Just returned from watching Monster House in “Digital 3D”. This tech seems a bit different from previous 3D movies I’ve seen. The glasses are very transparent with no color distortion. When you place two pair back-to-back and rotate 90 degrees, they don’t go completely opaque like regular polarizing filters. Anyone have a clue what’s behind this technology?
I haven’t seen the glasses, but I’ll bet they are partially polarized, at least. From http://news.com.com/Chicken+Little+gives+peek+at+digital+3D/2100-1026_3-5933692.html
Yeah, I read that article, but I don’t think it’s accurate. I’m looking through the glasses now and they’re definitely not normal polarization. When you hold a second pair in front of the first, there is no difference between the eyes (i.e. if you place the right lens from the 2nd pair in front of the left lens of the first pair, there is no change in transparency). Similarly, if you rotate the 2nd pair 90 degrees, there is a color change, but it doesn’t go opaque. Also, the same rotation by either lens in front of either lens produces the same effect.
Only possibility I can think of is that they use circular polarization instead of linear. I think it’s an expensive way to do it, but it would have the advantage of being insensitive to the angle of the filter (i.e. it’d work even when you tilt your head).
Have you tried flipping the 2nd pair (so you’re looking through it from the other side) and holding it in front of the 1st pair?
This would rule out circular as well as linear polarizations.
What do the glasses look like? Are they just cheap cardboard things with sheets of some sort of plastic in front of the eyes, or are they big, bulky plastic things? One technique I’ve seen used had both the lenses of the glasses and the screen blinking many times a second. The glasses were synchronized with the screen via radio, so that when the left frame was showing on screen, the right lens was opaque, and vice-versa. When they’re not receiving the synchronizing signal, both lenses stay transparent. Of course, this method is also somewhat more expensive, if you need a lot of pairs of glasses.
Chronos, I was about to mention that method, but the glasses would be more elaborate than cheap throwaways. I also assume they would be connected to the sync generator by wires, but do they use wireless or are they on an accurate clock? Either one would increase the manufacturing cost way above the simple polarization scheme.
There were wireless stereo glasses over a decade ago, but I can’t imagine that manufacturing costs have gone down enough in the interim to use them for viewing movies like that.
Yes! Yes! That’s it! If you flip the 2nd pair around backward and hold in front of the first pair, there is some definite opacity. Also, when watching the movie, head tilt had no effect on the 3D perception.
So, explain what is the difference between circular and linear polarization.
The glasses are not cheap cardboard, but are definitely not electronic in any way. They charged us an extra $1.50 for them and they look like ordinary drugstore plastic sunglasses, except that they are nearly completely transparent (they package had a warning not to use them as sunglasses :rolleyes: ).
Ah, thought so.
The filter on the glasses consist of a 1/4-wave waveplate and a linear polarizer. A waveplate is an optical element that has a different index of refraction depending on polarization, and “1/4-wave” means the thickness is adjusted so that the vertical polarization is retarded by 1/4 of a wave relative to the horizontal polarization (or the other way around). This converts a circularly polarized light into a linearly polarized light; a right-hand circular polarization becomes a horizontal polarization and left-hand circula becomes vertical polarization. Then one of the two is blocked by the linear polarizaer.
That means although it’s a “circular polarizer filter”, the light coming out the back side of it is linearly polarized. That’s why it doesn’t become opaque when you look through 2 layers of it. But when you flip it, it becomes a true circular polarizer (i.e. it creates circular polarized light). Incidentally, that’s what you get when you go to a camera shop and ask for a “circular polarizer” - a filter that selects one linear polarization, then converts it into a circular polarized light.
Yes, I’ve noticed this, too. The filters are actually paired linear and circular polarizers (or possible linear plus quarter wave plate), either of which take incoming linearly polarized light and turn it into circularly polarized. If you place two such filters with the linear polarizers back-to-back you can make them get more opaque by rotating one against the other, but in the wrong combination that won’t work. Only a couple of years ago, the filters were pure linear polarizers, each set 45 degrees to the horizontal (but perpendical to each other). We had a few filters like this from Edmunds long before they started doing this in the movies.