The Remarkable Fresnel Prism

 

A thin sliver of plastic has repaired my outlook and restored my sanity.

I had spectacular vision, once. (There is a pun there, so faint that it will be missed altogether by all but the most astute if I don’t point at it.) My Optometrist was always most impressed and said that with spectacles (referencing above pun) my vision was significantly better than 20/20. Do you know, by the way, what 20/20 vision actually is? The top 20 refers to the smallest line on an eye chart you can clearly see at 20 feet distance, compared to the maximum distance at which a person with normal sight can read the same line … or something like that.  Odd, isn’t it? It sounds a rather messy way of measuring to me.   Until Easter I could read signs at long distances and the finest print (except for fine print, which of course nobody reads); but it all ended in less than a fortnight, replaced by fuzziness and double vision.

The Ophthalmologist (= eye specialist, although one almost needs a mouth specialist to pronounce it) had two things to say.
The expected: he concluded it was “probably systemic”, meaning whatever is wrong with me is also wrong with my vision, (in other words, no idea) and
The unexpected: he said he thought he could help.
He sent me down the corridor to the Orthoptist (= double vision specialist, a word that would be easy to say if you hadn’t first been practicing saying Ophthalmologist). The Orthoptist had me peer through a great many holes in various gadgets, and then took a flimsy piece of plastic from a sealed box, cut it to size with scissors, and stuck it on the back on my glasses. This curious film, less than 1mm thick, is the remarkable Fresnel Prism.  It puts double vision back together again. While it doesn’t solve my problem altogether – I still find much of the world too fuzzy, and occasionally too two-y – it does greatly reduce eyestrain and makes the unbearable quite tolerable.

You might have seen a Fresnel Lens if you have visited a Lighthouse. I remember when I first saw one, and in a rare moment of scintillating insight I actually understood how it worked.

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The Lighthouse lenses designed by French physicist Augustin-Jean Fresnel

Each ring of glass is a triangular lens or prism, all working in chorus to send the light from the lamp in a parallel beam far out to sea, greatly enhancing the intensity of the light. If a single lens had been used it would have been of impossible size and weight, so a series of smaller lenses are used instead. I now have a miniature fresnel prism on the inside of my glasses: many tiny, vertical triangular prisms lined up across the lens.

 

Without the prism I think I may have fallen into some sort of depression, I could feel my eyesight dragging me to a dark place. Perhaps the suddenness with which it arrived is a factor, but I find it much harder to accept than other problems. A bit like the flu, which always seems to me worse than a broken bone, vision problems are right inside your head; intrusive, unavoidable, overwhelming, horrible.

Yet again I find myself the beneficiary of recent innovation. Although  Fresnel built his lenses in the early 19th century, the PVC prism I have is a modern application. Every day I rely on technology to breathe, to propel my wheelchair, and now to see, that is only a few years old.  Some of it just a couple of years old!  I commented on this to my GP recently, saying that I thought that a decade ago I would be much worse off; and she added something like, “or not here at all”.  That’s true I think. It is astonishing to live today in a first world country, benefiting constantly from science and invention.

Rejoice!


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Rejoice! from 2009

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