Re: [photo-3d] Re: Ghosting / Cross-cancellation

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

> > The amount of light reflected off of a screen is a very small
> > percent of that emitted by the source.
>
> Another fact I overlooked. Is the reflected fraction typically a
> quarter or so? Even less?

Much less.  Even a high quality front surface mirror reflects only about 90%
of the light.  A high quality diffuse surface (like a projection screen)
bounces back less than 4% of the light emitted by the source.  Part of this
is due to the magnification (every time you double the size of an image you
divide the light by 4, part by distance (light falls off at the square of
the distance), and partly due to scatter and absorption (not all of the
light that does reach the screen is bounced straight back; some is reflected
up, some down, some to the sides, and some is just absorbed).  Other factors
contribute, such as absorption by the lenses, the polarizers, and mostly, by
the color dyes in the film.  Amazing that anything at all is left.

> Though I've heard of circular polarizers, I can't visualize how
> they work. If they are clearly superior, is the reason they're not
> "standard issue" their higher cost or poorer extinction?

Both.  Although cost is the real factor.  Linear polarizers are slightly
superior in extinction, but, depending on other conditions, not that much
better.

> In the projector I'm using, the polarizers are between the lamps
> and film, so I imagine they have no effect upon image clarity.

They do have an effect, but it is very small.  Usually no different than if
mounted in front of the lenses.  If anything, front mounted polarizers
should yield even less degredation, since they are further from the slide
(where the back focals of the lenses are focused) than when between the
lamps and the film.  I say "usually" somewhat tongue-in-cheek, because the
real problem with external polarizers is keeping them clean and free of
dust, and even worse, fingerprints.  This can be the real daunting task.

> Is
> there any consensus, or pooled data, concerning which
> polarizing material specifications or catalog items are "best?"

What is best depends upon your projection system, requirements, application,
and level of satisfaction (what YOU and YOUR AUDIENCE consider "adequate").

> Or
> is this always a case by case fit of what's readily available?

Pretty much so.  I am hopeful that this situation will change once
stereoscopic imaging grows out of the "niche" category it has been relegated
to at this time.  Regardless of what anyone may think of digital imaging,
computers, et al, I think the one single best hope for this happening lies
with the Internet.

JR


 

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