Image Processing Reference
In-Depth Information
matter very gently, over much longer distance scales than is the case with medium
light. Examples of these phenomena appear in this chapter, which explores the
long-wavelength end of spectrum, and in the next chapter, which explores the short-
wavelength end of the spectrum.
The long wavelength end of the spectrum is, for the purposes of this discussion,
light with wavelengths on the long side of the extreme infrared band. This region of
the spectrum covers wavelengths from about100 mto 1 mm (thesub-millimeter,
orsub-mmWband), wavelengths around several millimeters (millimeter-wave,
ormmWband) and the 10 mm to 1 m region (themicrowaveband). Figure 3.1
illustrates this part of the spectrum. Beyond microwaves lies theradiowaveband,
which most people associate with wireless transmission of information rather than
imaging technology. The farthest this visual exploration will venture in the long
wavelength direction of the spectrum is a wavelength of73 cm, which lies in the
microwave band. At wavelengths longer than about 1 m, imaging becomes prob-
lematic. We can certainlydetectradio waves with wavelengths longer than a meter
with a simple antenna, but making a highly directional sensor (like a dish antenna)
that can generate what we consider to be images (distinguishing objects and repre-
senting their shape) is quite difficult to do. 1
The images we will examine are generated by both passive and active imaging
systems. Millimeter-wave energy is emitted naturally from objects at terrestrial
temperatures in much the same way as long-wave infrared energy, though at a much
lower intensity. This natural emission rapidly becomes weaker at increasingly
longer wavelengths; as a result, microwave energy emissions from terrestrial
objects cannot easily be used for imaging, since any signal tends to get buried in
the inherent noise of the imaging sensor. Microwave imaging of terrestrial objects
requires active illumination: the technique ofradar(radio detection and ranging).
The discovery of the long-wavelength region of the electromagnetic spectrum
happened about the same time as the discovery of x rays. Heinrich Hertz was
a professor of physics in Germany in the late19 th century. He is credited with
discovering radio waves with a simple experiment involving a spark gap to generate
radio waves and a loop antenna to detect them. Hertz correctly surmised that the
waves were light waves with very long wavelengths (on the order of a meter),
having observed many of the wave-like properties of visible light in these waves,
including reflection and refraction. The Hertz is a unit of frequency (cycles per
second) named in his honor.
The sub-millimeter, millimeter-wave and microwave bands are remarkably
useful for scientific and industrial imaging applications, since they can be used
to see through material that is opaque to more conventional imaging wavebands
(visible, IR, and UV). For instance, microwave imaging has been used to see
through the permanent and visibly-opaque cloud cover on the planet Venus.
Millimeter-wave imaging technology can see through clothing and other thin
1 This is due to the wave-like properties of light;diffractioneffects drive the requirement that a highly
directional sensor be many wavelengths in diameter. There are ways to synthesize a large antenna out
of an array of smaller, interconnected antennae.
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