What lies beneath

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The house in which I live was built in 1926. Over the years, as we discovered recently when we had the walls of a couple of rooms repainted, several layers of wallpaper and paint have accumulated.

Peeling back those layers is a bit like going back through time (and reveals quite a bit about the decorating sense of the original owners). And really old buildings sometimes have much more exciting things to find beneath the current paint and plaster than old wallpaper.

Archaeologists in France examining a 12th-century church, for example, recently discovered an ancient mural beneath five layers of plaster.

Buildings aren’t the only places were interesting stuff may be hidden underneath layers of other stuff. Artists often sketch their subject onto their painting surface first, then paint over top of that sketch. Unless they also drew preliminary sketches that have survived, it’s hard to know how they turned sketches into finished paintings, or how the final product differs from their original conception, because the original sketch is lost beneath several coats of paint.

Nor is it only the artistic world in which peering beneath layers has its uses. Non-invasive ways of looking inside the body, peering beneath layers of skin and tissue, are obviously of immense value medically. And if your goal is making sure someone doesn’t get on an airplane with a weapon or explosives, having a way of peering beneath layers of clothing is a plus.

Researchers are now working on doing all of these things utilizing a portion of the electromagnetic spectrum that up until now hasn’t been put to much use: terahertz radiation.

T-rays, as they’ve been dubbed, fall on the electromagnetic spectrum between infrared and microwaves.

Even though T-rays are all around us in nature, they’ve been hard to put to use because they’re hard to generate, falling between the established capabilities of electronic devices and lasers. Bianca Jackson, a doctoral student in applied physics at the University of Michigan, calls tetrahertz radiation “quasi-optical. It is light, but it isn’t.”

Jackson is part of a team of engineering researchers who have successfully used T-rays to detect colored paints and a graphite drawing of a butterfly through four millimeters of plaster. They’ll be taking their equipment to France in March to study that aforementioned 12th-century church.

The T-rays pass easily through the plaster, but some reflect back when there’s a change in material. How thoroughly they’re reflected and how much energy they have when they return depends on the material they encounter. Different colours of paint reflect rays with different levels of energy. Graphite also produces a reflection at a specific energy level. By measuring these reflected waves, scientist can produce an image of what lies beneath the plaster, or the top layer of paint on a canvas.

Meanwhile, scientists at the U.S. Department of Energy’s Argonne National Laboratory, along with collaborators in Turkey and Japan, have created a compact T-ray generator they believe could lead to a better kind of security system. T-rays can’t penetrate metals or water, but they can penetrate leather, fabric, cardboard and paper. And the same characteristics that enable researchers to use T-rays to see sketches and murals through paint and plaster could allow security personnel to identify all kinds of hazardous substances that X-rays would miss because they zip right through them without stopping.

T-rays can also penetrate the human body by almost half a centimeter, and have already been used to detect and help treat skin and breast cancers. Dentists could use them to image teeth, instead of relying on X-rays.

Even better, they’re non-ionizing: that is, they’re not powerful enough to knock electrons off of atoms (which X-rays), which can cause damage to materials and tissue. (No more lead aprons when you’re getting your teeth examined!)

Remember that movie of a couple of years ago entitled “What Lies Beneath”?

Thanks to T-rays, in the near future you won’t have to guess: you can find out for certain.

Even if it’s just old wallpaper.

Permanent link to this article: https://edwardwillett.com/2008/02/what-lies-beneath-2/

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