Lens Materials and Designs
New advances in lens technology have made it possible for spectacles to have better optics be thinner lighter and more comfortable. When choosing a new pair of spectacles most time is spent selecting the right frame – but available lens options should be of equal importance. New advances in lens technology have made it possible for spectacles to have better optics be thinner lighter and more comfortable. Lenses have also become more fashionable and fun. There are many different types of lens material that are specifically created with certain wearers in mind so work with your eye care practitioner to help you make the right choices that best suit your needs.
Lens materials fall into two categories: glass and plastic.
LENS MATERIALS
Glass
Glass was the first material to be used for ophthalmic lenses and for many years was the only option available to those with refractive errors. They provide good vision correction and are scratch resistant but compared to plastic material they are heavier and not as impact resilient. Although glass lenses can be chemically treated to increase durability they can shatter if accidentally dropped or hit by blunt force. As a result this material is not recommended for children people who need safety eyewear or those who play sports.
CR-39 Plastic
The most common type of plastic spectacle lens material CR-39 is made from a hard resin that is lighter and more impact-resistant than glass. Compared to glass this material has a softer surface so a scratch coating (preferably applied at the time of manufacturing) makes the lenses more resistant to scratches. One of the problems with CR-39 is that strong nearsighted prescriptions are quite thick at the edges while strong farsighted prescriptions have thick centres.
High Index Plastic
A variety of new plastic materials bend light more than conventional CR-39 lenses. High index lenses provide significant advantages over traditional plastic or glass. They are made of a denser resin material that makes high nearsighted prescriptions thinner flatter and lighter. It eliminates the thick “coke bottle” effect of high nearsighted prescriptions and the “bug eye” effect of high farsighted prescriptions. The process improves peripheral vision filters out UV light (both UVA and UVB) and makes the lenses more cosmetically appealing. The higher refractive index* allows the material to bend light to a greater degree meaning that less curvature is needed to achieve a specific prescription power. Since less material is being used the lenses are consequentially lighter. In order to maximize the benefits of high-index lenses they should always come with a scratch coating for protection and an anti-refection coating for better clarity. A simple rule of thumb: the higher the index number the thinner the lens.
Aspheric Plastic
Aspheric lenses provide visual and cosmetic benefits for stronger prescriptions. Unlike spherical lenses aspheric lenses flatten progressively from the center to the edge. This results in enhanced overall vision reduced thickness and decreased peripheral visual distortion. When used for high farsighted prescriptions they make eyes look more natural instead of magnifying them into “bug eyes.” Those with strong nearsighted prescriptions benefit from thinner lightweight lenses. Aspheric lenses can be ordered in high index materials for the ultimate in attractive thin lenses. ASPHERIC
NON-ASPHERIC
Polycarbonate
These lenses are one of the most impact-resistant materials available and they naturally filter out UV light. When vision has been significantly impaired in one eye polycarbonate is used to better protect the good eye from impact damage. These lenses are the optimal choice for children industrial safety and sports glasses. Polycarbonate lenses are considered a high index material as they are thinner than CR-39. Polycarbonate lenses should always have scratch-resistant coating on both the front and back surfaces to protect its surface and prolong the life of the lens.
Trivex™
Trivex™ is a new performance lens material that combines superior optics impact resistance and lightness. Because of their strength Trivex™ lenses can be ground thinner resulting in cosmetic benefits and greater comfort. These are great for rimless frames because they don’t distort or break at drillholes and they give an excellent jewel-like appearance to edges that are polished. They are a good choice for high-performance safety eyewear for both children and adults who are highly active. The excellent impact resistance makes Trivex an a possible alternative to polycarbonate. In its uncoated form it is easier to tint and lighter in weight than polycarbonate.
LENS DESIGNS
Single Vision These are all-purpose lenses for people who have normal accommodation. They offer correction for myopia hyperopia and astigmatism. Persons who normally wear bifocals or trifocals can use single vision for distance or near glasses. Single vision lenses can also function as occupational glasses for certain types of work (e.g. computer knitting). Single vision lenses can be ordered in all materials.
Straight Top Bifocals Bifocals incorporate two different points of focus usually one for near and the other for distance correction. These lenses are often prescribed for people over 40 with presbyopia The majority of the lens provides distance viewing while the bottom of the lens provides for near vision. Straight top lenses called that because of the line separating distance and near zones are the most commonly prescribed bifocals. The bifocal portion of the lens can be made in a variety of widths depending on the wearer’s near vision requirements. Those with presbyopia sometimes prefer progressive lenses since they do not have any visible lines and thus don’t reveal the patient’s age.
Trifocals Trifocals incorporate three different points of focus usually distance intermediate and near areas. Like a bifocal each viewing area is separated by a line. Trifocals have an added segment to provide clear vision at arm’s length distance (e.g. computer screen). They come in a variety of widths depending on the wearer’s intermediate and near vision requirements.
Progressive Lenses (Multifocal) These modern lenses provide all the benefits of bifocals but with some added features. Progressives are no-line multifocal lenses that provide uninterrupted visual clarity for those with presbyopia. The design provides a smooth transition from distance to near viewing making all fields of vision available including mid-ranges (arm’s length). Advances in progressive lens materials have allowed companies to produce thin lightweight lenses without the distinctive line found in bifocals. These provide wearers with a gradual change in prescription strength between distance and near vision. Progressives have become the lens of choice because they look like single vision lenses and the stigma of “looking old” with a bifocal is no longer applicable.
Progressive Computer (Occupational Multifocals) Extended computer use can increase eyestrain and vision problems especially for those over 40 who require a reading correction. Multifocal computer lenses are the ideal solution – they provide extra-wide intermediate and near vision. Wearers have a broad clear uninterrupted view of their entire workspace from reading the keyboard to a monitor up to eight feet away. The extensive visual fields and smooth transitional zone make it easier for users to adapt quickly and minimize strains. These lenses often have special tints and anti-reflection coatings to enhance contrast and minimize glare.
- The refractive index measures the angle of deviation of light as it travels through the lens material. The higher the index the wider the angle. It is possible to make a thinner lens with a higher index material. A simple rule of thumb: the higher the number the thinner the lens.
