Moving Up
The lure of thinner, lighter lenses sparks the growth of high index
By Joseph L. Bruneni
Photography by Peter Baker

Lenses made of materials that have a higher refractive index than glass or CR 39 recently passed an interesting milestone. The Vision Council of America reports that members' shipments of conventional hard resin lenses last year were down 7.3 percent while polycarbonate and high index shipments rose 10.5 percent during the same period. As a result, it's estimated that sales of all high index lenses now surpass the market share for CR 39 lenses.

High index lenses are traditionally used for myopes, so the initial impression from these statistics might be that eyeglass wearers are getting more nearsighted over time. A closer examination of this increased use of higher index lenses reveals the growth has little to do with the refractive corrections of eyeglass wearers.

Development of lenses made from materials with a higher refractive index was prompted initially to produce thinner, more attractive eyewear for people wearing high minus corrections. The first high index lens material was developed back during that period when all ophthalmic lenses were made from glass. The availability of high index glass was welcomed primarily by those who wore corrections of -7.00 diopters and over. To increase the index of glass requires adding lead to the molten glass. As a result, patients wearing high index glass lenses pay a penalty of increased weight. For those people, however, the advantages of thinner, more attractive edges compensate for any added weight.

Shortly after the introduction of plastic lenses following World War II, a quest began for a higher index in plastic. The first plastic with a higher index was 1.56 and, for some years, that remained the only high index in plastic. When polycarbonate became available as a lens material in the 1980s, it offered an index of 1.59. In spite of this advantage, poly's status as high index was largely ignored, and the total emphasis was on the material's superior impact resistance.

THE H.I. REVOLUTION BEGINS

The availability of high index plastic in 1.56 was interesting to ECPs and the lenses were used somewhat, but not in significant quantities. The next advancement in high index came in the early 1990s to meet growing consumer interest in thinner, lighter eyewear. When a new Japanese 1.60 material was introduced by Optima, it turned out to be the ideal time for a thinner, lighter lens and marked the beginning of the "high index revolution."

Within a few years, an even higher index, 1.66, became available, and a 1.71 high index plastic was developed Though 1.74 is available abroad, 1.71 is currently the highest index plastic material in the U.S.

There's been growth on the lower end of the index spectrum as well. Mid-index materials (under 1.59) have become popular and are generally less expensive than high index, except when they provide additional special properties, such as aspheric Spectralite (1.54) or impact resistant Phoenix, Trilogy, and Excelite in Trivex (1.53).

Glass lens manufacturers haven't been idle. There have been developments in high index glass, including 1.60, 1.70, 1.80, and 1.90. The 1.80 and 1.90 materials are distinguished from the more often used 1.60 index because these higher index glass materials cannot be tempered or heat-treated.

As a result, lenses made in 1.80 or 1.90 index will not meet the FDA impact resistance requirement. The government has made special provision for these materials. The lenses can be legally dispensed as long as patients sign a waiver stating they understand their new lenses are not impact resistant. When ordering these lenses, laboratories require a signed waiver from the doctor or dispenser acknowledging that the lenses will not be tempered or heat-treated to meet FDA requirements.

A RISING TREND

High index materials don't provide as much benefit to patients wearing plus corrections as minus. Plus lenses made in high index will be thinner, but the thinning takes place in the center of the lens where it is not visible and does little to improve the cosmetic appearance of the eyewear. For plus patients, aspheric lenses with their flatter, slimmer profile offer the greatest cosmetic benefits for plus lenses. Most aspheric lenses are made of higher index materials, and they provide improved cosmetic benefits and better vision for plus patients.

If high index offers less cosmetic improvement for plus lens corrections, do the increased sales of high index lenses mean there are more nearsighted patients than there used to be? No, there has been no noticeable increase in nearsighted patients. The increased use of high index materials has been prompted by other considerations altogether.

A SIGN OF THE TIMES

These are not the best of times from an industry economy standpoint. The industry has been flat, and now we have another factor that is concerning economists: The length of time between eye examinations is lengthening.

One of the suspected reasons behind this trend has been the rising cost of eyewear at the retail level. Patients remember spending more than $400 at their last visit and may tend to put off that exam. Eyecare professionals are seeing fewer patients, but are able to offer improved-- and more profitable--lenses and frames.

This isn't crass commercialism, however, because there is another side to the situation that is even more important to ECPs. In recent years, the optical industry has developed a wealth of new lens technology for eyewear consumers. Patients now have an opportunity to order lenses that will outperform what they have been wearing visually and cosmetically speaking.

Every new lens material introduced in the last 12 years has had a higher index than glass or CR 39. As a result, we tend to think of these and describe them as high index lenses. But, the reason they are being used often has more to do with factors other than just refractive index.

Often rimless mountings, for example, are ordered with lenses that have properties that will stand up to the added stress on lenses in rimless eyewear. For three-piece mountings, laboratories often prefer Trivex or polycarbonate, as well as such materials as the MR-10 1.67.

There is another reason for using improved materials. When the patient's new glasses are made from the same CR 39 as their old glasses, there's no opportunity to share the information about the most advanced, highest quality options in lenses.

Consumers like getting the latest and best, and if you don't offer it, they'll hear about it from another eyecare practitioner who does.