Image Processing Reference
In-Depth Information
imaging systems use special lenses and mirrors which reflect or refract acoustic
waves in order to focus them onto sensor arrays, just like the human eye or a
conventional photographic camera. Other systems include the acoustic equivalents
of radar systems discussed in Chapter 3. These acoustic radar systems transmit
pulses of sound toward a target in a beam pattern, detect the reflected pulses, and
process them into a visual image.
Like ultraviolet vision, acoustic imaging is an ability that humans do not
naturally have but that exists in the animal kingdom. Animals such as bats and
dolphins live in conditions where available visible-light levels are often so low as
to be insignificant, making acoustic imaging an alternative means of navigating the
environment and locating prey. 2 This particular ability, known as echolocation, is
an active imaging behavior involving specialized organs for the transmission and
reception of sound and neural processing within the animal's brain. Echolocating
animals emit short pulses of acoustic energy from specialized organs. These waves
propagate through air or water, strike objects, and are reflected back to the animal's
ears. The brain processes these perceived echoes into something like a picture of
the environment in ways that are not well understood. The “picture” that bats
perceive must be fairly detailed, since bats can fly around in a completely dark
room and avoid obstacles such as thin wires with uncanny ability. 3
UnderwaterAcousticImaging
Water is transparent to light only within a very narrow range of wavelength,
which (not coincidently) corresponds to our visual spectral range. But even within
that range, water is nowhere near as transparent as air. Even the purest natural
waters absorb light to such an extent that it is difficult to see an object more than
50 m away even under ideal conditions. Often water is much more turbid, cutting
visibility to zero or near zero. It can be almost impossible to see submerged objects
when the surface is choppy or even slightly disturbed by waves and wind. Yet the
need to see what lies beneath bodies of water is great, since many things get lost in
oceans, lakes, and rivers—things like ships, cars, planes, bodies, and cargo.Sonar
imaging offers a solution to this problem. It is the acoustic counterpart of radar,
standing for “sonic navigation and ranging.”
Sonar has its roots in the invention of the underwater microphone or hydrophone
during World War I. Multiple hydrophones lowered from ships enabled the
detection of submarines by their engine noise. This passive sonar evolved into
systems that rely on active illumination of an underwater target with short pulses
2 Recall that all known animal species that have eyes cannot see light with wavelengths that are longer
than about 750 nm. The photoreceptors in the eye are insensitive to longer wavelengths (infrared) so
the eyes of cave-dwelling animals cannot utilize thermal infrared energy that is naturally emitted by
materials at terrestrial temperatures.
3 For more information on bat echolocation, see Kunz, T.H. and P.A. Racey (Eds.),BatBiologyand
Conservation, Smithsonian Institution Press, Washington, D.C. (1998). For dolphin echolocation
information, see W.W.L. Au,TheSonarofDolphins, Springer-Verlag, New York (1993).
 
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