The purpose of this blog is to provide a brief resume of
the types of spectacle lenses available to the patient
for better vision (visual needs).
Here we can discuss and share all about ophthalmic lenses, theories, individual findings, inventions, optical standards, tolerance, errors, complex optics and more..
Tuesday, July 5, 2011
Major lens materials & Indexes!
Glass has historically been the material of choice for ophthalmic lenses. Glass is most stable, scratch-resistant, and provides the best optical quality of all lens materials. However, since glass is more brittle than most materials, lenses made of glass must be tempered or heat-treated to give them more strength and make them safer to wear. Glass is available in a number of indices of refraction. As mentioned above, the higher the index, the thinner a lens will be for a given power. However, the specific gravity also increases dramatically with the index making high-index glass lenses much heavier. Chromatic aberration is also more pronounced in high-index glass.
Index: Crown 1.52; High index 1.60, 1.70, 1.80
Pros Superior optics Stable material Scratch resistant
Cons Does not accept tint Not impact resistant Heavy
High chance to break
Developed by PPG during WWII, CR-39, also known as plastic or hard resin, serves as a much lighter lens material (approximately 50% lighter) than glass. CR-39, however, is far less scratch resistant and often must be coated to improve its scratch resistant characteristics.
Pros Lighter than glass Readily tintable Less likely to fog
Cons Susceptible to scratching (correctable by coating) Lower index of refraction makes it less suitable for higher powered prescriptions
While its optical characteristics are less than ideal, polycarbonate, the same material used for bullet-proof glass, is the most impact resistant of lens materials. Consequently, polycarbonate is the material of choice for safety and children’s eyewear. With an index of 1.59, polycarbonate also produces thinner, lighter lenses than glass or plastic. These factors along with polycarbonate’s inherent UV protection and pricing make it a popular material.
Pros Thinner and lighter than glass and plastic Highly impact resistant (used for safety glasses) Inherent UV protection
Cons Poor optical quality Susceptible to scratching (correctable by coating) Susceptible to stress fractures in drill mounts Does not readily accept tint
High index lenses polymers typically refer to products with an index higher than 1.58. High index lenses require flatter curves than their lower index counterparts, resulting in thinner and lighter lenses. Furthermore, aspheric curves come standard in many high index products, particularly 1.66 and 1.70 products, and are available in 1.60. Asphericity reduces spatial distortion, reduces magnification or minification, and further helps maintain a thin and flat lens profile. High index material, however, tends to have a lower Abbe value which could potentially affect patients who are sensitive to chromatic aberration. Also higher index and flatter curves tend to result in more backside and inner-surface reflections. AR coatings are usually recommended for high index lenses to eliminate these reflections.
Index: 1.60, 1.66, 1.70
Pros Thinner and lighter than glass and plastic Better optical quality than polycarbonate
Cons Susceptible to scratching (correctable by coating) Susceptible to backside and inner-surface reflections (correctable with AR)
Developed in 2001 by PPG, Trivex combines impact resistance of polycarbonate, exceptional optical clarity, and a specific gravity of 1.11 (the lightest available). Trivex’s tensile strength makes it ideal for drill mount frames. Trivex is available from Younger as Trilogy and from Hoya as Phoenix. All Trilogy products are aspheric and guaranteed for life against stress fractures and drill mount cracking. Some Phoenix products are now available with aspheric curves.
Pros Impact resistance of polycarbonate Better optical quality than polycarbonate Tintable Lightest material on the market Inherent UV protection High tensile strength (ideal for drill mounts)
Cons Susceptible to scratching (correctable by coating)