How to choose the best lenses for your glasses gas after eating meat

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The following information applies to all prescription lenses for glasses — whether you need single vision lenses to correct nearsightedness, farsightedness, and/or astigmatism, or you need progressive lenses, bifocals or other multifocal lenses to also correct presbyopia. Eyeglass Lens Materials – Features And Benefits

Plastic lenses. In 1947, the Armorlite Lens Company in California introduced the first lightweight plastic eyeglass lenses. The lenses were made of a plastic polymer called CR-39, an abbreviation for "Columbia Resin 39," because it was the 39th formulation of a thermal-cured plastic developed by PPG Industries in the early 1940s.

Originally developed for helmet visors for the Air Force, for "bulletproof glass" for banks and other safety applications, polycarbonate is lighter and significantly more impact-resistant than CR-39 plastic, making it a preferred material for children’s eyewear, safety glasses and sports eyewear.

A newer lightweight eyeglass lens material with similar impact-resistant properties as polycarbonate is called Trivex (PPG Industries), which was introduced for eyewear in 2001. A potential visual advantage of Trivex is its higher Abbe value (see below).

High-index plastic lenses. In the past 20 years, in response to the demand for thinner, lighter eyeglasses, a number of lens manufacturers have introduced high-index plastic lenses. These lenses are thinner and lighter than CR-39 plastic lenses because they have a higher index of refraction (see below) and may also have a lower specific gravity.

The higher the refractive index of a material, the slower light moves through it, which results in greater bending (refracting) of the light rays. So the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index.

The refractive index of current eyeglass lens materials ranges from 1.498 (CR-39 plastic) to 1.74 (a specific variety of high-index plastic). So for the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest lens available, and a 1.74 high-index plastic lens will be the thinnest. Abbe Value

The Abbe value (or Abbe number) of a lens material is an objective measure of how widely the lens disperses different wavelengths of light as light passes through it. Lens materials with a low Abbe value have high dispersion, which can cause noticeable chromatic aberration — an optical error visible as colored halos around objects, especially lights.

Aspheric designs — where the lens curvature changes gradually from the center of the lens to its edge — enable lens manufacturers to use flatter curves when fabricating eyeglass lenses, without degrading the optical performance of the lenses.

Because aspheric lenses are flatter than conventional (spherical) lens designs, they cause less unwanted magnification of the wearer’s eyes, for a better appearance. In some cases, aspheric designs also improve the clarity of the wearer’s peripheral vision.

Most high index plastic lenses are made with aspheric designs to optimize both the appearance and the optical performance of the lenses. With polycarbonate and CR-39 lenses, an aspheric design usually is an option that increases the cost of the lenses. Minimum Center Thickness (Or Edge Thickness)

Because of their superior impact resistance, polycarbonate and Trivex lenses that correct myopia can be fabricated to a center thickness of just 1.0 mm and still pass the FDA impact-resistance standard. Myopia-correcting lenses made of other materials usually have to be thicker in the center to pass the standard.

The size and shape of your eyeglass frames also will affect the thickness of your lenses, especially if you have a strong prescription. Choosing a smaller, well-centered frame can significantly reduce the thickness and weight of your lenses, regardless of the lens material you choose.

Anti-scratch coating. All lightweight eyeglass lens materials (see table) have surfaces that are significantly softer and more prone to scratches and abrasions than glass lenses. The softest eyeglass lens is also the one that is the most impact-resistant: polycarbonate. But all plastic and high-index plastic lenses require a factory-applied anti-scratch coating for adequate lens durability.

Most of today’s modern anti-scratch coatings (also called scratch coats or hard coats) can make your eyeglass lenses nearly as scratch-resistant as glass. But if you’re hard on your glasses or you’re buying eyeglasses for your kids, ask about lenses that include a warranty against scratches for a specific period of time.

Anti-reflective coating. An anti-reflective (AR) coating makes all eyeglass lenses better. AR coatings eliminate reflections in lenses that reduce contrast and clarity, especially at night. They also make your lenses nearly invisible, so you can make better eye contact and you and others aren’t distracted by reflections in your lenses. AR-coated lenses are also much less likely to have glare spots in photographs.

Anti-reflective coating is especially important if you choose high-index lenses, because the higher the refractive index of a lens material, the more light the lenses reflect. In fact, high-index lenses can reflect up to 50 percent more light than CR-39 lenses, causing significantly more glare, unless AR coating is applied.

For this reason, people should protect their eyes from UV beginning in early childhood. Thankfully, polycarbonate and nearly all high-index plastic lenses have 100 percent UV protection built-in, due to absorptive characteristics of the lens material.

Photochromic treatment. This lens treatment enables eyeglass lenses to darken automatically in response to the sun’s UV and high-energy visible (HEV) light rays, and then quickly return to clear (or nearly clear) when indoors. Photochromic lenses are available in virtually all lens materials and designs. Cost Of Eyeglass Lenses And Eyeglasses

To get the best value, it’s essential to understand the features and benefits of the products you are considering and to choose wisely with the help of a reputable eye care provider and/or eyewear retailer. When Buying Eyeglass Lenses, There’s No Substitute For Expert Advice

For greatest satisfaction with your eyewear, in addition to using this guide, follow this advice echoed by Consumer Reports: During your eye exam, ask your eye doctor which eyeglass lenses and lens treatments are best for your specific needs and eyeglass prescription.