Microwave and Millimeter-Wave Imaging: Piercing the Veil

Abstract

The appearance of the world changes dramatically when we image using light wavelengths radically different from the wavelengths of visible light. Infrared and ultraviolet images of familiar objects such as people and butterflies are often different from their corresponding visible-light images (as we have seen in the last two chapters), but the objects or scenes are nearly always recognizable, and most materials that are opaque in visible light are also opaque in these wavebands. These similarities in the general appearance of objects in both visible and invisible light break down sharply at both ends of the electromagnetic spectrum. As we saw in the introduction, visible light falls roughly in the middle of the spectrum plotted in a logarithmic scale. For the purposes of this discussion, we can think of infrared, visible and ultraviolet wavebands of light as medium-wavelength wavebands, or medium light, to coin a term. Medium light is strongly emitted and absorbed by the atoms and molecules in matter, and thus is not able to penetrate through more than a few millimeters of solid material. Very short and very long wavelengths of light (which one could call extreme light) can sometimes penetrate much deeper into the surface of materials that are absorbent to medium light, and therefore the surface one actually ends up imaging is sometimes quite different from the surface that is apparent to visible-light imaging. A good example of this is x-ray imaging. We cannot see the bones in our hand with visible light—the skin is all we see. But x rays (light with very short wavelengths) pass through the skin with very little absorption, until they encounter the bones beneath. The reason for this is that x rays have such high energy that they tend not to interact with the outer electrons in atoms and molecules. Instead they transfer energy to the inner electrons, which they are much less likely to encounter. The outer electrons in atoms and molecules give matter its physical volume while the inner electrons are localized within a tiny region near the core of atoms. Extreme light at the other end of the spectrum (the long wavelength region) interacts with matter in a very different way from x rays, but with some of the same effect—some materials become transparent. Microwaves (light with very long wavelengths) can pass through many meters of dry sand or kilometers of water ice before they reflect off what lies buried beneath. The reason is that microwaves have such long wavelengths and their photons correspondingly lower energy that they tend to transfer energy to matter very gently, over much longer distance scales than is the case with medium light.