Choosing the Right Index
This OLA-sponsored article offers tips for selecting the best high index option for every patient
By Alex Yoho, ABOM
Photography by Peter Baker

What a time to be in the optical industry. We have more options available to us for creating eyewear than at any time in history. Yet with more choices comes more responsibility. And, one of the things that we are responsible for is selecting the best index of refraction for each patient and each use.

How does one determine which index is best? Do we just divide all the possible lens prescriptions into groups and assign a higher index for higher prescriptions? Maybe, but there's a lot more to choosing the index than that, according to OLA leaders.

Since lenses are all about bending light, the higher the index of refraction, the more we can bend (refract) light without using steeper (translate thicker) curves. This means a CR 39 lens form that has an edge thickness of 10mm can be ground with less curvature, making it one-third thinner in a higher index.

Sure, there are general prescription guidelines, says Henry Bode, operations manager at Hawkins Optical in Topeka, Kan., but there are many materials making up the different offerings. For example when we refer to high index, we usually think that it starts at 1.60. But which 1.60 do you choose--glass, plastic, Photogray extra 1.60, or one of the photochromic plastics in a 1.56 or 1.67? Then, of course, the lens can be aspheric or spherical--what role does that play with high index or low index lenses? And then there are the so called mid-index lenses.

So where do you start? Don't limit a low prescription to a low index. There are patients whose prime motivation is to get the absolute thinnest lens possible, so don't disappoint them. You might actually insult a patient who overhears you presenting high index to someone else. That being said, does this mean you should offer the highest index to all patients? Well, not exactly.

Reflectivity. A more reflective material combined with these flatter curves begs for anti-reflective coatings. Patients can be sensitive to base curve changes, and high index lenses are normally made in a flatter lens form to reduce other aberrations that occur in all types of lenses.

Don't forget that there are the patients who are willing to put up with what many would consider poor vision for the sake of the look they want to achieve. This is another reason why it's important to know the different characteristics of each lens.

GETTING STARTED

If we always selected the highest index for all patients, there would probably be many more rejects. Those who are sensitive to chromatic distortion would complain of blurriness or rainbow colors at the edges. Those sensitive to base curve changes might complain of wooziness, and those bothered by reflections would tell you, "They weren't there on my old glasses." The key is to find a balance between thin and good vision.

That requires finding out more about each patient. Let them know that they have many lens options to choose from and that it will help to know their goals.

Here are other things to consider.

Peripheral vision. If the patient tells you that the highest priority is to make his lenses thin, you will obviously use the highest index possible. Nonetheless, a gentle reminder to the patient might still be in order about the possibility that his peripheral vision may not be as clear in certain cases as with some other lens designs. Bringing this up is also a good idea if the patient wants thin lenses in a wrapped frame.

Manufacturer recommendations. According to Mary Walker, president of Superior Optical Labs in Ocean Springs, Miss., the most important criterion is to keep everything within the lens manufacturer's recommendations.

Diagram charts. Walker says using the lens diagram charts that many manufacturers have printed can make it easy to show a patient exactly how thin their lenses will be. These charts can also show a patient that lenses are flatter in higher indices, and this can be a jumping off point for discussing AR.

Stability concerns. Ron Stene, the co-president of EPIC Labs in St. Cloud, Minn., adds that it's especially important to consider center thickness when it comes to some high index options. "Dispensers want to order lenses with thin centers, but as the power increases, the center actually needs to thicken slightly for stability. These are softer materials we're working with, especially the mid-indexes. There are safety limits as well."

A BALANCING ACT

Kevin Bargman, president of Hawkins Optical, says that mid-index materials seem to lend themselves well to manufacturing the blanks that labs begin with. For example, explains Bargman, "The first higher index photo-chromic plastics were what we consider mid-index. Their ability to accept different treatments may eventually lead to mid-index materials being the 'normal' plastic. Processing is not difficult until people push the limits."

Finding that balance between thinness and good optics can be aided by a number of factors.

Aspheric curves. One is the use of aspheric curves. These curves have been used so often with higher index lenses that some dispensers assume that all high index lenses are aspheric. This is not the case, however. Generally speaking, the higher the index, the greater the benefit of aspheric curves. They alleviate some of the problems encountered when base curves are changed. If your patient is notorious for sensitivity to base curve changes, you will definitely want to consider an aspheric version.

Anti-reflective coating. Since the "clarity" is improved as much as 10 percent when AR is applied, patients really appreciate the difference. AR is the best solution for chromatism while staying with a high index lens.

Frame selection. When it comes to frame considerations, Jim Schmidt, finishing expert at EPIC Labs, says, "You can't counter a poor frame selection with high index lenses."

Everyone interviewed for this story agreed that highly wrapped frames are not a good choice for high index. They say that a frame with too much curve will force a flat lens to pop out and a flat lens will force the wrapped frame to unwrap, making it difficult to adjust.

What about rimless frames and high index?

Bode suggests, "When putting low powers into drilled rimless frames, it is better to use polycarbonate. Trivex also shows great promise. As powers get above -3.00 or so it becomes safer to use other high index materials, since the prescription itself creates enough thickness for support."

So after taking into consideration everything from patient motivation and lens science to frame styles, it becomes obvious that eyecare professionals and their staffs must be aware of the myriad options when it comes to high index lenses.

Your best resource in finding out the details of these lenses is your OLA laboratory.

The labs' staffs are waiting, and they are always glad to share their expertise with you.