P3D Re: 3D Video with rear projection

[Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

"Rafael Ramirez" <globalimages@xxxxxxxxxx> wrote:
> >From: Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>
> >Actually, I don't think the video was projected at 120Hz.
> >Here's what the equipment was and what it did:
> >The Titanic footage came as a pair of Betacam SP tapes (one left tape
> >and one right tape).  The left and right video feed from the two
> >Betacam SP machines went to a pair of QD7500 line doublers.
> >The left and right line doubled signals were then sent to a pair of
> >3 gun CRT projectors (with circular polarisers fitted to the output lenses)
> >for projection onto the 3D Black Screen.
> 
> Do you know what type (brand) of polarized filters were used and how well
> did they worked?  

I'm pretty sure they were polaroid circular polarisers (left and right 
handed) being used on the projectors.  Polaroid's "Polaroid 2" glasses
are circular polarised glasses.  The quality was good although I believe
it is possible to achieve slightly better extinction using linear polarisers.

> I considered circular polarization for a permanent attraction in a zoo park
> running off dual CRV laserdisks (a special component-format videodiscs made
> by Sony), dual CRT projectors & front metalized screen, but the (higher)
> cost of the glasses and my inability to obtain any better extition with them
> led me to use linear instead.

Linear can achieve slightly better extinction than circular but circular
has the advantage that alignment isn't a critical issue.  

> Incidentally, anybody knows sources for circular glasses (polaroid still
> sells them?, Stereographics?, Vrex?, 3DTV? any comments as to which are
> best?)

The only source I know of for circular polarised glasses is Polaroid.
http://www.polaroid.com   As far as I can tell their web page doesn't
list the glasses but they are available.

> I'm also interested in details about the QD screen itself.  Can you
> elaborate
> (was it foldable, "plastic"-like?, rigid? Lenticular? etc?  Do you know how
> large in size can it be made?)  I tested Polacoat rear screen material and
> found it usable (it depolarises only slightly, it's foldable & washable).
> Also tested some "shower curtain" materials, with similar results. Any
> experiences as to which rear material depolarizes the least?

The QD 3D Black Screen is not foldable.  It's flexibility is sort of
like a sheet of 3mm thick polycarbonate sheeting.
The size screen we had at the conference was about 1.5m tall by
about 2m.  Larger sizes are apparently available.

The big advantage with the QD screen is its dark appearance
which I believe is due to its narrow acceptance angle which does
a good job of rejecting ambient light.

> Actually I run a mini film festival in the Canary Islands several years ago
> using a Vrex projector and a Polacoat screen.  I also run another festival
> (this time in 35mm film), but had to use video for the first one due to the
> inability to obtain movies dubbed in Spanish in film format.

Interesting.  What sort of footage to you project - movies, documentaries...?

> By the way, be warned that the Vrex projectors, at least mine, had its
> micropol filter fade (=lose polarizing efficiency) *tremendously* after just
> little use, and to the point of becoming unusable for stereo in just 8
> months (too much crosstalk).  Obiously the polarizing properties of filters
> made through this "iodine printing" process are not that stable.  Polarizing
> efficiency was never too great, although certainly good enough, if it would
> only last at least a couple of years...  This makes me wonder if anybody
> knows the expected lifespan of transparencies made by the Rowland institute
> printer inks.

Thanks for the insight.

Cheers,
Andrew Woods.       http://info.curtin.edu.au/~iwoodsa

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