BACK TO BASICS

When ONLY THIN AND LIGHT Will Do

A Back-to-Basics primer on high-index lenses, and when to dispense them to patients

BY JENEAN CARLTON, ABOC, NCLC

When it comes to prescription eyewear, all patients have a common wish list: they want fashionable frames that look good and fit well, lenses that are as thin as possible, and lens designs and treatments that meet their visual needs and lifestyles.

Creating eyewear that looks great and meets patients’ visual demands doesn’t happen by chance. Experienced optical professionals know there’s more to producing exceptional eyewear than just selecting a frame. After all, what does it matter if a frame looks stunning on a patient but the lenses don’t provide good vision?

Thin and lightweight lenses have always been at the top of patients’ desires: This is especially true of patients whose prescriptions are high powered and who, at some point in their lives, have endured wearing thick, heavy lenses.

Making use of high-index materials is a sure way to provide patients with thinner and lightweight lenses that are cosmetically appealing and comfortable to wear.

Top: A +3.00 Rx in 1.74 index vs. plastic. Bottom: A -6.00 Rx in 1.74 index vs. plastic. Image courtesy of Seiko Optical Products

MATERIAL BASICS

High-index lenses are an integral part of every optical professional’s tool box. These materials are patient-pleasers because they result in finished lenses that are thinner and lighter when compared to conventional plastic lenses.

VS. PLASTIC. Standard plastic lenses are fabricated from CR39. Composed of allyl diglycol carbonate, this polymer was invented by Columbia Resin Chemical Company in 1942; it was the first plastic material to be successfully used as an optical medium. While there is certainly a place for CR39 in the optical world, there are several newer lens materials that offer good vision yet are thinner and lighter in weight.

DENSITY. Higher-index materials are thinner because they have a denser chemical structure than standard plastic resins. Because of this, it takes less high-index material to refract light to the same degree as lower-index materials. Think of it this way: the higher the refractive index, the less material it will take to bend or refract rays of light.

INDEX OF INDICES. There is no official standard that categorizes lenses by index of refraction, but the optical industry tends to follow these guidelines:

• Low-index: < 1.53

• Mid-index: 1.53 to 1.5

• High-index: 1.59 to 1.66

• Super-high-index: 1.67 and up

FITTING BASICS

When reviewing a patient’s prescription, take a look at the sphere power, cylinder power, and axis. Try to imagine where the thick and thin zones of the lenses will be once the eyewear is fabricated. To do this takes some skill and experience, but at some point in your career you will be able to visualize how finished lenses will look once fabricated.

After you have an image of the finished lenses in your mind, consider ways to enhance the cosmetic appearance of the lenses. Which frame shapes will make this prescription look worse and which will make it look better? Which lens materials will provide the best vision while also being thin and lightweight? Is the eyewear for a child or someone who plays sports?

Before we get into some specifics about these questions, let’s go over some facts about prescription lenses:

1) Minus-powered lenses are very thin in the center and thicker on the edge.

2) Plus-powered lenses are thick in the center and wafer thin on the edges.

3) The thickness of the cylinder power is noticeable 90 degrees away from the axis.

With these facts in mind, you can steer patients to frame shapes and sizes, as well as lens materials, that will have the best possible cosmetic appearance. Let’s walk through a few examples to fully understand how each of these decisions affects the outcome of finished eyewear.

THE Technical Side

ABBE VALUE

The Abbe value of a lens material indicates how readily the material will refract light without causing white light to disperse into its color components, also known as dispersion. It also produces undesired chromatic aberration in lenses. The closer the Abbe value is to 100, the better the material will refract light without causing light to disperse into its color components. Different lens indices will have different Abbe values.

Just as white light will separate into its color components when traveling through a prism, light can also separate into colors when traveling through high-index materials. Such chromatic aberration is sometimes noticeable in the periphery of high-index lenses. Should this occur, and to such an extent that it will bother the patient, fabricate the lens with a lower-index material. Even a slight change in refractive index can alleviate this phenomenon.

SPECIFIC GRAVITY

The specific gravity of materials refers to the weight of the medium. The lower the specific gravity of a material, the lighter in weight the lenses will be when fabricated. The specific gravity of a material is the ratio of its mass compared to the mass of an equal volume of water (at a temperature of 4°C). Water has a specific gravity of 1.0, so if a lens material has a specific gravity of 2.54, it is 2.54 times heavier than water. With a specific gravity of 1.11, Trivex is currently the lightest weight material in the industry. Polycarbonate runs a close second with a specific gravity of 1.22.

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Image courtesy of Essilor of America

The patient wears her glasses full time and her Rx indicates she is moderately myopic and also has astigmatism correction. Her PD is 62 and she is interested in an oversized rectangular shaped, full-rimmed metal frame with an “A” measurement of 56mm and a “B” measurement of 34mm.

Considering the patient’s correction, we know that the temporal edge thickness of the lenses will be significant. Likewise, because of the cylinder correction, the temporal edges will be thicker than the other edges of the lenses. With these challenges in mind, it’s wise to inform the patient that her lenses will be much thinner, more cosmetically appealing, and lighter in weight if she opts for a smaller frame which results in less decentration. Excessive decentration always makes minus lenses thicker on the edges.

FRAME CHOICE: A good idea would be to choose a frame that is oval in shape, or at least a softened rectangle that doesn’t have sharp corners. Minus lenses always run thicker on angular frames with squared corners. To balance the edge thickness as much as possible, select a frame that has the patient’s eye positioned in the center of the lens opening.

LENS CHOICE: Opting for high-index lenses further decreases the edge thickness and weight of her lenses. Which high index is the best for this prescription? Is it necessary to choose the highest index lens possible, such as a 1.74? Or will the end result be satisfactory if a 1.66 material is used?

The final decision about which material to select is determined by the patient’s desires and by the optician’s knowledge level of materials. The fitting method described above works with the high-index material to result in an overall pleasing outcome.

Conversely, high-index lenses mounted into frames that are oversized, and require a significant amount of decentration, will likely disappoint patients hoping for very thin lenses. The best outcome is achieved when careful fitting techniques are used to ensure that the added expense of high-index lenses is justified.

The patient has chosen a rimless frame that requires the lens edges to be grooved. Because plus-powered lenses are ultra-thin on the edges and very thick in the center, fabricating this prescription for a rimless frame requires the edge thickness to be increased so the lab has enough edge thickness to groove the lenses. Unfortunately, when the edge thickness of plus lenses is increased, the center thickness of the lenses also increases.

FRAME CHOICE: The end result of mounting this prescription in a rimless frame is that the lenses would be thicker and heavier than if mounted in a full-rim frame, regardless of the lens material used.

LENS CHOICE: High-index lenses would result in thinner and flatter lenses, but the patient would be able to see much more of a cosmetic improvement if the lenses were fabricated for a full-rimmed plastic frame. If the patient opted for a full-rim metal frame, it still might be necessary to build up the edge thickness of the lenses so they don’t easily chip or fracture. The best outcome with this prescription would be to select a full-rimmed plastic frame. This way the edge thickness wouldn’t need to be increased to securely mount the lenses in the frame. High-index lenses would flatten and thin the lenses while also making them lighter in weight.

SERVICE BASICS

Being able to predict the cosmetic appearance and visual performance of a new pair of glasses is very gratifying. It can take years to reach this level of knowledge and confidence: but knowing that your patients are receiving the best the industry has to offer makes the effort worthwhile. Just ask any veteran optical professional!

Pro Tip:

Most high-index materials absorb 100% of UV wavelengths. Because of this, it isn’t necessary to add a UV coating to these materials. Be sure to check with your lab or lens supplier concerning the amount of UV protection offered by lenses dispensed in your practice.

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Image courtesy of HOYA Vision Care