preliminary summary on ghosting

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

There has been a good amount of input on ghosting, and this is a first
attempt at summarizing it for the archives.  Comments and additions are
solicited.
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Projection of stereo photographs is almost universally based on polarizing
the right and left images so that the orientation of the polarization of
the right side is at a ninety degree angle to that of the left.  Glasses
for the viewer have filters for each eye that are polarized in the same
directions.  The attempt is made to completely block the left image from
reaching the right eye, and vice versa, but this is never completely
achieved.  When the right eye sees a faint 'ghost' of the left image, it
can greatly interfere with the viewer's immersion in the presentation.  

There are many causes of ghosting.  It is most noticeable with slides that
have hard edged objects, with strong contrast between adjacent areas; or
with titles, and can also be seen when the viewer tilts their head from
vertical.  Moderate ghosting is usually imperceptible when viewing typical
stereo photographs.

Using one side of a stereo projector to illustrate, light from the
projector bulb passes through a condenser lens assembly, then the film,
through the projection lens, is reflected from the screen, and into the
viewer's eye.  One polarizer is placed in the spectacles worn by the
viewer, and the other can be placed anywhere in the projector.  

If placed between the condenser & the film, as in the Realist and TDC
projectors, the polarized light can be 'scrambled' as it passes through the
film, and will not be completely blocked by the polarizer over the opposite
eye.  This does not occur with modern E-6 based films, but many other types
of film depolarize to varying degrees.  Piper writes that estar based films
can depolarize, but acetate film bases do not.  If the slide is protected
by a clear cover, it must be of a strain free glass, since some glass
contains internal stresses that can depolarize, as can some clear plastic
covers.  This location is also getting close to the light bulb, which emits
great heat, which can damage the polarizer.

If the polarizing filter is placed between the film and the projection
lens, any depolarizing quality of the film will have no effect.  It can
also be placed on the outside of the projector, where it is subject to
scratches (which depolarize as well as scatter light), if it is not encased
in glass, like a camera filter.   Many projectors focus by rotating the
outer lens, and the polarizer cannot be mounted on a rotating surface.
Some care must be taken in the placement of the filter,since if it is at
one of the focal points of the optical path, any speck of dust or scratch
will appear on the screen.

Screens made of tiny glass beads or of white paint will scatter and
depolarize the light.  Metallic, silver screens are required.  Ghosting can
be stronger at the corners of a screen, or to a viewer who is not directly
in front of the screen.

When using Polaroid filters, some amount of ghosting is inevitable.  When
two sheets of the best Polaroid material are sandwiched so that the
orientation of the polarization of one is at 90 degrees to the other, a
maximum of 99 percent of the light will be blocked, and one percent will be
transmitted.  Commonly used Polaroid material transmits more than one
percent when crossed in this way, and this quality can degrade with time,
causing worse ghosting.  If the Polaroid is not held flat, but is curved
during manufacture, or bulges in the center from the method of mounting,
extinction is less complete.  Because the plastic is heated during
projection, flatness is best maintained if the filter is sandwiched between
glass.

A small amount of depolarization is caused by the curved surface of any
lens.  This is probably inconsequential for projection, but is a problem
discussed in texts on polarizing microscopes, where the lenses are spheres
of very small radius.  Condenser lenses can have quite steep curves.
Coating the lens also minimizes the effect.

There are more exotic methods for polarizing light.  
Circular polarizers have been used for stereo projection, but probably do
not give more complete extinction.  The typical circular polarizer is a
linear polarizer adjacent to a quarter wave plate.  I believe I've heard
that the problem with circular polarizers is that you can tilt your head
with no noticeable effect, but over time the rotation of the eyes that you
have to maintain with a tilted head will cause eyestrain.
Nicol prisms of calcite were used for stereo projection at the turn of the
century.  For a long time, I searched for Nicols for my microscopes, hoping
that they would give greater extinction (thus a blacker background).
Nicols deteriorate with time, and it took me much effort before I finally
got a good pair.  They did not give greater extinction than quality
Polaroid material.  
Finally, there are grades of polarizing material, and it is possible that
some laboratory grade Polaroid gives more complete extinction.

Questions for the list:
--what films are known to depolarize?
--the screen used at NSA is a very unusual lenticular screen.  Is it
possible that this optical element, with very small refracting elements,
introduces a depolarization?  I imagine the lenticular elements
(lenticles?) are not hugely larger than the glass beads used on screens.
--anybody tried 'research grade Polaroid', or is there such a thing?
--what am I missing?

_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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