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Medical Forum / General / Vision / December 2004Polarized spectacle lenses - this sounds like BS to me
Following is a description of a recent problem I had with 1.6 high index Zeiss G15 polarized polycarbonate lenses. I have a query in with Zeiss but another USENET reader suggested I ask here as well. Others have suggested that the problem has to do with clamp tightness combined with cutting speed. As I know nothing of the edging process I can't have an opinion. I hope someone on this list can point me toward some useful information that I can use to educate the option as to causes and solutions for htis problem, as they don't seem willing to be proactive without much prodding. I'm also wondering if the problems affecting my sunglass lenses can, to a less noticeable degree, affect my clear lenses, which are also Zeiss 1.6 high index poly; I replaced my daily-wear spectacles at the same time I updated my shades. As an FYI, the optician is not a Big Box outlet, but a small local establishment with a 40-year history, has been owned by the same family the entire time and does its own cutting/edging in-house. A couple of weeks ago I updated my 6yo sunglasses prescription. I ordered the lenses from the optician from whom I originally purchased the shades. However, in the intervening 6 years the optician switched to new customer management software and didn't bother to migrate any of the old data. So, the nice clerks at the shop kibbitzed and decided through committee that the existing lenses were G15 polarized with a mirror tint. Implicit UV protection, too. Lenses came in, I dropped the spex at the optician's shop for the afternoon. About a week later I started noticing black blobs impinging on my field of vision. Checked the lenses against white paper, didn't see anything untoward. Held the shades up to a clear blue sky and WHOA MOMMA! The entire outer edge of each lens was exhibiting a weird black reverse corona - the outer edge of each lens was black, with coronal fingers reaching toward the lens centers. I also observed several points at which there appeared to be neither tint nor polarization and around which hte discoloration was much more noticeable. I returned to the optician, where I got a patronizing run-around. I was then forced to agree to have the lenses re-edged in an effort to eliminate this problem. Re-edging made the problem worse. I was then told that this is how all polarized lenses are and I need to live with it. I of course wasn't having any of that! I dug the original lenses out of my purse, plopped them on the counter, and asked why the original lenses didn't exhibit this problem. More pooh-poohing, a chat with the management, and the optician is in fact ordering new lenses for me. They're also ordering some "test" lenses, which I understand to mean they're asking the manufacturer for some seconds so they can try to duplicate the problem (or prove it isn't an edging artifact) without ruining another set of customer lenses. Concoidal shaped ghosting images in a polarized lens are usually due to stress points in a lens. Points on the circumference that have higher levels of pressue will show a clam shell shaped dark or colored ring radiating from the pressure point to the center. The rings can reach a substantial distance into the lens if the stress is severe enough. High index lenses make it more pronounced by nature. Polycarbonate which is a 1.58 index material shows such stresses very well. Poly even has a set of stresses internal due to the injection molding process. This can set up patterns across the entire lens with highlights around the edge stress points. THis can show up in clear glasses as well as polarized sunglasses. The polarizing filter just makes it far more noticable. Add a second polarizing filter and cross it 90 degrees from the lens and you'll see a major show since most light is now effectively blocked. You'll see the true extent of the stress. No stress patterns mean that a lens is too small and will fall out when temperature changes cause a sizing differential between the frame and lens. (BIG problem in Alaska,) Large stress patterns mean the lens is too big or shaped improperly for the frame (Or the frame is too angular for an edger to match.) This leads to annoyance like you mention, or even chips, cracks, or delamination. THere will always be stresses in lenses, but there needs to be a change in fit or material to satisfy your keen eye. Carl > Following is a description of a recent problem I had with 1.6 high > index Zeiss G15 polarized polycarbonate lenses. I have a query in [quoted text clipped - 53 lines] > isn't an edging artifact) without ruining another set of customer > lenses. > filter just makes it far more noticable. Add a second polarizing filter > and cross it 90 degrees from the lens and you'll see a major show since > most light is now effectively blocked. You'll see the true extent of the > stress. With crossed polarizers, you can see transparent areas around the edges before they're ever mounted. It's hard to blame those on stress. Is it the heat of edging that depolarizes the film? -MT >> filter just makes it far more noticable. Add a second polarizing filter >> and cross it 90 degrees from the lens and you'll see a major show since [quoted text clipped - 6 lines] > > -MT In terms of the polarizer/analyzer sandwich itself, I have some informed guesses. If they are formed out of aligned dichroic needle shaped crystallites like those that made up the original Polaroid iodine based film produce by Land, transparency can be produced either by some degradation of the crystallites or by changes in crystal alignment. I have difficulty understanding how stress in the crystallites can have significant effect on the dichroism. On the other hand, the material in the middle of the sandwich is likely to be nominally isotropic. In that case, stress/strain can easily introduce enough birefringence to affect what is seen between polarizers. One way of checking this out would be to look at the transmission of light as a function of radius through the polarizing film with a spectrophotometer. A chemical change should be readily observable. Bottom line: I would look at the sandwich filling as producing the light transmission rather than at the slices of bread. Bill > in article zUVxd.7557$yK.201@newsread3.news.atl.earthlink.net, Mike Tyner > at [quoted text clipped - 9 lines] >> the >> heat of edging that depolarizes the film? A diamond wheel or a overpriced router bit chewing away at the edges of a lens introduces alot of stress and heat. The laminated lenses often separated after being subjected to such torture and then clamped into a metal frame and exposed to environmental variations. The new cast in mold designs are better, but I believe even they display some stress /heat issues at the edges due to the grinding. I've seen too many issues of mid and high index clear lenses being strange to look through near the edges when there was an issue like a sharp angle or overly tight fit involved. >> -MT >> [quoted text clipped - 8 lines] > on > the dichroism. The filter is still iodide crystals aligned in a PVA film. Manufacturers used to laminate the blanks like a sandwich, but the mayo proved to be the undoing. They delaminated often and caused problems in edging when the glues were sometimes significantly harder than the CR-39 lens. The old Weco 440 edgers used to have a devil of a time with some brands that will go unmentioned. The newer CR-39 and mid index lenses are "cast in place" where the film is positioned prior to the introduction of the monomer/accelerant mix. This has eliminated most of the delamination problems and even reduced stress levels in the finished product. Some high index materials and poly still use the lamination method. The rainbow (including clear and black, which are just localized extremes,) patterns produced in lenses usually are attributed to stresses in the surrounding lens material and not the polarizing film itself. Polycarbonate in particular is known to have a severe birefringence issue due to it's manufacture process of injection molding. Add the stress patterns from the lens being 1/20th of a mm too large or too rounded for an angular frame and we get a new set of stress waves. Out of true wheels or bits on the edger can add to the stress or heat. > On the other hand, the material in the middle of the sandwich is likely to > be nominally isotropic. In that case, stress/strain can easily introduce > enough birefringence to affect what is seen between polarizers. We've used a polariscope to check the annealing/ tempering patterns in glass lenses for decades. A properly heat tempered glass lens produced a dark Maltese cross pattern with a dark edge around the perimeter of the lens. Extra artifacts would often show pressure points that needed to be addressed before shipping the glasses to prevent future failure due to flaws. > One way of checking this out would be to look at the transmission of light > as a function of radius through the polarizing film with a > spectrophotometer. A chemical change should be readily observable. Probably definitive on the issue if someone has the resources to pursue it, but beyond the local ophthalmic shop. I still lean towards the polarizing filter showing the stresses inherent in the medium it's suspended in. > Bottom line: I would look at the sandwich filling as producing the light > transmission rather than at the slices of bread. Maybe even consider the Mayo if the bread is fine! This would only apply to laminated lenses, however. Carl > Bill Ok, so if I boil it down to something I can understand (I'm intelligent but uneducated in this field despite 30-odd years of wearing spectacles): The poly lenses themselves are subject to various stressors during manufacturing. These stressors are introduced during the injection molding process, the polishing process, any coating processes, and of course during the edging process. All of these stressors will combine to produce visible flaws in a tinted polarized poly lens. So how does one go about troubleshooting? The optician insists that "they all do that", which is patently untrue. My original lenses, some 6 years old, don't show the flaws that the new lenses do. Both sets were ordered through the same optician, are presumably from the same manufacturer, and were edged in the same on-site lab, which, by the way, is not a Big Box Spectacles place. Working on the assumption that hte two sets of lenses are identical save for the Rx, I'm forced to assume that the lion's share of the optical flaws I'm seeing were introduced AT THE OPTICIAN, presumably during hte edging process. If that's the case, what does the optician need to be troubleshooting in order to correct the problem? Wooly <nobody@nun.ya> wrote in news:elqhs0p6jhln0ocj6vav7feeaicmvhkhn4@ 4ax.com: > So how does one go about troubleshooting? The optician insists that > "they all do that", which is patently untrue. My original lenses, > some 6 years old, don't show the flaws that the new lenses do. Both > sets were ordered through the same optician, are presumably from the > same manufacturer, and were edged in the same on-site lab, which, by > the way, is not a Big Box Spectacles place. Have you considered the possibility that your new lenses are more strongly polarized than your old lenses. I'm not in the business, but I do wear polarized glasses. The ones I've been happiest with have come from Action Optics. I'd recommend browsing them up. Scott
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