What is a circular polarizer? at DVinfo.net

Jeff Miller · December 8th, 2004, 11:01 PM

FIrst off, I'm an idiot, and don't know the details of polarized light sources. Second, I got a c. polarizer with the XL2 since a DP I work with swears by his.

But, this polarizer is only one spinning filter. I might be wrong, but isn't a circular polarizer made of two elements, one fixed to the filter mount and the other rotating free? I haven't tried it on the camera yet since I've been afraid it's missing a piece. Can someone set me straight?

A. J. deLange · December 9th, 2004, 10:01 AM

When I first looked at your post I said "Can't be!" but after thinking about it for a minute decided that yes, one rotating piece will do it. This piece will have a polarization filter towards the subject followed by a quarter wave plate. The polarization filter preferrentially passes light of one linear polarization and you rotate it to select or reject the component you want or don't want. The quarter wave plate is fixed at 45 degrees to the preferential linear direction so half the lineararly polarized light leaving the filter is subject to a 90 degree phase shift and the other half, at right angles to the lagged light, isn't. This results in conversion to circular polarization which is necessary for use with cameras which contain beamsplitters which discriminate based on polarization.

Perhaps you are thinging of a polarimeter which consists of a linearly polarizing filter with a second "analysis" filter behind it. A sample is placed between the two and the relative rotation of the analyzer relative to the polarizer gives the rotation caused by the sample.

December 8th, 2004, 11:01 PM	#1
Jeff Miller Major Player Join Date: Nov 2004 Location: St Louis, MO Posts: 227	What is a circular polarizer? FIrst off, I'm an idiot, and don't know the details of polarized light sources. Second, I got a c. polarizer with the XL2 since a DP I work with swears by his. But, this polarizer is only one spinning filter. I might be wrong, but isn't a circular polarizer made of two elements, one fixed to the filter mount and the other rotating free? I haven't tried it on the camera yet since I've been afraid it's missing a piece. Can someone set me straight?

December 9th, 2004, 10:01 AM	#2
A. J. deLange Major Player Join Date: Jun 2004 Location: McLean, VA United States Posts: 749	When I first looked at your post I said "Can't be!" but after thinking about it for a minute decided that yes, one rotating piece will do it. This piece will have a polarization filter towards the subject followed by a quarter wave plate. The polarization filter preferrentially passes light of one linear polarization and you rotate it to select or reject the component you want or don't want. The quarter wave plate is fixed at 45 degrees to the preferential linear direction so half the lineararly polarized light leaving the filter is subject to a 90 degree phase shift and the other half, at right angles to the lagged light, isn't. This results in conversion to circular polarization which is necessary for use with cameras which contain beamsplitters which discriminate based on polarization. Perhaps you are thinging of a polarimeter which consists of a linearly polarizing filter with a second "analysis" filter behind it. A sample is placed between the two and the relative rotation of the analyzer relative to the polarizer gives the rotation caused by the sample.