The Glasses Lens
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
Ophthalmic 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.
Parts of Lenses
There are three main parts to lenses: Designs, Materials, and Coatings. Click on the links below to find out more about each:
Many different lens shapes can achieve maximum optical correction for your eyes. Here are a few basic designs that deserve discussion:
- This type of lens is designed to adjust the focus of the eye in only one specific way. The design does not vary across the diameter of the lens. It is not adjustable or adaptable for different focus points if the eye looks through a different part of the lens.
- This type of lens has two adjustments for the focus of the eye. Usually the upper part of the lens is designed to focus for far away viewing and the lower part of the lens is for viewing much closer.
- 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.
Multifocal or Progressive:
- This type of lens has a a variable focus. The upper portion of the lens is usually designed to improve far viewing, and the lower portion focuses increasingly closer as the eye looks more through the bottom of the lens. There are no obvious divisions or lines within the lens, so it looks just like a single vision lens, but performs like a trifocal, but with even more adjustments of focus than any trifocal.
Glasses 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. What this means is if your eyes have a really hard time seeing, you will want a higher index material so your glasses can be thin, rather than an inch thick.
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. 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.
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.
Lens Coatings and Treatments
Modern ophthalmic lenses are often coated to enhance their performance, click on a link below to find out more about each:
- No eyeglass material is completely scratch-proof. However, a lens that is treated front and back with a clear, hard Scratch-Resistant 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. Children’s eyeglass lenses especially benefit from a scratch-resistant coat
Smudge Proofing and Dirt Deposit Resistance
- Oils and smudges can reduce visibility. There are now coatings to inhibit dirt from sticking, and make oil turn into beads rather than smearing. This can be especially helpful if you are in an environment with oil based aerosols, such as cooking or high dust environments.
- Reflections on the front and rear surface of a lens can reduce visibility, particularly in night driving conditions or bright artificial 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). Since sunglass lenses are so dark, the AR coating can look smeary if placed on the front surface. Coating the back surface helps reduce the reflections of light that enter from behind you and bounce off the surface into your eyes.
- 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 do not require UV treatment since they naturally block UV.
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 a variety of plastics and glasses. Each type offers a slightly different performance.
PhotoGray and PhotoBrown
- 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. Silver halide causes the lenses to transform 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.
- ColorMatic Extra from Rodenstock is another mid-index plastic photochromic line. ColorMatic Extra’s uniform molecule distribution allows lenses to activate consistently when moving between indoor and outdoor locations. ColorMatic Extra lenses also 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 tinted. Only ultraviolet light breaks the bonds to change the molecular structure, therefore the lenses will not darken when exposed to artificial light. 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.
- The concept of polarizing light with man-made film was invented by Edwin Land. These lenses reduce glare from sunlight by limiting the light waves that are allowed to enter the eye. Compared to normal tinted sunglasses, polarized greatly reduces glare from reflections. Polarized lenses can be beneficial for those who spend long hours outside or for drivers: these lenses cut down the glare from water, ice, the road surface, or everyday light. However, polarized lenses may cause difficulties reading electronic devices, so they are recommended for mainly outdoor activities.
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. 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.