Re: [photo-3d] Re: How Big is Your club's Screen

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

----- Original Message -----
From: <gccampos@xxxxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Thursday, March 29, 2001 11:28 PM
Subject: [photo-3d] Re: How Big is Your club's Screen


> Is the greater gain necessary to
> make it "easier" for a polarized lite to be bright from a greater
> distance on the plus side, but on the down side make the screen more
> prone to hot spots (shiny glaring reflections) when closer to the
> screen?   Would I be better off with a lower gain for a smaller screen
> where we sit closer to the screen?

"Gain" is one of those misleading terms, since no screen can bounce back
more light than hits it.  Even a mirror reflects only a bit more than 90%
(depending upon the efficiency) of the light striking it.

Gain, as defined by the screenmakers, means the amount of light reflected
back toward the projector, ON AXIS, as compared to a standard magnesium
white block that has been assigned the rating of 1.0.

All screens have a reflectivity pattern that varies from "specular" (highly
directional, very bright on axis, and prone to "hot spots") to diffuse
(matte screens).

In general, a specular screen will tend to appear much brighter on axis, and
much dimmer from extreme angles than a diffuse screen.

For normal front projection of stereo slides by polarized light, a
nondepolarizing screen is necessary.  Metallic surfaces tend to be
nondepolarizing, while nonmetallic surfaces (glass beaded, matte, and most
"pearl" surfaces) are depolarizing and will not work.

Also, just because a screen appears to be silver in color does not
necessarily mean that it is metallic.   For example, a surface that is
painted with ordinary nonmetallic silver paint will look silver, but will
depolarize and cause stereo images to "ghost".

Screen manufacturers that make silver screens for stereo projection actually
use a coating that has very fine aluminum flakes in suspension.  (Some
paints are now available that have a similar composition, and do work, but
it is best to test them).

Any screen intended for passive polarized projection should be tested before
purchase.  An easy test can be done with a flashlight (electric torch to our
British friends), a polarizing filter, and a pair of polarizing 3-D viewing
glasses.  Tape the polarizer over the flashlight, and aim it at the screen
material.  Wear the 3-D glasses while rotating the flashlight.  The light
beam reflecting off of the screen will appear to darken and become invisible
in one eye while it gets brighter in the other.  It should completely
extinguish for a good stereo screen.  You may have to close one eye to make
sure.

Since a true stereo screen has real metal on the surface, these tend to be
very high gain, and very directional, especially for the smooth silver
surface types.

In an effort to make these less directional, screen manufacturers have
embossed screen material with a "lenticular" (resembling cylinder lenses in
general shape) or ribbed surface.  These reflect the light over a wider
angle.  Early lenticulars, designed for level seating, had just vertical
lenticule stripes.  Today, they often have horizontal lenticules as well, to
accommodate stadium seating, as well as a larger front-to-back distance.

Typical depolarizing matte surfaces have gains from about 0.8 to about 1.3.
"Pearlescent" surfaces, also depolarizing, have typical gains of about 1.5
to 1.8.  Some specialized "high gain" depolarizing surfaces (including glass
beaded and composite silver/beaded) have gains of 2.0 or slightly higher.
None of these are suitable for passive polarized stereo.

True stereo screens tend to vary from about 2.0 (lenticular) to over 4.0
(smooth).  These should be tested for angle as well as depolarization.  Tape
a screen sample to the wall, and compare the light falloff from the center
seat to the extreme corner seat in the front row.  You may find that you
want to eliminate some chairs in the extreme front corners of your seating
area.  A good idea anyway, since these seats usually give a distorted view
even if it is bright enough.  A trapezoid shaped seating pattern, with the
narrowest rows in front and the widest rows in the back is usually best for
stereo projection.  That way, most people in the audience will be able to
view an acceptable image.

JR


 

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