Personalized
PALs
A
look at the latest developments in progressive addition lenses
By Joseph L. Bruneni
Photography by Peter Baker
Tracking the evolution of progressive lenses since they first appeared reveals a steady progression of design refinements. It's interesting to note how much these sophisticated lenses have changed over the years.
The first progressives were symmetric in design. Today, this is considered a rather primitive concept since the right lens design was identical to the left. To produce the normal inset required for reading, labs simply rotated the lens right or left. This worked but ignored a number of factors designers later found crucial for satisfactory vision. Examine the plot of most modern progressives and you'll notice that, as a general rule, peripheral blur is more concentrated on the nasal side of the channel and often rises slightly above the 180 line on the nasal side. When left and right lenses are identical, those differences are ignored.
Modern progressives are asymmetric and produced in matched pairs, right and left. This means larger inventories, but it also means a significant improvement in PAL design.
The next design upgrade occurred when the design for a +2.00D add was made slightly different than the design for a +3.00D add. This improvement made sense since the reading area of a strong add is considerably smaller than that of weaker add powers. Progressives that change design based on add power are called "multi" design. Those that don't are "mono" design.
Another refinement was added when a manufacturer concluded a progressive design for plus corrections should differ from the design for minus powers. Another progressive design built on the concept of achieving a balance of visual fields based on the patient's basic type of correction (myopia, hyperopia, emmetropia) and their amount of add power (presbyopia). These types of design enhancements were not difficult to put into practice because they were built into the front surface of semi-finished blanks.
As labs select the proper semi-finished blank for each prescription, they automatically get the correct design for that patient, based on the design philosophy used for that brand. By the end of the 20th century, these design refinements brought progressive designs as close to the ideal as seemed possible.
Designers knew, however, that progressives could be better if they could be personalized to each patient, based on the frame selected and how that frame was positioned on the face. This individualizing concept has produced new customized progressives that provide personalization.
Lens manufacturers are always searching for ways to improve their progressives. Progressives represent their most profitable product and, for most companies, progressives are the company's flagship premium product. This explains why lens producers continually seek to advance their progressives to the next level.
The newest advancement in progressive design philosophy is based on a simple assumption that the best performing progressive lens is one that has been customized based on the patient's individual data.
Producing customized progressives, however, introduces several major problems. The first and most basic issue is that, at least for now, ordering personalized progressives requires bypassing the local lab. The lenses are still ordered from a local lab, but fabrication takes place in a factory lab. The most highly personalized progressives require complicated computer software and advanced processing equipment that labs do not yet have. This situation is changing, however, and equipping labs to fabricate customized progressives lies in the near future, probably within a year.
The second issue is even more daunting. Personalizing progressive designs to each patient requires additional data from the doctor or dispenser. The customary process of sending monocular PDs and fitting cross height to the lab does not always provide enough data to truly customize a progressive.
Explaining the category of customized progressives is difficult because manufacturers take differing approaches in creating their customized progressives. In some cases the differences are minor, in other cases not so minor.
The important point to know is what is required from the dispenser in fitting the customized brand selected. In general, customized progressives represent the state-of-the-art in progressive addition lenses. These lenses are also generally more expensive and require additional turnaround time because they are produced in factory labs, usually outside the U.S.
Looking ahead, be on the lookout for major changes in the entire field of progressive addition lenses. You can be reasonably confident of seeing more customized progressives in the future as progressive manufacturers explore ways to individualize their PAL designs.
This continues to be no small feat, as there are major considerations when designing such a lens. Merely designing these lenses is a massive undertaking that could never happen without today's powerful computers. Manufacturing such highly complex lenses is the next obstacle. Designing and developing production equipment to do it is a huge task that, to some extent, has been accomplished.
The greater task, however, will be getting individual laboratories equipped to process these lenses. Providing prescription service from a factory lab outside the U.S. can work well in this day of overnight air freight, but private practitioners always prefer dealing with their local labs. Fortunately, it now looks like even that is a possibility in the near future.
There's nothing wrong with the dozens of brand-name progressives now in general distribution. Cost factors will keep a percentage of consumers reordering these conventional progressives.
There's also no question that there is a very profitable market for substantially improved progressives. A lot of companies are betting big dollars that building a better progressive is going to bring more customers to their doorstep.
In the future, dispensing these advanced progressives will ultimately require gathering more data than doctors and dispensers are used to obtaining. The additional profits will make that effort worthwhile. The future for progressive lenses is very bright, indeed.
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Personalized PAL Products |
Rodenstock: The first customized progressive was launched several years ago in the U.S. by Rodenstock with its Multigressiv� progressive. It uses what Rodenstock calls "position of wear." Using special software, the patient's prescription is altered to overcome issues that compromise the effectiveness of a progressive. Rodenstock changes the doctor's prescription slightly to overcome inherent problems in ophthalmic lenses. No additional patient data is required from the dispenser. Rodenstock considers three situations: The ray path and lens power determined during the refraction (in the phoropter), the ray path and power when lenses are checked in a lensometer, and the ray path and power required when the lenses are in a frame positioned on the patient's face. The position of the lens is slightly different in each case, and Rodenstock believes compensating for those differences improves wearing comfort. The lenses are returned to the doctor with labels indicating what was ordered and how the correction was changed for each eye. With the Multigressiv, the back surface is aspheric or atoric. Rodenstock has refined the design and produced an advanced version called Multigressiv 2. Multigressiv lenses can be ordered from any lab distributing Rodenstock products but are produced in Europe. Pentax Vision: This company calls its customizing process "Retina Forward Design." The doctor's prescription is recalculated to compensate for factors similar to those considered in Rodenstock's position of wear. No special patient data is required from the dispenser. The doctor gets back lenses with two sets of values--one showing prescribed values and the second showing verification values. This process is used with all Pentax AF� and AF Mini� lenses. A number of labs have been designated "AF Labs" and have installed computer software needed to compute and produce the dual labeling. Carl Zeiss: Zeiss calls its process "Zeiss Measurement Value System" and incorporates it in all Gradal� lenses. Gradal PALs incorporate variable seg inset values, dependent on the patient's Rx and add power. They also use more base curves than conventional progressives. In Europe, where Zeiss operates factory labs, Gradal lenses are returned with two printed sets of values--what was prescribed and what the lenses actually read. In the U.S. where Gradal lenses are produced by authorized Zeiss laboratories, the lenses have the same customization, but because of the variety of software used in labs, a printed readout of the actual lens power is not returned with the lenses. In most cases, the Rx changes are relatively small and the power variations are well accepted even without the printout. Coming from Zeiss is a totally new progressive called Gradal Individual, which has been produced in Europe for several years and will be available here by the Spring of 2003. Each lens is produced from a raw slab of plastic so that the progressive curves on the front surface can be totally customized to each patient's prescription, frame adjustments, and frame positioning. This also means that the base curve and seg inset can each be individualized. When the dispenser can provide added data such as vertex distance, pantoscopic tilt, and frame wrap, the maximum in personalization takes place. Zeiss says that even when only fitting height and PD are provided, the patient still gets a personalized progressive. The current plan is to set up a factory production lab in Virginia to supply the lenses on Rx to authorized Zeiss laboratories. Basing the factory lab in the U.S. should expedite delivery of the lenses. Shamir Insight: This company is both a design house, producing lens designs for other manufacturers, and a lens manufacturer, producing and selling lens blanks to laboratories. It is currently involved in a roll-out test in laboratories of a new progressive that is individualized to the patient. What distinguishes Shamir's new lens is that the lenses can be fabricated in the wholesale lab, not at the factory. It has developed advanced software for labs that takes into consideration the patient's variables and then designs a customized progressive specifically for that patient. Developing the proper software was only the first step. To produce this new type of progressive, labs need generators capable of cutting progressive curves, something most labs do not currently have. Equipment capable of polishing such a complex surface is also required and will be available shortly. One important measurement factored into Shamir's software is the height of the fitting cross. Each progressive will be custom designed to produce a progressive corridor specifically for the length required to fit the frame chosen by the patient. Shamir believes this advancement will produce substantially improved wearing comfort. The design works with no additional patient data other than fitting height and PD, but when additional data can be provided, it can be incorporated into the software, producing an even more personalized progressive. |
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Backside Progressives |
There are currently two backside progressive lenses on the market. Seiko Optical: This company's top-of-the-line progressive is called Super Proceed Internal. The term "Internal" refers to the fact the progressive curves are positioned on the backside of the lens. Seiko believes this produces several distinct advantages. There are less size variations between distance, intermediate, and near zones. There is less distortion or "skew" on either side of the progressive corridor and a significant expansion of the visual fields. With conventional frontside progressives, off-center astigmatism and power errors increase as the refractive index of the lens increases. This issue is minimized with internal progressives. The final benefit is that the back surface can be as complex as required. All Seiko Super Proceed lenses have aspheric or atoric back curves, depending on whether there is cylinder in the patient's correction. Rodenstock: Taking its "position of wear" a step further, Rodenstock is currently introducing a new customized progressive called Impression that incorporates a process the company calls ILT (Individual Lens Technology). Like Multigressiv lenses, the prescription curves on the backside are aspheric/ atoric, but Impression ILT lenses also include the progressive curves on the back. Rodenstock believes another important variable with progressive patients is how the eyes converge. Narrow PD patients do not converge as much as those with an average PD (63mm). Myopes converge less, while hyperopes have increased convergence. Higher add powers also converge more than weaker adds. In conventional PALs, convergence is always a standard 2.5mm for each eye. Impression ILT creates a progressive channel customized to each patient. Positioning the progressive corridor on the backside also provides Impression ILT lenses with a broader field of view and reduces peripheral distortion. To create these highly personalized lenses, Rodenstock needs additional data from the fitter, who provides pantoscopic angle of the patient's frame, the amount of frame wrap, and the fitted corneal vertex distance. To help dispensers determine these measurements, Rodenstock has created a measuring tool. Laser markings on these lenses are found on the backside rather than the customary front surface. |
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