Special Lens Materials
It is important for the ophthalmologist to be aware of the variety of spectacle lens materials available. Four major properties are commonly discussed in relation to lens materials:
Index of refraction. As the refractive index increases, the thickness of the lens can be decreased to obtain the same optical power.
Specific gravity. As the specific gravity of a material decreases, the lens weight can be reduced.
Abbe number (value). This value indicates the variation in refractive index with wavelength, which governs the degree of dispersion of light, which is responsible for chromatic aberration. Materials with a higher Abbe number exhibit less chromatic aberration and allow higher optical quality.
Impact resistance. All lenses dispensed in the United States must meet impact-resistance requirements defined by the FDA (in 21CFR801.410), except in special cases wherein the physician or optometrist communicates in writing that such lenses would not fulfill the visual requirements of the particular patient. Lenses used for occupational and educational personal eye protection must also meet the impact-resistance requirements defined in the American National Standards Institute (ANSI) high-velocity impact standard (Z87.1). Lenses prescribed for children and active adults should also meet the ANSI Z87.1 standard, unless the patient is duly warned that he or she is not getting the most impact-resistant lenses available.
Glass lenses provide superior optics and are scratch resistant but also have several limitations, including low impact resistance, increased thickness, and heavy weight. Once the standard in the industry, glass lenses are less frequently used in current practice; many patients select plastic lenses. Without special treatment, glass lenses may be easily shattered. Chemical or thermal tempering increases the shatter resistance of glass, but if it is scratched or worked on with any tool after tempering, the shatter resistance is lost. Farmers appreciate photoreactive glass for its scratch resistance and easy care. Welders and grinders are better off with plastic, as small hot particles can become embedded in glass. Persons with myopia who desire thin glasses may choose high-index glass. The highest-index versions cannot be tempered and require that waivers be signed by patients who accept the danger of their breakage. High-index glass does not block UV light unless a coating is applied. (Characteristics of standard glass lenses are as follows: index of refraction, 1.52; Abbe number, 59; specific gravity, 2.54; impact resistance, pass FDA 21CFR801.410 if thick enough and chemically or heat treated.)
Due to its high optical quality and light weight, standard plastic (also known as hard resin or CR-39) is the most commonly used lens material and is inexpensive. Standard plastic lenses are almost 50% lighter than glass lenses owing to the lower specific gravity of their material. They block 80% of UV light without treatment, can be tinted easily if desired, and can be coated to resist scratching and to provide further UV-light blocking. The index of refraction is not high, so the lenses are not thin. CR-39 lenses do not have the shatter resistance of polycarbonate or Trivex. (Characteristics of standard plastic lenses are as follows: index of refraction, 1.49; Abbe number, 58; specific gravity, 1.32; impact resistance, pass FDA 21CFR801.410.)
Introduced in the 1970s for ophthalmic lens use, the high-index plastic material polycarbonate has a low specific gravity and a higher refractive index, which allow for a light, thin lens. Polycarbonate is also durable and meets the high-velocity impact standard (ANSI Z87.1). One disadvantage of this material is the high degree of chromatic aberration, as indicated by its low Abbe number (30). Thus, color fringing can be an annoyance, particularly in strong prescriptions. Another disadvantage is that polycarbonate is the most easily scratched plastic, so a scratch-resistant coating is required. Also, if polycarbonate is cut too thin, it can flex on impact and pop out of the frame. (Characteristics of polycarbonate lenses are as follows: index of refraction, 1.58; Abbe number, 30; specific gravity, 1.20; impact resistance, pass FDA 21CFR801.410 and ANSI Z87.1.)
Introduced in 2001, Trivex is a highly impact-resistant, low-density material that delivers strong optical performance and provides clear vision because of its high Abbe number. Its impact resistance is close to that of polycarbonate, and it blocks all UV light. Its index of refraction is not high, however, so the lenses are not thin. Trivex is the lightest lens material currently available and meets the high-velocity impact standard (ANSI Z87.1). Trivex material allows a comparably thin lens for the ±3.00 D prescription range. A scratch-resistant coating is required. (Characteristics of Trivex lenses are as follows: index of refraction, 1.53; Abbe number, 45; specific gravity, 1.11; impact resistance, pass FDA 21CFR801.410 and ANSI Z87.1.)
A lens with a refractive index of 1.60 or higher is referred to as a high-index lens. High-index materials can be either glass or plastic and are most often used for higher-power prescriptions to create thin, cosmetically attractive lenses. The weight, optical clarity, and impact resistance of high-index lenses vary depending on the specific material used and the refractive index; in general, as the index of refraction increases, the weight of the material increases and the optical clarity (Abbe number) decreases. None of the high-index materials passes the ANSI Z87.1 standard for impact resistance. Plastic high-index materials require a scratch-resistant coating.
Strauss L. Spectacle lens materials, coatings, tints, and designs. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2005, module 11.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.