By Palmer R. Cook, OD • Illustration by Iris Johnson
Manufacturers now offer some great new materials for the eyewear designer/dispensers' armamentarium. These are the ultra-highindex materials with lots of light bending power and high potential for thinner, lighter weight spectacles. Used appropriately these materials can give spectacular results.
We all know how unforgiving optical science can be and we spend plenty of time, effort and expertise making eyewear design trade-offs involving curvature, index, specific gravity and lens size to assure that our patients' needs are well met. Ultra-high-index materials broaden our range of choices when turning a lens power formula into finished eyewear and they present us with additional and complex tradeoff possibilities.
Just where does "ultra-high" start? The range of indices for ophthalmic lens material is almost a continuum rather than a series of steps. Patients don’t neatly fit into one or another index. A patient who does well in a 1.60 index material in a 48-20 frame with a 28 B measurement, could overlap to a 1.67 index if a 50-22 frame with a 38 B measurement is selected. To facilitate our thinking it probably makes sense to arbitrarily divide the index range, starting at 1.67 as the low-end index for the term "ultra-high."
Various lens manufacturers including Seiko, Essilor, Zeiss, Hoya, Tokai and Nikon offer lenses with refractive indices ranging from 1.67 to 1.74. On the horizon are even higher index lenses such as Tokai’s 1.76, which has been released in Japan, but is not yet available in the U.S. Not all lens materials are available in multifocal or PAL form yet, but you can bet that plans are being laid for making them available.
Unfortunately advertising claims of "X" percent lighter or thinner than another material have little meaning unless we are given tables comparing lenses of different materials at various specific powers and diameters. Such tables would allow us to make our design trade-offs with considered judgment rather than basing our choices on advertising and guesswork. (Manufacturers please take note.)
Ultra-high-index materials have high potential for thinner and lighter-weight lenses. They also have equally high potential for performance problems if they are misused. Our tasks are further complicated by the fact that a "best result" may be obtained with one material for one patient, while another material may be needed for a "best result" for another patient with the same Rx and even the same frame. This is a problem lamented by many students and optical apprentices as they earn their stripes with the most complex component of every eyewear design—the patient.
Know Your Patient
As professionals we often think "best optics" above all else. However, patients don’t always think that way. They expect to see well, but they also want comfort (e.g. lighter weight) and good appearance (e.g. thinner, less "glassy" lenses). These issues have gradations that present us with difficult decisions. Your patient may accept a little less optical performance, but there is no scale of reference to tell the patient, or you, how much is lost or gained when a particular index is selected. Experience and knowledge must come into play and that's what makes you a professional. Your judgments cannot be dictated by hard data alone.
Questions To Guide You
Is the patient’s previous Rx so strong that they have already become a nose pointer? Simple observation can give you a good clue. Does the patient have a history of easy adaptation to previous Rx changes? Just ask and you will hear about it. When balancing vision, appearance and comfort, your patients will give answers, but the most important information can be difficult to extract. Patients often say what they think you want to hear instead of expressing their real priorities. Often what the patient says must be weighed in the light of your experience and your skills of observation.
One of my own patients, a pharmacist, threaded soft, red-rubber catheters onto his temples as a temporary cushion to maintain comfort until he could return for a frame re-alignment. He left several inches of the catheters dangle below his earlobes rather than bothering to trim them shorter. Appearance was not at the top of his list, but comfort surely ranked high for him. Some patients have appearance as a top priority, but they will not appear in your office wearing sweats or a flowered Hawaiian shirt as this pharmacist patient typically did. The way a patient speaks and the way he or she dresses and moves can give important clues to your patient's priorities and potential tolerance to optical trade-offs.
All lenses of a given power perform about the same through the optical center regardless of curvature or material. It’s only when the line of sight passes through the lens away from the optical center that optical issues begin to arise. Chromatic aberration, caused by a low Abbe value, is largely hidden until the line of sight moves away from the optical center. When using ultra-highindex lenses with relatively low Abbe values in the range of 32 to 36, you must consider chromatic aberration or color dispersion. Problems related to chromatic aberration are generally less well tolerated by patients with lower and intermediate power Rxs. These patients are accustomed to having clear vision across a larger area of their lenses and they are troubled when they find a drop in clarity when their line of sight strays away from the central lens area in their new eyewear. Usually patients with strong prescriptions already know to preferentially use the central area of their lenses.
Lens design cannot alter chromatic aberration, but patient counseling ("point your nose at what you are looking at when you need the most clarity") and using a smaller lens size can both be of help. For other aberrations we depend largely on the lens manufacturer's skills in picking the best curvature, hoping all the while that the patient's vertex, pupil size, location of center of rotation and pantoscopic tilt are all close to the same values the manufacturer used in working out his design.
The Reflectance Factor
As more light is reflected from the front of lenses, there is a "more glassy" appearance and there will be more masking of the patient's eyes—both effects being cosmetically undesirable. Using a high-quality, anti-reflective lens whenever higherindex materials are prescribed significantly reduces these problems. Additionally, back surface reflections, which are usually only a problem with sun lenses, can be bright enough with higher- index lenses to become problematic even in non-tinted lenses.
Reflectance always increases as index increases. A 1.70 index lens reflects 6.7 percent of incident light compared to only 3.8 percent for a 1.49 index lens. Most ultra-high-index materials have specially designed anti-reflective treatment. This assures the best optical results by correctly matching the index of the anti-reflective layers to each material and improves patient satisfaction by creating the best adherence and smudge and dust particle resistance for that material. Manufacturers of most ultra-high-index lenses have made the wise decision of only supplying ultra-high-index in an anti-reflective format.
Most ultra-high-index materials are only available in aspheric design. This reduces lens volume compared to the volume that the lens would have in corrected curve form. Weight is further reduced if the selected material has a lower density or specific gravity. Density (specific gravity) as used here refers to the weight of a cubic cm of the material in grams. The density of PPG's CR-39 monomer is 1.32. Many high-index lenses are lighter because of both an index that allows smaller volume lenses and a low specific gravity.
Lens Curvature and Thickness
From an optical standpoint, corrected curve designs except in the higher plus powers, may give superior performance. Yet patients in higher power ranges probably do not use the periphery of their lens much for detailed (foveal) viewing and the flatter aspheric curves allow a shorter vertex and better PAL performance. Except with very flat or very steeply curved lenses, most people outside the ophthalmic industry probably don’t relate curvature to "strong lenses." However, thickness is reduced by aspheric design and this can give better appearance especially with strong plus and mid-range or higher minus lenses.
When to Use Ultra-high-index
There is no clear-cut formula, but a +3.00, 1.70 index lens will still have about the same volume and curvature as a +2.00, lens even though the index has been raised from 1.49 to 1.70. This appearance difference, to an untrained eye in terms of thickness and curvature, may be difficult to detect in lower powers even though the index is increased by what you might consider "a significant amount." The reflectance (without AR) will be 68 percent higher in 1.70 than in 1.49 and the ghost images during night driving will be 2.8 times brighter. Although AR reduces reflectance and ghost image problems, the lower Abbe remains an issue, as does the increased cost.
The same +3.00, in 1.70 index would have about the same volume as a +2.50 in polycarbonate. Again, the desired decrease in thickness and curvature going from 1.58 to 1.70 will not be easily apparent even though minus lenses are typically surfaced .8mm thinner and plus lenses are typically surfaced .25mm thinner than lenses with less tensile strength. Changes in eyewear design should yield patient-perceptible differences. The advantages of going to ultra-highindex for patients with low or even mid-range prescriptions may not be patient-perceptible, especially in smaller eyesizes.
Ask the Lab
Your lab should be able to answer some of your most critical questions, such as whether the ultrahigh material is available in a tint or polarized or with an add of one or several design(s) or whether the material is suitable for drill mounts.
A Final Note
Don’t be too quick to assume that cost is a prime factor. It seldom is, unless you make it so. The patient who complains about paying a few hundred dollars for eyewear may be the same one who spent three or four thousand dollars on a big screen TV, even though he could watch the same program on a fine set for a couple of hundred dollaars. And most women spen much more for shoes or cosmetics over 1,000 days-the useful life of most eyewear- than they do on the glasses that cover the most expressive part of their face: their eyes. If you establish good perceived value, cost becomes a non-issue-a fact of life that is all too often overlooked in our industry.
Palmer R. Cook, OD is director of education for Diversified Ophthalmics in Cincinnati..