------Original message------
>Date: Sun, 22 Dec 2002 18:43:36 -0500
>From: "Walt Wayman" <hiwayman@xxxxxxxxx>
>Subject: Re: [OM] circ. polarizer question
>Circular polarizers only work in one direction.
That is true; but they polarise in both directions.
>Looking through it at a shiny surface should give you the answer.
No, it won't. A circular polariser consists of two "glass" elements. The
first element (the one nearest the subject) is the polarising element.
Light from the subject strikes the polarising element and emerges polarised.
The polarised light then strikes the second element, which is a
randomiser -- it takes the polarised light and randomises the polarisation,
so that the light emerging from the second element is unpolarised This is
necessary to prevent autofocus sensors and spot meters giving incorrect
readings. So:
Polariser fitted correctly:
Unpolarised light from subject strikes first element, emerges polarised,
strikes second element, emerges unpolarised, then strikes front element of
camera lens. This gives correct spot readings.
Polariser fitted reversed:
Unpolarised light from subject strikes second element, emerges
unpolarised, strikes first element, emerges polarised, then strikes front
element of camera lens. This gives incorrect spot readings.
Now, the whole point of using a polariser is selectively to reduce the
amount of light in certain parts of the picture (e.g. the sky) by
eliminating polarised light originating from that part of the picture. So of
course you have polarised light striking the first element of the polariser.
But light with the incorrect polarisation is eliminated by the first
element, so that the light striking the second element is still polarised,
but there is less of it (e.g. the sky is darker than it otherwise would be).
The way to determine which way round a loose polariser should be fitted is
to use a second circular polariser. Fit them together and then rotate one
with respect to the other whilst looking through them (you don't need to
look at polarised light). If there is no (or little) darkening as one
polariser is rotated with respect to the other, they are fitted correctly.
If the image darkens to complete blackness, one polariser is fitted the
wrong way round relative to the other.
Although a circular polariser consists of two elements, they may be cemented
together, so that you actually have only one piece of "glass".
-- from Cy in the UK
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