Thursday, 29 October 2009

Holographic Diamonds

Diamond CutBack in January 2000, the Millennium Dome exhibition opened to what was supposed to be a display of the best of British achievement. Unfortunately the people in charge of setting it up didn't seem to have a clue how to run this sort of exhibition or what to put inside the dome, and it ended up as a bit of a national embarrassment.

One of the last-minute additions to the show was the Millennium Star diamond.

To see the diamond, you had to walk though an angled passageway that was completely pitch black apart from some slightly odd (monochromatic?) blue light, and there, in the middle, you'd see a case walled with bulletproof glass, containing the blue-lit diamond. You walk past it, perhaps pause, and then make your way out. No loitering, no photography.

Something struck me when I was in there. The thickness of the cabinet's glass meant that the diamond appeared be in different places, depending on which pane you viewed it through - that's completely normal, you usually see a similar effect with fishtanks. But the blue light confused me, because normally you only see blue-lit rooms when someone's trying to hide something. Okay, so it was a blue diamond, but still ...
The human eye is pretty bad at seeing sharp details in blue light, which is why Windows has traditionally had a blue-themed startup screen - the old splash screen used crude dithering to recreate the effect of a smooth variation in tone using the default 16-colour VGA pallette, and by doing this in blue, the eye was fooled into not noticing the effect too much. If Windows 3.1 had tried that trick in red or green or yellow, the result would have been bitty and grainy and would have looked awful. In blue, you can't see the fine detail that gives the trick away.

Now, the glass.
Bulletproof glass
uses a "sandwich" of alternating toughened glass and shock-absorbing plastic sheets, so that even if you shatter every layer of glass, the shatter-patterns are different, and the pieces stay stuck together by the plastic. If someone had simply added an additional sheet of plastic film with a with a hologram of a diamond ... then how would you be able to tell? You couldn't look for alignment errors between the sheets on different panes, because the diamond woudl appear at differtent positions when viewed through the different panes anyway, due the the thickness of the glass.
Does a holographic diamond appear to refract light in the same way as a real diamond? I don't know, but if someone wanted to look for an "anomalous" spectrum effect that didn't correspond to real diamond, the use of monochromatic blue light might be a good way to stop them. And with single-colour light source, we'd also find it difficult to see any interference fringes due to misregistration of the holographic films. Optical theory says that to see those coloured fringes, the colours already have to be present in the original lightsource, andf in our "blue room", that light wouldn't be there.

Of course, for all this to work, de Beers would have to have their own in-house holography R&D department aligned with their security people, which sounds pretty unlikely. But in fact, deBeers do have very strong links to holographic reseach: They have laser systems for checking diamonds, and for laser-etching holographic security marks onto them, and slightly more peripherally, Lucent have been researching diamond as a potential holographic storage medium. DeBeers also have a holographic diamond passport scheme. So diamonds and security and holography research and lasers and de Beers all have a pretty strong overlap. There probably aren't that many companies that know more about certain sorts of holography than de Beers do.

So here's a fun, harmless little conspiracy theory to ponder that's worthy of Sherlock Holmes or Jonathan Creek: What if this diamond, which thieves tried to steal from the Dome in November 2000 in a ram-raid using a mechanical digger, nailguns and a getaway speedboat, was protected by the ultimate "stage magic-based" security system? What if the diamond, that perhaps many thousands of people would swear on oath to having seen in person ...
... was never actually there?

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