Introduction to Thermal Infrared Remote Sensing

Thermal infrared radiation refers to electromagnetic waves with a wavelength of between 3.5 and 20 micrometers. Most remote sensing applications make use of the 8 to 13 micrometer range. The main difference between thermal infrared and the infrared (color infrared - CIR) discussed is that thermal infrared is emitted energy that is sensed digitally, whereas the near infrared "photographic infrared" is reflected energy that causes a chemical reaction in film emulsion.

Thermal Infrared ranges from 5.6 um to 1cm
- water and other gases in the atmosphere restricts aerial systems to two wavelength windows; 3 to 5 um and 8 to 15 um (absorption bands)

All photos can be called images but not all images should be called photos - images that are acquired digitally are usually not referred to as photographs ... there is no such thing as a thermal photograph!

Thermal IR imagery is difficult to interpret and process because there is absorbtion by moisture in the atmosphere.  Block out a small range of wavelengths along the bottom of a graph of atmospheric transmission in the thermal wavelengths, then look at the total area under that section.  Doing this under one of the peaks in the percent reflectance lines gives you way more area under that line in some short wavelength ranges than in others.  Some surfaces will emit predictably but have "peaks" in certain narrow wavelength ranges due to material and/or structure differences and, most importantly, the water content, this makes for confusing results sometimes unless you know for certain what covers the area you are looking at or have very precise control of the wavelengths sensed by the instrument (which makes in expensive).  Being that and Thermal IR image is digital, using "false color" really helps interpret them because you can select certain temperature ranges and classify them with a color while leaving the rest gray.

Camoflage Detection

Thermal Map - Click to Enlarge

Houston Galvinston Shipyard Boston Harbor 1969Sheldon Michigan

Limitations of thermal imagery:

It can be very expensive to acquire and process
Most thermal imaging systems have strict operational/technical parameters.
- detector materials must be kept extremely cold during use (because the emitted radiation being sensed is very weak)
Thermal infrared imaging systems are notoriously difficult to calibrate
- because temp differences can be very subtle and interactions with atmospheric moisture are unpredictable
The data collected is computationally expensive due to the iterative nature of filtering software
Thermal images can be difficult to interpret compared with other types of imagery, it takes some getting used too (false color helps)
Thermal images of water measures only the very top layer of the water surface
- because those wavelengths are attenuated/absorbed very rapidly, especially in water

Thermal IR - Geologic Mapping, Click to Enlarge
Aerial Photography (Top) and Thermal Imagery (Bottom), the faulting and subsurface hydrology are apparent due to differences in thermal inertia caused by water content (wet dirt cools at a different rate than dry dirt)
Thermal IR Application - Fire Mapping, Click to Enlarge

AVIRIS imagery show different wavelengths, notice the smoke obscures the fire's location in visible wavelengths while the thermal wavelengths are unaffected by the smoke. Thermal imaging can be used to locate hot spots in wildfire areas where smoke reduces visibility.

Why are the runways wavy? - Click to Enlarge
Thermal IR Image of San Francisco, notice the wavy runways caused by heated air above the surface.
Day vs. Night Thermal IR - Click to Enlarge
Day vs. Night Thermal IR Images
which is which? why?
- compare the roofs and the grass in the park.
Kilauea Volcano Panchromatic Photo - Click to Enlarge
Kilauea Volcano Panchromatic Photo
Kilauea Volcano Thermal IR Image - Click to Enlarge
Kilauea Volcano Thermal Infrared Image
Thermal IR Image of Houston Galviston Ship Channel - Click to Enlarge
Nighttime Thermal Image of the Houston Galviston Ship Channel.
Atmospheric Moisture and Temp Gradients - Click to Enlarge
Thermal IR imaging systems have big problems with atmospheric moisture and temperature differences. 
Thermal Images of Aircraft Carriers
Notice that there are two sets of apparent jet heat trails, these are called "ghosts". They are errors and result from lense reflection or instrument calibration error which introduce artifacts.

~0 Kelvin

This is an excellent website with a lot of information that is relatively easy to read and understand.