 250 m AGL 1 mrad IFOV 6:45 am Jan 10, 1980 0.25 x 0.25 m

11. Daytime Optical and Nighttime Thermal Infrared Imagery of the University of South Carolina Campus April 26, 1981 4:56 am 1 x 1 m 2x reduction

12. Blackbody – Theoretical construct that absorbs and radiates energy at the maximum possible. Wien’s Displacement Law – Dominant wavelength is inversely proportional to temperature. Thermal Radiation Laws Thermal Radiation Laws

13. Blackbody Radiation Curves for Several Objects including the Sun and Earth

14. Wein’s Displacement Law For example, the average temperature of the Earth is 300 K (80 ˚F). We compute the Earth’s dominant wavelength as: max = 2898 m K T max= 2898 m K= 9.67 m 300 K

15. Wein’s Displacement Law • The dominant wavelength provides valuable information about which part of the thermal spectrum we might want to sense in. For example, if we are looking for 800 K forest fires that have a dominant wavelength of approximately 3.62 m then the most appropriate remote sensing system might be a 3-5 m thermal infrared detector. • If we are interested insoil, water, and rock with ambient temperatures on the earth’s surface of 300 K and a dominant wavelength of 9.66 m, then a thermal infrared detector operating in the 8 - 14 m region might be most appropriate.

16. Emissivity • The world is not composed of radiating blackbodies. Rather it is composed of selectively radiating bodies such as rocks, soil, and water that emit only a fraction of the energy emitted from a blackbody at the same temperature. Emissivity, , is the ratio between the radiant flux exiting a real-world selective radiating body (Fr) and a blackbody at the same temperature (Fb): Fr  = ______ Fb

17. Emissivity • All selectively radiating bodies have emissivities ranging from 0 to <1 that fluctuate depending upon the wavelengths of energy being considered. A graybody outputs a constant emissivity that is less than one at all wavelengths. • Some materials like distilled water have emissivities close to one (0.99) over the wavelength interval from 8 - 14 µm. Others such as polished aluminum (0.08) and stainless steel (0.16) have very low emissivities.

18. No objects in the world are true blackbodies; rather, they are selectively radiating bodies. Emissivity (є) is the ratio between the radiant flux exiting a real world selective radiating body (Mr) and a blackbody at the same temperature (Mb). A graybody outputs a constant emissivity that is less than one at all wavelengths. Emissivity Emissivity

19. Spectral emissivity of a blackbody, a graybody, and a hypothetical selective radiator Spectral Emissivity, e Spectral radiant exitance distribution of the blackbody, graybody, and hypothetical selective radiator Spectral Radiant Exitance W m-2 um-1 2x reduction

20. What is the difference between thermal capacity, thermal conductivity, and thermal inertia?

21. Thermal Properties of Terrain • Thermal capacity(c) is the ability of a material to store heat. It is measured as the number of calories required to raise a gram of material (e.g., water) 1 ˚C (cal g-1 ˚C-1). • Thermal conductivity(K) is the rate that heat will pass through a material and is measured as the number of calories that will pass through a 1-cm cube of material in 1 second when two opposite faces are maintained at 1 ˚C difference in temperature (cal cm-1 sec-1 ˚C).

22. Thermal Inertia •Thermal inertia(P) is a measurement of the thermal response of a material to temperature changes and is measured in calories per square centimeter per second square root per degree Celsius (cal cm-2 sec -1/2 ˚C-1). Thermal inertia is computed using the equation: P = (K x p x c)1/2 where K is thermal conductivity, p is density (g cm-3), and c is thermal capacity. Density is the most important property in this equation because thermal inertia generally increases linearly with increasing material density.

23. Thermal Infrared Remote Sensing There is an inverse relationship between having high spatial resolution and high radiometric resolution when collecting thermal infrared data.

24. Forward Looking Infrared (FLIR) Examples