Image Processing Reference
than the background around them can be detected with thermal imaging, provided
they do not have shiny surfaces. Shiny surfaces such as polished aluminum emit
much less IR light than the surfaces of organic materials (wood, skin, cloth) at the
same temperature. Molecules emit and absorb infrared light readily-metals simply
reflect it. Thermal imaging has found its way into the popular culture, especially
into science fiction movies such as Predator and Hollow Man. The alien creature
in Predator hunts a band of mercenaries led by Arnold Schwarzenegger through
the jungles of Central America. This predatory and grotesque alien uses thermal
imaging technology to see people and animals in the dense undergrowth. In the
sequel, the Predator appears to use UV imaging as well, circumventing the thermal
IR camouflage the human hunters employ (they apparently saw the first movie).
The IR world looks different than the visible world in bright sunlight.
Conventional visible-light photographs taken in bright sunlight can be washed out
by scattered visible light, which can reduce the sharpness of shadows. The sky
looks blue because shorter wavelength blue light is scattered by the atmosphere
more strongly than red light, and thus the blue component of sunlight lights up the
air with a blue glow. This phenomenon is called Rayleigh scattering and it is caused
by tiny density fluctuations in the air at the nanoscale level. Rayleigh scattering is
a very strong function of wavelength, and it affects the propagation of infrared and
ultraviolet light too. In the preceding chapter, we saw how the daytime sky appears
bright white when imaged with a near-UV camera—this is scattered UV sunlight.
Infrared light scatters even less than red light, and thus the sky appears nearly black
when viewed in the IR wavebands. Near-IR images of a sunlit scene are reminiscent
of visible-light photographs taken while on the surface of the moon: a black sky
and harsh shadows. In both cases, scattering of light is absent: in one case because
the long wavelength of IR light means little Rayleigh scattering, in the lunar case,
because of the lack of an atmosphere. In a thermal image, the clear sky will appear
black, but objects and scenery tend not to cast shadows. This is because every solid
object in the scene emits thermal IR light.
This effect is very apparent in the following example. Figure 2.3 shows an IR
image of one of the great pyramids and the Sphinx on a sunny day. Note the dark
sky, which appears “cold” to the IR camera that imaged this scene in the 3-5
mwaveband (MWIR). The infrared intensities in the image are represented by
pseudocolor, with white being the most intense and dark blue the least intense.
The strongest infrared intensities represent the hottest temperatures within the
scene. This particular pseudocolor scheme (called fusion in the IR camera industry)
makes white the hottest pixels of the image, then yellow, then red, magenta,
and finally blue as the coldest pixels. The addition of pseudocolor is a common
technique used in thermal imaging to enhance slight temperature differences within
a scene, as the eye is much more attuned to slight changes of color than to slight
changes in grey level.
In this image, the sun is nearly overhead, and its rays strongly heat upward-
facing surfaces such as the top of the pyramid (where the original facing is intact),
the top of the Sphinx's head, and the ground around them—as shown by the white
and red pseudocolor. This heating is known as “solar loading,” an effect that can