B&W Polarizer question

[newmoon2night]

I have a 105mm B&W Kaesemann Polarizer that I'm planning on selling.
I'm trying to verify that it's a linear and not a circular polarizer.
How do I tell?
Do the circular ones have 'circular' included in the description around the filter ring?
Is there a way of telling by looking into the filter?
Thanks for any advice.

[Bob Salomon]

What exactly does it say on the rim?

[newmoon2night]

What exactly does it say on the rim?

'B+W 105 KSM POL' inside rim
'B+W F-PRO Made in Germany' outer rim

Thanks

[Peter K]

The only difference between a linear and a circular polarizer is the lambda/4 foil. This foil is mounted between polarizing foil and front lens resp. mounting thread. So one needs two polarizers arranged as a sandwich to distinguish between linear and circular.

If one of a two-polarizer-sandwich is rotated against the other there are two positions no light, or almost no light, goes through the two polarizers. But if there is no "black" position there is at least one circular polarizer. So look through the first polarizer from the male thread and put the second polarizer behind the first in the same kind.

With two circular polarizers one can only get "black" with the two female threads against each other. But looking through the linear polarizer one can get black from both sides.

[Bob Salomon]

'B+W 105 KSM POL' inside rim
'B+W F-PRO Made in Germany' outer rim

Thanks

It is a linear pol.

[newmoon2night]

The only difference between a linear and a circular polarizer is the lambda/4 foil. This foil is mounted between polarizing foil and front lens resp. mounting thread.

If one of a two-polarizer-sandwich is rotated against the other there are two positions no light, or almost no light, goes through the two polarizers. But if there is no "black" position there is at least one circular polarizer. So look through the first polarizer from the male thread and put the second polarizer behind the first in the same kind.

With two circular polarizers one can only get "black" with the two female threads against each other. But looking through the linear polarizer one can get black from both sides.

Interesting thank you. One lives and learns!

[newmoon2night]

It is a linear pol.

Thanks Bob. Much obliged.

[Helen Bach]

In the absence of helpful and knowledgeable people like Bob, you can find out with a mirror because a circular polarizer has a two elements, and what happens depends on which order the elements are in. Look through the polarizer at your reflection in a mirror, then turn the filter around - ie male threads towards you (a) vs threads away from you (b). If your reflection is significantly darker in b than in a, you have a circular polarizer. The circularly polarized light travelling from the filter to the mirror in b has its direction of rotation reversed by the mirror, so when it gets back to the filter the 1/4 wave plate turns it into light which is cross-polarized for the linear polarizer it then passes into before arriving back at your eye.

Best,
Helen