Hi Terje,
I think the circular polarizers are really linear polarizers with a 1/4-wave
retarder (which make the polarized light circular again) sticked to the back.
The reason for them selling the CPs these days is because that digital cameras
will have problem metering with LPs. But for human eyes, both should work if
you use it properly - meaning looking from the 1/4-wave plate side. And if you
are using another one for generating the polarized light, you should make sure
the polarizer side is facing the sample, and the 1/4-wave plate side is facing
the light source.
(a) If you place it so that the polarizer side faces the light source (this is
the normal photographic setup):
normal circular light---|- polarizer-|-linearly polarized
light-|-1/4-wave plate-|-circularized light again.
(b)If you place it so that the 1/4 retarder is facing the light source:
light---|-1/4wave plate-|-circular
light-|-polarizer-|-linearly polarized light
As you can see, with setup (b) you can generate polarized light, while with (a)
you can filter the light to only let light with certain polarized angle to pass
through.
Normally the CP filters you get from photographic stores has the polarizer side
facing outside, and the 1/4-wave plate facing the camera. If you are not sure,
you can use a mirror to check it: put the filter between you and the mirror,
look through the filter, if it looks transparent, then the polarizer side is
facing the mirror, if the light looks significantly darkened, then the
polarizer side is facing you. There is a youtube video that shows it:
http://ca.youtube.com/watch?v=6lVIo9C0NDA
Zhijie
- Original Message -
From: Terje Dokland
To: CCP4BB@JISCMAIL.AC.UK
Sent: Wednesday, January 21, 2009 12:16 PM
Subject: [ccp4bb] Polarizing filter
We are looking to buy a polarizing filter for our Leica stereomicroscope that
we use to observe crystals. However, I am a little confused about the types of
filters that are available from photography stores; specifically there are
linear and circular polarizers. I would guess that what I need is a linear
polarizing filter that can rotate, though these are harder to find than the
circular polarizers that seem to be more common for general photography. Any
advice? Thanks,
Terje
---
Terje Dokland, PhD
Associate Professor
Department of Microbiology
University of Alabama at Birmingham
845 19th St South, BBRB 311
Birmingham, AL 35294
Tel: (205) 996 4502
Fax: (205) 996 2667
Do. Or do not. There is no try. -- Master Yoda
