The Other Side of Free-Form

By Michael Walach

There’s much more to “free-form” (FF) than just the progressive lenses that you have begun to use. Free-form is a process that is used to manufacture wide varieties of lenses: single vision, aspherics, atorics, bifocals and specialty lenses in addition to progressives. The category of free-form is composed of equipment, software, patient adjusted power lenses, varieties of lens designs, as well as new ways to control the technical attributes of lenses and their quality.

FREE-FORM BASICS

Surfacing — The essential difference between free-form surfacing and conventional surfacing (CS) is that free-form equipment has the capability to produce greater precision (three axes or more) and smoother surfaces as well aspheric, atoric or progressive lens surfaces. This more akin to high definition (HD) because the complex curved surfaces (not excluding circular surfaces) are produced versus conventional surfacing where only circular curved surfaces of much higher surface roughness (0.03 to 0.1mm). HD surfaces after cutting have surface roughness of approximately 0.3 microns (0.0003mm), which is similar to a surface conventionally processed after fining. Therefore, HD equipment generated lenses do not have to be fined and can go straight to polishing.

Lens surfaces processed on conventional equipment have to be fined on laps before polishing and it is the lap fining process that restricts the lens curves to circular surfaces—spherical or toric. For this course, HD will be used generically to mean precision cut, free-form surfaces.

Equipment — There is a common misunderstanding that it is the free-form equipment that makes lenses free-form. To produce free-form HD lenses, the equipment is just the starting point. HD equipment is a necessity to produce FF lenses but requires free-form software. Some labs use HD equipment to produce conventional circular design lens surfaces only. That process is commonly called cut-to-polish or direct surfacing. The main advantages of cut-to-polish are a significant improvement in lens power accuracy, reduced rejects, improved production efficiency, no laps are necessary and a reduction of production labor.
Software — Typically, free-form software consists of three parts:

1. the Lab Management System (LMS),
2. a Lens Design System (LDS), that creates
3. a Surface Definition File (SDF).

The LDS can create either a brand name or generic lens design. The SDF is a three-point file of the X, Y and Z coordinates that describe the surface height across the lens surface. It is like a 3D picture of the surface. This file is then cut by an HD generator and reproduces the surface, typically on the backside of the lens though this equipment can cut convex and concave surfaces. So, the key component of the free-form software is the lens design system. It is here that lens design systems are also very different. At the present time, most LDS software produces surface files with circular back surfaces for conventional lab surfacing. However, the free-form buzz is the most exciting advancement in Rx lens production technology in over a century. This gives us the opportunity to shift from off-the-shelf type prescriptions to the personalization of corrective eyewear.

Labs are no longer restricted to circular surfaces. Instead labs are only limited by imagination, technical sophistication and the capabilities of lens designers and their design development environments. Therefore, all we have to look forward to are better executions of advanced lens designs, new varieties of lifestyle and vocational lenses, added economic benefits for the entire eyecare profession and most important, real benefits for the patient.

FREE-FORM LENS VARIETIES

Another common misunderstanding is that free-form lenses are synonymous with back surface progressives (PALs). That’s definitely wrong. The following suggests potential designs available that can be produced in virtually any index and sometimes as photochromic or polarized.

The opportunity exists to free-form lenses for these two cases. Teresa F., age 32, +1.75-11.50 x 180, wants a larger, more fashionable 52-eye frame, but requires 5mm decentration. For Timmy D., age 6, an Rx of +18.50-0.75 x 95, Add +2.50, 44 eye and decentered 4mm is needed.

For Teresa F., lenses were made on a +0.50D front curve. Using a 1.6 index, 42 Abbe blank, cribbed to a diameter of 66 mm knife-edge, a 38mm visual field backside Myodisc lens was produced with a saddleback curve (+ and – curves crossed at 90 degrees) on the back surface. The lens edge thickness was about 2.5mm, with back curves of +1.00D base and -9.45D. cross curve (see the blended Myodisc example below).

For Timmy D. a 1.67 9.50 base FT 28 semi -finished lens blank was used. On the back, a +4.50 curve was cut and cribbed to a diameter of 55mm @ knife-edge, 37mm visual field. The backside was also blended and lenticulated. The center thickness of 9mm creates an edge thickness about 1mm. The results were a thinner pair of glasses than Timmy had before.

PATIENT ADJUSTED POWER LENSES

The most significant aspect of HD FF technology that will make the biggest impact on the quality of eyewear is the capability to produce patient adjusted powered lenses. This may eventually make circular surfaces obsolete.

There is an inherent discrepancy between refracting conditions and the actual lens wearing conditions. During the refraction, the refractor lens in front of the patient’s eye has no pantoscopic tilt, no facial wrap, is typically of different shape and at a different vertex distance from the eye than the actual lens the patient will be wearing in the eyeglasses. For example, a -5.00D refracted power, when fitted at 9 degrees tilt, 5 degrees wrap, 2mm closer vertex distance, sees as a -4.76 Sphere -0.15 cyl x 64. In order for the patient to see through -5.00D lens under those fitting conditions the measured power of the sphere must be corrected by 0.24D and the cyl power correction is -0.15D, which is more than the relevant power tolerance standards allow. That may result in reduced visual acuity, fatigue and discomfort. The new lens design systems take all that data into consideration. Also, other information is considered like vocational requirements, lens shape, index and Abbe value. Using ray tracing analysis, a sophisticated software and the findings of vision scientists, the system will create complex surface files for cutting. These surfaces are then produced on lenses for personalized prescription eyewear and unsurpassed visual performance.

LENS QUALITY, INSPECTION AND RX VERIFICATION

The lab’s biggest challenge is to assure the lens is exactly what was ordered in the way that it was calculated, designed and delivered. For free-form lens manufacturers (the laboratory), they must implement new lab inspection equipment since standard instruments are unsuitable for patient adjusted power lenses. A lensometer alone cannot tell the lab whether the design, across the surface was created as intended. In standard cast lenses, the through power was sufficient in single vision and progressives to verify the lens. In progressives, the manufacturer makes sure the design is consistent from semi-finished lens blank to lens blank. For a lab-produced lens, the design changes for each lens created so the lab will need to know the targeted design map (from the software system) and be able to compare it to a lens map of the finished lens. Then a comparison can be made between the two maps and determine go/no-go. While the optician will measure the lenses as they did before; distance power, prism at the PRP and add power by the engraving, the lab will have verified the lens’ surface for the ECP. Therefore, to have confidence in free-form lenses, the optician must trust the design author (Essilor, Zeiss, Shamir, etc.) but more importantly their lab, the manufacturer of the lens.

For example, a lens might be over polished in some areas from an equipment malfunction during the grinding process. The result is an inaccurate reproduction of the intended surface design so the optical performance is altered. Instruments from Rotlex or Automation & Robotics are capable of scanning an entire lens, reconstructing the surface through power data and comparing them to the target. This allows the lab to verify their production.

CONCLUSION

There’s much more to free-form than just progressive lenses. Free-form allows the lab to manufacture a wide variety of new lenses for the ECP. New forms of single-vision, aspherics, atorics, bifocals and specialty lenses, in addition to progressives opens the door for better patient solutions.

The category of free-form is broader than just lens styles. It is composed of equipment, software, patient adjusted power lenses, lens designs in all materials and treatments. It is also new ways to control the technical attributes of lenses and their quality. This is one of the finest advancements in the eyewear field for the benefit of our patients and our profession.

There is still much to learn about this technology and many other new products in the pipe for patients. Look for new inspection devices and automated dispensing devices to effectively measure the way that consumers wear their glasses. This will provide consistent data for vertex distance, pantoscopic tilt and facial wrap needed for personalized lenses. Combined with lifestyle needs, we can move closer to providing an even more custom and personalized eyewear than we do now.