I may have misunderstood Chronos in my response in post #19…
It may simplify matters to consider only one channel - let’s say the one intended for the left eye. The right eye channel is switched off.
An image of full movie brightness, polarised a certain specific way needs to be on the screen. The left eye is able to see all (barring small losses) of this image, via the filter in front of it. The right eye is not able to see any (barring small leakage etc) of this image.
The filter in front of the left eye is not purposely discarding any of the left-eye-channel image, but this left-eye channel image does need to be at full brightness, so the projector lamp needs to be brighter than normal to compensate for the polarising filter at the projector.
If that’s what you meant, Chronos, my apologies for not reading it properly.
You can semantically put the losses anywhere you feel like, but the net result ends up being that you need to start with four times as much light as a regular movie, not twice as much, and all the rest of that light ends up getting thrown out somewhere or another. Personally, I would say that some of that loss is at the glasses, since if you had a scene in a 3D movie with no depth (and where the two images where therefore identical), a person without the glasses would see an image twice as bright as would a person with the glasses.
You mean twice as much for each of the two projectors, totalling four times as much as for a conventional flat movie from one projector with (theoretically) no filters, right? If so, I agree.