Image Processing Reference
Figure2.23 Propane gas leak imaged with a special 3.3 micrometer midwave-IR camera.
Test Range Radiometry
Because thermal infrared camera systems are used extensively on the battlefield,
it is crucial to characterize the appearance of military targets in the thermal IR
band. This is known as signature measurement, and it is accomplished using
special calibrated IR camera systems that perform radiometric measurements.
The signature of an aircraft, for example, will consist of measurements of the
shape of the exhaust plume, its radiance, and sometimes its spectral signature.
Figure 2.24 shows a Harrier Jump Jet hovering in mid-air at Nellis Air Force
Base in Nevada. The exhaust plume is barely visible in the first photo, but is very
apparent in the MWIR image because the plume is loaded with hot water vapor
and hot carbon dioxide. I happened to take a NIR image of the plane at the same
time, and it shows the plume slightly more than the visible-light image, probably
because the water vapor in the plume absorbs NIR light. The water vapor is hot
enough to emit MWIR light, but cold enough to act as an absorber to NIR light.
Helicopters show various sources of heat in thermal infrared images, but by far
the brightest part of the Chinook helicopter in the midwave IR band is the exhaust
system in the back, as seen in the MWIR image in Fig. 2.25. The exhaust manifolds
are not quite hot enough to glow with visible light; in fact, the IR images were taken
in almost complete darkness. A reference image of another Chinook helicopter is
included to illustrate the exhaust system.
Sometimes a scientist or engineer wants to measure the temperature of something
that can't be touched or otherwise measured with a temperature probe. Bullets
and cannon projectiles heat up as they move down a gun barrel due to friction.
The frictional heating leads to barrel wear which is useful to characterize. Sniper
detection systems will track bullets in flight by their thermal infrared signature.
Figure 2.26 shows two ultrahigh speed pictures of a 30 caliber bullet in flight, about