P3D Re Round or Circular (polarisers)

[Peter Homer <P.J.Homer@xxxxxxxxxxxxx>]

>>These filters to nothing of the sort.  A circular-polarized filter is
>>actually a linear polarizer with a quarterplate (a depolarizing layer)
>>behind it.  Perhaps a more correct term would be >>"polarizer-depolarizer

A quarter plate (quarter wave plate) or 90 degree retarder definetly
converts linear to circular polarisation it does not depolarise however it
does de-planepolarise and I am begining to see where the confusion is
coming in and perhaps the camera manafacturers are partly responsible.


>> If you have a circular polarizer and a linear polarizer, try this
>>experiment: First, hold the linear between you and a light source.
>>>>Now, place the circular between your eyes and the linear filter and
>>>>turn either filter.  You will observe that the light can be almost
>>>>completely blocked out.  This is because the light is being polarized
>>>>in one direction by the linear filter, and can't pass through the
>>>>polarizing layer at the front of the circular polarizer - there's no
>>>>light left the quarterplate layer to depolarize.  Now, hold the
>>>>circular between you and the light, and place the linear between you
>>>>and the circular. You will see that the light can only partially be
>>>>blocked by turning the filter.  This is because the circular filter is
>>>>polarizing the light, but then depolarizing it, and the linear filter
>>>>can't completely block the light because it is not strongly polarized

In the first instance the linear polariser layer of the circular polariser
comes first  so it acts like a normal plane polariser . In the second
instance unless you also turn the filter round you have got the 1/4 wave
plate after the first polariser so it is converted to circular polarised
light . The second polariser should not be expected to give total
extinction with circular polarised light because it is a linear polariser.
To do that you should expect to use a second circular polariser of the
opposite rotation which requires the retarder to be turned through 90
degrees not the polariser. They are generaly laminated to the polariser so
its not easy to do you need to get the other hand filter a given camera
manafacture probaly only provides them in left or right it does not matter
to them. But if you use different manafacturers filters you may be able to
get a left and right pair. You need to hold the first filter from the light
source with the polariser towards the source the second the retarder. But
the easiest way to show extinction with a circular polariser is probaly to
lay it on a mirror which will reverse the rotation so that the light does
not pass back through the filter.      P.J.Homer



------------------------------