Eyeglasses
Lenses Frames Page
There are many types of eyeglass lenses, and literally thousands of eyeglass frames to choose from.
So, what do you need for your two eyes?
On this page you can explore lens designs and applications as well as learn about eyeglass frame designs and materials.
Lenses
Eyeglass lenses are called Ophthalmic Lenses. This means that they are designed specifically to correct optical errors of the human eye, and not for any other purpose.
History: Ophthlamic lenses have been in use in various forms for many centuries. For as long as there have been translucent material such as amber or glass, there have been visual uses for these materials.
For detailed information about the history and development of eyeglasses, we suggest you click the link below to visit Wikipedia, the open source internet encyclopedia:
http://en.wikipedia.org/wiki/Eyeglasses_and_contact_lenses#history
Basic Designs
Although there are thousands of different shape combinations to achieve the ideal optical correction for an eye, are a few basic designs that deserve discussion here.
Single Vision:This type of lens is designed to adjust the focus of the eye in only one specifiic way. The design does not vary across the diameter of the lens. It is not adjustable or adaptable for different focus if the eye looks through a different part of the lens.
single vision
Bifocal:This type of lens has two adjustments for the focus of the eye. Usually the upper part of the lens is to adjust the eye's vision for far away viewing and the lower part of the lens is for viewing much closer.
bifocal
Trifocal:This type of lens has three adjustments for the focus of the eye. The upper portion is usually designed for far viewing, the middle portion is designed for viewing at approximately arm's reach and the lower portion is designed for even closer viewing.
trifocal
Multifocal or Progressive:This type of lens has a a variable focus. The upper portion of the lens is usually designed to improve far viewiing, and the lower portion focuses increasingly closer as the eye looks more through the bottom. There are no obvious divisions or lines within the lens, so it appears just like a single vision lens, but performs much like a trifocal, but with even more adjustments of focus than any trifocal. In the diagram below, the grey lines mark the approximate area of the lens that is devoted to variable focus. The grey lines are for illustration only.
progressive
Lens Materials
Ophthalmic Lenses are made of clear plastic or glass. For safety reasons, glass is seldom used these days. An ophthalmic lens works because of the ability of the material to bend light in a predictable way. By manipulating the shape of the material, we can create an individualized corrective lens for the human eye. Different plastics and glass materials have been created that have differing abilities to bend light. Each material that has been developed can be measured in the way it bends light and assigned a performance number called the refractive index.
The refractive index measures the angle of deviation of light as it travels through the material. The higher the index, the wider the angle. As a result, it is possible to make a thinner lens with a higher index material.This matters when the prescribed shape of the lens would require it to be very thick if the more common low index materials are used. In this case, it may be wiser to choose a high index material.
CR-39, developed in 1945 by PPG, has become the dominant plastic used for ophthalmic lenses. It has a relatively low refractive index (1.45), so lenses made with this material will be thicker than those made with other materials.
Polycarbonate, is another popular material developed by the aerospace industry for use in helmet visors worn by astronauts. It has an index of refraction of 1.59. Polycarbonate is also used in bulletproof windows, safety goggles, airplane windows and riot police shields. The material is 10 times more impact resistant than most other plastics, and exceeds the ANSI eyeglass lens breakage resistance requirements by over 40 times. This makes polycarbonate ideal for situations that may cause lens breakage such as industrial use, sports or children's play. Preventing the lenses from shattering means your eyes are protected from shards of plastic or glass entering the eye. Polycarbonate is also lightweight, and the eyeglass frame won't slide down your nose as easily because of this. They are also naturally protective from ultraviolet light (UV rays). About 99 percent of potentially damaging UV rays are filtered out by polycarbonate, whether the UV rays come from sunlight, fluorescent lights or a computer screen. Polycarbonate lenses that are uncoated are especially susceptible to surface scratches. For that reason, these lenses usually come with a special scratch-resistant coating.
Other materials: There are other plastics available with even higher indexes. One of the highest is Essilor Thin & Lite with a refractive index of up to 1.74. This high index material offers the thinnest possible ophthalmic lens. Please click on the red link above for more information on all Essilor products.
Lens Coatings and Treatments
Modern ophthalmic lenses are often coated to enhance their performance.
Scratch-Resistant Coatings
No eyeglass lens material — not even glass — is scratch-proof. However, a lens that is treated front and back with this clear, hard coating does become more resistant to scratching, whether it's from dropping your glasses on the floor or occasionally cleaning them with a T-shirt or a paper towel. Children's eyeglass lenses, especially, benefit from a scratch-resistant hard coat.
Smudge Proofing and dirt deposit resistance
Oils and smudges can certainly reduce visibility. There are now coatings to inhibit dirt from sticking and make oils bead up rather than smear. this can be especially helpful if you are in an environment with oil based aerosols, such as cooking or high dust environments.
Anti-Reflective Coatings
Reflections on the front and rear surface of a lens can reduce visibility, especially under night driving conditions and bight atificial lights. To improve both the vision through the lenses and the outward appearance of the glasses, an anti-reflective coating (also called AR coating) can be used. AR coatings are similar to the coatings found on microscopes and camera lenses. They consist of several layers of metal oxides applied to the front and back lens surfaces. Because of the layering effect, AR coatings sometimes have a hint of green or purple color, depending on the individual manufacturer's formula. With sunglass lenses, an AR coating is usually applied only to the back surface of the lens (the surface nearest the eye). Because sunglass lenses are so dark, the AR coating can look smeary if placed on the front surface. Coating the back surfaceonly helps reduce the reflections of light that enter from behind you and bounce off the surface into your eyes. A back-side coated sun lens is much more comfortable than an uncoated sunglass.
An excellent example of a lens coating that combines all 3 of of the above features is Crizal by Essilor. Please click on the red text to go to the Essilor web site and learn more about this great product.
Ultraviolet Treatment
Just as we use sunscreen to keep the sun's UV rays from harming our skin, UV treatment in ophthalmic lenses blocks those same rays from damaging our eyes. Exposure to ultraviolet light contributes to skin cancer of the eyelids, cataracts, and even retinal damage. An ultraviolet treatment is simple and quick to apply to most plastic eyeglass lenses, and does not change the outward appearance of the lens. One exception is polycarbonate lenses, which don't require UV treatment because they naturally block UV.
Photochromics
Photochromic lenses change from light to dark depending on the amount of ultraviolet light they are exposed to. Early photochromics were strictly of glass material, but today you can choose from CR-39 plastic, polycarbonate and high-index plastic and high index glass. Each type offers a slightly different performance.
Thirty-five years after their invention, glass PhotoGray and PhotoBrown lenses from Corning Medical Optics are still on the market. As their names suggest, the lenses are available in either gray or brown colors that are light enough to wear indoors and darken to a sunglass shade when exposed to ultraviolet light. The active ingredient that causes the lenses to transform is called a silver halide and is mixed evenly throughout the lens. This means the whole lens will change when exposed to light. It also means that if a particularly high powered prescription is made, the thickest part of the lens will be darker than the thinner parts. One difficulty with this lens is if there is a large difference in prescription between the two eyes, the lens with the thicker material will be darker than the weaker one. Corning has overcome this difficulty with some of its newer designs.
ColorMatic Extra from Rodenstock is another mid-index plastic photochromic line. To quote the Rodenstock web site: "Rodenstock’s unique In Mass technology disperses the ColorMatic Extra photochromic molecules uniformly throughout the lens material and is activated to a depth of 0.3 mm at a time to ensure uniform, consistent color, regardless of the prescription. Traditional plastic photochromic lenses only contain a thin coating of photochromic molecules on the front surface. ColorMatic Extra’s uniform molecule distribution allows lenses to activate consistently when moving between indoor and outdoor locations. ColorMatic Extra photochromic technology will also allow the lenses to tint up to 50% behind the windshield of a car. ColorMatic Extra lenses have a much longer lifespan than competing photochromic lenses - up to 4 years of consistent color. Once a layer of color molecules begin to fatigue, another layer beneath it will kick in and activate to its full potential. If a single lens is damaged or the prescription changes, both lenses in a pair of spectacles do not have to be replaced to achieve a color and performance match."
Transitions lenses by Essilor are a photchromic technology for plastic ophthalmic lenses that change the lens from clear to dark in the presence of ultraviolet light. To achieve this effect, photochromic dyes are applied to the lenses. The molecules of the dye appear colorless when no UV light is present. When exposed to UV light, the molecular bonds break, causing the molecules to change shape and appear as a color. Visible light does not bring about the change. Only ultraviolet light breaks the bonds to change the molecular structure, and therefore the color, of the photochromic molecules. When UV light is removed, the molecular structures quickly return to a colorless state. This changeable coating means that the color darkens evenly regardless of lens prescription or thickness. Transitions are available for CR-39 plastic, as well as in high-index plastic and polycarbonate. For more information on Transitions, click the red link to visit the company website.
Polarized Lenses
The concept of polarizing light with man-made film was the idea of Edwin Land. Land would later become famous as the inventor of instant photography with his famous Land Polaroid Camera. As plastic lenses grew in popularity, eventually laminated polarized lenses made in plastic became the most popular material for making polarized sunglasses. They were sold as over-the-counter drug store glasses and eventually became available in prescription form from eyecare professionals.
Tinted Lenses
Tinted Lenses are available on plastic lenses as well as glass and can be produced in almost any color. Lighter, fashion tints are used primarily for cosmetic purposes to enhance the wearer’s looks. Typically, fashion tints are applied in light pink, blue, green, amber, brown or gray. They are often added in a gradient, meaning that they fade from color at the top to clear at the bottom, or from one color at the top to another at the bottom.
Darker tints are used for sunglasses. Sunglasses are usually gray or brown. About Colors: Gray tints will not alter color perception, but other tints can skew color perception significantly. Yellow, sometimes referred to as a "blue-blocker" because the color keeps blue light from entering the lens, is often the color of choice for target shooters because it decreases atmospheric haze and makes objects appear sharper, with more contrast.
Green, or its cousin G-15 (the Ray-Ban lens color), is sometimes used as a sunglass, though brown and gray are more popular. Red is a very uncomfortable color to look through, however, some people enjoy seeing the world through "rose-colored glasses." Although Blue is may people's favorite color, it really doesn't makes a good sunglass. Tints are applied to plastic lens materials through a process of absorption. The lenses are immersed into a warm color bath, and depending on the length of time they sit in the tank, emerge in varying shades of darkness. A lens that requires only a light tint will go into the bath for just a few minutes, while a lens that is meant to be sunglass-dark will stay in for quite some time. Plastic tints will fade over time. Glass lenses may be manufactured with the color distributed throughout the lens material, or a tint may be applied as a coating in a vacuum chamber.
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