In the days of large eyesizes just jumping a rung or two up the index ladder was both an easy and correct choice. But the formula of bigger eyesize equals stronger Rx may not always be the best solution, especially with today’s smaller frames.
With many frame and lens combinations, increasing the index of refraction is the best way to create attractive, comfortable eyewear with superior optics. Yet other options can eliminate the need for increasing the index. It’s important to understand how a lens material’s refractive index interacts with other lens properties before automatically increasing the index of a lens.
As index increases other lens qualities change. The amount of light reflected always increases with an index increase. A polycarbonate lens at an index of 1.59 reflects almosot 34 percent more light than a hard resin lens at 1.49. Higher-index lenses also tend to have more chromatic aberration and higher cost. So what’s an eyewear designer/dispenser to do?
Reflected Light Is Useless Light
For ophthalmic lenses, reflected light is useless light. It is light lost from the major ray bundle that forms the retinal image. Worse, an increase in reflected light makes the bulls-eye rings of minus lenses more prominent and contributes to annoying back surface reflections. Ghost images that can interfere with night driving also become more prominent as the amount of reflected light increases.
Higher-index additionally causes more light to bounce from the front surfaces of lenses. Remember the science fiction movies of the ’50s? A bespectacled mad scientist was a common figure in these features and shots showing eye-obscuring reflections were almost obligatory. “Mad scientist” effects are not what most patients want from their new eyewear.
Patients like to perceive increased value for every increase in cost. Typically, options and benefits increase cost. But is it really a benefit if the difference is not patient-perceivable? A tint is easily perceivable, as are anti-reflective properties and multifocals without the lines. Not only are these options perceivable, but their benefits are easily patient-recognized and patient-appreciated.
Increased index decreases edge thickness in minus powers and center thickness in plus powers. Increased index also decreases lens volume and, unless the specific gravity dictates otherwise, the weight of the lenses is decreased. The big question is whether the decreased thickness and weight is patient-perceptible. For practical purposes a change that is not perceptible is not cost justified in the minds of our patients.
Increased reflectance and increased chromatic aberration (i.e. lower Abbe value) are negative effects that may be patient-perceptible. They can evoke complaints of poor optical performance and poor appearance. Even if chromatic aberration and reflectance don’t give rise to perceptible optical problems, increased cost may be an issue.
Guidelines and Answers
Patients with a history of sensitivity to eyewear weight or who have medically predisposing conditions such as diabetes or who complain of past problems with eyewear comfort and fit, are often good candidates for higher-index materials.
Most patients are appearance conscious. Using a higher index is one way of making lenses more attractive in terms of decreasing thickness. However, decrease in thickness is related to lens size, decentration, and Rx. Using a frame with a smaller lens size, both horizontally and vertically—especially one that requires little or no decentration—may may eliminate the need for increased index. If an increased index is the right choice, reflectance issues can be made better by the use of AR lenses. If the index is increased, it is a good rule to switch the patient to AR lenses.
If weight and thickness issues are a primary concern, consider using aspheric or atoric lenses. An aspheric bonus, especially with larger eyesizes, is that there may be a shortening of the vertex and a concomitant increase in PAL performance. It’s important to look at the power in or near the horizontal meridian if lateral edge thickness is a concern. If the Rx is -1.00 = -3.50 x 180, lateral edge thickness is little affected by an increased eyewear size. If the Rx is -1.00 = -3.50 x 090 the power in the 180 meridian is -4.50 and a relatively small jump in eyesize may cause a noticeable increase in lateral edge thickness. This effect is due to both the increased lens size and the increased decentration.
As every clinician knows, there is a wide range of sensitivity among patients. The clinician’s judgment is important in identifying the patients who will benefit from an increased index or other lens options. Careful choices are especially important because those who are sensitive enough to appreciate an option will also tend to be sensitive to option-induced problems.
Patients who are “eye-turners” or who have a prism prescription are more likely to be sensitive to problems arising from chromatic aberration (i.e. low Abbe). Most patients who have worn higher power lenses or who have worn PALs have learned to stay near the lens centers. They may be less sensitive to problems induced by chromatic aberration.
When impact resistance is a prime concern, polycarbonate has in the past been the only material of choice. Now PPG’s Trivex offers impact resistance with lower reflectance and less chromatic aberration. Rule of thumb: If increasing the index is needed, use polycarbonate; for lower prescriptions and smaller eyewires use Trivex to get optimal optical performance along with impact resistance.
Consider All Options
The Rx is stronger and the patient insists on bigger frames. Using a higher-index material may be a great choice, but a knee-jerk jump up the index ladder is not a good idea. First, consider using aspherics or atorics. Increasing the bridge size for eyewear width may allow a smaller eyesize and less lens thickness and weight. If you do increase the index, using AR lenses will tend to make most patients more tolerant of the problems of chromatic aberration.
Higher-index materials are very good technology. Used properly they can be an important part of creating patient pleasing eyewear, but consider all available options and how they interact when creating your patient’s eyewear.
Palmer R. Cook, OD, is director of eduction for Diversified Ophthalmics in Cincinnati.
Index – All lens materials have an index of refraction, commonly just called the index, which indicates the light bending muscle of the material. The higher the index of a lens material, the more light bending muscle it has. A higher index means the material slows down light more and therefore doesn’t have to be curved as much to bend light to the extent called for by the Rx. That’s why higher index materials can make flatter, thinner lenses.
Chromatic Aberration – Not all wavelengths (i.e. colors) of light are bent to the same extent by lens materials. If a lens material bends all wavelengths about the same, the material is said to have a little chromatic aberration. If a lens material bends some wavelengths a lot more than others, it is said to have a lot of chromatic aberration. If the chromatic aberration is greater, the image formed by the lens is less clear and color fringes may become apparent when the patient looks away from the center of the lens. Chromatic aberration is more troublesome with large lenses, prism prescriptions and in mid-range and higher Rxs.
Abbe Value – The Abbe value of lens materials tells how much chromatic aberration a lens will generate. Low Abbe values indicate more chromatic aberration and high Abbe values indicate less chromatic aberration. Common ophthalmic Abbe values range from a low of about 28 to a high of about 58.