See the Future in Digital PALs
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The Rx was
tough, a high minus with oblique cylinder. The patient needed to use the progressives for a full range of distances. This demanding situation called for a progressive in its best form: a digital PAL. But which digital PAL? The office used a number of suppliers, and each manufacturer made ‘digital” claims about a few of their PALs. There were several price points.
Some discussion took place about which PAL was really the best. Things began to get confusing. The Optician called the supply lab. All agents were busy assisting other customers.
Have you ever called the supplier with the patient waiting, only to find no one available to answer your question? It certainly feels like Murphy’s Law. Calling the supplier and coming back without an answer never instills confidence in anyone.
More proactive measures are in order. A better relationship with the Sales Consultant assigned to the office might have avoided the problem. Most lens companies support their customer well, and that customer is you, the eyecare professional. It only makes sense to keep in contact with your sales consultant (lens rep), and to learn all you can about progressives, particularly digital designs. It’s a safe way for you to voice any uncertainty about the best match of a lens.
Progressives, or PALs experience refinements every few years. Presently, we are all experiencing the effects of a leap in PAL technology on a grand scale. Digital progressives involve a new process.
New technology, applying CNC (computer numerically controlled) machining to Ophthalmic lenses allows optics to be applied to the surface of the lens more accurately, and truer to the correction called for by the prescription.
In some instances, there is a point-by-point mapping of the lens surface, reducing imperfections, enhancing the design for each individual prescription, and delivering a more true prescription.
Digital PALs promise to more accurately deliver the prescription to the surface of the lens, and to account for and minimize higher order aberrations. Wave front technology from corneal mapping has lead to a leap in PAL technology. By measuring the waves reflected from a surface, a sort of topographic map is created, as reflected waves reveal imperfections on a surface.
Digital PALs Deliver Improved Optics to the Wearer
Traditionally, lenses have been surfaced by generating the back of the lens to cut the power into the lens blank. This ground the power, but left irregularities needing to be smoothed out. The next step took care of this, where the lenses were fined and polished, using a lap tool determined by computer. Lap tools are accurate to within .10 Diopters.
Potential drawbacks to conventional surfacing have included the amount of space taken up by a rack of lap tools and that fact that several lens blanks need to be stocked.
Instead of assigning prescriptions to a series of limited base curves, (and stocking these lens blanks in a supply lab) digital PALs can be made on a much wider range of base curves, sometimes even on a base curve unique to the particular prescription.
Traditional surfacing produced only spherical or toric surfaces on the lens blank. Digital methods produce aspheric, complex curves, using a small diamond tip tool. The older methods applied only simple curves to the back of aspheric blanks.
Just what is a digital PAL?
Digital PALs Involve a Combination of Three Processes:
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Lens designs
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Software, for digital manufacture
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CNC machining, involving as many as five axes
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Digital lenses are made on a free form generator and polisher. The cutting tool can be thought of as operating like a stylus. Flexible tools, instead of laps are used to polish. This equipment measures three or more planes, and complex curves can be produced on the lens surface. There is a reduction in imperfections on the lens surface from the lap tool drifting during conventional methods.
Important Distinctions
As you begin to separate all lenses “digital,” you will enhance your own understanding of the manufacturing process, something essential before you can separate the very good/ better/ best. There are several forms of digital technology working to improve PALs. It’s important to understand which improvement is at work.
Digital Improvements to Conventionally Surfaced PALs
All things labeled “digital” are not equal. Sometimes, conventionally surfaced progressives benefit from the fact that they were made in a digitally created mold, improving the front surface of a PAL, which is surfaced using traditional methods.
When you hear claims that digital manufacturing will replace conventional methods, remember that this has already begun: for example, some progressive lens blank molds which were once made by slumping are now made digitally.
In the slumping process, glass is heated to melting, or “slumped” over a ceramic lens “puck,” to create the mold, whereas a lens mold formed digitally and CNC machined produces a more accurate mold.
Digital Processes:
Software
Digital designs use software that accurately takes into account index of refraction. The software creates a map of the lens surface, so digital lenses account for beams of light, not just single rays, hence the reduction in higher order aberrations.
The software calculates complex curves on the lens surface, reducing unwanted astigmatism in the intermediate zone. The polishing process for digital PALs accounts for the paths taken by the polishing pad on the lens surface.
Design
Digital PALs may be CNC machined on the front and back of the lens, an uncompromising feature of the best digital PALs. Add powers may be backside, and closer to the wearer’s eye, or split front and back of the lens, both strategies work with software to reduce the astigmatism as the eye moves through visual zones.
Digital PALs account for vertex distance, pantoscopic angle, and angle of frame (degree of wrap/ flatness). Sometimes, manufacturers provide proprietary measuring devices for obtaining these subtle measurements more accurately than any felt tip pen.
Manufacturers may refer to their digital PALs as “wavefront,” “W.A.V.E.,” ” free form,” “direct to surface,” or “digital.”
Machining
CNC machining tracks the stylus across the lens surface in a spiral motion, while the lens rotates. The whole procedure is more complicated than back and forth and up and down, and may involve as many as five axes. Surfaces are as accurate as .01 Dipoters.
Again, “digital” technology may refer to digital lenses, or may mean molds for PALs surfaced conventionally.
Putting it All into Perspective
By reviewing the criteria of digital PALs, software, design, and machining, it’s apparent that the three are strongly interdependent. By separating digital PALs from conventional PALs that have been improved by digital steps, it’s easier to make informed decisions and select the right lens for the job.
Verification
Remember that we verify PALs through several points on the lens surface. Most of us are still doing so with trusty equipment, that could honestly be called “antique.” As a result, there are going to be some instances where verification is an issue. Suppliers may send you a compensated Rx for a digital PAL, as they do for a wrap frame. The reason is that digital PALs are optimized for the particular prescription.
Conclusion
Digital lenses are considered new technology at present, and it’s hard to think of them as anything other than dramatic and exotic. Manufacturers are assuring eyecare professionals that in the near the future, all lenses will be produced digitally, but that may sound far off.
There are at least two ways to consider digital manufacture. The old method may have been fine, but over time things evolve, and it’s important to recognize the latest opportunities to provide patients the best. On the other hand, it’s important to also recognize when “new” is becoming the norm. Be sure to take advantage of the best and new while it is still just that. And don’t be surprised –like the day the LP left the record store—when the latest technology becomes the current technology.
Optometrists openly refer to the standard of care in their profession as a changing, rising thing. Technology in our daily lives has gone from expensive to ubiquitous. Consider all the things patients do, the devices they use, the cars they drive, and the technology they routinely enjoy. Take notice of the patient, investigate their needs, and be instrumental in showing the best solutions. You’ll empower yourself, support the patient, and provide them the best vision.
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