Like for many people, 3D images (stereoscopy) are inherently fascinating to me, and I’ve previously blogged about this topic. Recently, some colleagues brought back some promotional material from a commercial fair. One of these was a poster that came with a pair of glasses.
Naturally, everyone tried looking at the poster through these glasses. There was a kind of 3D effect, but how did it work? The glasses were made with some kind of translucent colorless plastic, so it wasn’t a traditional anaglyph, but the poster looked like just normal ink on paper – was this some kind of polarized ink?
Looking at the glasses again today while having a cup of coffee, I think I figured it out. It’s not real stereoscopy after all! The glasses are fake! Kind of…
It’s still interesting because they produce a sort of three-dimensional effect where elements of the image seem to pop out from the page. One of the plastic pieces in the fake 3D glasses is just a transparent film. The other one is a diffraction grating. It introduces what is effectively chromatic aberration into one eye when you look through it. Reds are displaced slightly to the left, and blues are displaced slightly to the right.
Look through these glasses at a normal scene, and everything just seems a bit fuzzy around the edges. But when you look at the fluorescence micrographs printed on the poster, then the magic happens. These pictures are mostly bright reds, blues, greens and yellows on a stark black background. The chromatic aberration introduced to one eye by the grating causes the different colors to be displaced by different degrees, whereas the other eye sees things normally (all in their proper place). This displacement is what causes the stereoscopic effect, and gives the illusion of movement when you look at the pictures from different angles.
So it’s cheating because it’s not really stereoscopy. Nonetheless, it’s a cool gimmick to get attention to what is otherwise just another poster.