Page 1 of 2 12 LastLast
Results 1 to 25 of 35

Thread: Unwanted prism HELP!

  1. #1
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14

    Unwanted prism HELP!

    Hello everyone this is my first post so bear with me. I run the surface department in our lab. We have a loh pla blocker (not the prism blocker) and two v-75 generators. We have six toro cylinder machines (two finers, four polishers). Our LMS is labzilla/CCsystems with innovations calculations software. I keep getting unwanted prism in the low power lenses that we run. This problem is accounting for approximately 18 percent of my surface breakages (80/20 rule at its finest). Its always seems to me no more than .75 out and no less than .50 out. I just had a tech come out a check the generators and blocker and said they were fine.... What else can i do to fix this?

  2. #2
    Doh! braheem24's Avatar
    Join Date
    Feb 2001
    Location
    KOCF & 89ft ASL
    Occupation
    Other Eyecare-Related Field
    Posts
    3,843
    You sure it's only low powers or could it be that you may only notice it on low powers because the images does not decenter?

    surface and fine a plano on all finers and see if it's on all or some finers.

  3. #3
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    You make a good point would it be possible for the higher powers to "swallow up" the prism and it not be noticed. I will try your plano lens suggestion on Monday.

  4. #4
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Be sure to check the prism off the V-75 before you surface the lens. You should be able at this point, to tell if the prism is coming from blocking or the generator. It is rare, with surfacing systems of today to fine in prism. Your problem is most likely blocking or generating.

  5. #5
    OptiBoard Professional RT's Avatar
    Join Date
    Feb 2001
    Location
    CT
    Occupation
    Lens Manufacturer
    Posts
    879
    I pulled this from an Optiboard response from a few years ago...

    Do a little divide and conquer. You need to determine whether what you're calling "prism" is really prism or decentration. Different causes, different cures. Decentration would be if your guy was blocking the lens so the exact center of the lens is not over the exact center of the block. Prism would be if something in the process either tipped the lens in relation to the block, tipped the block in relation to the generator, or fined the lens unevenly to produce a thickness difference from one side of the lens to the other. Clearly blocking technique on an exec or a progressive can cause the lens to tip down toward the seg, creating prism.

    Inaccurate alignment of the lens would cause decentration. You'll need to rule out misalignment due to the inherent power of the lens blank. One problem with many surface blockers is that you need to look through the lens--you know, the thing that is supposed to bend light--to align the lens. The seg isn't really where it appears to be. That's why the Gerber Step One contained software to predict that misalignment and compensate for it. If you're seeing more problems on steep front curves, this may be the issue.

    Either way, you still need to be more precise than to lump "prism" and "decentration" together and call them both "prism".

    Do this test. Scribe 3 identical lenses with cross hairs, and block exactly on the cross hairs. On those lenses you should surface, fine, and polish a +1.00 power, plano power, and a -1.00 power. Be sure to mark the top of the lens so you can re-orient your results. Direction will be important.

    If you have a PRISM problem, you'll see the same amount of unwanted prism in roughly the same direction on each lens. Look for things that cause PRISM. (i.e. tip the lens back surface relative to the chuck face).

    If you have a DECENTRATION problem, you'll have the same amount of prism on the +1.00 and -1.00, but in opposite directions. And there will be no prism on the plano power. Look for things that cause DECENTRATION (misalignment in blocking, non-round blocks that sit offcenter in the chuck, etc.).

    If you have both, well, you're screwed.
    RT

  6. #6
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Decentration would show up as abundant prism in stronger powers and very likely go unnoticed in weak powers.

  7. #7
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    Im pretty sure it not a decentration issue

  8. #8
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
    Join Date
    May 2000
    Location
    Manchester, CT USA
    Occupation
    Other Optical Manufacturer or Vendor
    Posts
    2,976
    gemstone, why do think "[i]t is rare, with surfacing systems of today to fine in prism"? Today's finers and polishers are little different from yesterday's, except for the freeform/lapless systems, which, clearly, this fellow doesn't have.

    zhutchinscec, you didn't mention the kinds of lenses on which this occurs. Single vision? Bifocal? Progressive? Minus? Plus? All of them? Is it invariably base out? Are you measuring this after surfacing or in the finished specs?

  9. #9
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Quote Originally Posted by shanbaum View Post
    gemstone, why do think "[i]t is rare, with surfacing systems of today to fine in prism"? Today's finers and polishers are little different from yesterday's, except for the freeform/lapless systems, which, clearly, this fellow doesn't have.

    zhutchinscec, you didn't mention the kinds of lenses on which this occurs. Single vision? Bifocal? Progressive? Minus? Plus? All of them? Is it invariably base out? Are you measuring this after surfacing or in the finished specs?
    Most systems grind on mechanical center now days. There would have to be a very serious problem for a surfacing machine to create prism in a lens that is on mecanical center. In particulatly considering that the stock removal is around 4 points. Some computer systems are falty in that in some cases the computer will tell us to block on mechanical center , then after generating the lens it is no longer on mechanical center. (in cases of stronger lenses and smaller frames. )

    Yes, it is very rare and unusual to have prism problems due to fining, and non exsistant to have prism problems due to polishing.

  10. #10
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    To Shanbaum - The type of lens varies and is on minus powers. Its always..everytime base out and it is caught by my surface side inspector.

  11. #11
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
    Join Date
    May 2000
    Location
    Manchester, CT USA
    Occupation
    Other Optical Manufacturer or Vendor
    Posts
    2,976
    Quote Originally Posted by gemstone View Post
    Most systems grind on mechanical center now days. There would have to be a very serious problem for a surfacing machine to create prism in a lens that is on mecanical center.
    I can't say whether "most systems" were doing it, but we were surfacing on blank center when I started full-time in the lab 35 years ago. As was the case then, it's still possible to have an off-center grinding pattern that will induce prism, and that's something I saw frequently when I was doing consulting back in the 1980's (which is not to say that's not a "serious" problem - just a common one).

    One factor that may diminish the untoward effects of a badly maintained cylinder machine is lens size - back in the very old days (mid-60's for me), we blocked most lenses on PRP (though of course we didn't call it that back then), but the lenses were so small that they stayed fully engaged on the laps, even when the strokes weren't centered.

    So, zhutchinscec, we're down to minus lenses only (but not very minus), invariably base out. And that's OU?

    When you run your plano test, do three tests - one each with 4, 6, and 8 base lenses.

  12. #12
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    Shanbaum - Actually its hardly ever OU its usually one or the other. I will do the plano tests on Monday and let you know what kind of results i get.

  13. #13
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Quote Originally Posted by shanbaum View Post
    I can't say whether "most systems" were doing it, but we were surfacing on blank center when I started full-time in the lab 35 years ago. As was the case then, it's still possible to have an off-center grinding pattern that will induce prism, and that's something I saw frequently when I was doing consulting back in the 1980's (which is not to say that's not a "serious" problem - just a common one).

    One factor that may diminish the untoward effects of a badly maintained cylinder machine is lens size - back in the very old days (mid-60's for me), we blocked most lenses on PRP (though of course we didn't call it that back then), but the lenses were so small that they stayed fully engaged on the laps, even when the strokes weren't centered.

    So, zhutchinscec, we're down to minus lenses only (but not very minus), invariably base out. And that's OU?

    When you run your plano test, do three tests - one each with 4, 6, and 8 base lenses.
    Yes indeed. I learned to layout in 1973, before we started useing the mechanical center method. We had disk we would put in the all alloy blocks that would allow run the lenses to control or remove prism. Then, 65 mm was an oversize lens.

    Zuch, do you have the info to chart the power range and ammount of prism?

  14. #14
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    What chart are you referring to?

  15. #15
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Quote Originally Posted by zhutchinscec View Post
    What chart are you referring to?
    Is always the same amount of prism on all powers or is it a consistent amount?

  16. #16
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    Usually no higher power than -1.00 and it is consistantly .50D Base Out.

  17. #17
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Run the suggested test to determine if it is comming from the blockers or the generator. It may also be in some higher powers and going unnoticed. It funny .50 diopters would always kill a weaker power lens and go un noticed in a stronger lens.

  18. #18
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    I ran a 6 base plano lens and an 8 base plano lens (we were out of the 4 base so I'll run it this afternoon). Both tests came out fine. Could dirty block contribute to any kind of prism issues?

  19. #19
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    Quote Originally Posted by zhutchinscec View Post
    I ran a 6 base plano lens and an 8 base plano lens (we were out of the 4 base so I'll run it this afternoon). Both tests came out fine. Could dirty block contribute to any kind of prism issues?
    Yes. Make sure ths chucking surface is clean. The Flat part that goes against the flat part of the generating surface. Also the sides of the block that the generator grips.

  20. #20
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    One thought, you said minus lenses. The blank edge thickness is thicker on the lower bases (minus lenses). If the lens is being pulled from the chuck in the V-75 it is usually druing the cribbing cycle.

  21. #21
    Doh! braheem24's Avatar
    Join Date
    Feb 2001
    Location
    KOCF & 89ft ASL
    Occupation
    Other Eyecare-Related Field
    Posts
    3,843
    Depends on the blocks and where they're dirty.

    If they're dirty on the outside they can cause prism becausse they dont seat properly in the blocker.

    If your blocks are they type where the outer diameter touches the lens instead of the lens "suspended" on a bed of wax or alloy then inside dirt could also cause prism.

    You may want to also see if one person in the lab may be causing the problem, a poorly trained left handed person on a right handed chucked generator may not place blocks securly, poorly someone may not insert block into blocker securely before blocking.

    Did you surface the planos start to finish yourself or run it through the line and allow others to process it?

  22. #22
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    They are Loh blocks that we use. I have tried multiple operators (including myself)...it didn't make any difference. I did the test lenses myself.

  23. #23
    Banned
    Join Date
    Dec 2006
    Location
    On Top
    Occupation
    Other Eyecare-Related Field
    Posts
    1,662
    That blocking system forms the surface that sets against the flat front generator chuck? I would check for bumps on that surface. But they would also be a problem on plus lenses.

  24. #24
    Doh! braheem24's Avatar
    Join Date
    Feb 2001
    Location
    KOCF & 89ft ASL
    Occupation
    Other Eyecare-Related Field
    Posts
    3,843
    Did you run the planos through the software or manually set the generator?

  25. #25
    Bad address email on file
    Join Date
    May 2009
    Location
    st louis
    Occupation
    Optical Laboratory Technician
    Posts
    14
    Through the software

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Unwanted Prism on bifocals
    By Yomandinga in forum General Optics and Eyecare Discussion Forum
    Replies: 20
    Last Post: 10-22-2008, 03:45 PM
  2. where can one sell unwanted frames?
    By melvilletim in forum General Optics and Eyecare Discussion Forum
    Replies: 13
    Last Post: 11-04-2007, 11:24 AM
  3. unwanted prism on Bi-Focals
    By jameselex in forum General Optics and Eyecare Discussion Forum
    Replies: 21
    Last Post: 05-26-2006, 01:01 PM
  4. unwanted prism on bi-focals
    By Michael Walach in forum General Optics and Eyecare Discussion Forum
    Replies: 2
    Last Post: 05-23-2006, 10:52 AM
  5. Unwanted Tele-sales
    By sarahr in forum General Optics and Eyecare Discussion Forum
    Replies: 9
    Last Post: 06-10-2002, 08:28 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •