Image Processing Reference
In-Depth Information
Light becomes something quite strange and powerful in the region of the
electromagnetic spectrum in which wavelengths are shorter than in the near-
UV ultraviolet waveband. This region, shown in Fig. 4.1, includes theextreme
ultraviolet, x-ray and gamma-ray wavebands. X rays and gamma rays are
electromagnetic waves with such short wavelengths (and correspondingly high
energies) that they interact with matter very differently than do the longer
wavelengths discussed previously. For the purposes of the following discussion I
shall describe x rays and gamma rays in the context ofphotons, which are particles
of light. Light has a dual nature: it can behave as a wave sometimes and a particle at
other times. When it is a particle, or photon, it is best described by its energy; that
is more appropriate than describing this light as a wave of a particular wavelength.
This method of description reflects the way these high-energy photons interact
with matter: when an x ray or a gamma ray does interact with an electron or a
nucleus in material, there is a significant amount of energy transferred to a very
localized area, like a bullet hitting a metal target. That is why x rays and gamma
rays are considered radiation and are dangerous to living beingsā€”the energy
transfer often produces permanent chemical changes, which can lead to cellular
mutation in living tissue. At the same time, when x rays and gamma rays traverse
matter, the probability that they will interact with the matter is fairly low, and if
the material is thin or has a lowatomicnumber, then a significant fraction of the
light can pass through with little or no loss. The thicker or denser the material, the
more reduction in the intensity of a transmitted beam of x rays or gamma rays. This
is the principle behindradiography: the film or detector receives more radiation
along a path of lower density or thickness through the material being imaged, and
less along a thicker or denser path. Thus, the bones of the hand appear darker in a
radiograph than the surrounding flesh, since the photographic film is less exposed.
X rays and gamma rays were not initially recognized to be electromagnetic
waves at all. Their wave-like properties were hard to detect because their
wavelength (10 nm and smaller) is very short, and it was not until some years after
the discovery of x rays that it was conclusively shown that they are electromagnetic
waves, producingdiffractionpatternson a screen after passing through a narrow
metal slit or a crystal.
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