Image Processing Reference
In-Depth Information
Chapter3
MicrowaveandMillimeter-Wave
Imaging:PiercingtheVeil
The appearance of the world changes dramatically when we image using light
wavelengths radically different from the wavelengths of visible light. Infrared
and ultraviolet images of familiar objects such as people and butterflies are often
different from their corresponding visible-light images (as seen in the last two
chapters), but the objects or scenes are nearly always recognizable, and most
materials that are opaque in visible light are also opaque in these wavebands. These
similarities in the general appearance of objects in both visible and invisible light
break down sharply at both ends of the electromagnetic spectrum. As seen in the
introduction, visible light falls roughly in the middle of the spectrum plotted in a
logarithmicscale—for the purposes of this discussion, think of infrared, visible
and ultraviolet wavebands of light as medium-wavelength wavebands, ormedium
light, to coin a term. Medium light is strongly emitted and absorbed by the atoms
and molecules in matter, and thus is not able to penetrate through more than a few
millimeters of solid material. Very short and very long wavelengths of light (which
one could callextremelight) tend to penetrate much deeper into the surface of
materials that are absorbent to medium light, and therefore the surface one actually
ends up imaging is often quite different from the surface that is apparent to visible-
light imaging.
A good example of this property is x-ray imaging. We cannot see the bones in
our hand with visible light—the skin is all we see. But x rays (light with very short
wavelengths) pass through the skin with very little absorption, until they encounter
the bones beneath. The reason for this is that x rays have such high energy that they
tend not to interact with the outer electrons in atoms and molecules—instead they
transfer energy to the inner electrons, which they are much less likely to encounter:
the outer electrons in atoms and molecules give matter its physical volume—the
inner electrons are localized within a tiny region near the core of atoms. Extreme
light at the other end of the spectrum (the long wavelength end) interacts with
matter in a very different way from x rays, but with some of the same effect—
materials become transparent. Microwaves (light with very long wavelengths) can
pass through many meters of dry sand or kilometers of water ice before they reflect
off what lies buried beneath. Microwaves have such long wavelengths and their
photons have such corresponding lower energy that they tend to transfer energy to
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