Image Processing Reference
Figure3.4 Visible (top) and T-ray (bottom) images of a milk chocolate bar with almonds.
The region of the spectrum known as the millimeter-wave (abbreviated mmW) is
located in the wavelength range between 1 and 10 mm. Millimeter-wave images
are radically different from what our eye sees, since one millimeter is about
2,000 times the wavelength of visible light. Millimeter-wave imaging is only just
beginning to be used for practical purposes, as the technology that makes it possible
to synthesize vision with reasonable frame rates in the mmW band is a fairly recent
development. Early imaging work in this waveband during the 1960s and 1970s
utilized a single detector scanned over a scene with a moving mirror. This scanning
technique could take many minutes to generate an image, making it suitable for
static scenes only.
Applications such as covert imaging for security purposes or aeronautical
navigation require a fast frame rate (20-30 frames per second). For example,
airports, courthouses and other areas have devices to control the entry of
contraband materials, but there are limitations to their effectiveness. Metal
detectors are employed to scan people, but this does not prevent the introduction of
nonmetallic contraband (drugs, explosives, etc.) carried in under clothing. It is not
always practical or desirable to pat down everyone that comes through a security
checkpoint, and there are health and safety concerns associated with x-ray imaging
of people, although x rays of luggage are fairly useful in finding contraband. 4 A
waveband of electromagnetic energy is needed that penetrates clothing to image
what is underneath without any health risk. Ideally, the person being scanned would
emit these lightwaves naturally, eliminating the need to illuminate the subject.
4 I say fairly, since it can be difficult to interpret luggage x-ray images, and there is a large probability
for human error.