Re: [Leica] use of circular polarizer

[Dooley <dooley@iinet.net.au>]

This can be summarised to say that, no Hans, its not your eyes.  But read on if
you want a technical and quite possibly incorrect explanation.  (Its been a while
since I studied physics.)

Every beam of light has an electric field coming off it, vibrating in some
direction.  A linearly polarizing filter lets through light so that the electric
field of that light is oscillating in one direction only.  For example, up and
down relative to the direction of the light travel. This means that all light is
to some extent transmitted, but filtered so that only the up and down fields of
the light are transmitted.  _Except_ light which has a field travelling only left
to right, ie. at 90 degrees to the transmitting angle of the filter.  This light
shouldn't get through at all.

If we then have another polarising filter on the other side, oriented so that it
transmits light with a field going left to right, then theoretically no light is
transmitted, because all the light hitting it has a field perpendicular to it.  In
practice, it is impossible to make filters this effective, and impossible to get
the orientation perfect.  If the second filter is oriented so that it lets through
up and down light, then it should let through all the light.  This is what we
change when we turn one of the filters that make up the polarizing filter unit.
This is also why the orientation of _both_ pieces of glass is critical.

A circular polarizer lets through light which has an electric field where the
field in perpendicular directions are equal, eg if the light is traveling forward,
then the field in the up and left directions have equal magnitude, but are 90
degrees out of phase with each other.  I dont know what effect this would have on
photographs, but it should be uniform, regardless of the orientation of the two
pieces of glass.  (I'm not even sure if you need two pieces of glass.  I don't
know how a circulat polarizing filter is built)

Sean


George Huczek wrote:

> >>i have always thought that by turning this filter one would obtain different
> >>density at different settings. it seems that my P-cir 13406 has an even
> >>density across the entire filter even though the glass element can be
> >>turned.
> >>are my eyes getting bad ? If not, why can the glass be turned when the
> >>entire glass has the same density /shading of grey ?
> >
> >The extent that light is polarized depends on the angle of the light.  You
> >will have to look through the filter with the light coming in at an angle.
> >That's why polarizing filters have more of an effect when the sun is low in
> >the sky.  Take the filter outside on a sunny day and look at the sky
> >through it from different angles.  As you turn the filter, you will see the
> >sky darken when the light is most polarized.  Hope this helps.  Tina
>
> This doesn't quite get to the point.  With a linear polarizer there is a
> noticeable difference in the polarizing effect when the filter is rotated,
> viewing at the correct angle of the sun as Tina mentions.  Take off the
> linear polarizer, replace it with a circular polarizer, and rotate it at
> exactly the same subject, and the extent of the difference in polarization
> seems to be less noticeable.  I have noticed this effect as well and
> wondered about it.
>    Hans, I have no idea why this happens, but it does.  Your observation is
> correct, and your eyes are not failing you.  The answer probably lies in
> understanding the difference between linear and circular polarized light.
> For that, stay tuned.  There are likely some LUGers out there who have a
> better understanding of physical optics who might like to take a shot at
> the answer.
>
> -GH