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Thread: Marking/Blocking Polarized Lenses

  1. #1
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    Recently, a pair of polarizing sunglasses was submitted to us to build. I was told that polarizing lenses are not spotted in the same way as regular lenses, and that if there was even a minor error in the axis of the mounted lenses, the result would be to nullify their polarization effect.

    Can someone fill me in on these creatures?

    The only reference work currently available to me is Clifford Brooks "Essentials for Ophthalmic Lens Work" and it does not mention polarizing lenses.


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    Forever Liz's Dad Steve Machol's Avatar
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    Polarized lenses have an axis of polarization. This is usually indicated by reference notches at the edges of the lenses. The lenses should be layed out so the notches are aligned across the 180 degree line after finishing. Of course if you grind prism for decentration, you'll need to keep this axis in mind.

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    Master OptiBoarder Jeff Trail's Avatar
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    Originally posted by Kurt Brandon:
    Recently, a pair of polarizing sunglasses was submitted to us to build. I was told that polarizing lenses are not spotted in the same way as regular lenses, and that if there was even a minor error in the axis of the mounted lenses, the result would be to nullify their polarization effect.

    Can someone fill me in on these creatures?
    Kurt,

    You have to look at it as "two" lens in one .. what you do to the back of the lens (sphere & cylinder) has no bearing at all on the lens ability to de-polarize light.
    Think of the polar sheet as a set of blinds.. they run str8t across, it blocks out light at an angle but still you can see out of them when slightly open.. same thing with a polarized lens.. light strikes surfaces and becomes "polarized" (scatters) and the polar sheet "blocks" light waves that are not bouncing back to you in a str8t line .. now bi-focals and PAL's are a given on getting the polar sheet alligned correctly since the segs runs along the same "axis" (0-180) as the sheet.. but in a SV you have to allign those notches on the out side edge.. which is done when surfacing.. you lay the lens out with those tabs on the 180 line and so when you grind the power into the ocular surface , if it has cylindrical correction when you mark up the lens to edge they polar sheet should be in the correct place.. if not then the lens was ground incorrectly.
    One thing I always found interesting was that a "polarized" lens is actually de-polarizing polarized light..
    You have two functions in a pair of polarized correction lens.. the back surface (ocular) is for the correction of vision, while the front surface (polar shield) is for blocking polarized light reflections.

    Jeff " gee I thought you put the sheet on the 90 " Trail :)

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    1) On the polarized lenses there were two semi-circular indentations on the periphery, 180° apart. I assume that the axis of polarization lies along a line connecting the two.

    2) It sounds as though, when the lenses are spotted prior to being blocked, the three dots on the "spot line" must be coincident with the axis.


    Are the above items correct?

    Kurt

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    Forever Liz's Dad Steve Machol's Avatar
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    Originally posted by Kurt Brandon:
    1) On the polarized lenses there were two semi-circular indentations on the periphery, 180° apart. I assume that the axis of polarization lies along a line connecting the two.
    Correct.

    2) It sounds as though, when the lenses are spotted prior to being blocked, the three dots on the "spot line" must be coincident with the axis.
    Actually the three lensometer dots should coincide with the 180 line - not the axis. And these line should fall on the axis of polarization.


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    Master OptiBoarder Jeff Trail's Avatar
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    Kurt,

    Steve is correct.. seems like you are trying to connect the two "sides of the lens".. that is not exactly what you are doing, if it a spherical correction or plano then when you dot it up to edge make sure you ROTATE the lens so that the polar sheet is on the 180 (marked by the tabs) or if the lens is to thin and the tabs got cut away turn the lens till they cross polarize and put your own marks on it 90 degrees from each other then when you dot them make sure you keep those marks on the 180 :)
    As for a lens that has both spherical and cylindrical correction.. when you put it on the lensometer and you rotate then lens to the correct axis, if it was cut correctly, then those tabs will be on the 180 along WITH your three dots to block it to edge...
    If you think that it MAY be off axis (polar sheet) when you dot it up you should hold both lens up and rotate the three dots 90 degrees from each other and the lens should turn DARK (cross polirization)
    It can get a little confusing if you don't have a pair of lens in your hand to tinker with :)
    The thing to remember the axis the cylindrical correction runs along really has NO bearing on the axis of the polar sheet UNLESS it was cut incorrectly when grinding.
    The problem is that if you have to "shift" the polar axis to get the cylindrical axis correct then you defeated the purpose of having a polarized lens :) .. but if you keep the polar axis on the 180 and the cylinder axis is off then you end up with someone telling you "oh I feel dizzy" .. it all boils down to layout when surfacing, if the mistake is made there nothing can be done to correct the problem.. or if it is a spherical or plano power you have to make sure you keep the tabs on the 180 when you block it to edge.. sounds simple huh? ..
    Nothing beats having a pair in your hands to tinker with to really get the feel of what we are talking about though..

    Jeff "Grind'em if ya got'em" Trail

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    OptiBoard Professional Mike Fretto's Avatar
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    Kurt,one trick we always use is to have an old junk poloroid lens with the notches still in it and keep it by the lensometer. That way you can do as Jeff suggested and cross polorize with the new lense to make sure its right. We get them all the time where the notches are cribbed off in the generator, its not bad when theres a cyl. but you get a pair of spheres and it can be tricky. I hope this helps

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    What is the difference between the "180 line" and the axis of polarization?

    My present conception is as follows:

    1. On a spherical lens, you may safely rotate the lens so that the lensometer dot line coincides with a line connecting the two edge marks on the polarized lens.

    2. On a lens with cylinder correction, the alignment of the cylinder axis is done in the usual way. If, when the lens is oriented properly for spotting, the polarization-axis line is not coincident with the spot-line, the lens was manufactured wrongly and is unusable.

    Is this correct?

    Kurt


    [This message has been edited by Kurt Brandon (edited 08-13-2000).]

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