- Scratch resistance
- Smudge proofing and Dirt Deposit Resistance
- Anti-Reflective Coating
- Ultraviolet Protection
- Photochromic (lenses that darken outside, lighten inside)
- Polarization
- Tinting
Lens Coatings and Treatments
Scratch-Resistant Coatings
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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
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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.
Anti-Reflective Coating
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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.
Ultraviolet Protection
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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.
Photochromics
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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:
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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
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
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.
Polarized Lenses
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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
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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.