The Earth’s surface and atmosphere radiate thermal energy outward owing to heating by solar irradiation and by internal heat flow (generally a very minor contribution). Sensors that measure this emitted radiation in parts of the thermal region of the spectrum can produce very informative data (especially as images) that provide both distinctive signatures and ,indirectly, indications of properties of materials that are diagnostic. The atmospheric windows passing thermal radiation are indicated. The Planck Blackbody Radiation Law, the basis for thermal remote sensing, is presented.
Before you start this section, you might profit from a look at the brief vignette on thermal infrared imaging as produced by JPL. As always, access through the JPL Video Site, then the pathway Format-->Video -->Search to bring up the list that includes "Infrared: More than your Eyes can See", January 13, 2003. To start it, once found, click on the blue RealVideo link.
We have already shown several previews of imagery that depict the thermal state of the Earth's surface. The Landsat Thematic Mapper Band 6 produces images that show the relative differences in emitted thermal energy from surfaces of varying nature and orientation. On page 1-3, we showed this effect in the obviously warmer slopes in the Morro Bay scene and in Section 2, page 2-4 the surprisingly cool surfaces of the bright, white sandstone at the Waterpocket Fold. In seeming contradiction, the warmer surfaces associated with dark shales showed clearly in the Waterpocket Fold image. As we shall demonstrate, these thermal effects also differ, sometimes drastically, in images taken during the warmer days and cooler nights. This is clearly the case in this airborne thermal scanner image of a valley-mountain terrain in southern California, in which the same features may appear in sharply contrasting tones between the dawn and day scenes:
One usually obtains considerable information from thermal imagery, especially that generated as multispectral images. Some of this information supplements images obtained from reflected radiation and some stands alone as an information source. In this Section review, we delve rather extensively into the theory and practice of thermal remote sensing and examine some striking examples obtained from land and water targets. However, we postpone our discussion of using thermal sensors to obtain temperature profiles or find other thermal properties in the atmosphere until we consider meteorological satellites (Section 14). Remote sensing of direct temperature effects is carried out by sensing radiation emitted from matter in the thermal infrared region of the spectrum. Most thermal sensing of solids and liquids occurs in two atmospheric windows, where absorption is a minimum, as shown in this spectral plot taken from Sabins (Remote Sensing: Principles and Interpretation, 1987). Note that the segment of the spectrum labeled Reflected IR is considered to be non-thermal, although objects viewed in that range have discrete temperatures.
The windows normally used from aircraft platforms are in the 3-5 µm and 8-14 µm wavelength regions. Some spaceborne sensors commonly use transmission windows between 3 and 4 µm and between 10.5 - 12.5 µm. None of the windows transmits 100 percent because water vapor and carbon dioxide absorb some of the energy across the spectrum and ozone absorbs energy in the 10.5-12.5 µm interval. In addition, solar reflectance contaminates the 3-4 µm window to some degree during daylight hours, so we use it for Earth studies only when measurements are made at night.
Temperature is a measure of the motion of atoms and molecules in a substance. At absolute zero (-273 degrees Kelvin) all atomic/molecular motion has ceased. As a body is heated, its constituent particles begin to vibrate over very short distances (submicroscopic). This motion can be expressed in terms of velocity (speed); for gases, in particular, individual molecules move at various kinetic energies (K.E. = 1/2mv2). The Maxwell-Boltzmann Distribution function is a probability plot of the range of speeds. It can be applied to a specific gas for different temperatures (top plot) or to several gases of different composition at the same temperature:
Particle motions occur in gases, liquids, and solids which can be heated to various temperatures. For gases, temperature T plays a role as expressed by the Perfect Gas Law PV = nRT, where P is pressure, V is Volume, n is the number of moles of the gas in a specific sample (n = m/M in which m is the mass involved and M is its molecular weight), and R is the Universal Gas Constant (R = 8.31 Joules/mol/deg). Another relationship that directly involves the Kinetic Energy of a gas has two expressions, the second derived from the first: 1) T = 2/3(NmlnR)(1/2m0{v2}av), where Nm is the number of molecules in a given sample and ln refers to the natural log (of R); this equation states that the temperature of the gas (K) is proportional to the mean kinetic energy (summed from the range of velocities) of translational motion per molecule of the gas; 2) replacing (NmlnR) with k, the Boltzmann constant, one gets:
T = (2/3k)(1/2m0{v2}av); k = R/NA = 1.38 x 10-23(Joules/Kelvin)/molecule
(note: NA is Avogadro's Number given as 6.02 x 1023 molecules per mole of any gas).
Nothing specific about temperatures in solids and liquids will be discussed here but some information about this can be found at these Wikipedia and
UCAR sites.
The concept of a Perfect Blackbody (BB) relates to an ideal material that completely absorbs all incident radiation, converting it to internal energy that gives rise to a characteristic temperature profile. Therefore a BB does not participate in any transmittance or reflectance but emits (re-radiates) the absorbed energy at the maximum possible rate per unit area. The amount of this radiant energy varies with temperature and wavelength(s).
In 1900, Max Planck published a paper on the thermal properties of blackbodies that became one of the foundation stones from which quantum physics was built on. The Planck Radiation Law gives the rate at which Blackbody objects radiate thermal energy:
Depending on what terms (parameters) and unit systems are used, the Planck equation can be written in various ways. As often used in remote sensing calculations, this differential form is given as:
where Pλ = Eλ = spectral emission in W/m2/m at a wavelength λ.
