Results 1 to 6 of 6

Thread: pure cylinder lens oblique axis with prescribed prism

  1. #1
    OptiBoard Novice
    Join Date
    Nov 2009
    Location
    Bari
    Occupation
    Lens Manufacturer
    Posts
    3

    pure cylinder lens oblique axis with prescribed prism

    Hi all,

    This is my first post. Your discussions helped me a lot and I want to thank you all of you!

    We've got in our LAB the following prescription:

    Sph: 0.00 Cyl +0.50 Ax 75 Prism 6DP base up, Index 1.592
    By using the thin prism approximation formula I grinded in a prism of 5.80° at 270

    6/(100*0.592) and then to degrees

    The resulting lens at geometrical center is only 5.50D base up

    In 99% of the cases, even with torical lenses, this formula works correctly. It seems to fail only with pure and high oblique cylinders.

    How can I calcualte the correct prism angle for the generator, taking into account those cases ? This formula doesn't take care about the lens power at all.

    Thank very much for your help

  2. #2
    ATO Member HarryChiling's Avatar
    Join Date
    Apr 2005
    Location
    Nowhereville
    Occupation
    Other Eyecare-Related Field
    Posts
    7,765
    You'll find your answer in a paper by Don Whitney in the downloads section of the site, here's the link:

    http://www.optiboard.com/forums/show...by-Don-Whitney

    Another thread that may be of interest is:

    http://www.optiboard.com/forums/show...gree-meridian/
    Last edited by HarryChiling; 02-16-2011 at 10:11 PM.
    1st* HTML5 Tracer Software
    1st Mac Compatible Tracer Software
    1st Linux Compatible Tracer Software

    *Dave at OptiVision has a web based tracer integration package that's awesome.

  3. #3
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    If you are actually grinding in the specified prism at the specified angle, there are no additional calculations required, cylinder power or not. But keep in mind that your calculation, fabrication, or measurement of prism may still be off.

    The deviation D, in degrees, of a 6.0 diopter prism P is P = ATAN(6.0 / 100) = 3.43º. The thin prism approximation for the prism apical angle A, as you know, is given by:



    which is 5.80º for a refractive index of 1.592 and a prism of 3.43º. The exact formula for the apical angle A is given by:



    which is 5.76º for the same values. Obviously, this is not enough to make a meaningful difference. So, it is unlikely that your actual calculation is the problem.

    However, you need to confirm that you are using the correct refractive index. If your actual refractive index is weaker than the value you're using, this will result in a reduced prism error, although this seems unlikely with a refractive index of 1.592.

    And this assumes, of course, that your generator software does indeed require the prism apical angle in degrees and not a value in prism diopters.

    You can confirm the fabrication of your prism by using a thickness caliper at appropriate points at the upper and lower edges of the lens blank. The thickness difference T at a measurement diameter M is given by:



    which is 5.1 mm for a prism of 6.0 prism diopters at a measurement diameter of 50 mm. It is important that these measurements are taken relative to a point centered at the blocking center of the lens and along the correct meridian, especially when cylinder power is involved.

    Finally, you can look at measurement issues, such as verifying the prism of the lens at a point that does not represent the actual blocking center or prism reference point location. You may also observe some small differences depending upon your focimeter (that is, whether it is an "FOA" or an "IOA" instrument).

    For this particular job, the cylinder power is at an oblique axis, which means that you will induce both horizontal and vertical prismatic effects away from the prism reference point of the lens due to Prentice's rule, even though the lens has no sphere power.

    So, if the prism reference point location is not centered correctly in front of the lens stop of the focimeter, you may get an error. Similarly, if there were any blocking errors, you may throw the prism measurement off, once you position the lens at the desired cylinder axis and horizontal prism amount (i.e., 0.0).

    Some of the other OptiBoarders may also come along with some additional comments or suggestions.

    Best regards,
    Darryl
    Last edited by Darryl Meister; 02-20-2011 at 05:31 PM.
    Darryl J. Meister, ABOM

  4. #4
    OptiBoard Novice
    Join Date
    Nov 2009
    Location
    Bari
    Occupation
    Lens Manufacturer
    Posts
    3
    Dear Darryl,

    I can confirm:

    - The index is correct
    - The generator requires prism in degrees
    - We measure the prism at the blocking center

    I'll take the thickness measures you suggest and I'll let you know.

    Thanks for the answer

  5. #5
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Because the lens has very little power, and the cylinder axis is close to 090, lens positioning errors in front of the focimeter will not result in significant errors in prism for this particular job. The power is almost Plano through the vertical meridian, resulting in very little change in vertical prism. Verifying the thickness difference of the lens may reveal the source of the problem.

    Also, what type of focimeter are you using? Manual focusing? Automatic? If you have an automatic focimeter, especially a Humphrey-style instrument, there could be several factors at work here. I'm assuming with a refractive index of 1.592 that you're using a high-index lens material like MR-8. You'll want to ensure that the Abbe value and reference wavelength (mercury e-line verus helium d-line) settings of your device are correct.

    Once you have confirmed the settings of the instrument, if applicable, and the positioning of the prism reference point over the lens stop, you might rock the lens vertically a bit, bringing the top forward and backward by a few degrees, in order to see if this reuslts in an improvement due to the use of a focimeter with "infinity on axis," versus the "focus on axis." Your lens surfacing calculations are probably based upon having the focus on axis.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    By the way, it is probably worth noting that your error in prism would be very close to the error produced by attempting to use prism values in the standard tooling index of 1.530, instead of the actual 1.592 refractive of the material: (1.530 - 1) / (1.592 - 1) * 6.0 = 5.37 prism diopters, close to your actual result of 5.50 prism diopters.

    Since you are apparently entering in the apical angle in degrees, instead of prism diopters, this really shouldn't matter though, unless your software is converting your apical angle back into some other prism quantity using the standard tooling index. If so, the settings in your surfacing software should reflect this.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Calculating Cylinder Power in an Oblique meridian
    By musicvirtuoso in forum General Optics and Eyecare Discussion Forum
    Replies: 20
    Last Post: 01-20-2018, 01:32 PM
  2. Cylinder in an Oblique meridian
    By musicvirtuoso in forum Ophthalmic Optics
    Replies: 1
    Last Post: 08-14-2010, 09:25 AM
  3. Oblique Cylinder Chart Question
    By Strab in forum Ophthalmic Optics
    Replies: 9
    Last Post: 05-06-2010, 08:38 AM
  4. prescribed prism with slab-off
    By mwh1058920 in forum General Optics and Eyecare Discussion Forum
    Replies: 13
    Last Post: 07-27-2009, 03:38 PM
  5. measuring prescribed prism ...
    By mirage2k2 in forum General Optics and Eyecare Discussion Forum
    Replies: 9
    Last Post: 04-26-2006, 10:34 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
  •