Hi Francois,
A slight correction. The circularising part is usually bonded to and
rotates with the linear part.
One way to tell if your polariser is one of these circular sandwiches
is to view a reflection off glass, or the blue sky, rotating the
lens. If you look thru like a camera lens would, you will see
changes, just as with the linear polariser. If you look thru the
other way (with the male threads away from you), you will see no
change at all if you have a circular polariser.
Tom
...
> >(4) If you look at a linear and circular side by side you will
> >notice the linear has a single piece of glass which is mounted in the
> >rotating ring. The circular has two. One is mounted in the rotating
> >ring and the other fixed in the part that screws into the lens ring.
> >On a circular, the part that rotates is a linear polarizer, just as
> >with your linear one. The fixed, second piece of glass behind it is
> >a special filter that circularly polarizes the linearly polarized
> >light that was admitted by the linear polarizer, effectively
> >dispersing it into the same as linearly polarized in many directions.
> > Your eye doesn't care about the polarization. That is why when the
> >first one blocks the glare, it doesn't matter that the second one
> >effectively disperses the polarization into many directions again.
...
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