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Re: [OM] Detecting polarizer type?

Subject: Re: [OM] Detecting polarizer type?
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Wed, 06 Mar 2002 20:38:22 +0000
At 01:00 3/7/02, Daniel J. Mitchell wondered:
 Is there any way to distinguish a linear from a circular polariser?

Yes!
Equipment required:
  a.  Mirror
  b.  Polarizer
  c.  One eyeball (and *only* one eyeball; see procedure)
Procedure:
  a.  Look in mirror at your own reflection and close one eye.
  b.  Hold polarizer in front of the *open* eye.
  c.  Observe whether you can see your open eye behind the polarizer
        in your reflection.
  d.  Turn polarizer over and look through it again.
        (Keep the other eye closed!)
  e.  Observe whether you can see your open eye behind the polarizer
        in your reflection.

Evaluating Test Data:
  a.  If you can see your open eyeball looking through the polarizer
        in both directions, it's a linear polarizer.
  b.  If you can see your open eyeball looking through the polarizer
        one direction, but not the other (because the polarizer is
        pure black), it's a circular polarizer.

(is this a dumb question?

No!  There's no such thing as a "dumb" question.

the polarizer filter rotates, so does that mean it must be linear?

No!  Both linear and circular polarizers rotate.

Can I work this out by trying spot-metering with my om2s and seeing if it goes wrong in the way that linear polarisers cause?)

This would take more work with ambiguous results and may not detect whether or not it's linear or circular. The test above is simpler and quite dramatic with a circular polarizer if conducted properly. The problem with attempting to test "on camera" is the polarizer must be rotated carefully to see the effect it has on metering.

Difference between a linear and a circular polarizer:
A circular polarizer *is* a linear polarizer with the addition of a quarter-wave plate cemented to the back side of it. The front linear (that faces the subject) performs exactly as a pure linear does and passes through light that's polarized in only one orientation. The direction of that orientation is determined by the rotational position of the polarizer. The quarter-wave plate behind it circularly polarizes the light that has passed through the linear polarizer. To the semi-silvered mirror (which is much like a reflective linear polarizer), a portion of the circularly polarized light is admitted through it to the light meter sensor, and the rest is reflected to the focus screen and prism, just as if it were randomly polarized (i.e. as if you weren't using a polarizer).

 thanks,

You're welcome!

 -- dan

-- John



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