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: [email protected] 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
