Image Processing Reference
Figure5.10 (a) ANI target and (b) an acoustic noise image of the target. (Courtesyof
Acoustic imaging and light-wave imaging are complementary technologies.
When one fails to work, the other often succeeds, since many materials that are
opaque to light waves are transparent to sound waves, and vice-versa. Acoustic
imaging can only work in situations where sound waves can travel from the target
to the detector. It is thus not very useful in air, where sound is highly attenuated,
and does not work at all through the vacuum of space, unlike light waves, which
need no dense medium in which to propagate. Yet it allows us to see through
materials opaque to light. We have seen images made through water and living
tissue, for example, but acoustic imaging can see through denser materials like
metal and rock. The Earth's internal structure has been mapped with seismic waves
generated by earthquakes—an interesting example of acoustic noise imaging on a
grand scale. Just as light-wave imaging allows us to look out into the universe,
away from Earth, so acoustic imaging allows us to look inward: into our own
bodies, into the depths of the ocean and the core of the planet.
The next chapter examines a single object imaged with light from a wide range
of the electromagnetic spectrum. Each image shows a different aspect of the object,
aspects that are not apparent with visible light alone.