P3D Re: polarization

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Bob,

> Is there some kind of similar analog to follow for circular
> polarization? Could you get less ghosting if you right hand
> polarized one side and left hand polarized the other?

If the projector uses circular polarization and the viewer looks through
corresponding circularly polarized lenses then the result, if your head
is level, is the same. With linear polarizers you must keep your head
level so that the right lens polarization is parallel to the right image
polarization. With circular polarizers you can rotate your head and the
images stay seperated.

So neither the projector's polarizers nor the viewer's polarizers are
sensitive to rotation. On the other hand, right and left polarizers are
not interchangeable. They are made from different stock. So you cannot
buy one sheet of right circular polarizer and cut both left and right
polarizers from it. You must have both right and left circular material
to make both sets of polarizers.

Linear polarizers can be oriented a number of ways. As long as right and
left polarizations are perpendicular to each other. Somebody made the
choice that they should be polarized up and out. So that is the standard
for stereo projection. 

Somebody has chosen the circular polarizations for projection also.
These are sometimes used in virtual reality action games where you might
want to tilt you head at significant angles. I don't know what standard
is used.

One way to experiment is to use quarter wave retarder sheet. You can get
this from Edmunds Scientific but it is cheaper from American Polarizer's
in PA. You can cut a single sheet with the retarder principal axis at 45
degrees from the linear polarizer axis and lay it over a pair of common
linearly polarized lenses. The result will be right circular for one
image and left circular for the other image. How perfect this is will
depend on the quarter wave working well over the visible spectum and the
linear polarizers being good ones and oriented properly. 

You want the projector light to pass through the linear polarizers first
and the quarter wave retarder second. For the viewer, the light must hit
the quarter wave retarder first then the linesar polarizer. So the
quarter wave retarder can be hung over your projection lenses and hung
over your viewing lenses.

Another oddity. When circularly polarized light reflects from a mirror
or a polarization preserving screen it reverses polarization. Right
circular changes to left circular and left changes to right. So the
quarter wave retarder on the viewing lenses must be rotated 90 degrees
from the quarter wave retarder on the projector or you will see pseudo
stereo.

You can get the circular polarizer sheet and circualrly polarized
viewing glasses from American Polarizer also.

> I have a reason in asking this beyond photo-3d. Another of my
> mailing lists is talking about using polarized light to bring
> out faded/bleached signatures and artwork outdoors  - this is
> done via a polarizer over a flash with another polarizer over
> the lens. Since it's outside, this is done at night. I'm
> wondering if using circular polarizers might behave significantly
> differently than linear (if that's the correct terminology).

Yes. It will be significantly different. It may be better or worse. I'm
not sure how you are using the linear polarizers so I'm not sure how the
circular ones will behave. Any literature on how the linear polarizers
help? Are the flash and camera polarizers parallel or perpendicular to
each other. Are the light source and camera nearly perpendicular to the
painting or signifcantly off axis.

I would guess that the camera and light source are both way off axis.
Probably 45 degrees each so that they are 90 degrees apart. Then the
reflected glare from the glossy surface is highly polarized. A linear
polarizer can practically remove this light. The dull or diffuse
surfaces will randomly polarize the incident light and a significant
part of that will make it through the camera polarizer. Just a guess
though.
 
> And another question - if you place a polarizer over the lens of
> a digicam (probably one of the 2 mil pixel models) will the resulting
> image be the same as if you did this with film (I'm *thinking* so,
> but have no ready way to test it - I suppose you could just take
> a polarizer and rotate it in front of the lens and see if the
> usual blue sky darkening occurs on a digicam similar to what you
> get with an film based camera).

A common dichroic linear polarizer, when oriented properly, reduces the
highly polarized blue sky light to much less than 38% while passing 38%
of the randomly polarized cloud light. The recording medium is not
important. The sky polarization depends on the location of the sun and
how the light is scattered from the sky to your camera. The northern and
southern skys are nearly always highly polarized because you, the sky,
and the sun form a significant angle. When you shoot the sky with the
sun directly in front of or behind you the sky is not especially
polarized. A linear polarizer will not have much of an effect in this
situation.  
  
Same with reflections from glass or glossy surfaces. The reflection is
highly polarized if the angle of reflection is near the Brewster angle.
That is the inverse tangent of the index of refraction of the glass.
About 57 degrees for window glass. About 53 degrees for water. But if
the angle of reflection is nearly perpendicular or parallel to the
surface then the reflected light is not polarized and a linear polarizer
won't help.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design



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