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Thread: evaluating lenses with a polariscope

  1. #1
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    evaluating lenses with a polariscope

    I opened up an independent optometry practice and haven't sold glasses in 14 years. I want to provide excellent material to compliment my excellent care!

    I ordered a PAL lens identifier and didn't receive the unit I anticipated but thought I'd see how well the unit I received performed and it happens to have a polariscope evaluator as well. Trying out the polariscope was illuminating! I've started comparing the amount of distortion, apparent through the birefringence revealed through the the polariscope, of all the lenses that I get back from the different labs I send my frames out to. It varies, I guess, based on many factors but what I wonder is with the manufacturers claiming to reduce unwanted distortion in their PALs via digital surfacing, how much difference can it make if the frames are inducing distortion in the lenses?

    I measure base curves and recognize that the frames are capable of inducing measurable warpage of the lenses and I understand that 1D is ANSI standard tolerance. That seems like a lot.

    How many opticians use a polariscope or a colmascope when inspecting frame mounted lenses?
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ID:	12759cheap 'polarized' WM sunglasses; no distortion
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ID:	12760trivex Younger Image PAL; moderate amount of distortion, some in the near reading zone
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ID:	12761demo lens w/AR; small amount of distortion inferiorly
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ID:	12762hi index flat top; some small amount of distortion around the mountings but pretty clear
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ID:	12763SV poly; a moderate amount of distortion in the corners. Looks crisp through the lensometer. There is 0.75D of warpage of the BC. Is the birefringence inferior nasal and superior center a function of frame tension and an indication of distortion or related to being poly?

    Perhaps I'm too compulsive. I'm here to learn, so don't hold back.

  2. #2
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    Short of re-edging the lenses, is there anything I can do to relieve some of that unwanted tension?

    I took the lens out of the frame and scratched the lens but this is what it looks like out of the frame:
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    and this is what it looks like after re-installing the lens:
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    So the distortion has nothing to do with it being poly....
    Last edited by pearson1662; 06-09-2016 at 05:39 PM. Reason: addition of pics

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    Welcome to the forum, pearson1662!

    Stress is ok if it is uniform, not localized. Check out the stress in a glass hardened lens.

    You could ensure that the eyerim is curved to follow the curve of the lens in your lens, and a feathering of the bevel peak may reduce tension adequately.

    Keep in mind that stress can be internal pressure, denoting strength(not all stress is bad).
    Eyes wide open

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    Hi Pearson, I recently opened an independent office myself after 17 years, and was wondering the name of the unit you received which has worked well? Thanks!

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    Quote Originally Posted by Polliwog View Post
    Hi Pearson, I recently opened an independent office myself after 17 years, and was wondering the name of the unit you received which has worked well? Thanks!
    I don't remember the name of the unit. We received it from a lab in Lawton OK, Dunlaw Optical (580) 355-8410

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    Thanks, appreciate the info, will look into it!

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    Ghost in the OptiMachine OptiBoard Silver Supporter Quince's Avatar
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    Quote Originally Posted by pearson1662 View Post
    Short of re-edging the lenses, is there anything I can do to relieve some of that unwanted tension?

    I took the lens out of the frame and scratched the lens but this is what it looks like out of the frame:
    Click image for larger version. 

Name:	2016-06-09 17.32.19.jpg 
Views:	23 
Size:	51.4 KB 
ID:	12764

    and this is what it looks like after re-installing the lens:
    Click image for larger version. 

Name:	2016-06-09 17.37.38.jpg 
Views:	25 
Size:	32.5 KB 
ID:	12765

    So the distortion has nothing to do with it being poly....

    It may not be that poly automatically has the distortion, but because of its flexibility, it is more likely to have distortion once pressure from the frame is applied. In regards to what can be done; there are a couple different routes that can be taken. In some cases the lens can be very finely brought down in size to help relieve pressure or a switch into a plastic frame can be made. Aside from that, a lens material change could be made.
    Have I told you today how much I hate poly?

  8. #8
    Master OptiBoarder rbaker's Avatar
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    Your polariscope does not measure "distortion" it measures internal stress. Excessive stress can cause distortion. The stress can be relieved by judicious work on the hand edger. It ain't got anything to do with the lens material.

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    Quote Originally Posted by rbaker View Post
    It ain't got anything to do with the lens material.
    I totally does though, Poly disperses light much more under the same amount of "stress" than any other material

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