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The Light Guide Hiding In Your Extrusion

There should be a line of jokes that start “A physicist and an engineer walk into a bar…”. In my case I’m an engineer and my housemate is a physicist, so random conversations sometimes take interesting turns. Take the other day for example, as one does when talking she picked up a piece of aluminium extrusion that was sitting on our coffee table and turned it over in her hands. It has a hole down its centre and it’s natural to peer down it, at which point her attention was caught by the appearance of a series of concentric rings of light. Our conversation turned to the mechanism which might be causing this, and along the way took us into cameras, waveguides, and optical fibres.

The light reaching us after traveling along a straight narrow tube should at a cursory glance be traveling in a straight line, and indeed when I point the extrusion out of my window and look down it I can see a small segment of the tree in the distance I’ve pointed it at. It didn’t take us long to conclude that the concentric rings were successive reflections of the light coming into the end hole from off-centre angles.

In effect, the extrusion is a pinhole camera in which the image is projected onto the inside of a cylinder stretching away from the pinhole rather than onto a flat piece of film, and we were seeing the successive reflections of the resulting distorted image as they bounced to and fro down the tube towards us. It’s likely the imperfect mirror formed by the aluminium wall allowed us to see each image, as light was being diffused in our direction. Adding a piece of tape with a small pinhole at the end accentuated this effect, with the circles becoming much more sharply defined as the projected image became less blurry.

The Annals of the Bleeding Obvious

Total internal reflection of a HeNe laser beam in a piece of acetate.
Total internal reflection of a HeNe laser beam in a piece of acetate. Sai2020 (Public domain).

So great, we’ve invented the pinhole camera, where’s our Nobel Prize? Our place in the esteemed scientific journal Annals of the Bleeding Obvious is evidently assured. But on the other hand, we’ve explored the mechanism used by light pipes and fibre optic communication, namely the passage of light along a waveguide through successive reflections from its internal walls.

It’s not much use with a piece of extrusion because it’s bulky and not flexible, but when the medium is switched to a piece of laser-cut acrylic the same technique can be used to great effect. Edge-lit acrylic displays are hardly a new idea, but in the age of the LED and perhaps more pertinently the expensive Nixie tube, they appear to have made something of a comeback.

Of more interest is the idea that each of those concentric rings is a successive reflection of the image projected by the pinhole. In our idle discussion around the coffee table we surmised that with some image processing or suitable lenses it could be mapped from a cylindrical reflection to a flat one in the manner of a photograph we’re all used to. Could such an image travel round curves in the light pipe and be used for remote sensing? That scientific journal just called again, we’ve just invented the endoscope.

What have you reinvented lately?

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