LENSES

The Other Side of Free-form, Part Two of Two

FREE-FORM LENS VARIETIES

Another common misunderstanding is that free-form lenses are synonymous with back surfaceprogressives (PALs). That’s definitely wrong. The following suggests potential designs availablethat 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 wearingconditions. During the refraction, the refractor lens in front of the patient’s eye has nopantoscopic 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 itwas 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 confidencein 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.