The best choice if you want the most economical lenses possible.
Excellent optical clarity (comparable to glass lenses).
Due to thickness, not recommended for higher prescription powers.
Can be easily tinted to dark sunglass shades.
The most popular lens material in the United States, accounting for over 50 percent of all eyeglass lenses sold.
The best choice if you want the safest and/or lightest lenses possible.
Over ten times more impact resistant than regular plastic lenses.
Used in safety eyewear and sports goggles as well as general purpose eyewear.
Recommended for children’s eyewear and whenever eye safety is a primary concern.
Up to 40% thinner and 30% lighter than regular plastic lenses.
Provide 100% protection from the sun’s ultraviolet (UV) rays.
The second most popular lens material in the United States, accounting for approximately 20 percent of all eyeglass lenses sold.
HIGH INDEX PLASTICS
The best choice if you want the thinnest, most attractive lenses possible.
Range from 20% to 65% thinner than plastic lenses (depending on refractive index).
The higher the refractive index, the thinner the lens (and the higher the cost).
Most high index plastic lenses provide 100% protection from the sun’s UV rays.
Anti-reflective (AR) coating is essential for the best optical performance and appearance.
Seek Expert Advice Before Choosing
A number of additional factors can influence the overall thickness, weight, and optical performance of your eyeglass lenses. Some of these factors include the center thickness of the lens, the edge thickness of the lens, the lens design, and the size and shape of the frame(s) you select.
Keep in mind that to satisfy all of your visual and lifestyle needs, it’s usually best to purchase more than one pair of eyeglasses.
Trivex, also known as Phoenix or Triology, is a relatively new material that is similar to polycarbonate lenses but with higher quality optics, and thus provides clearer vision. Trivex is lighter in weight than standard plastic but not quite as thin as polycarbonate. Trivex is a more rigid material, making it a better selection for rimless or drill mount frames. Trivex is just as impact resistant as polycarbonate and may also be prescribed for childrens lenses, sport lenses and safety lenses.
An aspheric lens design gives several advantages to eyeglass lenses. An aspheric lens has flatter peripheral curvatures than a regular spherical lens. This reduces the amount a lens may bulge out of an eyeglass frame, reducing magnification of the eyes, which improves cosmetic appearance. Flatter peripheral curves also reduce distortions that occur when looking away from the center of the lens, making vision much crisper. Aspheric lenses are also much lighter in weight than standard spherical lenses.
Anti-reflective coating is applied to eyeglass lenses to reduce the amount of internal and external reflections on a lens. This increases the amount of light transmitted through the lens, which improves quality of vision. Anti-reflective coating also decreases unwanted glare and halos at nighttime. It also makes the lenses appear somewhat invisible and very thin. While everyone could benefit from an anti-reflective coating, it is especially beneficial for people with high prescriptions, people who have a decrease in vision at night, and professions in which cosmetic appearance is important.
Scratch-resistant coatings are applied to the front and back of lenses in the manufacturing process. Although it is important to realize that no lens is scratch-proof, this special coating does make lenses harder to scratch when dropped or rubbed against a hard surface. While most lenses are made with scratch-resistant coating, sometimes the term scratch-resistant coating indicates a type of scratch warranty. These warranties guarantee against scratches, ensuring the lenses will be remade if a scratch does occur. Be sure to clarify any warranty of this nature.
Ultraviolet treatment is applied to lenses to protect against harmful UV sunrays that can accelerate the development of cataracts and macular degeneration. It is extremely important to protect eyes from the damaging effects of the sun. UV treatment is easy to apply to lenses and is often included with the purchase of eyeglasses.
Polarized lenses are usually used to make sunglasses. They are available most commonly in grey or brown tint but many other colors are available. Vertically polarized lenses decrease bright glare and reflections by blocking horizontal polarized reflected light. Polarized lenses have been used by fishermen for years to better deal with bright light being reflected off water and to see deeper into the water.
Photochromatic/ TRANSITION00 Lenses:
Photochromatic lenses have a special chemical coating that makes them change to a dark tint in the sunlight and turn clear indoors. Photochromatic lenses are great for people who do not wish to carry a separate pair of prescription sunglasses. It is important to recognize that these lenses do not darken as well while driving a car. The windshield prevents most of the UV light from reaching the lens.
THE PRESCRIPTION AXIS:
The axis in a prescription describes orientation of the axis of the cylindrical lens. The direction of the axis is in degrees measured anti-clockwise from the horizontal line through the centers of the pupils when viewed from front side of the glasses (i.e. when viewed from the point of view the person making the measurement). It varies from 1 to 180 degrees.
In the illustration below, viewed from the point of view of the person making the measurement, the axis is 20 degees if written in minus notation or 110 deg (at 90 degrees to the negative axis) if written in plus notation.
The DV portion of the prescription describes the corrections for distant vision. For most people under forty years of age, this is the only part of the prescription that is filled in. The NV or near-vision portion of the prescription is blank because a separate correction for near vision is not needed.
The NV portion is used in prescriptions for bifocals.
In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate.
This is called “presbyopia”; the presby- root means “old” or “elder”. (It is the same root as in the words priest and presbyterian.)
The hardening of the lens is a continuous process, not something that suddenly happens in middle age. It is occurring all along. All that happens around middle age is that the process progresses to the point where it starts to interfere with reading. Therefore almost everybody needs glasses for reading from the age of 40-45.
Because young children have a wider range of accommodation than adults, they sometimes examine objects by holding them much closer to the eye than an adult would.
This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm. (4″) from the eye, a task for which an adult needs a magnifying glass with a rated power of about 3.5X.
The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such a prediction is sufficient in many practical cases, especially when the total correction is less than 3 diopters. See also the following calculator for computing this correction.
When someone accommodates, they also converge their eyes. There is a measurable ratio between how much this effect takes place (AC/A ratio, CA/C ratio).