Image Processing Reference
In-Depth Information
Electromagnetic energy in the 3-mm waveband fulfills this requirement. It is
naturally emitted by skin in sufficient amounts to produce a passive image (like
thermal infrared), and it freely penetrates thick clothing (unlike thermal infrared). 5
Cloth is composed of molecules that absorb strongly in the visible, ultraviolet and
infrared wavebands, and most cloth is thick enough to absorb all or most of the light
in these wavebands. If we want to see through clothing, we need to use a waveband
like mmW that can image through clothing with enough spatial resolution to image
some small concealed object like a plastic bag full of cocaine. Figure 3.5 shows a
schematic diagram of this phenomenon. The contraband becomes a dark blob on
the skin, because it emits less millimeter-wave light than skin. X-ray imaging can
certainly see through clothing (and denser materials), but it requires a source of
x-ray light to illuminate the target.
Figure 3.6 shows two views of a man. The left image, made with visible light,
shows a man who does not have the appearance of being armed, but the mmW
image on the right reveals that he is concealing two handguns under his sweater.
This striking image is the weak glow from his body in the 3-mm waveband. The
high degree of contrast in the mmW image is due to the optical properties of
the dissimilar materials present. The person appears much brighter than the guns
because of differences inemissivity. Human skin emits more mmW energy than
metal or plastic at the same temperature, so the guns appear darker in the image
than the skin, and stand out in sharp relief in the mmW image. Clothing is thin and
passes the skin's emission of mmW light easily, but the thick metal of the guns
completely blocks the emissions, creating a shadow. Plastic explosives and drug
packets have similarly lower emissivities relative to human skin, and are usually
thick enough to block mmW light from the skin, making them easy to detect as
well.
Millimeter-wave imaging is also known in military circles as all-weather
imaging, since it has the ability to produce picture-like images in conditions that
defeat other imaging systems. Radar imaging, which will be discussed shortly, can
“see” through weather and atmospheric conditions, but the imaging technology
does not easily allow for high-resolution imaging at high frame rates, which is
what one needs to land a plane in heavy fog.
Many plane crashes have resulted from poor visibility due to inclement weather
conditions during landing. A pilot running low on fuel has little choice but to
attempt a landing in fog or drizzle; these conditions are very difficult to see through
with visible or IR sensors. Visible light has a wavelength around 500 nm, or 0.0005
mm, which is comparable to the size of water droplets in fog. When light waves
pass through a medium with scattering particles that are comparable in size to
the wavelength of the light waves, there is strong scattering, which is why fog
appears as a uniformly white substance to our eyes. In the case of mmW imaging
systems, the wavelength of light detected is several millimeters. There is very little
5 The 3-mm waveband is widely used because it corresponds to a “window” in the atmosphere, with
little absorption by water compared to wavelengths on either side of the 3-mm waveband. This region
of the spectrum is also known as the W band.
 
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