Fecking 3D glasses: Why don't they work they way I expected?
August 2, 2010 3:05 PM Subscribe
I thought I knew how the current crop of 3D glasses worked, but an experiment with my glasses and my laptop screen proved that something weird is going on. Why does the direction I'm looking through the glasses change what I see, and why do the polarising filters appear to be oriented the same way?
posted by metaBugs to science & nature (7 answers total) 2 users marked this as a favorite
I've recently been to see a couple of 3D films in a UK, non-IMAX cinema. I was given a fairly boring-looking pair of shades to make the 3D magic work. My assumption, based on A-level physics and some fiddling during the boring bits of Avatar* is that the 3D works by simultaneously projecting two images using light polarised into different planes, then bog-standard polarising filters in the glasses ensure that each eye only sees one image. Simple.
However, I was playing with the glasses when I got home, looking through them at my laptop screen and the face of my LCD watch. From a vague understanding of optics and liquid crystals, and from trying the same thing with an old pair of polarising sunglasses, I was expecting each lens of the glasses to allow normal viewing when held at the "correct" angle, but black out the screen and watch face when held at the "wrong" angle.**
Instead, something really weird is going on.
(a) Holding the glasses between me and the laptop screen, looking through them in the normal direction, slowly rotating them clockwise:
0, 90, 180 and 270 degrees rotation: No obvious change.
45, 225 degrees rotation: strong blue tint to screen (but red areas of screen still look red)
135, 315 degrees rotation: strong red tint to screen (but blue areas of screen still look blue)
(b) Holding glasses between me and the laptop screen, looking through them backwards, slowly rotating them clockwise:
0, 90, 180 and 270 degrees: Slightly dimmed
45, 225 degrees: Screen completely blacked out!
135, 315 degrees: completely clear
(c) For the conditions in (a) and (b), both lenses behave identically. So when one is giving a red tint, the other is too. And when one is blacking out the screen, so is the other.
So, to summarise:
(a) Looking through the lenses forwards changes the colour tint but not light intensity;
(b) Looking through the lenses backwards can black out the screen but not change the colour tint;
(c) Whatever effect is causing this, the lenses are oriented the same way.
Optics wonks of metafilter: Srsly, WTF? Why does light's ability to travel through the glasses depend on the direction it's going? And how can the 3D effect possibly work if the two lenses are aligned the same way?
To pre-empt an obvious question: I have used this pair of glasses in the cinema and they seemed to render the 3D effect very well. Either these glasses have been built correctly (otherwise my first assumption would be that a manufacturing defect had alignined the lenses incorrectly) or I'm susceptible to a 3D-placebo effect to an astonishing degree.
*Almost the entire film. Still, it was very pretty.
**I can't reproduce this now to test this memory, because that old pair of polarising sunglasses is currently sitting at the bottom of the Solent. I had to remove them to read my diving computer's screen without dislocating my wrist and a stray bit of spray washed them overboard. Damn physics.