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Thursday, 10 June 2010


Holograms are extraordinary things. Just by using constructive and destructive interference, a two-dimensional photographic plate recreates the entire pattern of light in space bouncing off a three-dimensional object.

The diagram of the way they are made is a staple of every physics text book:

(That one is courtesy of Wikipedia.)

A hologram is a rather complicated diffraction grating. Imagine leaving the three-dimensional object out, changing the angles in the diagram a bit, and just recording the interference between the raw illumination and reference beams. When you do the interference sums (which for this case require no more than school trigonometry) you find that that interference pattern forms a - this time simple - diffraction grating. If you re-shine the reference beam at that grating, trigonometry again shows that the angle of the diffracted light that it gives off is just right to recreate the original unhindered illumination beam.

The hologram of the three dimensional object is just this process going on all over the place to recreate the much more complicated light pattern bouncing off the object.

For someone used to the Roman alphabet like me, walking round an Oriental city can be a confusing experience. You have to work out every sign by consciously and deliberately comparing bits of geometry. And heaven help you when the font changes. The same must also be the case for someone used to reading, say, kanji hiragana and katakana, when they find themselves in Europe or America.

What you would like to be able do is to buy a pair of glasses for a couple of dollars from a concession stand at Tokyo (or London) Airport with a British (or Japanese) flag printed on their cardboard mount. You put them on, and suddenly all the signs are in your own language.

(Image adapted from this original.)

One of the things that a hologram must recreate is light polarisation.

So - you start by making a sign for Ueno Station in Japanese that reflects polarised light in one orientation. Simply placing a sheet of polaroid in front of the sign should do the trick.

You half-expose the sign to create a white-light hologram. You then do the same for the sign in English with a different angle of polarisation and do the other half of the exposure. The result should be a sign that, when viewed through polaroid sunglasses with the appropriate angle, lets you read the sign in just Japanese or just English. The other language should be (almost) invisible. It may even be possible to do more than two languages at once, depending on how finely the polarisation angles can be distinguished.

Of course, it doesn't just work for station signs and street names. It should be possible to implement the whole scheme for free in any city, simply by insisting that advertisers who use the technique also pay for some of the public signs...

1 comment:

Dean Reese said...

I had no idea that you could just put a sheet of polaroid in front of the sign to help with it. I've been looking for a way to do that and I haven't found anything. I'll have to try this out and see if it works. Hopefully I'm able to figure everything out!

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