polarized lenses,question for darryl

[harry a saake]

The other day while checking out a polarized job, i took a sample lens and rotated it 90 degrees, as i always do to make sure they are polarized correctly. The reaction instead of the lens turning from light to darker was instead kind of a streak. I then realized that the lenses in the frame were cr-39 and my tester was a polarized poly lens. question is why did i not get the normal reaction you get, when you turn one over the other and there the same material?

[Darryl Meister]

Hi Harry,

I would be guessing with this one -- without actually seeing the phenomenon, myself. I will say that the materials shouldn't have a drastic effect on the Polaroid film performance... But strain in the materials may produce some side-effects that become apparent when examining the lenses like you do. (This discussion also assumes that both Polaroid filters are designed to be 100% polarized.)

I would say that your streak is probably related to strain in one of the lenses (or both). When you cross two Polaroid filters at a right angle (90 deg), you basically make an "analyzer" -- like a Polariscope. These two crossed filters will detect strain in any lens between them -- just like a Polariscope would. When plastic lenses are under stress, they become "optically active" and cause the waves of light to vibrate in different orientations as they pass through the areas of strain. This is why you can use a Polariscope to determine whether or not a plastic lens is mounted too tightly in a metal frame, for instance. Anyway, when you use two polarized lenses in this fashion, you are basically analyzing the stress of the two layers of lens materials on the inside of each of the facing Polaroid films. That is, you'll have one layer of poly and one layer of CR between your two polarized lenses when you hold them together like that. Since either a polycarbonate lens or a plastic lens mounted in a frame may exhibit strain, you are likely seeing the net effects of both when you look at them in this fashion. This strain manifests itself as funny light and dark regions, bands of color, unique patterns, etc.

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 08-11-2000).]

[harry a saake]

Hello Darryl, yes i am familiar with the polariscope and strain patterns, etc as believe it or not we use to have to put every pair of safety glasses under a polariscope back in the old B&L labs. This was a case of cr-39 mounted in a metal frame. took my poly sample lens that i always use to see if there polarized correctly, and turned it to the right 90 degrees, which will normally make the lens turn completely dark. In this case it did not and the only difference i can think of is the two different materials, but i am trying to reconcile in my mind why this would make any difference. Perhaps you could run a few tests there at Sola and let us know what you found