HIGH INDEX LENSES
The Lowdown on High Index
Not sure which refractive index is right for which patient? Here's what the varying indices have to offer
By Joseph L. Bruneni

The original reason for using lens materials with a refractive index higher than glass or CR 39� was to provide cosmetic enhancements for strong corrections, primarily myopes with corrections of -7.00 diopters or higher. The first high-index material was developed at a time when all lenses were glass, and impact resistance of lenses was not yet a consideration. The primary disadvantage of those early high-index lens materials, apart from their increased cost, was substantially increased weight. The primary ingredient for increasing the refractive index in glass turned out to be lead. High-index glass was considerably heavier than crown glass, and this is still true with high-index glass.

As plastic lenses became popular, the situation changed in several regards. Today, eyecare professionals can choose from plastic lens materials in 11 indices, ranging from 1.50 to 1.74. In glass, they can select from five indices, from 1.52 to 1.90. Strangely enough, in certain ways, deciding which lens material to use today is often only marginally related to the material's refractive index. This article will review the factors governing lens material selection in today's ophthalmic marketplace.

The industry continues to call these newer substrates "high-index" lens materials, but a better choice of words would be to call them "alternative" lens materials since the rationale for choosing lens materials other than conventional glass or plastic often has little relationship to the corrective power of the lens. In this story, we'll discuss what's currently available in "alternative" lens materials and offer suggestions on how to choose the lens material that will best meet each patient's needs.

Current Choices

Any lens material with a refractive index higher than CR 39 (1.50) or glass (1.52) is considered to be high index. As the range of high-index materials expanded over the years, a new term was required. Materials with an index higher than glass (1.52) but lower than polycarbonate (1.59) are now considered to be "mid-range." Everything higher, from polycarbonate at 1.59 to 1.90 in glass, falls in the high-index range. A listing of available mid- and high-index materials can be found in the accompanying chart. There are certain caveats to keep in mind associated with alternative lens materials. For one thing, higher index materials are often heavier. The weight of high-index plastic materials doesn't increase nearly as much as in glass. Increased weight is not a major consideration with high-index plastic materials.

There is another general rule of thumb applying to alternative materials. As the refractive index increases, so does cost. There are at least two exceptions to this rule: Polycarbonate (1.59) is less expensive than might be expected from its relatively high index, and costs less than other mid- and high-index lens materials. Another exception is a mid-index lens line produced by Sola called Spectralite�. These lenses have a low index but carry a higher cost because of the lens design: All Spectralite lenses are aspheric, a sophisticated design that is more costly to manufacture.

Selecting Index

This ought to be a simple task. Stronger corrections require a higher index than weaker prescriptions. If it were only that simple. Four things complicate the decision.

• Lens design. Lenses made of a mid-index material and produced in aspheric form can produce cosmetic benefits comparable to lenses in a much higher index. Mid- or high-index materials and aspheric designs make good companions. In fact, most aspheric lenses are produced in mid- or high-index materials for that reason.
• Cost. One of the major aspects that made polycarbonate the most-used high-index lens is the fact that this material has a relatively high index but costs less than other high-index materials.
• Availability. Not every lens design is available in all high-index materials. This is another reason why poly outsells all other high-index materials combined. Currently, more lens designs are available in polycarbonate than any other material except for CR 39.
• Center thickness. Some high-index materials can be safely produced in stock lens form with centers .5mm thinner than conventional high-index materials. Lenses with 1.0 centers will have edges half a millimeter thinner than those with conventional centers. Several high-index lenses (1.60 and 1.66) are now produced in stock lens form with 1.0 centers. These are made possible through use of special "cushioned" hard coatings that permit the lens to flex rather than break. These hard coatings are proprietary to the manufacturer and not currently available to laboratories. At this point in time, polycarbonate is the only high-index material that can be surfaced by laboratories to 1.0 center thickness.

Alternative Lens Materials

Earlier, it was suggested that a better term for high index might be "alternative" lens materials. To better understand what's happening in the lens field, let's consider why there was such a stampede away from conventional CR 39 during the 1990s. That trend continues to this day with the market share for CR 39 now less than 50 percent. At International Vision Expo East in New York, one of the premier events was the introduction of a new lens material from PPG, the company that made plastic lenses possible some 50 years ago. Their development of CR 39 launched the plastic revolution in lenses. Their newest plastic is called Trivex� and this new lens material illustrates what's happening with today's lens materials.

In certain ways, the index of a lens material has become less important than other attributes. Advantages of the new Trivex material, for example, center around the material's impact resistance (comparable to polycarbonate), its high Abbe value (43) and its low specific gravity (1.11, lightest of all materials). The material's refractive index is 1.53, only marginally qualifying it as a mid-index material. Trivex lenses will be produced by Hoya under the name Phoenix and Younger Optics will produce Trivex lenses they call Trilogy�. Advanced information from both companies indicates they will be marketing their Trivex lenses to compete directly against polycarbonate, currently the leading alternative lens material.

An Underlying Reason

There's another reason why alternative lens materials are taking over the lens market, and it has little to do with index, weight, or Abbe values. It falls in the realm of economics. CR 39 proved to be an outstanding lens material, providing valuable service to the eyecare professions for more than 50 years. Today, however, CR 39 lenses are only marginally profitable-for manufacturers, for labs, and even at the retail level. Today, virtually all sin gle vision CR 39 stock lenses are produced outside the United States, and much of the semi-finished lens production is also moving off-shore. Laboratories that produce nothing but CR 39 lenses would have difficulty keeping their doors open.

The profits generated by lenses made of alternative lens materials have become essential to the entire industry. When was the last time you saw a full-page ad for lenses made of CR 39? These lenses still provide value and service, but no one seems to be advocating their use. One of the most discussed topics of conversation at the recent International Vision Expo was the continuing flat state of the optical market. For the past 18 months, there has been little growth in the ophthalmic industry, and unit sales in many sectors have declined.

Fortunately, during this same period, the selling price of eyewear has continued to increase-primarily because of the more sophisticated lenses currently being dispensed.

There's another aspect to alternative lens materials that shouldn't be overlooked. Eyewear consumers are always impressed with new technology, and convincing consumers to order lenses made of improved space-age materials is not difficult if the doctor or dispenser takes the time to properly explain and demonstrate them. The explosion of new lens technology in recent years has been a substantial factor in keeping the average time between eye examinations relatively stable in spite of a declining economy.

Once we understand the importance and value of alternative lens materials, what's the best way to utilize them? It's difficult to provide exact guidelines on how to determine which lens material will work best for each patient. It requires the doctor or dispenser's careful professional interpretation of each patient's lifestyle to make that decision. In many cases, the choice of material is best determined by the doctor during the eye examination. The doctor's recommendation has a powerful influence on patients, and it's always smart to take advantage of this fundamental fact. Here are the major considerations when determining which lens material should be recommended.

• Safety. When the patient is a youngster, sports enthusiast, or anyone with an active lifestyle, eye protection will be the key issue. Polycarbonate will be the lens of choice in these cases, although the new Trivex lenses ultimately may fall into this category.
• Required corrective power. There is little advantage in using conventional high-index materials for plus corrections. These patients are always better served with aspheric lenses, most of which are made of high-index materials. Weak minus corrections (less than 2.00 diopters) are viable candidates for polycarbonate. The added cost is modest, and they can be ordered with 1.0 centers when edge thickness or weight is important. When the correction is over -2.00, the decision may depend on the amount of minus correction, keeping in mind the greater cost for the higher-index materials and the advantages of super thin lenses such as polycarbonate and certain high-index stock lenses.
• Lens design. For plus corrections, aspheric or atoric lenses are the lens of choice, and the index plays a much less important role. Very high minus corrections (-5.00D and up) gain additional thinning of the edges when ordered in the new atoric form. Aspheric/atoric lenses are expected to become the norm in the future, and some entire lens lines are now produced only in this more advanced design. Optima's new polycarbonate line, Resolution, is all aspheric or atoric, another sign of the times.
• Budget considerations. When cost for their new lenses is an important issue to patients, polycarbonate will be the usual choice.
• Weight. Lighter weight has become a valuable aspect of modern eyewear. Materials with a higher index have less bulk, and this will lessen the weight of the lenses. When the material has a low specific gravity, this reduces the weight even further. Aspheric design can also reduce lens weight, and lens materials that permit reducing center thickness to 1.0mm also contribute greatly to weight reduction.

A Finishing Touch

Regardless of which high-index material is selected for the patient, there is one other important issue that should be considered. Here is one more rule-of-thumb that applies to high-index materials. They all transmit less light than CR 39 or glass. Generally speaking, the higher the index, the less light is transmitted through the lens. For this basic reason, an AR coating should be suggested to all high-index patients. The interesting thing about AR and high index is that AR coatings bring all high-index lenses up to the same maximum light transmission as AR-coated CR 39 or glass: 99.5 percent transmission. As a result of this basic scientific fact, high-index wearers benefit more from AR coatings than anyone else.

This is why many offices add the cost for AR coating into the fee they quote for high-index lenses. When quoting cost for high-index lenses, they explain that the fee for the patient's new lenses includes a high-tech, anti-reflection coating, creating the most advanced eyewear possible for their correction. Assuming the patient receives a proper demonstration of AR coatings, few consumers will turn down the opportunity of getting the "best." Patients are inevitably impressed with advanced lens technology, and following these guidelines will make sure they know that's what they are getting in your office.