THE TRUTH ABOUT NO-LINE BIFOCALS

     It was wheat harvest in 1964 on the eastern edge of the Dust Bowl. A hundred yards north was an old homestead abandoned during the Great Depression. The red bulb thermometer on the shady side read 102 degrees.

     A dust devil twirled some wheat chaff and dark brown loam past my feet and into Dad’s face. I was handing wrenches, bailing wire and W-40 under the combine. He was a Christian man, but a half hour of lying next to 200 degree combine transmission while a blast furnace southerly blows top soil and chaff into your face as you fight sweat bees will make you as unpleasant as Satan’s segundo. Occasionally a blue streak flared from under that metallic mutant scorching the nearest ear, “Boy they sure don’t make them like they used to, dad-blame-it. Everything is so complicated, and Dang, I can’t see a cottonpickin’ thing under this dark somebeast.”

     Some of the Greatest Generation remember the past a little rosier than reality, and almost every generation questions the advancements of the next. Texas Bix Bender in his second edition of Don’t Squat with Your Spurs On says, “If you’re wonderin’ what this ol’ world is coming to, you’re in the same shoes as your daddy, your daddy’s daddy and every other daddy that’s come down the line.”

     You see my daddy was threshing ten times the grain per hour he had 20 years earlier with a team of Percheron. By1964 he had crossed the 40 year mark and, as all quadragenarians discover, seeing anything inside arms length can be darn near impossible. His glasses were lying in the usual place – the pickup dash. Such highfalutin fixins were just a nuisance. Course, they would have kept the chaff out of his eyes and improved the clarity of the hydraulic snafu that was ruining the day, reducing income, and increasing work.

    One thing’s for sure. When it comes to new-fangled eyeglasses they’re nothing like the old. Thank God. In the glasses lens industry, even the change over the past two years approaches the miraculous – especially in the arena of no-line bifocals. If you tried them several years ago and found them impossible, it may be time to try again.

 Lens materials:

     Spectacle (glasses) lenses are made from either glass or some acrylic polymer, i.e. plastic. Glass has two advantages over plastic:

1. Scratch resistance. But, it is just a matter of time before that changes.

2. Color dispersion. When white light bends through glass, it isn’t broken into the colors of the rainbow as badly as with acrylics. It’s a trivial distinction seldom noticed.

    But plastics can be worn more safely, and one glass lens weighs as much as two plastic lenses. Many new acrylics are also much thinner than standard optical glass. Because plastic lends itself better to special configuration, glass no-line bifocals are an inferior product. Consequently this article addresses only acrylic lenses. 

How power is made in lens material:

     Power is produced by first molding the plastic in a front surface convex curved cast. These cast blanks are made in a large variety of front base curves. The final power is determined by grinding and polishing a cast blank with the appropriate back side concave curvature with a ball shaped tool called a lap. It’s laborious and time-consuming process.

     Simple lined bifocals are made exactly the same as non-bifocals, except for one variation. The front mold has a small section with a steeper base curve, producing an abrupt area of power change or a line. Such a bifocal has a side profile like this:

 Front View:

  Advent of PAs:

     People have always complained about having to jump across the bifocal line. And, with a lined bifocal when a person reaches fifty they have distance and near vision, but nothing in between. In this day of computers, that’s not good. Computers are used at distances too close for the distance section and too far for the near section, so are pianos, recipes, blueprints, card games, and rifle sights. Trifocals are one solution, but then there’s another line. Even with trifocals, at fifty-five the wearer starts noticing more gaps in range. Frustrating.

     Back in about 1907 Owen Aves came up with the idea of making a slow transition in power to replace the line. He even made a few no-line bifocals, but none were marketed until 1959, and the first commercially successful no-line was the Univis Omnifocal in 1965.

     The gradual addition of near power gives rise to the name commonly used for no-line bifocals, progressive add. Somewhere along the road some wise-acre decided everything technical and/or medical must be written as initials. We all know about UV, IR, WMDs, ICBMs, IVs, CPR, MRIs, and ICUs. Well now you’ve been introduced to PAs – progressive adds.

     These gradual power arrangements make things complicated by causing twists and warping. The wearer sees bends, waves, and displacement, i.e. distortion, which causes swim and disorientation. Scientific types call these visual distortions aberrations. Since the Omnifocal, the PA industry has worked diligently to reduce aberrations.

Early PA production:

     PAs were first produced just like standard lined bifocals. The near power on the bottom of the lens was added by gradually increasing curvature on the bottom of the cast giving it a smooth profile.

     This rather simple front surface results in more complicated back curvatures for the compensation of nearsightedness, farsightedness, and astigmatism. There are no exact curvatures. Rather, there is an array of continuously changing curvatures that’s difficult to diagram.  This “front view” depicts power: not shape topography.

   The progressive power transitions were made smoother and the design improved until 1993. Between 1993 and 2006 advancements in PAs stagnated and not a lot happened.

Enters from left stage, refractive surgery:

     In the 1980s refractive surgery became an accepted procedure. Refractive surgery is the process of reducing the need for glasses by surgical manipulation of the front part of the eye called the cornea (the little clear dome sitting on the front of your eyeball).

     Initially refractive surgery was done by radial incisions that make a disturbing spoke pattern. In the 1990s the refractive surgery industry began using lasers to remove tissue. No incisions. No scars.

     The name for laser eye surgery has almost become a household word. LASIK (laser assisted intrastromal keratoplasty) is a computer numeric controlled (CNC) process incorporating the excimer laser, an ultraviolet laser. Without increasing temperature, the excimer removes corneal tissue like the vanishing coin in a magician’s hand. Now you see it. Now you don’t. It is a truly miraculous evaporation process called ablation.

     In today’s world everything from motors to mico-circuits is now made by CNC production. LASIK may be one of the most exciting CNC procedures in the world. A surgeon feeds a computer the proper information, and the excimer laser produces a cornea in the shape of a donkey, if desired.

     Refractive surgeons began reshaping corneas in to perfect works of art. The resultant corneas had the perfectly round shape and tolerance of a Mercedes-Benz ball bearing. Unfortunately, people were complaining, “I can see the smallest line, but it sure doesn’t look clear.”

     Why?

     Because the human eye and brain were not made by an engineer. Perfect corneal symmetry produces aberrations and visual images with all the human character of Mozart’s bronze bust on the mantel. It seems human only if the light’s dim.

     Refractive surgeons knew they had a problem with aberrations. Vision science sought the use of an instrument used by astronomers a century earlier to detect aberrations in telescopes. Its name? An aberrometer. By bouncing light off the back wall of the eyeball and reading the wavefronts emitted through the pupil vision scientists could determine the light patterns to which the human eye was accustomed – even reduce aberrations within the eye.

     They had an advantage over the early astronomers – laser light. Lasers are monochromatic (single wavelength or frequency) and much easier to read and manipulate than the diffuse multicolored light sources dealt with by the old star gazers. These new laser aberrometers are known as wavefront sensors. With wavefront sensors surgeons have diminish post operative aberrations and the complaints that come with them.

     How does all this talk about LASIK relate to PAs? Somewhere, within the spectacle industry, someone posed the question, “Why haven't we been using some of this sophisticate optical science?”

  Aberrometers, Algorithms and PAs:

     Just like in the television FBI series Numbers, the spectacle industry is now solving many of the mysteries of progressive bifocals with algorithms.

     Certainly spectacle lenses cannot correct intra-eye aberrations like refractive surgeons, but they can reduce light aberrations entering the eye. Aberrometer information allows development of algorithms so computer assisted design (CAD) and computer assisted manufacturing (CAM) can be employed in another CNC process making production practical.

     Instead of using grinders and laps the lens industry devised free-form generators which use a single diamond cutting point to quickly lathe lenses. This saves time, labor, and machinery, and it makes large numbers of front base curves unnecessary. These efficient generators will eventually reduce the cost of production of all spectacle lenses.

     Free-form production increases the optical accuracy six times, from 0.06 optical units (diopters, abbreviated D.) to 0.01 D. It can precisely remove lens material down to 0.1 microns (1/10,000th of a millimeter). And any company can devise and patent its own software establishing a form of competition that maximizes innovation and quality.

     Programs have now been developed that address eyeglass frame structure. PAs can be adapted to almost any frame size and shape – a process called frametization.

     Some companies have made fabrication software to customize lenses to accommodate how the glasses are worn, addressing face wrap, tilt, and distance from the eyes. The optical provider must provide significantly more information to the optical lab, and fibrication is more costly, but the results are worth the time, cost and effort.

PA Brands:

     Two companies that have taken customization to new levels of genius are Shamir and Zeiss. Depending on face shape, a frame may sit high or low. Common bifocal heights are 16 to 28 millimeters (mm), usually with in an accuracy of 1 to 2 mm. Zeiss, for example, is able to free-form heights from 13 to 35 mm in 0.1mm increments. Zeiss takes customization one step further by allowing the wearer’s initials to be laser micro-etched into the PA.

     Shamir makes free-form PAs of several levels of sophistication. If the wearer cannot afford the Cadillac, a nice Buick is available. Both near and intermediate zones can be customized independently.

     Many if not most of the lens producers cast their front surfaces from ceramic molds, but a better front surface can be made with glass molds. Shamir makes a glass mold from a ceramic master, and the ceramic masters are digitally generated just like the back surface of the final lens. So technically both the front and back of Shamir’s PAs are CAD designed, computer controlled and digitally produced.

     Shamir and Zeiss are considered by many to be at the top of the heap. Essilor, Hoya, Seiko and Signet (Kodak) are some more good producers of PAs.

     Essilor is a merger of two companies one of which engineered the Varilux lens which has become the common or generic term used for PAs. They are to be commended for kicking the PA industry out of stagnation with genius marketing of the Physio-360 -- making it one of the first well known and accepted free-form PAs widely used throughout the industry. 

     It’s an accepted fact that curvature variation is best left on the back of spectacle lenses increasing field of view and reducing aberrations. But Hoya has separated itself from the pack with an innovative two sided implementation of the progressive power. How valid is such a theory? Time will tell.

     Currently some clinics throughout the United States use aberrometers on every eye. iZon, a pioneer in this arena, develops algorithms and builds highly customized spectacle lenses from that information. Theoretically, glasses could be made to correct aberrations even though a patient might not otherwise need a vision prescription. How well does it work? Again, time will tell.

Adapting to PAs:

      Laser analyzed, digitally produced PAs dramatically reduce swim and disorientation while significantly improving sharpness. Adaptation has become much easier. That is not to say there is no adaptation. After all, with modern PAs you have the same range of focus as a twenty year old, but it takes a set of algorithms that boggles the mind. You must accept some areas of visual compromise. Adaptation may still take four or five weeks of persistent wear.

     Adaptation does not mean “you just get used to them.” Your brain actually straightens your world and loosens neural tolerances, allowing you to see in a more true and useful fashion with more comfortable and efficient vision. Once adapted, putting them on and taking them off is not a problem. Give your brain some time.

     Remember, PAs are the vast majority of bifocals sold. If you cannot adapt, you are in a slim minority. Further, a curmudgeon’s attitude can wreck anything from metal to matrimony. Adapting to glasses is like faith, love, and marriage. They are all choices. If you tell yourself that you cannot tolerate your spouse’s colloquialisms, leaving the cap off the toothpaste, and failing in expressions of unfailing fondness, well then …you’re right.