Image Processing Reference
In-Depth Information
Chapter5
AcousticImaging:Seeingwith
Sound
By imaging with the appropriate wavelength of electromagnetic energy, materials
considered completely opaque become transparent, revealing hidden wonders.
Objects the human eye perceives as dark look light, and light objects look dark;
indeed the whole notion of dark and light becomes very subjective, as it depends on
the wavelength response of the imaging technology used and not what the human
eye sees. However, imaging with light waves is not always possible or desirable
because of basic imaging limitations imposed by the optical properties of matter
between the object and the imaging system. As we turn the imaginary knob on our
head and sweep the response of our eyes through the electromagnetic spectrum,
we may never find a waveband of light capable of imaging through a particular
obstacle. For instance, we cannot see through even the purest ocean water for
more than about 100 m at any wavelength of light, yet the ocean floor is full
of interesting things worth imaging. 1 Imaging with light is not always desirable
because sometimes the light we need to see through intervening matter may be
harmful. Imaging of a fetus can be done with x rays—in fact doctors used to x ray
pregnant mothers as a means of diagnosing prenatal conditions—but this practice is
now discouraged due to the potentially harmful effects of x rays on the developing
child.
Acoustic or sonic imaging is an alternate way to see through intervening matter.
This is literally “seeing with sound,” made possible because sound waves are very
similar to electromagnetic waves in their properties, yet they travel freely through
fresh or salt water, tissue, and a variety of other materials that are opaque at many
or all wavelengths of electromagnetic energy. Acoustic pictures are different from
pictures made from light in that they are essentially maps of density variations.
Sound waves will reflect from boundaries between different materials and carry
back information about changes in density and object shape to a sensor. There
are several different ways to image with acoustic waves, and these methods are
all analogous to techniques used to image with light. For instance, some acoustic
1 Very long wavelength radio waves can penetrate through ocean water, but building an imaging
system that would operate in the kilometer-scale waveband is extremely impractical and would have
very limited resolution, i.e., the image would be blurry.
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