Image Processing Reference
In-Depth Information
Figure4.1 The short-wavelength region of the electromagnetic spectrum.
X rays and gamma rays are best known for their ability to enable imaging
through opaque material, since they are not absorbed by matter nearly as much as is
light with longer wavelength. While microwave light can be used to image through
certain materials such as dry dirt or sand quite well, it cannot penetrate effectively
through conductive materials such as water or metal. X rays and gamma rays,
however, can penetrate all known materials. While visible light may only penetrate
a few millimeters deep into human tissue, x rays at modest energies will easily
pass all the way through a person's chest, and a person could easily look nearly
transparent in a gamma-ray image - bones and all. Electromagnetic waves at these
high energies are generally not encountered in daily life except when generated by
medical or scientific instrumentation, or (in the case of gamma rays) by radioactive
isotopessuch asCobalt-60. Luckily for us, the atmosphere blocks nearly all x-ray
and gamma-ray light impinging on Earth from extraterrestrial sources. 1
We can imagine what it would be like for a hypothetical creature that could see
only in the x-ray or gamma-ray wavebands to be on Earth. Such a creature would
have a very hard time passively imaging anything in our atmosphere at sea level,
since there would be virtually no available x-ray or gamma-ray illumination from
the sun or other celestial sources, and none at all from ordinary (non-radioactive)
objects. This situation is quite different from what a creature with thermal infrared
vision (like the Predator in the movie of the same name) would encounter, for
example. People, animals, trees and machines all emit thermal infrared light and
can be observed without any active illumination being present. Observing people
or trees or other objects using light in the x-ray and gamma-ray bands would
require active illumination of a very specialized nature. Ordinary light sources
such as lightbulbs emit invisible light (infrared and ultraviolet), but they do not
emit x rays or gamma rays because the filaments in lightbulbs are simply not
hot enough. A lightbulb filament would have to be heated to millions of degrees
to glow incandescently with x-ray light. Making incandescent x rays with even
modest energies in a terrestrial laboratory is very hard to do. 2 It takes extreme
conditions such as those encountered in a nuclear explosion or nuclear fusion
reactor to achieve these temperatures.
1 X-ray and gamma-ray astronomy requires detectors mounted on high-altitude platforms such as
rockets or satellites.
2 Incandescent means that the material producing the x rays is doing so by heating alone; this requires
enormously high temperatures ( > 1 million degrees centigrade).
 
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