Image Processing Reference
In-Depth Information
Figure3.1 The long-wavelength region of the electromagnetic spectrum.
materials, enabling detection of explosives and drugs concealed on a person. In
both cases, the materials (clouds or cloth) appear opaque in the visible waveband
(or IR and UV) because the size of the “scatterers” (clouds or cotton fibers, in these
two examples) is similar to the wavelength of the light. Much shorter wavelength
visible light is scattered so many times as it traverses the material that the original
direction of the light wave (and thus any image) is lost. We can regain our ability to
image through these materials by going to increasingly longer wavelengths where
the waves we use average over the scatterers and so suffer only slight deflections
in their path. If the wavelength of the energy is sufficiently long, we can even use
it to image through materials as opaque as many meters of dry sand in the desert,
like Superman's famous “x-ray vision.” All the above examples are presented in
this chapter.
The sub-millimeter waveband is one of the newest wavebands to be used for
imaging. This region of the spectrum falls into a technology gap between
the photodetectors used in the LWIR waveband and antenna-type detectors for
millimeter-wave and microwave imaging. Detectors based on tiny thermometer
elements working in the submillimeter waveband have to be cooled to very low
temperatures and suffer from low sensitivity, while conventional thermal sources
and special lasers used for active illumination are low in brightness. Recent
technology developments in solid state device physics have now made it possible to
generate and detect extremely short pulses of light with wavelengths of a fraction
of a millimeter. Sub-millimeter imaging is also known as T-ray imaging, since the
frequency corresponding to lightwaves with sub-millimeter wavelengths is in the
terahertz range. 2
This imaging technology has interesting applications in packaging and materials
inspection, and it is in the early phases of commercialization for this purpose and
others. 3 Extremely short pulses of sub-millimeter light are transmitted through
a sample and detected on the other side by a tiny antenna. The detected signal
contains both density and composition information, since the chemical properties
2 1 Terahertz = 1 trillion cycles per second.
3 David Zimdars and J.V. Rudd, “Opening the terahertz window,”PhotonicsSpectra(May 2000).
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