Image Processing Reference
Figure3.22 Visible (left) and MIR (right) images of metal structures in concrete slab.
containing metal rods and plates. The concrete has yet to be poured in the visible
Light in the long-wavelength region of the electromagnetic spectrum has the
ability to penetrate through certain non-conductive materials that are very opaque
to visible, infrared and ultraviolet light. This penetrating power is most apparent at
the longest wavelengths, in the radio and microwave bands, and is a consequence
of its gentle interaction with matter—the photons impart very little energy to the
material they encounter. The shorter wavelengths in this region of the spectrum
(sub-mmW, mmW) are not as penetrating, but they can be used to generate images
with much higher resolution than radar images. Millimeter-wave imaging has
some of the penetrating power of radar combined with the advantage of much
higher frame rates, making real-time, long-wavelength synthetic vision available.
Sub-millimeter-wave imaging gives us the ability to see through thin, low-density
materials and, in some cases actually determine the chemical composition of the
material. Astronomers use microwave light to detect and image celestial material
too cold to emit at shorter wavelengths. The penetrating power of microwaves also
gives us the ability to see exotic celestial bodies invisible to conventional visible-
light imaging due to the obscuring effects of intervening matter.
The next chapter will consider extreme light at the other end of the spectrum—
lightwaves with wavelengths so short and frequencies so high that they interact
with matter more like particles, an interaction characterized by deep penetration of
matter combined with abrupt transfers of energy.