Image Processing Reference
Typically, denser materials will retain heat longer than less dense materials. Solar
loading can supply the heat to a scene—once the sun sets, differential rates of
cooling can lead to x-ray-like images. Figure 2.18 shows two views of a living room
on a winter's day. The MWIR image shows that the studs behind the sheet rock are
conducting the cold in from outside. The drywall screws are apparent as well—
they are “probing” colder interior volumes of the studs and “conducting” the cold
into the house. These volumes are closer to the exterior on the winter's day. The
ceiling is warm because of solar loading and the lack of decent insulation. Hidden
compartments or contraband will also retain heat, making it possible for law
enforcement to detect their presence with thermal imaging devices under certain
conditions. Police have used thermal imaging to detect excess heat generated by
lighting used for indoor marijuana cultivation. They would fly over a suspected
building and image it from the air. The infrared evidence was then used to obtain
search warrants for a physical search of the premises, though that is no longer
allowed, since it is considered unreasonable search and seizure.
The phenomenon of differential heating and cooling also makes it possible to
locate buried objects with thermal imaging. Figure 2.19 shows a LWIR image
of unmarked shallow gravesites that are undetectable to the eye. The horizontal
pseudocolor bar along the top edge of the image indicates IR intensity, with red
being the highest intensity, followed by white, light blue and dark blue. The graves
could be imaged by a thermal imaging camera because they contain voids (dead air
spaces) caused by body cavities (in the case of burial without a coffin) or because
the soil surrounding the coffins did not collapse as they deteriorated. The resulting
air spaces act as insulation, thereby changing the surface temperatures above the
voids as the flow of solar energy is blocked from entering the soil. 4 This same
process has been used to locate hidden contraband, terrorist explosives, landmines,
buried storage tanks, and other objects that create a void or space underground that
changes the way heat from solar loading flows.
Industrial processes often require quick and precise measurements of the
temperature of a surface at many different points without having to place a
thermometer at each point. A thermometer works by thermal conduction, where
thermal “vibrations” in a material are transmitted to a temperature probe. Heat
is also transmitted by radiation, and the surface temperature of an object can
be determined by observing the intensity of IR light radiated from the surface.
A MWIR thermal imaging system is excellent for temperature measurement in
the 0-200 degrees Celsius range. Temperature information can be displayed by
assigning a temperature-dependent color value to each point in the image. This type
of inspection is known as thermography. Some types of thermal cameras can be
calibrated to measure temperatures to accuracies of around one degree centigrade.
Thermosense XIV, Vol. 1682, SPIE (1992).