Continuing Education

The Thin Lens Category

Lens material is a key factor in the quality of vision and wearing comfort for patients. Knowledge in the "thin lens" category—polycarbonate (1.59), 1.60, 1.67, and 1.74 indices—is crucial to your helping patients achieve their best eyewear experience

Continuing Education for Opticians Sponsored by Essilor

According to the Vision Council, patients cite the most important lens qualities overall as lightness, scratch resistance, thinness, and impact resistance. Each material has strengths in different areas, and by understanding which material is most appropriate for your patients, you can offer improved optics and patient happiness with their vision correction. There is an appropriate material solution for each patient. Understanding the differences is key to patient satisfaction.

The thin lens category defined in this course is polycarbonate (1.59), 1.60, 1.67, and 1.74 indices. Prescription lenses in this category are thinner and lighter weight than the same Rx in standard plastic. There are other materials on the market; however, the materials defined in this course are specific to "thin lenses" materials.

Materials matter when selecting the best lenses for patients. Details also matter; knowing primary and subtle differences between a 1.60 and a 1.67 lens can determine a patient's satisfaction with their eyewear and your practice.

By understanding the features and benefits of the thin lens category and recommending them appropriately to patients, eyecare professionals can build a lifestyle-centric practice by fulfilling patient needs.

QUALITIES OF THIN LENSES

By offering the cosmetic benefit of thinner lenses, high index satisfies what many patients want in an appealing lens. Aesthetically speaking, high index lenses lessen eye magnification for patients with high-plus powers and also lessen minification with high-minus powers.

High index thermoset plastics used in high index lens production begin as a liquid and require a processing activator such as heat. Higher refractive powers result in a flatter base curve, a desirable trait that allows lenses to be thinner, flatter, and more cosmetically appealing. However, the reflection coefficient will be increased with refractive index, causing glare and making the lens less appealing. Therefore, it is critical to recommend an anti-reflective (AR) treatment with high index materials. Today's premium high index lenses come automatically with AR.

High index materials bend light more efficiently. Because they require less curvature than lower indices for the same Rx power, they can be thinner than other lens materials, oftentimes making them lighter.

Other lens qualities to consider are:

Index of refraction: Refractive index indicates lens thickness. Light travels through different materials at different speeds, and high index materials bend light more, taking less material to achieve the desired refraction. The higher the index, the more light reflects, so AR treatment is a must to help combat unwanted reflections and increase transmission.

Abbe number: The Abbe number characterizes the dispersive power of the lens material. The lower the Abbe number, the greater the dispersion; a low number disperses light more than a high number. However, clinical study has demonstrated that even at low Abbe value, chromatic aberration is not perceived by the wearer.

Optical quality: In essence, optical quality is a complex function of transmitted light, absorbed light, haze or scattered light, and birefringence (the difference in two axes). Optical lens materials typically exhibit second-order influences on all the factors used to determine optical quality.

Photochromics: Photochromic properties offer additional visual comfort by adjusting automatically from clear indoors to sunlens dark, depending on ultra-violet light exposure. These lenses are a solid recommendation for everyday wearers seeking visual comfort and convenience.

AR: The higher the index, the more light is reflected, making AR necessary to help combat unwanted reflections. Anti-reflective properties increase the amount of transmission of the lens material. This improved transmission elevates the wearer's visual clarity and comfort.

TECHNICAL ATTRIBUTES OF LENS MATERIALS

Today, 1.50 standard plastic lenses are generally viewed as the value option by the optical marketplace and patients.

Thin lens category lenses, which come in refractive indices of 1.59 (polycarbonate), 1.60, 1.67, and 1.74, are considered technologically advanced and better able to meet the lifestyle demands of each patient.

Note that the absolute difference in weight between many of the high index materials is very similar. However, there is a large step change when comparing 1.50 standard plastic lenses to thin lens materials. Each material offers varying benefits, and patients, depending on their lifestyles, may need a specific lens material.

The category of thin lenses could be defined as a family of products with an edge thickness reduced by at least 30 percent as compared to 1.50 standard plastic when using high-minus lenses such as -6.00D (for plus lenses it will be center thickness).

The following materials can be found in all general-purpose progressive lenses, premium lenses, single-vision (SV) lenses, and many bifocals. These thin lens materials are great for presbyopes as well as SV wearers.

■ Polycarbonate 1.59
Specific gravity: 1.20
Abbe value: 31

Polycarbonate (poly) lenses are 43 percent lighter and significantly flatter than 1.50 standard plastic lenses and lighter than most materials. As the material with the most impact resistance, polycarbonate lenses are also 10 times more impact resistant than 1.50 standard plastic and meet ANSI and ISO safety standards.

All polycarbonate comes with 100 percent UV protection as well as scratch protection and can be surfaced to a 1.0mm minimum center thickness, qualifying it as thin lens material.

Because of its superior impact resistance and UV protection, children under the age of 16 should be prescribed polycarbonate lenses.

In addition, poly is a good choice for those wanting durable lenses, and it satisfies Duty to Warn requirements.

Polycarbonate is a unique optical material, a thermoplastic rather than a thermoset resin such as 1.50 standard plastic, 1.60, 1.67, and 1.74. Thermoplastics begin as a solid material, then are melted and pressed into the lens shape. Today's thermoplastics are highly refined products such as CDs, DVDs, and airplane windshields.

Always coated, poly meets or exceeds the scratch resistance of 1.50 standard plastic and other lens materials. Denatured alcohol is recommended for use around poly lenses rather than isopropyl alcohol to help avoid cracking. Ensure that poly lenses are cleaned using only alcohol-free products. With today's advanced AR technology, most lenses no longer require a liquid, but simply a cleaning cloth.

A lens introduction timeline. Image courtesy of Essilor of America

■ 1.60
Specific gravity: 1.30
Abbe value: 41

1.60 lenses are up to 30 percent thinner and 20 percent lighter than standard plastic lenses. The lenses are also 100 percent UVA and UVB protective. Recent improvements to the 1.60 monomer, termed MR-8, allow 1.60 lenses to offer the benefits of high index at an affordable price, including lower density than previous Thin&Lite 1.60 lenses for lighter weight and improved tensile strength for easier edging, drilling, and mounting into rimless frames.

1.60 lenses are a notable option for patients who want to upgrade their lenses from 1.50 standard plastic, and as a value-orientated high index option. These lenses are ideal for patients within the low-to-mid prescription range.

In addition, 1.60 lenses are easy to edge and mount, and work well for patients who want rimless or semi-rimless mountings.

■ 1.67
Specific gravity: 1.35
Abbe value: 32

With 100 percent UVA and UVB protection, 1.67 lenses are recommended for patients with higher prescriptions. Known for high tensile strength, 1.67 lenses are the best choice for drill mounts and rimless frames because of tensile strength. The lenses are also thinner, making them cosmetically appealing for people with higher prescriptions or those wanting to look better in their eyewear.

Currently the leading high index lens, 1.67 index is a preferred upgrade from 1.50 standard plastic; they are recommended for patients with a mid-to-high prescription range. 1.67 lenses are 34 percent thinner and 32 percent lighter than 1.50 standard plastic lenses.

Comparison of lens characteristics. Image courtesy of Essilor of America

■ 1.74
Specific gravity: 1.47
Abbe value: 33

Designed with built-in 100 percent UVA and UVB protection and systematically included anti-reflective technology, 1.74 lenses are the thinnest on the market today. They are also light weight, creating a cosmetic and comfort duo that appeals to those with very high prescriptions and those who want to look their absolute best.

The highest index available in the United States today, 1.74 lenses offer extreme thinness and flatness.

They are the ideal upgrade for patients currently wearing 1.60 or 1.67 lenses who want to go thinner and lighter, or for those who are image-conscious or want the latest or most technologically advanced lens. Anyone expecting the best lens performance will appreciate and enjoy 1.74 lens technology.

1.74 lenses are up to 50 percent thinner and 50 percent lighter than 1.50 standard plastic lenses and include AR treatment.

PATIENT PROFILES

Here are six hypothetical patients, complete with mini-profiles and prescriptions. Lens design and material recommendations for each patient with Rx and lifestyle clues follow.

PATIENT 1
A man in his early 30s
Rx: OD -7.25 -.50 × 135
OS -7.50 -.75 × 108

Recommendation: This patient is highly myopic (nearsighted). Since his Rx is greater than -6.00D, a single-vision high index 1.74 lens is the best choice for optimal thinness and flatness. Systematic AR helps to improve appearance and visual acuity.

PATIENT 2
A 12-year-old boy
Rx: OD +1.00 sph.
OS +.75 sph.

Recommendation: Impact resistant polycarbonate lenses are the best choice for any child needing corrective lenses. Duty to Warn requires that parents or guardians of a minor child are informed of lens impact resistance qualities. 100 percent UV protection is also a key poly feature that benefits kids, as the majority of a lifetime's UV exposure occurs before 18 years of age. Photochromic lenses with durable AR can increase visual clarity and wearing comfort and compliance.

PATIENT 3
A woman in her late 50s who is interested in rimless frames
Rx: OD -4.50 -1.00 × 023
OD -4.75 -.75 × 178
ADD +1.50 OU

Recommendation: A progressive addition lens (PAL) in 1.67 lens material is thinner and flatter than 1.50 standard plastic and is the best material for use in drill mounts. AR neutralizes reflections and improves visual comfort and appearance.

Photochromic lenses offer indoor-to-outdoor comfort and convenience for everyday wear.

PATIENT 4
A man in his mid-40s who currently wears bifocals and also plays racquetball and golf
Rx: OD -2.00 sph.
OS -1.75 sph.
ADD +2.50 OU

Recommendation: Since he is a current bifocal wearer, he is a good candidate for conversion to PALs.

Due to his active lifestyle, polycarbonate is the lens material of choice. It offers lightweight impact resistance, as well as 100 percent UV protection.

Other recommendations include photochromic lenses for everyday and polarized lenses for increased safety and visual comfort in the sun.

PATIENT 5
A woman in her late 50s
Rx: OD -5.75 -1.00 × 115
OS -5.25 -1.00 × 132
ADD +2.50 OU

Recommendation: PALs are a good choice for this patient.

A high index 1.67 is a thin, flat lens material for this patient's Rx, and offers enhanced cosmetic appearance and increased visual wearing comfort.

A thinner, flatter, and even more lightweight option is 1.74 high index.

PATIENT 6
A public relations manager in her late 40s with an active lifestyle
Rx: OD -1.75 -.50 × 106
OS -2.00 -.25 × 169
ADD +2.25 OU

Recommendation: PALs are a recommendation here. In the public eye, this patient is likely to want the no-line cosmetics that PALs can offer.

Polycarbonate lenses deliver lightweight, comfortable, and attractive lenses with the best protection during physical activities.

Also recommend photochromics with AR for increased visual performance, comfort, and appearance.