Thanks to all, and especially Darryl, Pete, Awtech and QDO1, for a great education.
Thanks to all, and especially Darryl, Pete, Awtech and QDO1, for a great education.
Andrew
"One must remember that at the end of the road, there is a path" --- Fortune Cookie
This thread is over my head but very interesting. A few people have asked questions about the abbe value of the lens material used for these designs and either the questions have been unanswered or I've not understood them!
Are the newer designs attempting to correct abberations that are inherent in the lens material itself? - is this even possible? If not, then what is the point of designing a great lense with superior (peripheral) optics and then to go ahead and make that lens in a material that has inherently poor (peripheral) optics?
Some (slightly off topic) examples are the Hoya Nulux EP (bi-aspheric) eyry 1.7 - abbe value = 36! and the Nikon SeeMax - abbe value = 32!!!
Unless these designs overcome the poor abbe value of the lense they dont make much sense! The Hoya Nulux EP is also available in eyas 1.6 - abbe value 42 - this makes sense - premium lense, thin, flat and still has a reasonable abbe value!
Can any of the experts make sense of all this?
ps: I'm not looking for free advice - I'm genuinely interested in this stuff!
Are the newer designs attempting to correct abberations that are inherent in the lens material itself? - is this even possible? If not, then what is the point of designing a great lense with superior (peripheral) optics and then to go ahead and make that lens in a material that has inherently poor (peripheral) optics?
Abbe value is only one (relatively minor) component of a material's optical properties. The abbe value of the eye's internal structures is only around 45 or so, which limits the impact of a material's abbe value on vision. Also, a good deal of prism is required to create chromatic aberration sufficient to significantly impact vision.
I've attached a chart based upon the data collected by Meslin & Obrecht (sp?) regarding relative visual acuity and abbe value. As noted on the chart, a wearer needs to look through approximately 6-8 diopters of prism to experience a significant drop in relative acuity. So, in a -6.00 polycarbonate lens, you would need to deviate about 10mm from the optical center/axis of the lens before you may notice the aberration. For the -1.00 sph patient, chromatic aberration is almost completely negligible.
As you correctly note, high index materials have low abbe values. So, the dispenser who avoids polycarbonate in favor of 1.67 due to abbe concerns is perhaps a bit misguided.
Perhaps of more importance, from a processing point of view, is the rigidity of the material. Look through an Rx glass lens in a lensometer, and the crispness of the mires is amazing. That's because the material is harder (and in some regards, easier to grind/polish to a precise curvature). Crown Glass is largely unaffected by heat during processing. Conversely, high index materials (and polycarbonate in particular) are sensitive to temperature deviations during grinding, fining, polishing. I believe this is the real issue with many "non-adapts" due to material.
Fortunately, the equipment and processes used by most labs are far more advanced today compared to the past. Additionally, the new processes (discussed ad nauseum in this and other threads) is subject to far tighter controls- which can only improve the optical performance of ophthalmic materials.
Hope this is helpful,
Pete
Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74
thanks for the response. So abbe value only really comes into play in very high rx. Now what if this -6 poly lense in your example had 4 diopters of base in or base out prism - am I right in saying that you wouldn't have to look far from the oc at all to see abberations/drop in visual acuity?
Also, on the point of the rigidity of the material, is it fair to say then that a softer material with a higher abbe (42), may not necessarily produce a better lense than a harder material with a lower abbe (33/36), since the softer lense is more susceptible to problems during processing?
The term ray tracing really applies to just about any method of analytically calculating the optical performance of a lens system for an arbitrary principal ray. A principal ray (or sometimes chief ray) is the ray passing from the object point through the either the entrance pupil or center of rotation of the eye after refraction through the lens. A typical ray tracing process often begins by calculating the refraction of the principal ray at each intersection with the lens surface and determining how the surface affects the vergence (or wavefront) of light at each of these intersections.Originally Posted by Pete
This can be done in more than one way. You can, for instance, ray trace a lens using a bundle of rays (e.g., 4 rays arranged in a square representing the pupil) centered on the intersection of the principal ray with each surface, calculating refraction by computing the angle of each ray with the normal to the surface. Or you can use the surface characteristics directly at each point of intersection of the principal ray, calculating the change in vergence at that point. You guys may use even a different method, still.
Keep in mind that the eye is a fixed and somewhat centered optical system, so it is less susceptible to its own lateral chromatic aberration. However, lateral chromatic aberration -- which is a prismatic displacement that causes the infamous color fringing -- is the most bothersome effect produced by chromatic aberration in spectacle lenses.
That said, monochromatic aberrations will certainly compound the blur and other negative visual effects produced by chromatic aberration. Consequently, a free-form process that improves optical performance will improve the overall performance of the lens and, consequently, wearer acceptance. If a wearer is going to purchase high-index anyway, which is usually the case for the prescriptions often used with free-form lenses, he or she will certainly get the best possible optical performance from the free-form version of the product if it has been fully optimized.
There aren't many that include comparisons of higher order aberrations like coma and trefoil, probably because most lens designers really haven't been too concerned about them in the past. Higher order aberrations due to large pupil sizes, like coma and spherical aberration, aren't an issue in single vision lenses until you get into extremely high powers.
In progressive lenses, they are a necessary evil in regions of changing (or progressive) power. Moreover, they are already relatively low in the stabilized viewing zones of the lens, unavoidable in the progressive corridor, and completely overwhelmed by astigmatism in the periphery. But that's not to say that Essilor hasn't reduced them in the distance zone of Physio even further compared to many other lens designs.
Nevertheless, not too long ago I ran across one paper that compares the wavefront aberrations of various progressive lenses. Look for Comparison of aberrations in different types of progressive power lenses, by Eloy A. Villegas and Pablo Artal, in a 2004 issue of Ophthalmic and Physiological Optics. It may be out on the Internet somewhere.
My guess is that minimizing coma in a progressive lens probably doesn't demand all that much precision (at least no more precision than any other progressive), since you are really just minimizing the gradient, or rate of change, in surface curvature. It would probably be analogous to smoothing out medium spatial frequency differences in the free-form process.
Darryl J. Meister, ABOM
Sorry I didn't look at the chart, but the above begs to be challenged. The human visual system does NOT need a "good deal of prism" to significantly impact vision. Most people would notice the aberration in far less than 10 mm off center in a -6 polycarb lens if given the opportunity to compare it with the same off center vision in say a trivex, cr39 or any other kind of lens in the world. Why do the manufacturers always downplay the effects of their distortions? (I'm more invlolved in this same argument in IOLs than in SRx lenses, but the same logic applies!!!).
and add about 4 base out prism in one lense and you can see color seperation right on the oc!
So if a patient has these kinds of problems in their lense, can these new designs do anything about it??? If not then the only option is a lense with a higher abbe!
There are a few other threads around that discuss chromatic aberration and Abbe values. You might have a look through a few, if you haven't already.If not then the only option is a lense with a higher abbe!
Darryl J. Meister, ABOM
yes but do these other threads discuss aberration with respect to the types of lenses being discussed in this thread?
This thread is about wavefront corrections, and wavefront corrections do not reduce chromatic aberration.yes but do these other threads discuss aberration with respect to the types of lenses being discussed in this thread?
Darryl J. Meister, ABOM
I'm sorry, its confusing because this thread used to be two seperate threads that have merged, one about Wavefront corrections and the other about freeform, Individualized and WAVE, etc. Anyway, a few posts were questioning the value/effects of new hi-tech design when combined with low abbe lenses.
WAVE refers to a "wavefront" correction. Neither free-form nor wavefront technology will reduce chromatic aberration.I'm sorry, its confusing because this thread used to be two seperate threads that have merged, one about Wavefront corrections and the other about freeform, Individualized and WAVE, etc. Anyway, a few posts were questioning the value/effects of new hi-tech design when combined with low abbe lenses
Darryl J. Meister, ABOM
Thanks Darryl for all your help! I've read the article you wrote on chromatic aberration (on opticampus) - it is excellent and very clear. I'm just a novice so forgive me if my questions are out of context. All I've really been trying to find out is if there is ANY technology out there that can reduce chromatic aberration in a low abbe lens. I think you've answered my questions - higher abbe less chromatic aberration. Thanks again.
Unfortunately, for a given prescription or prism power, the only way to reduce chromatic aberration is by selecting a material with a higher Abbe value. (You can also minimize the prism power by selecting a smaller frame that fits close to the eyes.) However, minimizing the total blur produced by the lens with technologies like free-form optimization should reduce overall wearer discomfort in high-index materials by significantly reducing the effects of non-chromatic (i.e., monochromatic) aberrations.All I've really been trying to find out is if there is ANY technology out there that can reduce chromatic aberration in a low abbe lens.
Also, as I note in my article (and as Pete noted earlier in this thread), there are sometimes other factors that contribute to wearer discomfort in high-index materials, including processing aberrations, proper base curve selection, and material quality. Fortunately, these have all improved over the years. Further, the MR-7 and MR-10 materials used for 1.67 high-index are very robust.
Darryl J. Meister, ABOM
Excellent. Thank you very much!
There's been a lot of (healthy) skepticism on this forum about the application of wavefront technology in spectacle lenses, especially WRT correcting the higher order aberrations that are unique to each patient's eyes - ala Ophthonix, for example. Far be it from a layman like me to speculate about how well such lenses would work, or for what particular visual conditions/diagnoses they might best be prescribed. But I would like to comment again on the theory of this invention - as much as I've been able to glean it from the overview provided by the vendor, via Ophthonix Wavefront-Guided Vision Technology, and from my own research, which has led me to US Patent 6,942,339 "Eyeglass manufacturing method using variable index layer".
This is the part of the patent text that made the most sense to me:The present invention utilizes the technology developed by the wavefront aberrator in which a layer of variable index material, such as curable epoxy, can be sandwiched between two plane or curved glass or plastic plates. This sandwich is then exposed to the curing radiation (i.e., UV light) that is modulated spatially or temporally in order to create spatially resolved variations of refractive indices. This will allow the manufacturing of a lens that is capable of introducing or compensating for low and high order aberrations.
In the simplest form, two lens blanks are sandwiched together with a layer of epoxy such that the lenses used in conjunction approximately correct the patient's refractive spherical and cylindrical correction to within 0.25 diopters. Subsequently, the epoxy aberrator would be exposed to curing radiation in a pre-programmed way in order to fine-tune the refractive properties of the spectacle lens to the exact spherical and cylindrical prescription of the patient's eye.
Another application of the present invention is to manufacture multi-focal or progressive addition lenses constructed with a layer of variable index material sandwiched in between the two lens blanks. The drawback of progressive addition lenses today is that, like regular spectacle lenses, a true customization for a patient's eye cannot be achieved due to the current manufacturing techniques. Using the two lenses and epoxy, a customized progressive addition lens or reading lens can be manufactured by appropriately programming the curing of the epoxy aberrator.
The present invention provides an opportunity to manufacture lenses that give patients "supervision." In order to achieve supervision, higher order aberrations of the patient's eye need to be corrected. Since these higher order aberrations, unlike the spherical and cylindrical refractive error, are highly asymmetrical, centering of the eye's optical axis with the zone of higher order correction ("supervision zone") is important. To minimize this effect, one could devise a spectacle lens that incorporates a supervision zone only along the central optical axis, allowing the patient to achieve supervision for one or more discrete gazing angles. The remainder of the lens would then be cured to correct only the lower order aberrations. An optional transition zone could be created between the supervision zone and the normal vision zone allowing for a gradual reduction of higher order aberrations. Again, all of this would be achieved by spatially resolved programming of the epoxy aberrator's curing.
In order to cover a larger field of view with supervision, a multitude of supervision "islands" might be created. The supervision islands then are connected by transition zones that are programmed to gradually change the higher order aberrations in order to create smooth transitions.
Credit: http://www.warpax.com/graphics_for_b..._graphics.html
Figure 1: Ophthonix spectacle lenses are pixelated. Each lens is an array of pixels or "microlenses", each of which has its own independently variable index of refraction, within a range that is centered on a nominal index (of 1.6). The index of refraction is set for each pixel as the lens is fabricated, using laser technology.
Credit: http://www.perret-optic.ch/optometri...ulux-ep_gb.htm
Figure 2: Gaze angles. The Ophthonix lens surface could be approximated as a two-dimensional grid, using discrete gaze angles, stepped across the full surface of the lens. At each gridpoint, a "supervision island" could be created, by programming the pixel at that gridpoint with an optimized refractive index, taking into account the sphere, cylinder and higher order (wavefront) corrections, as determined from the Z-View Aberrometer (autorefractor) data, for optimal vision at that exact gaze angle. The supervision islands could then be connected by transition zones, by interpolating for the refractive index at each intermediate pixel (intermediate pixel: i.e., the off-grid pixels) to gradually change the higher order aberrations in order to create smooth transitions.
Credit: http://en.wikipedia.org/wiki/Linear_interpolation
Figure 3: Transition zones. There are supervision islands (see Figure 2; caption) at pixels (x0,y0) and (x1,y1); also (but not marked here) at (x0,y1) and (x1,y0) - four pixels which correspond to four of the gridpoints that were calculated (Figure 2) by stepping the gaze angle. A transition zone is created by interpolating for the index of refraction at each intermediate or off-grid pixel (x,y). I guess you could call it a "four-point interpolation." I'd go dust off my old math textbooks at this point (or do some more web research) - but I think that I've made my point.
Why not have "supervision" at every single pixel, and do away with the transition zones altogether? Maybe it would take too much computing power - so many exacting calculations that it would be impractical to make the lens that way, even if it were theoretically possible. It might take too much time for the computer program to execute.
I'd be pleased if this post prompted some readers to look at the Ophthonix website material and then at this patent application. I haven't been able to view any of the images that would seem to be available with the patent text. I think that's because my Mac is so out of date - but I'm not sure. (Friend with Windows PC says she can't see the images, either.) If it seems that another layman posting on OptiBoard has just gone off the "tracks" again (relates to my rather obliquely selected post title ...) - please articulate. I'm not trying to pull any wool over anyone's eyes. I just wanted to report on what I read, and what I thought I could make of it.
There's another company working on the development of pixelated eyeglass lenses; see PixelOptics: Auto-focus spectacle lenses for presbyopes.
What happens when optical science and software engineering collide? VirtualOptician™. You may have seen the liftoff - but did you see the landing?
The term "pixel" is very misleading as it is used here. Pixel generally refers to the smallest bit of light output or color from a monitor. However, Ophthonix is reducing the wavefront over a finite region of the lens roughly the size of the pupil diameter, which covers thousands and thousands of the cones or receptive fields in the retina (i.e., the "receiving pixels"). Further, each of these so-called "pixel" regions of supernormal in an Ophthonix lens is asymmetric, or vary in different meridians, since the higher-order wavefront aberrations of the eye generally are also asymmetric.
To make a long story short... Ophthonix is basically creating a honeycomb-like array of these supernormal viewing areas across the lens by varying the refractive index (sort of like a gradient-index lens). However, because of the asymmetrical shape of the power profile of each these supernormal viewing areas, these regions wouldn't normally be continuous with each other. That is to say, the edge of one region would have a different power profile than the edge of the adjoining region. (It gets a little more complicated than this, but this explanation should do for now.)
Because the power profiles of these viewing regions aren't continuous with each other, the variation in refractive index would have to change abruptly from one region to the next. This probably isn't physically possible with gradient index technology, and would most likely create a less pleasing visual transition from region to region, even if it were. This is where the interpolation comes in, which essentially provides a smooth transition from region to region. Besides, it would require a lot more time and cost a lot more to make if you zapped every single 3 or 4 mm zone across the lens with the laser.
Darryl J. Meister, ABOM
My question as well. I have read through the entire thread, and still remain puzzled about why an apparently great new Physio design was not applied to high ABBE materials (or even reasonably good ABBE materials), but only to the worst of the lot: Poly and 1.67? I mean, would the resulting lens not be significantly better if made in CR-39 or Trivex, especially in hyperopic distance corrections?
Second, why go only for mid/high index materials when the majority of prescriptions are between +/- 3 diopters?
thats an interesting point, especially as the concept of free-form technology would come into its own in the higher RX's. Whats ironical is that the lenses are generally only available on a limited range
Can you shed some additional information for this thread regarding ABBE valve.My question as well. I have read through the entire thread, and still remain puzzled about why an apparently great new Physio design was not applied to high ABBE materials (or even reasonably good ABBE materials), but only to the worst of the lot: Poly and 1.67? I mean, would the resulting lens not be significantly better if made in CR-39 or Trivex, especially in hyperopic distance corrections?
I know you referred previous posts to Opticampus which does contain a great deal about ABBE values.
My impression is that many opticans are shooting their guns at ABBE value, when I do not know if there is a way for the optician to know what the patients complaint actually is, other than the lens does feel comfortable.
If a patient says the lens does not feel comfortable, I have heard many opticians first thought is to look for the index. Once they spot poly they jump to ABBE value as the problem. Where this theory really got traction for me was the number of opticians whose next recommendation was to put the patient in 1.67, which has about the same ABBE value as poly.
I ask Darryl and others with specific knowledge on this to comment.
For starters, Physio will be available in Hard Resin later this year. I imagine that Essilor started with 1.67 and Polycarbonate because these are their "premium" lens materials and Physio is their new "premium" lens design.
As for free-form lens materials, higher prescriptions will benefit the most from free-form optimization of progressive lenses. And, since higher prescriptions are generally ordered in a High-Index or Polycarbonate lens material, it stands to reason that a free-form lens design (which is also the most "premium" option) should be available at the very least in higher index materials. Further, in low prescriptions, it really doesn't matter either way what the Abbe value is, so it wouldn't matter whether you're using High-Index or Hard Resin in the -3.00 to +3.00 D range. Consequently, there isn't much rationale behind offering low-index materials as free-form options.
Free-form is generally not positioned (by dispensers or anyone else) as an improvement in optics over low-Abbe lens materials. Again, more often than not these wearers are going to get High-Index either way. And Free-form will improve their quality of vision in higher prescriptions, compared to conventional semi-finished High-Index lenses. Besides, as you noted, many dispensers switch their patients into 1.67 High-Index -- a low Abbe material -- with a great deal of success. (I addressed the possible reasons why earlier in this thread.)
Darryl J. Meister, ABOM
I am currently wearing lenses that have bad peripheral vision and the problem is largely down to the ABBE value of the material (chromatic aberration)! I have high +rx with bags of prism and this rx in 1.74 material in not nice!
Here is the point, this thread covers all the latest technologies that are emerging and they all have the same goal which is to provide the best vision possible. But for someone like me (and my optician) who is chasing around a problem that is material based, how do these technologies help at all!
There are lenses out there that could provide me, and lots like me, with great vision, i.e. atorics, however, most of these lenses are only available in high index - low ABBE material - so the design is somewhat flawed! I could spend lots of $ on lenses like this just to find the vision is not much better since my key problem is a material problem! High rxs dont always look good in high index material and some of the newer lenses on offer target higher rxs and yet are being produced in high index material!!!
The reference to Opticampus was regarding an article that Darryl wrote on chromatic aberration. An interesting point in the article was the mentioning of special lenses (too expensive for opthalmic lenses) that reduce chromatic aberrations by using a combination of high index and low index materials. I wonder if this is a technology that is becoming affordable? This type of material would compliment all designs mentioned in this thread.
The most significant thing I have learned from this thread is that in addition to ABBE value, the rigidity of a material is a determining factor in the quality of lense produced. As Pete has stated and Darryl has confirmed, a more rigid material will suffer less from the "stresses" of processing (grinding, polishing, etc.) and likely produce a better quality lense. Perhaps this is partly why glass provides superior vision to plastic(?) and probably explains why my 1.8 glass lenses provide superior vision to my plastic 1.74.
But these technologies were not meant to reduce chromatic aberration. And you could always choose a material that has a higher Abbe value...?Here is the point, this thread covers all the latest technologies that are emerging and they all have the same goal which is to provide the best vision possible. But for someone like me (and my optician) who is chasing around a problem that is material based, how do these technologies help at all!
Darryl J. Meister, ABOM
a higher ABBE material is my only option, but its a shame that it is - and its annoying when you read marketing that suggests that this atoric lense or some other design lense is going to provide "the best peripheral vision".
On the topic of rigidity, is there much to choose between a 1.74 and a 1.67? There is only a small difference in ABBE (33 vs 36) but do 1.67 tend to produce better lenses as a result of combined ABBE and rigidity? And would agree that glass is better than plastic - from a purely optical point of view? My optician back in the UK would ONLY make my lenses in glass because of my rx. Now I'm in Australia and they dont want to touch glass! They also tell me that nowadays there is not a lot of difference between the two.
But they do for a given lens material. Since you're a software engineer, I'll pose an analogy: Increasing the RAM in your Pentium II computer may improve its overall performance, but it's still not going to perform as well as a Pentium 4 computer with more RAM. And you wouldn't blame your RAM if your Pentium II computer wasn't as fast as a Pentium 4. ;)its annoying when you read marketing that suggests that this atoric lense or some other design lense is going to provide "the best peripheral vision".
Rigidity is certainly no guarantee of "better optics." It just reduces the likelihood of certain processing aberrations. And there are really several factors to consider in terms of processing, including heat deflection temperatures, hardness, and flexural strength.On the topic of rigidity, is there much to choose between a 1.74 and a 1.67? There is only a small difference in ABBE (33 vs 36) but do 1.67 tend to produce better lenses as a result of combined ABBE and rigidity?
Also, 1.74 High-Index actually has a slightly higher Abbe value than 1.67 High-Index (at 32). You would probably have to move into MR-8, Finalite, or some other 1.6 High-Index material with an Abbe value in the 40s to see a meaningful difference.
Darryl J. Meister, ABOM
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