Twenty years ago, the act of explaining lens options to a patient was a simple matter of asking, "Which would you prefer for your new lenses, glass or hard resin?" Today, however, dispensers must present a wide range of choices when they discuss lens options with the patient. Two of the most misunderstood lens choices remain aspherics and high index.

Before breaking out these two options, however, it's important to look at the lenses as a whole. Today's lens choices can be broken down into three primary categories: lens materials, lens designs, and lens treatments. The first two divisions are self-explanatory, while lens treatments are made up of options such as coatings, including anti-reflection, UV, and mirrored.

Consumer needs also fall into three basic categories: Cosmetic (thinner, more attractive lenses); comfort (lighter lenses); and visual (better optics). In fact, most of the lens innovations introduced in the last 15 years were developed specifically to address one or more of these concerns (or desires) of eyeglass wearers, and it's important to view options such as high index and aspherics in terms of those consumer wants.

For starters, there's little doubt that these two lens developments -- high index and aspheric -- have done more than any other in answering the consumer concerns listed earlier. The first innovation was the development and introduction of lenses made of high index plastics, a trend that is still evolving. There were seven different indices from which doctors and dispensers could choose, and an eighth variation (1.71 index) was introduced this year, giving eyecare providers the highest available index in plastic.

Addressing the three primary patient concerns in a totally different way, some lens designers felt more could be achieved by concentrating on the design of the lens rather than the material from which it was made. These innovators believed patients would be better served by lenses made with aspheric curves. As a result of this alternative approach to improving modern eyewear, some eyecare providers are finding it difficult to decide which approach works best for answering specific needs of the patient: high index or aspheric.

Let's review in detail how the benefits provided by high index lenses compare with those offered by aspheric lenses.

High Index Lenses

The desire for better looking eyewear was first addressed through development of plastics that had a higher index than conventional CR 39. This made it possible to fabricate strong minus lenses with thinner, more attractive edges. High index materials are seldom used for plus corrections because making plus lenses in high index simply means thinner centers, and this benefit is not readily apparent in finished eyewear. Basically, the advantage of making lenses in high index materials is that they end up with thinner edges, a cosmetic concern. Lenses made in a high index material have slightly less bulk, and this could be interpreted to mean lighter lenses (answering the comfort need). Most high index materials, however, have a higher specific gravity so that weight savings, if any, are minimal. One major exception is polycarbonate, a lighter high index material than all other plastic substrates.

Labs report that many orders for high index lenses are for weak corrections (less than one diopter), even though it is difficult to detect any visible thinning of the lens edges in those cases. The additional profits that come from high index lenses are enticing and influence some dispensers, but there is risk of buyer's remorse when high index is suggested where any improvement in cosmetics will be difficult for the patient to see.

Aspheric Lenses

Aspheric lenses appeared on the scene several years after high index plastics came into use. Initially made in plus powers only, aspheric lenses enjoyed modest success. It was unfortunate that many doctors and dispensers overlooked the fact that using an aspheric design addresses all of the important patient concerns.

Aspheric lenses are flatter and thinner, definitely a cosmetic asset. The lenses have considerably less bulk, and this contributes considerably to comfort (lighter weight). Lastly, the unquestioned superior optics of aspherics provides enhanced vision for wearers. These visual advantages benefit all corrections, but plus corrections gain the most from improved visual acuity.

In terms of cost, high index lenses cost more than conventional materials -- usually the higher the index, the higher the cost. Lenses with aspheric curves also justify higher prices. Though differences in price between high index and aspherics has not influenced which is recommended to the patient, there is one consideration, probably more than any other, that has influenced whether high index or aspheric lenses are recommended. That consideration has much to do with the experience or expertise of the dispenser.

Dispensing high index lenses requires no particular skill. If you can spell "high index," you can explain and dispense these
lenses. Aspherics are somewhat different. They require a more in-depth explanation (or demonstration) and, for a successful fit, they require greater care and proficiency in dispensing.

As a result, people who don't fully understand aspheric lenses tend to shy away from recommending or using them. That's unfair to the patient and a disservice to the practice, as well.

Making a Choice

So, how do you decide which to recommend to your patient -- high index material or aspheric design? Normally, lens manufacturers can be relied on to provide advice on which they believe should be used. But because each lens manufacturer produces high index lenses and most of them also produce aspheric lenses, they often don't specify which type of lens should be used.

Looking at recent developments within the industry, however, it's clear that aspherics are garnering the lion's share of attention. For example:

When Sola introduced a proprietary line (Spectralite�) a few years ago, it chose to produce the entire line in aspheric design.

When Seiko introduced a totally new plastic mid-index photochromic line (Changers�) two years ago, it made the entire line in aspheric design.

Most progressive addition lenses introduced in the last two years have featured an all-aspheric design throughout the lens.

When 2C Optics designed a totally new mid-index "molded to prescription" lens in 1998, it chose to produce the entire line of single vision, bifocal, and progressive lenses in aspheric form, designed by Rodenstock.

A sophisticated new type of lens is now produced by a number of manufacturers in a variety of high index materials. These lenses fall into a totally new lens category called "atoric." Atoric lenses are simply aspheric lenses taken to the extreme by positioning the aspheric curves on the backside of the lens. When there is cylinder in the lens, each meridian of the cylindrical surface is aspheric. This change makes a dramatic improvement in visual acuity.

Future Developments

It doesn't take a rocket scientist to conclude that most, if not all, lenses in the future will most likely be both high index and aspheric. By combining the two technologies in this way, these newest ophthalmic lenses will be marrying the best of both worlds. The offspring of this marriage will be state-of-the-art lenses that specifically address each of those three importantissues for eyeglass wearers: improved cosmetics, greater wearing comfort, and the ultimate in 21st century visual optics.

Many aspheric lenses are already being produced in some form of high index material. For example, Gentex (now part of Essilor) scored a spectacular coup when it introduced a polycarbonate aspheric lens created by John Davis, their chief lens designer. It featured a very flat aspheric design, combined with the light weight and relatively high index (1.59) of polycarbonate. The result of this combination was a lens the company claimed to be the ultimate in a fashionable lightweight lens.

To prove its claim in a dramatic manner, Gentex sent a mailing to 10,000 eyecare providers. Each customer received a sample +4.00 uncut lens housed in a traditional plastic container used for packaging CD disks. Anyone who received one of those dramatic samples will never forget the wonder of finding an uncut plus lens fitting in a flat box no more than 3/8 of an inch in thickness.

Two years ago, in an Eyecare Business article about aspheric lenses, a prediction was made that, much like camera lenses, all eyeglass lenses would ultimately be produced in aspheric form. As the industry approaches the millennium, it's becoming obvious that aspheric lenses will indeed become the standard for ophthalmic lenses. It is also obvious that most of those lenses are going to be made from materials with a higher index than conventional glass or plastic. EB