Visibility Cloak

It says here that scientists at the University of Birmingham have invented a sort of prism, made out of something called calcite, which when placed in front of a small object, such as a paperclip, renders it invisible.  Calcite is itself transparent (and visible) but has the property of somehow bending light so that whatever is behind it, under very restrictive controlled conditions, can't be seen in the human visible light spectrum.

I foresee a few difficulties.  For a start, if what's behind the calcite prism can't be seen, how do they know it's transparent?  More importantly, even if the technology can be developed to encompass much larger objects than paperclips, the fact that the calcite itself is visible is a bit of a giveaway, isn't it?  I mean, if you see a big lump of what presumably looks like glass walking around, once you know about this stuff you're going to think 'mm, that's a person', aren't you?  And if a spy plane sees a big sheet of glass in the desert, with apparently nothing behind it, it's a strong clue.  Probably a uranium enrichment plant.

No, I can't see this one catching on.  I'll continue to disguise my uranium enrichment plant as a disused compost bin - which is itself disguised as a toasted Dalek.