I recently asked a room full of well-educated, licensed ECPs who was recommending and dispensing digital lenses, and about half the room raised their hands. I asked the same group who was recommending and dispensing free form lenses and about half the room raised their hands.
Then I asked the real question. “Who thinks digital and free form lenses are the same thing?” Not everyone raised a hand, but a significant number did and provided the beginning of a 2 hour discussion on the differences between the two technologies.
It’s understandable, really. The marketing we read every day is often misleading and until we have a basic understanding of the lens production process, we don’t know what questions we need to be asking.
What does DIGITAL mean?
To answer that question, we need to understand how a conventional lens blank is made. Traditionally, lens designers created software to design and produce the ceramic molds that contained the lens design. These ceramic molds, sometimes called analog molds, were used to create the glass molds used to manufacture lens blanks. Then the lens material (crown glass, CR-39, 1.60 et. al) is poured or injected into the glass molds, cured, cooled and removed from the molds for inspection. Like using a fancy cake pan over and over again, the molds would begin to lose some of their accuracy and need to be replaced.
These conventionally manufactured lenses have their designs, both PAL and lined multi-focal, on the front surface of the lens blank. The actual Rx power is traditionally surfaced on the back side of the lens, followed by fining and polishing the lens to the exact Rx specifications and full clarity.
We knew what a generator was. It was big, loud and messy. It cut curves in two directions (up-and-down, side-to-side). We understood fining and polishing. We understood base curves and ANSI standards. We knew to fit a PAL by dotting pupil center and taking monocular PD’s with a PD stick or a
This is the “off the rack” approach to lens design, production, fitting and dispensing.
The new digital manufacturing processes and equipment allow those same lens designers to skip the ceramic mold process and create the glass mold directly. By removing that extra molding process, the design begins to increase in accuracy. The lens designs are still on the front surface of the lens blank, but the design is created more accurately.
Still “off the rack” but better than the old analog process.
This is the point in time where we began hearing about digital and free form lenses. We heard of the mystical “Schneider” machines in Europe that were capable of creating the most highly developed lens designs ever available. These magical machines were CNC (Computer Numerically Controlled) milling machines whose accuracy was almost unimaginable even just a few years before.
The digital lens age dawned. Lens designs were being introduced at an amazing rate; and some of our heads exploded.
At this point, because most ECP’s didn’t understand the digital process and had begun hearing about “free form” lenses, we began to use digital and free form interchangeably and very few marketers were going to clarify that confusion.
To simplify the explanation, digital is a production process. It’s used to produce the most accurate glass molds for conventional front-surface lens designs, including spherical and aspheric (multi-curve) single vision, conventional lined multi-focals and conventional front surface design PALs which are always aspheric. The digital production process cannot make an old design better; it can simply reproduce it accurately from one batch to the next.
What does FREE FORM mean?
Now that we have a better understanding of digital processing, we can begin to understand free form design.
Remember the old style generator?
Producing a free form design requires a digital (CNC) generator that can create curves in 3 directions; up and down, side-to-side and in-and-out. It does this by using a stylus rather than a diamond-toothed grinding wheel.
What is being created is not just the Rx power but the lens design as well, with pin-point accuracy, all on the back surface of the lens blank. Not only is this system able to produce powers within .006D of the required Rx, the lens only needs to be polished to remove any excess material prior to coating. It’s called Direct-To-Polish. This technology allows the lab to create a custom designed PAL from a single vision lens blank. It is important to remember that, even with this amazingly accurate combination of technologies, lens design is still the key to providing the best interpretation of the patients’ visual requirements.
These are not the old “off the rack” designs we are accustomed to using. These digitally-produced, free form designs are akin to “bespoke” or custom made clothing. Now, simply measuring a PD with a PD ruler and dotting the pupil center is no longer sufficient. Even the verification process has changed, requiring a separate “compensated Rx” form for easier check in.
These individually made, customized lenses require POW (Position of Wear) measurements, including monocular PDs, seg heights, vertex distance, pantoscopic angle and panoramic (face-form) curve.
The basic fitting requirements remain the same. The patient and ECP should be seated at equal eye level with the frame pre-adjusted. However, without the POW measurements the various lens design programs will revert to default values and the patient will not experience the added benefit of a customized fit.
In addition, ECPs now have the choice of fixed or variable corridor lengths. The advantage here is that the lens can be further customized, taking into consideration the shape of the frame and the distance from pupil center to the frame edge (Frametized).
There are several options for taking these measurements, from simple tools available from your lab at little to no cost, to high-tech electronic equipment costing hundreds if not thousands of dollars. Your lens and lab reps can help you sort through the options and choose the right tool for your practice.
In a nutshell, Digital is a production process. Free Form is a design process.
All free form lenses are digitally produced, but not all digitally produced lenses are free form designs.
When evaluating the potential performance of a lens ask yourself or your rep these questions:
If this is a PAL, is the design and add power on the front or back surface of the lens?
Are there specific measurements required for optimal performance?
What are the Rx limitations of the design?
What materials are available?
What lens treatments (A/R, etc.) are compatible?
When should POW measurements be taken and recorded?
My personal opinion is that ECPs should be taking these measurements for every patient fitted with eyewear of any kind. What better way to impress upon your patients the level of personalization that is required for all prescription eyewear.