Author Topic: 3D Projectors?  (Read 1813 times)

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Offline Ftiafpi

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3D Projectors?
« on: February 05, 2010, 12:50:44 pm »
So, I was wondering if someone could explain this. How do the 3D projectors work in the theaters? Specifically, do they use one roll of film or two? Two separate projectors or just one with two lenses? What about the difference between the IMAX and the RealD projectors? Do both use just one giant roll of film? Etc.

Offline HTML_Earth

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Re: 3D Projectors?
« Reply #1 on: February 05, 2010, 01:16:19 pm »
I'm sure they don't use film. They're digital.

Offline Eight

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Re: 3D Projectors?
« Reply #2 on: February 05, 2010, 01:27:37 pm »
If I remember rightly, (IMAX is probably different), there are two projectors.

Two methods of polarising the video - one projector displays an image that is vertically polarised, the other overlays an image that is horizontally polarised. One lense in your googles picks up the vertical, the other picks up the horizontal.

The second method is weird. The first projector puts out it's light in a weird clockwise motion (I don't understand it either). The second projector does it anticlockwise. Again, one lense in a pair of special glasses picks up one of the polarised streams, the other picks up the oppositely polarised stream.

Offline Eight

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Re: 3D Projectors?
« Reply #3 on: February 05, 2010, 01:30:19 pm »
Addendum: RealD uses the latter method but some clever sod figured out how to do it with only one projector. :D

Edit: Just checked, it alternates between left and right eye - so I'd guess it'd be possible to use one roll of film IF it had mixed the left and right eye frames at the same frequency as the projector was going to polarise.

E.g (no idea if they do this). 2 frames of left, 2 frames of right, 2 frames of left... etc. with the projector polarising 2 frames clockwise, next 2 frames anticlockwise etc. etc.
« Last Edit: February 05, 2010, 01:36:29 pm by Eight »

Offline Meuia te Stxeli Tstew'itan

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Re: 3D Projectors?
« Reply #4 on: February 05, 2010, 05:43:08 pm »
E.g (no idea if they do this). 2 frames of left, 2 frames of right, 2 frames of left... etc. with the projector polarising 2 frames clockwise, next 2 frames anticlockwise etc. etc.

In RealD 3D, they use one standard digital projector with a special TFT filter in front of it. It work the same as your LCD monitor but the transistors flip at 144 FPS. This cause light polarization to happen and display each frame three times altering with the left and right eye. F1L, F1R, F1L, F1R, F1L, F1R, F2L, F2R, F2L, F2R, F2L, F2R, F3L, ... To compensate for the polarizing filter, they also need special silver screens to reflect more light and keep the polarization.

IMAX uses two synchronous projectors with standard 70mm film going through a polarized matrix which is similar to the one used in RealD 3D except that it does not need to switch for different polarization angle because there's already two sources. IMAX also need silver screen for the same reason as RealD 3D.

Side note, a standard digital cinema projection is 2k resolution, IMAX digital is 4k and IMAX film has equivalent resolution of about 18k (using higher resolution to digitalize the film doesn't give more quality).
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Offline Eykyuan

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Re: 3D Projectors?
« Reply #5 on: February 06, 2010, 07:52:55 am »
Kaltxi!

I wrote togeter what i know about the most important 3d projection systems.(don't mind my bad english)
Some of that stuff was already mentioned in foregoing posts. I'm sorry that the text on the images is in german but i don't have them in english. (everything written there is mentioned in my text)

Polarisation with active polarisation filters: RealD


You have an active polarisation filter in front of the projector lens that aligns the lightwaves
of the two stereoscopic pictures different (it's called z-screen).The passive 3D glasses are oppositional polarised too
so that only the picture for the corresponding eye passes through. Matt white cinema screens scatter
the light from the projectors and polarisation is annihilated. That's why they have use a silver
plated screen.
RealD uses circular polarisation, only few very old installations still use linear polarisation. Circular polarisation
technology has the advantage over linear polarisation methods in that viewers are able to tilt their head and look around in
the theater naturally without a disturbing loss of 3D perception, whereas linear polarization projection requires viewers
to keep their head orientation aligned within a narrow range of tilt for effective 3D perception.Otherwise they may see
double or darkened images called artefacts.
The systems glasses are very cheap due to they are passive. The disadvantages of all polarising systems are the above mentioned
silver screen and a distinct sweet spot. Due to the inherent brightness losses of the polarisation filters (about 50%) the projectors
are equipped with brighter lamps. IMAX uses either linear polarisation (older installations) or circular polarisation (newer installations).


Polarisation with polarisation filter wheel: Masterimage


The only difference to realD is that the filters are placed on a wheel rotating in front of the projector lens.
It also needs a silver screen.


3D through color shifting: Dolby

The Dolby 3d system uses a rotating color wheel integrated into the projector in front of the lamp. It shifts the waves of the RGB elementary colors of the left and right image slightly against each other.(The filter wheel has sections that will filter the
projector light for different wavelengths for red, green and blue. There is one wavelength for red that only the left eye can
see.The other wavelength for red is only visible to the right eye. The same works for green and blue. To make sure that the
separation works as expected the glasses also have filters according to the different red, green and blue wavelengths different
for each eye.

For example :
Left eye: red 629nm, green 532nm, blue 446nm
Right eye: red 615nm, green 518nm, blue 432nm

Difference in color perception for the left and right eye are corrected within the glasses through additional
filters) With passive 3d glasses equipped with exact matching interferrence
filters each eye senses only the corresponding picture. You can use a matt white screen here.


3D Shutterglasses system: XpanD (NuVision)


The cinema Projector generates a sync signal corresponding to the projected left or right image. A special sync box
distributes it to several infrared transmitters placed above the screen. The active shutterglasses shade alternately
one side of the glasses that only one eye views the screen. (The shutterglasses have a "single pixel" LCD inside)
You can also use a matt withe screen here.


All these systems (except Masterimage and shutterglasses) can be used either with one or two projectors. Using two projectors results in brighter images,
one projector is easier to syncronize.
If only one projector is used left and right eye are projected consecutive. In order that the stereoscopic picture doesn't flicker
higher frame rates than for 2D projection are required (typically 70 fps). To achieve the high frame rates the same picture is projected
four times. ( 1L, 1R, 1L, 1R, 1L, 1R, 1L, 1R, 2L, 2R, 2L, 2R, 2L, 2R, 2L, 2R and so on)
All systems can be used either for digital projection (Motion Jpeg stream from a cinema server) or analog projection methods (35mm
or 70mm IMAX film).



To linear and circular polarisation:

At linear polarisation the wave vector alternates only about one axis of the coordinate system.



At circular polarisation the wave vector rotates around an axis of the coordinate system.

« Last Edit: February 06, 2010, 07:56:27 am by Eykyuan »

Offline Ftiafpi

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Re: 3D Projectors?
« Reply #6 on: February 06, 2010, 01:15:13 pm »
Excellent info, exactly what I was looking for. Irayo!

Offline Meuia te Stxeli Tstew'itan

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Re: 3D Projectors?
« Reply #7 on: February 06, 2010, 01:19:26 pm »
Kaltxi!

I wrote togeter what i know about the most important 3d projection systems.(don't mind my bad english)
Some of that stuff was already mentioned in foregoing posts.

*snip*

This is a very precise mine of informations! There's a lot of stuff in here that should get onto Wikipedia ;)
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Offline Nyx

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Re: 3D Projectors?
« Reply #8 on: February 07, 2010, 04:29:26 pm »
Irayo for the info, I've been curious about this for a while :)

 

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