Results 1 to 7 of 7

Thread: Understanding Equithining Prism

  1. #1
    OptiBoard Novice
    Join Date
    Jul 2012
    Location
    midwest
    Occupation
    Optical Laboratory Technician
    Posts
    3

    Understanding Equithining Prism

    Hello everybody.

    A few years back I asked for help and received quite a few extremely helpful answers regarding surfacing, so I figure I'd try for seconds.

    I'm curious if anyone can explain (with out to much technical jargon) what equithining prism in a PAL is used for besides cosmetics, and how (or if) it is of benefit to the patient.

    From what I understand a small amount of od/os matching equithin is acceptable due to processing difficulties, but I am curious everytime I see an RX come through with 3+ diopters prescribed.

    Thank you in advance!

  2. #2
    Master OptiBoarder
    Join Date
    Feb 2010
    Location
    ALBUQUERQUE
    Occupation
    Optical Laboratory Technician
    Posts
    442
    By design, a standard Progressive SF blank is molded like a wedge..upper distance portion thicker than lower addition portion. As you know, to balance base down prism is used on positive distance Rx's in affect balancing upper and lower thickness. To calculate the amount of prism the ADD power is mulitplied by a factor of .66 or 2/3 the add power. This seems to work well if the vertical decentration is in the 3 to 6 mm range. Higher vertical decentration results in increased amounts of Prism for thinning. Typical prism thinning is not prescribed so I'm assuming you mean the 3+ is when you layout.

    This resource has been posted in other posts and is the best explanation of prism thinning

    http://www.dicoptic.izispot.com/Files/thinpsm.pdf

  3. #3
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,327
    Benefit is pretty much just cosmetics. Although in certain prescriptions the OC may be moved giving the client an optical advantage and in other cases it may cause ill effects. The rule of thumb is that the amount of prism is generally 2/3 the add power however more sophisticated algorithms exists that take into account the fitting height, frame dimension, and surface geometry of the lens.

    I prefer the PRP to be as close to the center of the B as possible in PALs, prism thinning can cause the OC to shift up, sometimes high enough to loose a line or even two of visual acuity through the reading segment of the lens. I will often ask for no prism thinning in a PAL if I think it's going to effect the patients view.

  4. #4
    OptiBoard Novice
    Join Date
    Jul 2012
    Location
    midwest
    Occupation
    Optical Laboratory Technician
    Posts
    3
    Quote Originally Posted by PRECISIONLAB View Post
    This resource has been posted in other posts and is the best explanation of prism thinning

    http://www.dicoptic.izispot.com/Files/thinpsm.pdf

    I've actually used this information to teach my lab rats. =)

    I'm curious why I see the occasional order come into our lab asking for an odd amount of prism (over 3-4 diopters) on a progressive lens.

  5. #5
    Master OptiBoarder
    Join Date
    Feb 2010
    Location
    ALBUQUERQUE
    Occupation
    Optical Laboratory Technician
    Posts
    442
    Do you have an example rx in which the prism of 3-4 is prescribed. I'm guessing a high plus with high add may be the case. Perhaps the OD does not want any base up prism in the distance portion of the progressive. OC @ the PRP

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    This resource has been posted in other posts and is the best explanation of prism thinning
    Is this old article still floating around? Once it's uploaded to the Internet, I guess it lasts forever. Here are two more recent treatments of the subject:

    OptiCampus.com Prism-Thinning Course

    Prism-Thinning Article for Dispensing Optics

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  7. #7
    OptiBoard Novice
    Join Date
    Jul 2012
    Location
    midwest
    Occupation
    Optical Laboratory Technician
    Posts
    3
    Quote Originally Posted by PRECISIONLAB View Post
    Do you have an example rx in which the prism of 3-4 is prescribed. I'm guessing a high plus with high add may be the case. Perhaps the OD does not want any base up prism in the distance portion of the progressive. OC @ the PRP
    I'm afraid I'm relying on memory. Although after reading through the materials posted here, I'm getting a clearer understanding into the probable motivation of this kind of request on an order.

    In the past I've assumed the benefits are cosmetic, but I've never had anyone explain if equithinning is included as an additional benefit to a patient/RX the same way adding something like a bifocal would be.




    Quote Originally Posted by Darryl Meister View Post
    Parts of this explain things in a different light for me! Thank you very much for this, especially the last regarding verticle prism imbalance. I've had animated discussions with coworkers when we come across this kind of situation. I do believe this will help my argument quite a bit.

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. A Better Understanding
    By Hardworlder in forum General Optics and Eyecare Discussion Forum
    Replies: 20
    Last Post: 08-08-2014, 05:13 PM
  2. Help Understanding WTR/ATR Astigmatism
    By oxmoon in forum Progressive Lens Discussion Forum
    Replies: 10
    Last Post: 03-10-2011, 05:57 PM
  3. Help me understanding some new lenses
    By John Smith in forum Just Conversation
    Replies: 4
    Last Post: 01-03-2011, 07:36 PM
  4. Need Optiboarder Help understanding this...
    By Barry Santini in forum General Optics and Eyecare Discussion Forum
    Replies: 4
    Last Post: 12-01-2010, 03:08 PM
  5. Understanding Prism-Thinning by Darryl Meister
    By Steve Machol in forum OptiBoard File Directory
    Replies: 0
    Last Post: 05-07-2005, 08:58 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
  •