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Thread: Ray Tracing Free-form

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    Ray Tracing Free-form

    Ray Tracing of Free-form Progressive Lenses

    I have been told for years that Ray Tracing Free-form lenses was impossible. I think I found a way that will enable us to Ray Trace Free-form Progressives effectively, although imperfectly.

    Other Posts in this forum explain Ray Tracing better, but to make it simple Ray Tracing is basically a lensometer that measures multiple points of a Progressive lens at the same time, and then graphs them in a drawing that looks like a colored Topo map. Ray Tracing traditionally will provide 2 sets of data, variation in spherical power and variation in astigmatism. Since we use samples without astigmatism, its presence is one sign of distortion. Light is moving through the areas of higher astigmatism without a clear focal point. Spherical power is easier to interpret as it shows us where the lens fails to provide the intended correction, just like your lensometer does but at multiple points.

    The difficulty with traditional ground progressives is not getting the Ray Tracing data, but interpreting it. For someone to get better picture we have to overlay both sets of data which no software that I have seen actually does. The other problems with Ray Tracing are that there is no currently ability to gauge binocular symmetry or judge a corridor in reference to Listing Laws, having a wide corridor is one thing, placing it before the path of the eye another. Although these could be resolve with advanced software, I lack the resources to explore those options fully. So inherently Ray Tracing is an imperfect, but valuable tool to compare one progressive design to another. Again, this won’t solve the limitations inherent in Ray Tracing, but it will give new life to this great tool. I hope that someone can take this information and use it effectively to the benefit of the Optical community.

    Some of the newer Ray Tracers are doing work of lensometers after surfacing. Instead of just checking a lens at one point, they verify both the design and powers of a Free-form lens are as intended across its entire surface. Ray Tracing is the only way to do this. They work by Ray Tracing the finished progressive and comparing it to its own theoretical point file. The point file is the design as the manufacturer intended. It then resolves the two data, and ideally the picture should be blank. If it isn’t it reveals the variations between the actual lens and the intended, giving the ability screen a poor lens. An example of this Ray Tracer is at http://www.rotlex.com/ffv.asp. By using this technology, I believe we can effectively Ray Trace Free-form lenses.

    We would use comparative Ray Tracing but instead of comparing a Progressive to its own point file, we compare it to a SV Free-form lens by the same manufacturer. Although traditional Ray Tracing works best with Plano Distance Progressives samples, for this we would have to move into using powered lenses instead as they would be better representatives of the Aspheric (and other) compensations involved. Free-form lenses need some room to run so to speak, and we need to give it to them. For example, my sample lenses would be Brand A Progressive and Brand A Single Vision, with the same distance power (a -2.00 sphere would work) with a +2.00 Add on the Progressive (as its average). We would Ray Trace both, but then use the single vision point file as the comparison. Since they are built from the same algorithms, the Single Vision Lens would effectively neutralize everything but the actual progressive design. We would be left with an effective Ray Trace that would reveal only the Progressive attributes of that lens. In effect, we would have the same Ray Trace we have now with Grinders.

    This could be done with Free-form progressives that did not have a Single Vision equivalent as well, but with less accuracy. Most of the algorithms used to compensate for face form, panto etc, are fairly standard. In this case would we take a standard -2.00 Aspheric SV, Ray Trace it, create a point file, and then modify that file based on the standard algorithms and matching the power at OC. Although this would not be perfect, this would neutralized out most (but not all) of the fudge factors. It would still reveal the basic flavor of that lens.

    In the future, we would need to combine Comparative Ray Tracing with advance software that could analyze the lens against multiple criteria to form a better single picture. This is all possible to current technology; it’s just a matter of their being not enough market to justify the costs involved. I don't have access to the new Ray Tracers, but if someone does and wants to test this out, it would be terrific.

    Van Y. Rue -- 2/2/2012
    Sharpstick

    Single Vision Express – Director of Business Development
    Optical Consulting Group - Consultant
    Highline Community College - Instructor - Business and Optics
    Last edited by sharpstick777; 02-13-2012 at 07:34 PM.
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    Master OptiBoarder MakeOptics's Avatar
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    Fantastic idea, I would love to see the results of your project once completed.

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    Quote Originally Posted by PhiTrace View Post
    Fantastic idea, I would love to see the results of your project once completed.
    No will let me have their Ray-Tracers for enough time, they are all on busy prodution lines. So I hoping that someone with access could help it along? Tony, have you seen this thread?
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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    "Ray tracing" in the context of measuring progressive lenses typically refers to modeling the interaction between the spectacle lens and the eye. This requires calculating the path that light takes between the eye, the spectacle lens in its position of wear, and some set of coordinates in object space. The refraction of light along this path is then computed in order to determine the changes in optical powers at each point.

    Although conventional progressive lenses are designed to provide the correct "vertex" powers, as measured by a standard focimeter, some modern progressive lenses are designed using ray tracing. And, in some cases, free-form versions of these lenses are also designed using ray tracing with the design parameters set to the individual patient's specific frame fitting measurements and prescription data, instead of the average or "default" values commonly used in semi-finished lens design.

    It is possible to rig a focimeter to measure something akin to "ray traced" power by mounting the lens on a moveable jig that rotates at a fixed distance associated with the "vertex sphere" of the eye, but these are rarely available commercially. And most of the commercially available lens "mapping" systems that I am aware of do not offer true ray tracing capabilities. This makes it difficult, if not impossible, to evaluate the true optical performance of the lens as perceived by the wearer with the lens in its position of wear.

    The issue is that both surfaces of the lens must be characterized by lens mapping equipment before the device can accurately predict the refraction of light by the lens in some assumed position of wear. However, most lens mappers only measure the final power through the lens without any regard to the contribution of the individual surfaces to the final optical output. Neverthless, you can still compare the intended through power of the lens design to the through power of the lens sample as measured by one of these devices in order to estimate the deviations in power when assessing quality.

    Of course, if you physically measure both surfaces individually, you can mathematically reconstruct the lens in order to perform accurate ray tracing calculations. This can be done with a CMM (coordinate measuring machine), for instance. Alternatively, at least manufacturer of lens mapping equipment has developed a device that measures power through the lens as well as one surface in order to estimate the optical characteristics of the opposite surface.

    While the cost of these devices is still high enough to preclude their use at the dispensing level, quality free-form laboratories should already be investing in this type of equipment in order to ensure consistent quality and accuracy. This is especially important for free-form lenses, since significant optical errors may occur over the lens from the free-form surfacing process that are not readily appreciated by one or two power measurements by a standard focimeter.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Darrell, by my method, using the same brand A in SV and Prog, we effectively neutralize the POW because it would be the same. The comparative Ray Tracers I have seen only compare one lens to another, they don't take POW into account. Its still useful.

    And not all Free-form lenses use indiviualized POW measurments, just defaults like traditional grinders. So if is valuable for grinder is should still valuable for most FF.

    Or are you saying, that if we did this to some lenses, the result wouldn't look good?

    Sorry, but although you provided a wealth of incredible information I am a little lost as to what your conclusions are?

    Thanks
    Van
    Beer is proof that God loves us and wants us to be happy ~Benjamin Franklin

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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    First, when you say "ray tracer," can you clarify what type of instrument you are referring to, since most lens mappers (including the one you linked to) don't actually offer ray tracing? These devices can compare one lens to another, but that doesn't necessarily mean that you can derive any meaningful conclusions from this comparison. You will simply know whether Lens A looks like Lens B or not.

    Second, if your free-form lens is not customized for any patient-specific data at all, then simple lens mapping equipment that shows only through or vertex power would probably suffice, as you suggest. Of course, this also implies that the final lens will not be any better than a semi-finished lens either.

    In either case, however, you can still use one of these lens mappers to evaluate quality against a known target distribution from the original lens design, regardless of whether you can actually predict patient acceptance with them.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister View Post
    First, when you say "ray tracer," can you clarify what type of instrument you are referring to,
    I use the term Ray Tracer because if I used "Moiré Deflectometry Free-form Verifer" you would probably be the only person on this board who knew what is was (You would probably even know the people who filed the patent in 1994!). I am compromising terminology to make this discusion open to a wide variety of people. I believe more Opticians are familiar with that term than any single other, although Lens Mapper is good, we can switch to that instead.

    "
    Quote Originally Posted by Darryl Meister View Post
    Second, if your free-form lens is not customized for any patient-specific data at all, then simple lens mapping equipment that shows only through or vertex power would probably suffice, as you suggest.
    So it will work, just not as well with POW adjusted lenses.

    Quote Originally Posted by Darryl Meister View Post
    Of course, this also implies that the final lens will not be any better than a semi-finished lens either.
    Semi-finished in Free-form... being a hocky puck? Please explain more if you will.
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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    I use the term Ray Tracer because if I used "Moiré Deflectometry Free-form Verifer"
    Yes, it would be best to use a different term. "Ray tracing" has a very specific meaning in lens measurement technology. When you have been told in the past that free-form lenses could not be "ray traced," the term referred to a computer calculation of the eye and lens as an optical system. Moire deflectometry, on the other hand, is a physical measurement of light, not a computer calculation of an optical system.

    So it will work, just not as well with POW adjusted lenses

    Yes, it can work to verify quality, just not to compare the optical peformance of one lens to another, unless both lenses were designed without computer ray tracing.

    Semi-finished in Free-form... being a hocky puck? Please explain more if you will
    Free-form is a manufacturing process, not a method of lens design. Nor is free-form surfacing inherently superior to traditional lens molding in most cases. The true advantage to free-form surfacing is the fact that it allows you to design lenses in "real time" using data specific to the individual wearer. If you do not customize the lens for information specific to the individual wearer, however, you are not really offering in significant gain in optical performance over traditional semi-finished lenses.

    I can, for instance, cut a basic SOLA VIP using a free-form lens generator. But this free-form VIP isn't going to work any better than a traditionally molded VIP.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister View Post
    Yes, it would be best to use a different term. "Ray tracing" has a very specific meaning in lens measurement technology. When you have been told in the past that free-form lenses could not be "ray traced," the term referred to a computer calculation of the eye and lens as an optical system. Moire deflectometry, on the other hand, is a physical measurement of light, not a computer calculation of an optical system.
    Thanks, I was always told that Ray Tracing is the process, and Lens Maps are the result.
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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    Yeah, maps or "contour plots" are just graphs of data. These maps just represent various measurement quantities, which might be derived from through or vertex power measurements, mathematical ray tracing, surface height measurements, etcetera.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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