Glass: Glass lenses are heavy, can be dangerous, and because of their lack of use, relatively expensive. They are the most scratch-resistant and cause the least distortion of vision. Glass lenses block the least ultraviolet light in comparison with other lens. The type of glass most commonly used today is Crown glass.
Federal law requires that any glass lenses dispensed are at least 2.2mm thick, and are safety hardened. Because of the physics of lenses, 2.2mm thickness translates into a very heavy lens. Safety hardening is another step in the fabrication process, which adds to the expense and increases the fabrication time. The term “safety hardened” means that if any hard, fast moving object strikes your lenses, they will merely crack and stay in the frame.
The index of refraction is 1.523 and approximately 4% thinner than CR-39.
Out of concern for the safety of an eyeglasses wearer, and in view of the variety of first quality lens options available today, most laboratories will not accept nor process orders for any glass spectacle lenses in rimless, semi-rimless or drill-mounted frames.
Plastic: Plastic is a misnomer for this material. It is actually a hard resin known as CR-39. The very characteristics that make CR-39 lenses so popular today are the main reason it took plastic lenses some time to catch on, namely, the soft nature of the material. The lens softness (low density) makes them very susceptible to scratching and, in a comparable prescription, somewhat thicker than glass lenses. Scratch resistant coatings and high index (denser) lenses have solved those challenges. Plastic lenses are light, scratch resistant and very shatter resistant. The index of refraction is 1.49 and approximately 30% lighter in weight than crown.
These lenses are plastic sun-glass lenses that contain a grey/green/brown film embedded in the lens. This film contains a grid, unseen to the naked eye, which cuts glare by re-directing the light passing through the lens (similar to the slats in a mini-blind). To confirm if two lenses are polarized, hold one lens over the other and rotate. If they are polarized, all light at certain points will be blocked. These lenses are available in plastic, High Index plastic, Polycarbonate and Transition.
Polycarbonate lenses are virtually unbreakable and are lighter and thinner than plastic. Because of this, polycarbonate lenses have been used for athletic glasses. Most opticians and doctors insist that eyeglasses for children be made with polycarbonate lenses. All are scratch coated and have UV protection in the material. Polycarbonate lenses also have a 1-year warranty for scratches. Polycarbonate lenses are difficult to tint and therefore should not be tinted more than 40% to avoid peeling. The index of refraction is 1.586 and approximately 10% thinner than CR-39.
Polycarbonate lenses may cause a blur in vision and cause color fringes (chromatic aberration) on peripheral vision images, especially in strong prescriptions. Solvents like nail polish remover or dry cleaning fluid can damage them. They are not available in all styles including some progressives (refer to Lens Highlights' section of this chapter).
Hi Index Plastic:
A plastic material that is thinner, almost 1/3 less thinner than a normal plastic lens. It does not contain the impact resistant qualities of polycarbonate. High-index plastic is available with the following indexes of refraction: 1.54, 1.55, 1.56, 1.6, and 1.66. These lenses are 8% to 25% lighter and thinner than CR-39 (normal plastic), depending on the prescription. The approximate weight is equal to polycarbonate, which is lighter than CR-39, depending on the prescription.
Note - Comparison to Polycarbonate: Both materials have a high index of refraction and are lightweight. Both can be ground to thin profiles. However, high-prescription patients may experience more chromatic aberration with Polycarbonate than with 1.6 High-Index Plastic in the periphery of the lens. This is due to a higher Abbe value (standard measure of color dispersion) for 1.6 High Index Plastic (36 vs. 30), meaning it has less chromatism.
High Index Glass:
High Index glass comes in a 1.6 index and 1.7. HiLite glass is a trademark name for a high index glass lens with a 1.8 index of refraction.
This is a High Index (1.53) lens with impact resistant properties similar to polycarbonate. It was designed specifically for drill mount frames and is manufactured by Younger™.
Plastic Photosensitive Lenses:
Photosensitive or photochromic, refers to light sensitive, self-adjusting lenses (darken when exposed to light and lighten in low-light situations). This changeable tint offers comfort from glare. Plastic photosensitive lenses were introduced in 1991. The most popular photosensitive lenses are called Transitions™. Transitions™ were originally designed for people who normally wear a cosmetic tint (not to be true sunglasses). The Next Generation Transitions™ are the most recent transition lens and can serve as actual sun-glass lenses. Corning® has also introduced a plastic Photochromic lens that goes by the name of SunSensors™.
How do Plastic Photosensitive Lenses work?
Plastic Photosensitive Lenses contain millions of light-sensitive Photochromic molecules. When exposed to the ultra-violet sunlight, a chemical reaction quickly transforms these molecules into colored light absorbers. Outdoors, ultra-violet rays cause a chemical reaction, which darkens the lenses. This is the activation phase. Indoors, in the absence of ultra-violet rays, the molecules return to their original form and the lenses fade back to their clear state. This is the deactivation phase. The degree of darkening is directly proportional to the intensity of the ultra-violet rays.
Transitions, Next Generation:
This is a trade name for plastic photosensitive lenses. This lens will darken enough when out of doors to be used as a sun-glass lens and will lighten to a clear lens indoors. This lens is manufactured by Transition, Inc.
Developed by Corning, this unique plastic photosensitive material is ultra lightweight (lighter than polycarbonate), has a thin center (1.56 mid-index) and natural UV absorption. Photosensitive molecules are located throughout the lens material, resulting in a durable, consistent, and longer-lasting lens. (Transitions have the photosensitive molecules located on the front surface.) They are clear indoors, sun-glass dark outdoors, Available in single vision, bifocal (FT-28, FT-35), trifocals and progressives in either grey or brown. SunSensors ™ are a standard Photochromic style lens similar in properties and price to the Transitions ™ brand.
Instashade ™ by Amorlite:
According to the manufacturer these plastic photosensitive lenses will get as dark as the Transitions™ brand but will return to a much clearer state quicker than the Transitions™ do. This lens is available in a mid index 156 and high index 160. The reason is these new lenses are the latest in technology for photochromic lenses and are therefore more costly.