L&T: Lens Choices

Jul
2009

Winning the High-Index Numbers Game


Photograph by Ned Matura; Lenses courtesy of KBco and Vision-Ease Lens

By Timothy Coronis

High-index plastic lenses can greatly improve the eyeglasses you build. Using a high-index lens material—one with an index of refraction ranging from 1.60 to 1.74—can reduce lens thickness and weight, making eyeglasses comfortable and cosmetically appealing. High-index lenses can also be used to improve the fit of lenses in the frame.

Because of the ever-expanding range of high-index lenses on the market, many dispensers are unsure of how choose an appropriate lens for a patient. With so many lens materials and lens treatment combinations available, knowing when to recommend one high-index lens versus another can seem like a numbers game. To win that numbers game, you’ve got to master a few key optical concepts and facts.

Understanding Index of Refraction
Index of Refraction is the speed of light in a vacuum divided by the speed of light in a given material. The resulting number (1.5, 1.6, 1.67, 1.74) is referred to as the “index.” The higher the number, the more light is slowed down or bent. This means high-index lenses bend light at steeper angles than standard-index, hard resin lenses. Therefore, high-index lenses need less curvature to achieve the same focal power. That’s why a lens made from a 1.67-index material can be significantly thinner than a standard-index (1.50) plastic material with the same prescription.

An additional benefit of high-index lenses over standard plastic lenses is that they offer inherent UV protection as opposed to having to apply a UV coating, which may cause some standard-index lenses to turn yellow.

High-index lenses also have flatter curvatures than their standard plastic cousins. Aspheric design, when combined with high-index resin, will further thin and flatten both plus and minus powers. All these factors make high-index lenses an especially good match for today’s frames, which are of an increasingly flat design. And its high tensile strength makes a 1.67 material a good choice for drilled rimless.

Factoring in Abbe Value
When looking through a lens at a point other than the optical center, the component colors making up white light are displaced laterally by differing amounts resulting in a degraded image, so a challenge in high-index lens design is minimizing effects of chromatic aberration, or color dispersion occasionally noticed by the eyeglass wearer as a “halo” or color fringes around the edges of an object.

A scale used to rate the degree of chromatic aberration of optical materials is the Abbe value. A lower Abbe value indicates more chromatic aberration and a higher Abbe value indicates less.


Comparing High-Index Materials

Material Refractive Index Abbe
1.74 1.74 32
1.70 1.70 36
1.67 1.67 32
1.60 1.60 42
Polycarbonate 1.59 32
Trivex 1.53 45
Standard Plastic 1.50 58


A “sweet spot” exists at and around the optical center of lenses within which chromatic aberration is more tolerable. Minimal decentration of lenses in the frame and a smaller ED minimizes the effect of chromatic aberration. The idea is to position the best optics directly in front of the patient’s pupil and to reduce areas of the lenses in which the chromatic aberration is most noticeable. There is no practical way to eliminate chromatic aberration, but anti-reflective (AR) lenses can help patients tolerate the problem.

The AR Advantage
When dispensing high-index materials, AR lenses offer significant advantages over non-AR lenses. Because high-index materials are relatively dense, they have significantly higher reflectance than lower index lenses. This higher reflectance creates increased veiling glare under ordinary lighting and also creates annoying ghost images when bright light sources are introduced into an environment of generally low illumination (e.g. oncoming headlights during night driving or candles in a darkened restaurant). By reducing the higher reflectance that is otherwise going to be present whenever index is increased, the optical performance and the cosmetic appearance of these lenses are both improved.

Presenting High-Index Options
When recommending a high-index lens to a patient, remember to go easy on the technical features. Focus instead on the benefits the patient will receive (while always being prepared for questions and concerns). You’ll feel more like the expert when you act like the expert, recommending what’s best and keeping in mind the individual’s prescription and lifestyle needs, rather than presenting a laundry list of every option.

Always consult with your optical laboratory or lens sales consultant if a patient is having difficulty wearing a particular lens material. They can recommend other lens materials and treatments. For example, a patient who cannot tolerate a polycarbonate material or a high-index can still be given a highly impact-resistant material by using a lens made of PPG’s Trivex, which features a 1.53 index and an Abbe value of 44.

With knowledge, ability and a little patience, it’s possible to ascertain what your customer wants and then combine several elements of cosmetics and technology into a pair of glasses that are as attractive as they are functional.


Timothy Coronis ABOC-NCLE, is a certified optician and contact lens examiner based in Keene, N.H. He is also an American Board of Opticianry technical speaker. He may be reached at timothycoronis@hotmail.com.


Solving Patient Needs
Here are some examples of how choosing the right high-index material can solve a variety of patient’s optical and cosmetic needs.

Example 1
The patient’s prescription is -8.00 R –7.75 L and the PD is 58mm. The patient selected a fashionable, rectangular frame, 52-20, and is asking about polycarbonate lenses.

This prescription/lens/frame combination could spell trouble. The fact that the lenses are decentered 7mm each and rectangular shape means the outer edges of the lenses would be very thick. It’s better to avoid thick lens edges through careful frame selection and choice of materials whenever possible.

A useful tip when building glasses and anticipating how lenses will look in the frame is to remember that greater decentration causes thick lateral edges for minus prescriptions and thicker centers for plus lenses. The former case is exactly what we’re trying to avoid.

Solution
Recommending a smaller, 48-18 as something to complement the lenses, will result in a decentration of only 4mm each. A less severe frame shape such as an oval means there will be less thick lens edge in the eyeglasses. These decisions, combined with a 1.74 index and an aspheric design, mean there will be much thinner lenses in the frame. In our new example, each factor worked toward the same goal, thinner, more attractive lenses.


Example 2
The patient’s prescription is  +0.75 OD, +4.25 OS, PD is 65. The patient picked out a 48-20 zyl frame and then had eye exam. These are two very different lenses and two very different powers. Glazing a zyl frame with these lenses would result in the OS temple being angled inward because the left eyewire must be molded around the steep lens curvature, while the OD side would be more nearly at right angles to the front.

Solution
To achieve better symmetry and to get this prescription into a zyl frame in an artful and practical manner, the optician suggested a 1.67 aspheric lens for the OS and a standard plastic, non-aspheric for the OD. The high-index makes the OS lens thinner and being aspheric will makes it even flatter. The OS lens is a non-aspheric standard plastic. In spite of dissimilar powers, the lenses will look like a matched pair and will behave similarly in the frame, and the patient with dissimilar eyes can receive eyeglasses of the same cosmetic standard as anyone else. AR lenses are also recommended for low power lenses.

Patients with this much anisometropia will be more prone to spatial distortions if they have approximately equal acuity between the eyes, and moderate to good stereopsis. A consultation with the Rx-ing doctor would be a good idea before proceeding.

 

Example 3
The prescription is +1.00 OD and +1.25 OS, add power is +2.75 OU and the PD is 56. The patient likes a grooved and rimless frame design that measures 52/18. He wants to use his glasses for reading only and he likes the idea of standard plastic being the cheapest option. He mentions having gotten “eyeglasses-in-an-hour” last time.

This is a different type of potential problem. The full reading prescription here is +3.75 OD and +4.00 OS. Plus lenses are thickest in the center and thinnest at the edges. The patient’s PD of 56mm means the lenses would have to be decentered 7mm each, which would result in eyeglasses with lenses inordinately thick both near the bridge and in the centers. In fact, in this particular case, ordering lenses surfaced to get sufficient thickness for grooving at the temporal edge would make the overall lens thickness considerably greater.

After reflecting on similar situations, the dispenser explained to the patient that in order to get the lenses in their best form, another frame would have to be selected.

Solution
Use 1.67 aspheric lenses mounted in a zyl frame to conceal the thick nasal edge and a rounder frame design at 48/19 to reduce decentration. The lens and frame combination will harmonize quite well. The aspheric greatly reduces the lens thickness at the nasal edges.

Example 4
The prescription is -4.00 OD, -3.75 OS. Patient likes drilled rimless frames. Conversation revealed the patient works in a bank, uses a computer screen all day and deals with the public. Recommendations should include that AR lenses present a nice appearance, relieve eyestrain from prolonged computer use and improve the appearance of rimless designs. 

Solution
Rimless, eight-hole drilled, anti-reflective lenses for looking at the computer screen all day, fabricated with 1.67 drillable material with a high-tensile strength.

 

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