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THE LENS

Aim High: Fitting the Newest Lens Material

When a customer enters an electronics store, they are not offered an analog, black and white television and a VCR. Instead, they are offered the latest in technology. The same should happen when a patient enters an optical dispensary. He or she should not be offered uncoated, CR-39 lenses. Instead, the patient should be offered the latest in the technology that meets his or her needs. In many instances, this would mean offering the patient high index lenses.

What is a high index lens?

Although the technical definition for high-index lenses is any lens with a refractive index higher than 1.52, the high index lenses that will be discussed in this article are those with an index of 1.64 or higher.

First things first, what is an index? In simple terms, index is a measurement of how much light is bent by the lens material. The higher the number, the more the light is bent by the lens. What this means to the eye care professional and the patient, the higher the index, the less material that is needed to bend the light to fill the doctor’s prescription. For example, a -7.00D lens ground on a 1.70 index material will be approximately 50 percent thinner than a -7.00D lens ground on a 1.50 index material if all parameters are equal.

High index lens designs

As technology has improved, so has the availability of high index lens designs. High index is most commonly available in multiple progressive and single vision designs. However, lined multifocals can sometimes be found in the material. Aspheric and atoric designs are also becoming more readily available to the eye care provider in high index materials. In aspheric and atoric designs, the lenses are gradually steepened along the periphery in minus designs and the periphery is gradually flattened in plus designs. The benefit is greater peripheral vision as well as less magnification or minification of the eye. High index is also available with polarization or photochromic options. Glass is another high index lens option; however, it is not usually the best option due to weight and safety considerations.

When to recommend a high index lens

A high index lens is thinner and lighter (except in the case of glass lenses) than other conventional materials. Therefore, it is an ideal lens material for individuals with a higher prescription. A general guideline is to recommend high index materials for any patient with a prescription of +/-3.00 D or more. Anything less than this prescription and the weight and thinness benefits are usually minimal.

High index, especially the index of 1.67 and 1.70, is also an excellent choice for individuals who want the minimalist look of drilled rimless eyeglasses. Due to the thinness of the material’s edge, the lens does not look out of place or heavy when placed in a drilled rimless. In addition, high index materials do not crack as easily when drilled and the hole maintains its size and shape more readily than lower index materials such as CR-39.

High index is also essential for the newer, larger frame designs. With the increasing popularity of larger frames, edge thickness problems due to decentration are going to become more prevalent. High index can eliminate some of this but remember that it is very important to consider the lens prescription when fitting a larger frame.

Lifestyle can also be an important factor when recommending high index lenses. Certain professions such as politicians, lawyers, and those who engage in public speaking prefer the cosmetic benefits of high index lenses. Individuals who are fashion conscious are also good candidates for the thinner profiles that are provided by high index.

There are some exceptions to fitting a high index lens, however. One instance is children or individuals who have severely decreased vision in one eye. In these cases, the safety and impact resistance of polycarbonate or Trivex® is necessary to protect the patient’s vision. Also, the flatter base curves of high index lenses prevent the lenses from fitting into the wrap designs that are popular in some frame designs and are therefore not recommended.

The Importance of Anti-Reflective Coating

Do not sell a high index lens without anti-reflective (AR) coating. This may seem extreme, but an uncoated CR-39 lens reflects 8% of light, whereas an uncoated high index lens will reflect up to 50% more than CR-39. A rule of thumb is the higher the index, the greater the amount of light reflected. This can lead to increased difficulty with night driving as well as eye fatigue due to decreased light transmittance. With an AR coated lens however, the light transmittance can increase to 99.5%. As a result, reflections and chromatic aberrations are reduced, the patient has a clearer view of his or her surroundings, and the glasses have a better cosmetic appearance. By applying AR, the rate of non-adapt to the visual differences in high index material over lower index materials will decrease. Fortunately, AR coatings have really improved over the last few years and are an essential enhancement to high index with the new scratch, dirt, and oil resistant formulas available.

How to fit High Index Lenses

Although there are several factors that help to ensure patient satisfaction when fitting a high index lens, the main factor is taking accurate measurements. It is important that the eye care professional take monocular PDs and determine the optical centers. When taking measurements it is essential that the frame fits well, there is minimal decentration, and that the pantoscopic tilt is between 10 and 15 degrees. Once these measurements are taken, it is best to determine the lens thickness to ascertain if it meets the needs of the patient. This can be done by utilizing the sag approximation formula and adding the result to the predetermined center thickness or by applying optical calculators such as those found on OptiCampus.com. For reference, the sag approximation formula is:

Sag = ((d/2)2 X D) / 2000(n-1)

where d is diameter in mm
D is power
n is index

Everybody wants the look and feel that high index lenses can provide. Low index lenses are thicker, thinner lenses look better. Lower index lenses are heavier, lighter lenses are more comfortable. Therefore, it is no surprise that high index lenses are the wave of the future in the optical industry. High index lenses provide the best solution for most patients with higher prescriptions. With the proper understanding of the different designs of high index materials and fitting techniques, an eye care provider can give the best possible vision solution to their patients.

Carrie Wilson
BS, LDO, ABOAC, NCLEC

Comments
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Fezz
Posted: 1/12/2009 6:38:15 PM

Great article Carrie! Keep them coming!
claireros
Posted: 5/11/2009 2:40:53 AM

The industry is promoting high index lenses without pointing out the disadvantages. As a longtime user of glass lenses (the old photochromatic "Rapide" lens), I am very disappointed in the degree of chromatic aberration at the edges which, while tolerable with standard index lenses, quite unacceptable with high index. All dispensing opticians and the industry ignore overlook this either hoping the customer does not realise this or not realising that the effect is pronounced in my case iwth a high minus lens compounded with a 5 dioptre cylinder. Does the "Push" for high index comes from market surveys, or as I suspect, what the industry and this article thinks that we should have? I recently put on my "old faithful" glass (albeit photochromic) after months of wearing plastic high index and was pleasantly reminded of the clarity of the image. It seems to now to be a foregone conclusion with the industry "pushing" high index lenses for my benefit! Can Opticians and the industry point out the disadvantages of chromatic aberration/dispersion with high index lenses and high prescriptions (specifically high minus with a high cyl on top) instead of leaving it to the patient who in most cases will not have the technical vocabulary or unable to complain or exp[alin the effect they are seeing. I look forward to your comments! Regards, Eros.
johnny pratt
Posted: 1/29/2010 2:24:35 PM

Great article, question, can anti-reflective coating be applied after customer has taken delivery of lenses?
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