Results 1 to 21 of 21

Thread: Memo on Understanding the Position of Wear

  1. #1
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700

    Memo on Understanding the Position of Wear

    I recently wrote a white paper for the Lens Technical Committee of the Vision Council regarding the optical principles and practical implications associated with the position of wear of spectacle lenses, which some of you may find useful.

    Memo on Understanding the Position of Wear

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  2. #2
    Bad address email on file
    Join Date
    Dec 2006
    Location
    Florida
    Occupation
    Dispensing Optician
    Posts
    2,175
    i printed and will read later. Thanks for caring so much about what you do, you are rare.
    Craig

  3. #3
    Master OptiBoarder
    Join Date
    May 2000
    Location
    Fayetteville, NC, USA
    Occupation
    Dispensing Optician
    Posts
    1,843
    Outstanding as usual, Darryl. I have long admired your development and appreciate all you do for Opticianry and the field at large. It is indeed a shame that the vast majority of Opticians now are so poorly trained that they will understand very liitle about the topic. I continue to work to alleviate that situation. I wish you the very best.

    Warren

  4. #4
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Glad you found the information useful, Warren.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  5. #5
    OptiBoard Professional Randle Tibbs, ABOM's Avatar
    Join Date
    Dec 2012
    Location
    Alabaster, AL
    Occupation
    Other Eyecare-Related Field
    Posts
    170
    Great article, and by the way, will you be at SECO this year?

  6. #6
    Manuf. Lens Surface Treatments OptiBoard Gold Supporter
    Join Date
    Aug 2002
    Location
    Dorval, a suburb of Montreal in a park like environment on Lake St.Louis.
    Occupation
    Other Optical Manufacturer or Vendor
    Posts
    21,306
    What a job. I just see all the hours and weeks of work that went into this master piece.

    Congratulations Darryl
    Chris Ryser
    ________________________________________
    DLO. NA.IC.I.T.PO

    http://optochemicals.com............................. http://arcoatings.com

  7. #7
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Do you have any reference to the "effective tilt" angle you discuss in the beginning. I have gone back and forth between the arctan and arcsin and eventually settled on the arcsin as you have outlined in your white paper but never really had a reference, just the geometry to look at. If you also have any reference to aspheric and as you discussed PAL's being hard to measure lens tilt that would be an interesting read as well. I too agree that the deviation from spherical to aspheric surfaces causes the formula to deviate from a more accurate estimation to a good guesstimation. I wonder if the apsheric radius can be subbed in to the equation to gain a more accurate measure?

  8. #8
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    I'll have to compare more in depth later when off work, but I have another question:

    Past papers I have read have combined the tilt to create a combined effect that the dioptric matrix is rotated by, in this paper you outline a step by step process with the rotation matrix. How important is it to rotate around the x axis first and y axis second?

  9. #9
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Also I wish there was a better discussion of hte merit functions with an example, I guess that would be divulging trade secrets or "secret sauce" as they call it.

  10. #10
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Do you have any reference to the "effective tilt" angle you discuss in the beginning.
    If you are referring to the tilt due to meniscus decentration, Stimson acknowledges the effect in Ophthalmic Dispensing. I derived my own formula, however, many years ago for my old Rx compensation software, using the geometry of my figure and operating with the assumption that a modern, guided mini-bevel is generally positioned equidistant from the front surface of the lens.

    Also, you would use the arcsin function in this formula, not the arctan function, since you know the distance DEC and the length R1 to FP. It is difficult for me to explain without a drawing, but the arctan function would only apply if the distance DEC was measured in the plane of GC, with R1 extended past the surface, not in the plane of FP.

    That said, for the small angles involved, the dfferences between the two are negligible and, in fact, you could drop the inverse function altogether and simply compute the angle in radians as the ratio, DEC/R1.

    Past papers I have read have combined the tilt to create a combined effect that the dioptric matrix is rotated by, in this paper you outline a step by step process with the rotation matrix. How important is it to rotate around the x axis first and y axis second?
    By "past papers," I assume that you are specifically referring to Keating's 1995 article, since he is one of the few who treats that particular subject in detail. His calculations actually assume the reverse of the Euler rotation order that I am describing. My order is more consistent with the orientation of the lens as established by electronic frame tracing though.

    Nevertheless, I am certain that the differences between the two orders are small for the usual pantoscopic and face-form tilt angles involved. Neither way is necessarily superior to the other, although I feel that mine is more consistent with manufacturing practice. However, I haven't actually tried yet to work out numerical examples to see the actual differences, myself.

    Also I wish there was a better discussion of hte merit functions with an example, I guess that would be divulging trade secrets or "secret sauce" as they call it.
    I mainly kept this section short and simple in the interest of length, since this particular topic wasn't really critical to the discussion. At the very least, astigmatism and mean add power are included as merit function terms, but I imagine that a lens designer may also include additional terms associated with quantities such as gradients of power, image swim, and so on. However, the more terms involved, the greater the computational burden.

    Best regards,
    Darryl
    Last edited by Darryl Meister; 01-14-2013 at 03:37 PM.
    Darryl J. Meister, ABOM

  11. #11
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Quote Originally Posted by Darryl
    By "past papers," I assume that you are specifically referring to Keating's 1995 article, since he is one of the few who treats that particular subject in detail
    I should add that Keating's is certainly not the only good paper on the topic. Harris (Optom Vis Sci 2002, Optom Vis Sci 2006), Blendowske (Optom Vis Sci 2002), and others have published some nice treatments on this subject, although I have spent more time personally digesting Keating's.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  12. #12
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Quote Originally Posted by Darryl Meister View Post
    If you are referring to the tilt due to meniscus decentration, Stimson acknowledges the effect in Ophthalmic Dispensing. I derived my own formula, however, many years ago for my old Rx compensation software, using the geometry of my figure and operating with the assumption that a modern, guided mini-bevel is generally positioned equidistant from the front surface of the lens.

    Also, you would use the arcsin function in this formula, not the arctan function, since you know the distance DEC and the length R1 to FP. It is difficult for me to explain without a drawing, but the arctan function would only apply if the distance DEC was measured in the plane of GC, with R1 extended past the surface, not in the plane of FP.

    That said, for the small angles involved, the dfferences between the two are negligible and, in fact, you could drop the inverse function altogether and simply compute the angle in radians as the ratio, DEC/R1.
    That's interesting I ended up using the arcsin function I believe from past discussions that invloved your posts on this thread an even without images it makes sense that the tan function would have the hypotenuse extend beyond the front surface of the lens to a flat lens plane that would exist had the lens not been tilted, the sin function is definitely the more accurate. I'll have to look further into Stimsons book to see if I can find the inference.

    Yes for the small angles the ratio can be subbed out for the tilt altogether and tan and sin are very close in value at these small numbers but for the sake of accuracy and understanding I prefer to know which way is the most accurate.


    By "past papers," I assume that you are specifically referring to Keating's 1995 article, since he is one of the few who treats that particular subject in detail. His calculations actually assume the reverse of the Euler rotation order that I am describing. My order is more consistent with the orientation of the lens as established by electronic frame tracing though.
    I saw you mentioned that, so from a purist standpoint you are aligning your compensations to mimic the way in which the frame tracer is measuring the lens. I can see how that could eliminate or reduce inconsistencies in tilt measures.

    Nevertheless, I am certain that the differences between the two orders are probably small for the usual pantoscopic and face-form tilt angles involved. Neither way is necessarily superior to the other, although I feel that mine is more consistent with manufacturing practice. However, I haven't actually tried yet to work out numerical examples to see the actual differences, myself.
    I would have to say that if the compensation is more aligned with manufacturing methods then this is a more superior method. Conversions and rounding alone can cause loss in accuracy probably negligible given the accuracy required to dispense an average lens.


    I mainly kept this section short and simple in the interest of length, since this particular topic wasn't really critical to the discussion. At the very least, astigmatism and mean add power are included as merit function terms, but I imagine that a lens designer may also include additional terms associated with quantities such as gradients of power, image swim, and so on. However, the more terms involved, the greater the computational burden.

    Best regards,
    Darryl
    I would love to see more on the merit functions and specifically how each companies weighs their functions.
    Last edited by MakeOptics; 01-14-2013 at 12:14 PM.

  13. #13
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    I saw you mentioned that, so from a purist standpoint you are aligning your compensations to mimic the way in which the frame tracer is measuring the lens. I can see how that could eliminate or reduce inconsistencies in tilt measures
    Yes, this is also the order of Euler rotations assumed by SOLA and ZEISS free-form lens designers.

    I would love to see more on the merit functions and specifically how each companies weighs their functions
    There are really two separate stages of lens design involved with progressive lenses that are customized for the position of wear: 1) the initial design of the original target lens and 2) the optical optimization of the prescription lens against the target lens.

    For the initial design of the target lens, more effort generally goes into determining the metrics associated with merit functions or arriving at other numerical and analytical solutions that achieve the desired distribution of optical performance, including defocus, unwanted astigmatism, image swim, high-order aberrations, binocularity, and so on. It is also not uncommon to begin with an analytical approach (using formulas) to generate a starting surface, and then to rely on a numerical approach (optimization) to refine the design of the surface.

    For the free-form optical optimization process, on the other hand, we are only interested in replicating the ideal performance of the original target lens by minimizing the differences in optical power errors due to oblique astigmatism in the position of wear between the prescription lens and the target lens. So only mean power and astigmatism is important at this stage. It is assumed that the other performance metrics will be achieved if the power distribution of the lens is sufficiently replicated.

    For the initial design of the target lens, the weightings are often determined manually, requiring the lens designer to tweak individual values at various points or areas over the lens design until he or she achieves the desired result. For free-form optical optimization, on the other hand, it may be more computationally economical just to specify a weighting function that varies with position over the lens, generally weighting the central regions more heavily.

    Anyway, this is how the ZEISS and SOLA lens designers usually approach these problems. I'll leave it to the folks at the other lens manufacturers.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  14. #14
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Quote Originally Posted by Darryl Meister View Post
    For the initial design of the target lens, more effort generally goes into determining the metrics associated with merit functions or arriving at other numerical and analytical solutions that achieve the desired distribution of optical performance, including defocus, unwanted astigmatism, image swim, high-order aberrations, binocularity, and so on. It is also not uncommon to begin with an analytical approach (using formulas) to generate a starting surface, and then to rely on a numerical approach (optimization) to refine the design of the surface.
    I have heard this explanation before and appreciate it even though I would never be a situation to design it gives me great pleasure in knowing the process. So the 1st step is like a rough draft and further iterations refine. How many iterations of a design are customary when creating a new lens design or does it vary greatly?

    Also, to a give a weighing factor compared to defocus, oblique astigmatism would be worth 1/2 of a defocus error, (according to the spherical equivalent) is there a simple guesstimation that applies to other errors as well?

  15. #15
    Master OptiBoarder OptiBoard Gold Supporter
    Join Date
    Oct 2004
    Location
    Down in a hole!
    Occupation
    Dispensing Optician
    Posts
    13,045
    Thank you for all that you do for our profession Darryl!

    Although most of the topic is way above my understanding and grasp, I am enjoying the discussion between PhiTrace and Darryl immensely!

    Thanks guys!

  16. #16
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Quote Originally Posted by Fezz View Post
    Thank you for all that you do for our profession Darryl!

    Although most of the topic is way above my understanding and grasp, I am enjoying the discussion between PhiTrace and Darryl immensely!

    Thanks guys!
    I've been called crazy and I've been called brilliant, but only Darryl is crazy brilliant. Fezz, I really enjoy the posts and articles Darryl puts up, too bad he teases us with very little of his "optical crack" and with big gaps between.

  17. #17
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    So the 1st step is like a rough draft and further iterations refine. How many iterations of a design are customary when creating a new lens design or does it vary greatly?
    This is a fairly complex question, because it will dependly greatly upon the particular approach of the lens designer. What you're referring to as an "iteration" is probably slightly different from how the term would be used in lens design. There are actually a number of stages at which an actual "iteration" of some sort may be performed; a few examples come to mind:

    1. Prototype lens design iterations that each represent a specific combination of design parameters are utilized in wearer trials to evaluate the performance or satisfaction of one or more prototype lens designs under consideration.

    2. Numerical optimization iterations are utilized during the lens design process to minimize merit functions, perform finite element analysis, or to otherwise seek a lens surface that satisfies some mathematical criteria as closely as posssible.

    3. Design iterations are also performed for each base curve and add power combination, which often requires modifying the inset of the eyepath, fine-tuning the asphericity, or even varying the corridor and/or viewing zones for multi-design lenses.

    4. Finally, mold iterations are performed to fine tune the lens design based upon measurements of lenses produced from the initial set of molds in order to ensure that the final lens surface matches the intended surface after shrinkage occurs.

    But these iterations are actually distinct from the steps involved in developing a lens design using analytical and numerical tools. The "rough draft," as you put it, is often produced using an analytical or semi-analytical software tool that allows the lens designer to define the initial progressive lens surface by specifying various design parameters, such as the progressive corridor, size of the viewing zones, et cetera.

    Multiple tools or stages may be involved to define the shape of the progressive power profile, design the periphery of the lens, improve binocularity, apply asphericity to the base curve, and so on.

    This starting surface could then be fine-tuned using numerical optimization tools, such as finite element method, to improve certain optical measurements by minimizing a merit function, for instance. Bivariate splines are then frequently used to join smoothly the individually optimized sections or points of the lens. It is also possible to incorporate ray tracing for the position of wear into this optimization process.

    I actually discuss the basic of a typical progressive lens design process in my Optics of Progressive Lenses article at OptiCampus.com.

    but only Darryl is crazy brilliant
    Well, I appreciate the kind words but, in reality, I simply distill the work of men and women much smarter than I am into something that at least remotely resembles English.


    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  18. #18
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Quote Originally Posted by Darryl Meister View Post
    The "rough draft," as you put it, is often produced using an analytical or semi-analytical software tool that allows the lens designer to define the initial progressive lens surface by specifying various design parameters, such as the progressive corridor, size of the viewing zones, et cetera.
    Is this proprietary software to Zeiss, or are we talking about available "over the counter" software such as ZEMAX or OSLO?

    Quote Originally Posted by Darryl Meister
    Well, I appreciate the kind words but, in reality, I simply distill the work of men and women much smarter than I am into something that at least remotely resembles English.
    So you translate engineerese to neaderthalese.

  19. #19
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Is this proprietary software to Zeiss, or are we talking about available "over the counter" software such as ZEMAX or OSLO?
    I imagine that most of the major lens manufacturers probably develop their own lens design tools internally, but there are some private lens design firms out there that service a few lens suppliers and it is not uncommon for some of the major lens manufacturers to supply OEM designs to some lens suppliers.

    I know that Carl Ziess, SOLA, and American Optical each have (well, "had" for AO, since they're all retired) their own lens design teams, typically comprising several different lens designers, mathematicians, vision scientists, and computer programmers as well as various ancillary personnel.

    I don't know that popular commercial optical design packages like OSLO, Zemax, and Code V actually provide tools for designing progressive spectacle lenses though.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

  20. #20
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,300
    Quote Originally Posted by Darryl Meister View Post
    I don't know that popular commercial optical design packages like OSLO, Zemax, and Code V actually provide tools for designing progressive spectacle lenses though.
    I figured as much. I know that the ray tracing is pretty good with those packages and anlyzing lenses is great, but designing optics just seems a bit outside of the software's scope. Plus the parameters for the design I figure has got to come from scientists in the field applying the latest in geometric and physiological optics.

    In this day and age I would love to see the programmers, do you think they are aware of the science behind the optics or are these guys pure red bull drinking programming machines? I am guessing that free form software packages are the wave of the future and inputs have to be accepted from the various lab management systems in their respective formats and then feed to various machines in the the various formats that these newer CNC equipment requires. I have been boning up on my coding getting ready for the day and age that I am allowed to submit a file tot he lab and have them produce a design wholly my own. I don't think I would dable in the PAL market, yet anyway but I would love to cut my teeth on some free form SV lenses.

    Do you get access to any equipment that you can tinker with? That would be optical nerd nirvana.

  21. #21
    Carl Zeiss Vision Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    In this day and age I would love to see the programmers, do you think they are aware of the science behind the optics or are these guys pure red bull drinking programming machines?
    Honestly, with what appears to be so much derivative work in this industry, so it's hard to say.

    For Carl Zeiss Vision, at least, the computer programmers on our lens design teams are often mathematicians by education, so they often both develop and implement the algorithms and design tools, working closely with the lens designers. The lens designers are typically physicists who learn to specialize in optics or, particularly in Germany, optical engineers.

    Do you get access to any equipment that you can tinker with? That would be optical nerd nirvana
    I work with our lens design software on a daily basis, but since I am no longer based at facility with an on-site laboratory, I don't do much tinkering with equipment anymore. Years ago, when SOLA still had manufacturing, the R&D department, and a test surfacing lab in Petaluma where I used to work, I did get to play around with a lot of neat stuff. <Insert Sigh>

    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. Memo on Reference Wavelengths
    By Darryl Meister in forum OptiBoard File Directory
    Replies: 2
    Last Post: 04-12-2010, 07:50 PM
  2. What is a "position of wear" compensation meant to compared to?
    By Barry Santini in forum General Optics and Eyecare Discussion Forum
    Replies: 106
    Last Post: 12-25-2008, 02:53 PM
  3. Lab Coats on Sales Floor...to Wear or Not to Wear
    By Ladyoptician in forum General Optics and Eyecare Discussion Forum
    Replies: 32
    Last Post: 05-06-2008, 05:13 AM
  4. What's happen with PALs when the frame is not in the right position of wear ?
    By Bobie in forum Progressive Lens Discussion Forum
    Replies: 17
    Last Post: 12-25-2006, 02:30 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
  •  
OptiBoard is proudly sponsored by:
Younger Optics, Carl Zeiss Vision Inc., Vision Systems, Inc., Optitech USA and The Optical Group