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Thread: glass segs

  1. #1
    OptiBoard Apprentice
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    Have a customer who we gave ft28 PGX-add wouldn't darken same as the rest of the lens so he returned them-i asked the lab they said thats the way it is. Patient came back with new pair from his doctor and add was darkened same as the rest of the lens. Our lab heat treats would chem treating have helped?

  2. #2
    Bad address email on file John R's Avatar
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    If lens is a minus power from experence then the seg always is a bit lighter due to it being made from diffrent glass. Were the new lenses the same substance as the old pair as this can make a diffrence

    . Our lab heat treats would chem treating have helped?
    Is this a standard U.S thing to toughen all glass lenses as in the U.K we only do this to order?

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    [This message has been edited by john r (edited 05-21-2000).]

  3. #3
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    Air or heat tempering is not recommended for photo reactive lenses. The better choice is chemical tempering since the high heat used in the heat tempering process can destroy or reduce the darkening and speed of the photo reactive compound used in the lens.

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    Master OptiBoarder OptiBoard Silver Supporter
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    Call and find out what brand the other guys are using. I have never in 16 years of practice seen any PGX FT other than an Exec or round seg darken the same as the distance part. One brand may darken better than another brand due to the manufacturing process and index of the seg.

  5. #5
    sub specie aeternitatis Pete Hanlin's Avatar
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    Beth,
    I have never seen a PGX FT that had a seg that darkens, either... This is because the seg is made from flint glass, which does not contain the silver halide that makes the rest of the lens photoreactive. If the patient has a pair that does darken evenly, the lens may have been ground thicker (the surrounding crown glass would make the seg "appear" to darken).

    John had asked if we are required to temper all glass lenses... with the exception of "special" lenses (e.g., executives), all glass lenses manufactured in the US must be tempered and drop ball tested before given to the consumer. There are several glass products that are not available in the US for this reason.

    Pete

  6. #6
    Bad address email on file Darris Chambless's Avatar
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    Redhot Jumper

    I'm sitting here listening to my new CD by the group Third Door Down and basically digging the toonage that is this group. Not that any of that is relevant to anything, but I figured I'd share that with you all in the event you got the urge to run out and pick up a CD for no apparent reason (I have it cranked up on the sound system here in the shop for all to hear ;-)

    More than anything I wanted to post to make sure I was still a demonic minion, but I figured I'd add my verbiage to the question regarding glass lenses Dudes and Dudettes (I get into this grunge rock persona when I'm listening to my kind of music ;-)

    Glass PGX bifocals = lighter lens at the bifocal. No if ands or buts.

    I too have been in the biz for a long time and unless they are doing what Pete said in grinding the lens thicker you will always have this problem. Now, with that said over time the lens will of course pick up and maintain more of its color and can make the bifocal seg "look" a little darker but this usually doesn't take place until the lenses are 18 to 24 months old minimum, but by that time the lenses should be replaced anyway either for script change reasons or from chips or scratching that ruin the integrity of the lenses impact resistance.

    And now one of the lyrics from the first song on this album "If I go crazy then will you still call me Superman? If I'm alive and well will you be there holding my hand? I'll keep you by my side with my super human might Kryptonite." If you put the hard hitting toon with it you'll be hooked :-)

    Back to the glass thing, history dictates that one of the reason (but not the main reason) for making advancements in plastic changeable lenses was consistency in color change. The lenses were designed to be a) lighter weight b) less susceptible to temperature changes (Yeah right ;-) c) consistent changing in color over a predetermined period of time (e.g. darkening and lightening at the same rate for the life of the script without getting lighter or darker due to exposure) and d) consistency throughout the lens in overall color change (no lighter or darker spots or patches)

    If you take a single vision high minus glass photo gray-extra lens (uncut) and let it darken you can see a noticeable difference in how dark the outer edges are as compared to the center of the lens. Add to that a bifocal of a different molecular structure in-set into the surface of the lens and Viola! it will be lighter in color. Flattops and kryptoks alike will do the same thing, but a progressive is consistent in its makeup and will stay consistent in its color change as well.

    Okay enough of my backwoods gibberish and now on to see if I'm still being listed as a demonic minion :-) If I'm wrong in anything I've said then I apologize and will reiterate that the main reason for my post is to see if Steve changed my designation or not :-)

    Darris C.

  7. #7
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    Lightbulb

    You can minimize the difference in colors between the top of the lens and the seg by exposing the back side of the lens to the sun. Flip over and expose the front side to the sun after darkening on the back side occurs. If it is stubborn and doesn't give the exact density you are seeking alternate exposures to sun and then placing in then freezer usually works well. Lenses change quicker with chem tempering.

  8. #8
    Forever Liz's Dad Steve Machol's Avatar
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    One other thing to consider - the add power and seg size will also affect the relative "lightness" of a segment. This is because higher add powers and larger seg sizes will have deeper segments. All things being equal, a 3.00 diopter add will always appear lighter than a 1.00 add.

    Steve

  9. #9
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    Redhot Jumper

    WAIT A MIN. THERE IS NO LAW SAYING THAT GLASS LENSES HAVE TO BE HEAT OR CHEM-TEMPERED BUT IT DOES STATE THAT THEY DO HAVE TO BE DROP BALLED!
    C-R-A-C-K

  10. #10
    Bad address email on file Darris Chambless's Avatar
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    Originally posted by Bev Heishman:
    You can minimize the difference in colors between the top of the lens and the seg by exposing the back side of the lens to the sun. Flip over and expose the front side to the sun after darkening on the back side occurs. If it is stubborn and doesn't give the exact density you are seeking alternate exposures to sun and then placing in then freezer usually works well. Lenses change quicker with chem tempering.
    Hello Bev,

    I think I'm a little unclear here. How is exposing the back surface of the lens to the sun going to help in the darkening process for the patient? Since the patient wears the glasses on their face it might be a bit of a problem to the patient to have to take the glasses off and reverse them every time they go out into the sun. Or is it a one time deal?

    Also since inset segs will thin the area of changeable material thus lessening the darkening effect, how will it help in matching the darkness?

    Finally, have the companies manufacturing the chemicals for the chem-treaters changed the components in the chem-temp salt to allow less compression of the glass molecules? If I want a lens to have the maximum amount of impact resistance I chem-treat them, but with the chemicals I use it will sacrifice its changeability to a certain extent because of the compression factor. I use heat treating if my main objective is the darkening effect and heat treating always seems to allow the lenses I use to change faster and get darker than a chem-treated lens.

    Just curious. If all this works I'm all for it. Thanks for the tips.

    Darris C.


  11. #11
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    Big Smile

    Pete implied that glass execs need not be drop ball tested. This is not necessarily true. Z80.1-1995 (don't have the new one yet but this section is unchanged) states, "All lenses shall be capable of withstanding the impact test described in 6.6 Laminated, plastic and raised-edge multifocal lenses that may be damaged by impact test procedures may be certified by the manufacturer as conforming to the initial design testing or statistically significant sampling as specified by Title 21, Code of Federal Regulations, 801-410. "Many manufacturers USED to pass impact requirements thru to the lab if the lens was processed according to recommended procedures. They no longer do that. Ergo, statistical sampling with good records are required.

  12. #12
    Master OptiBoarder karen's Avatar
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    I know we are talking glass here (BTW-thanks Darris for all the cool info) but just a reminder that the seg on a Corning Sun Sensors does get aa dark as the rest of the lens

  13. #13
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    I like Steve's answer for the variance do to add power and segment size.
    The one I really question is Segments made of Flint Glass. The only lens left made with Flint segments is the Krytok. All others are made of Barium Crown.

  14. #14
    Master OptiBoarder Clive Noble's Avatar
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    Thumbs up

    Originally posted by karen:
    I know we are talking glass here (BTW-thanks Darris for all the cool info) but just a reminder that the seg on a Corning Sun Sensors does get aa dark as the rest of the lens
    Karen, you're correct, but you didn't explain why....Sunsensors material is like a solid tint, I've just made a pair of -8.00sph
    and the centre was lighter than the edges, you don't get this with Transitions.

  15. #15
    Master OptiBoarder Shwing's Avatar
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    Redhot Jumper

    Ehllo, all. I know it is a bit off the original question, but I prefer heat hardening.

    The reason (because enquiring minds want to know)...

    I can prove (under the polariscope) that the lens IS hardened.

    I would think that, being the litigious beings you are, that the U.S.of A. would also prefer this method, purely for C.Y.A. reasons...


    Having said that, yes, I do prefer chem hardening on glass lenses, though I think that I may sell about 5 pair per year...



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  16. #16
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    Hi Darris,

    I expose the back side of a lens to sunlight and expose the lens to cold in the freezer. The patient recieves them in a darkened state and they change back to indoor color as I complete the dispensing. This is a one time thing. I learned this from my college professor who was an engineer way back when....

    I never had a patient come back to me complaining of different colors between the upper and lower portions of the lens because the color difference was minimized. If you allow the lens to trigger from the front only you do have disappropriate color differences. By triggering the back side also you have exposed the underlying base glass.(When I had a complaint I could always identify which one of the dispensers worked with them.)

    I also found that chem tempering produces deeper color changes on PGX than heat. I believe it has something to do with the ion exchange. There were times when we couldn't resolve an issue of the new lenses being dark enough that we took heat treated lenses
    and rehardened them in a chenm unit. I like the new timed shortened chem unit for PGX.

    My current favorite glass are the thin and dark lenses.

    I am sorry I confused you oh gracious one.

  17. #17
    Bad address email on file John R's Avatar
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    Bev said

    My current favorite glass are the thin and dark lenses.
    Do they make a thin and dark bifocal? We do the S/V but do not shift many due to the extra cost, also dont ever see much advertising for them. do you find this a problem state side

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  18. #18
    Master OptiBoarder
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    There are limited seg styles. FT-28 and VIP are the ones I used if I remember correctly.

  19. #19
    OptiWizard
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    Thin & Dark available in D-25, D-28, 7 x 28, Freedom 5 PAL (Excel), Varivue PAL (Vision Ease) and VIP. I'm surprised that you haven't seen more advertising; Corning has developed a Corning Connections program with several labs, has had some national advertising on Corning Clear 16 and Thin & Dark. Both, by the way, can be surfaced to a 1.5 C.T. on minus Rx's. With today's smaller frames, the glass alternative becomes more viable. In particular, AR is MUCH more durable on glass.

  20. #20
    Master OptiBoarder karen's Avatar
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    Originally posted by Clive Noble:
    Karen, you're correct, but you didn't explain why....Sunsensors material is like a solid tint, I've just made a pair of -8.00sph
    and the centre was lighter than the edges, you don't get this with Transitions.
    Because the photchromatic dye is injected when the lens is cast the entire monomer is changing color in a SS lens. It would make sense that the thick edges of a high minus lens would be darker than the center- didn't this also happen with PGX?? Transitions is surface technology which works great in those high minus scenarios but will also wear out more quickly than SS. I am a huge plastic photochromatic fan and I wear both types of lenses but at this point I prefer the SS- just as dark as my extra active but much clearer inside-especially with AR!

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