Eye Glasses

Lens Materials

CR-39

CR-39 or a traditional plastic lens is the most widely available material and has the most lens options. It has low light dispersion and relatively good optics. However, it requires the addition of scratch resistant and UV coatings. For mild to moderate prescriptions its weight and thickness are acceptable, however, in high prescriptions it is better to use a lighter and higher index material. It cannot be used in safety frames or drill mounted lenses. Use in semi-rimless frames is possible, but the lens is more likely to chip. There are fewer reflections off the lens surface with CR-39 than most materials.

Polycarbonate

Polycarbonate is a very light and impact resistant lens material. It comes with a scratch resistant coating and inherently blocks UV light. It is a good material for use with drill mounted, semi-rimless and safety lenses. It has more light dispersion than a traditional plastic lens, which causes some adaptation issues because of color fringes that may be seen. Polycarbonate has the second most available lens options and is more expensive than traditional plastic. The use of an anti-reflective coating is recommended to limit dispersion and glare.


Trivex

Trivex is the newest material available for use in ophthalmic lenses. It is the lightest material available, most impact resistant, inherently blocks UV light and comes with a scratch resistant coating. It is the best material for use in with drill mounted, semi-rimless and safety frames. The tensile strength of Trivex makes it highly resistant to cracking around drill holes and is the best material for this and any high end fashion frame that depends on the lens to hold its shape. It has less dispersion than most lenses and good optical qualities. Because it is relatively new, it has less lens options and is more expensive. With high prescriptions it will have more thickness than polycarbonate or higher index materials.

Hi-Index

Hi-index lenses were created to reduce the thickness and weight of lenses with a high prescription. They should mainly be used for that purpose only. For a prescription less than +/- 2.00 the benefits are negligible, between +/- 2.00 and –4.00 there is a small benefit, but above +/- 4.00 there is a significant difference in thickness. They can be tinted and are acceptable for drill mount frames. They are inherently UV protective and come with a scratch coat. Reflections off the lens surface are high and it is strongly recommended that they come with an anti-reflective coating.

Crown Glass

Crown glass is rarely used to make prescription lenses now. The reason behind this has to do with the weight of the lenses and potential of ocular trauma from impact. The lenses can crack and cut tissue. Heat/chemically treated lenses offer increased protection, but if they are scratched they have been compromised. They do have the best optics and low dispersion. They are usually specially ordered and are in some high end sunglass frames.


Lens Additions

Anti-Reflective Coatings

Anti-reflective (AR) coatings are a microscopic layer of a special material applied to lenses that reduces the amount of glare that comes off of them. They give a lens a green or purple sheen. AR coatings are most beneficial for those who work on a computer screen all day and for those with difficulty driving at night or who frequently drive at night. An AR coating is also more beneficial for high index lenses because of increased surface reflections and polycarbonate lenses because of high dispersion. A good AR coat can increase light transmission into to the upper 90% range.

Polarized Lenses

Normal light is un-polarized meaning its electromagnetic fields’ orientation is in many directions. With the use of polarizing filters it can become polarized where the electromagnetic field’s orientation is only in one direction. This can be very beneficial for sunglasses because they can reduce a significant amount of glare without degrading image quality too much. Ideal candidates for polarized sunglasses are those that do water sports because of the way light reflects off of water. It is also good for general use as a sunglass. Polarized lenses usually come in a brown or gray color. Polarized lenses should be the preferred type of sunglass lenses.

Tints

Tints absorb light to decrease the intensity of incoming light. They come in many different colors and some seem to enhance vision in certain activities (i.e. a yellow tint in playing golf). A specific tint called FL-39 (a rose pink color) has been shown to help some people with migraine headaches

Photochromic Lenses

Photochromic lenses have a photochromic material added to them that will darken when exposed to ultra-violet light. A popular name brand is “Transitions.” The lenses will be clear when indoors because there is no ultra-violet light; however, when they are taken outside they act like a sunglass lens and darken because of exposure to the ultra-violet light. A drawback to these lenses is that they don’t work well in a vehicle because the vehicles windows absorb ultra-violet light.

Multifocal Lenses

Multifocal lenses are usually used for those over the age of 40 who have lost the ability to focus on objects at near distances. Lenses that are used are lined bifocals, lined trifocals and progressives.

Lined Bifocals

A lined bifocal provides distance vision and one fixed near distance. There are three commonly used types of bifocals. They are a Flat Top-28 (FT-28), Flat Top-35 (FT-35) and the Executive. The wider FT-35 and Executive give more working room in the lens.


Lined Trifocals

Trifocal lenses provide distance, fixed intermediate and fixed near vision. Typical types of trifocals include a 7 x 28, 7 x 35, 8 x 35 or an Executive.

Progressives (No-line Bifocals)

Progressive lenses provide clear vision at any given distance that is viewed, but “no-lines” are seen. They have a “corridor” or “channel” of clear vision that increases in plus power as an individual looks down through their glasses. The “channel” proceeds down and slightly in. A drawback of the lens is that when an individual looks down and in or out their vision is distorted because the way the lens had to be designed. Better progressives have a wider channel.

Frame Materials

Metal frames are usually made of nickel/nickel-plated, stainless steel, titanium or aluminum.
  •  Nickel/Nickel-Plated: it is inexpensive, but some people are allergic to it (a rash) and it causes a green build up on frame, temples  and nosepieces. It is very durable and weighs more than stainless steel and titanium.
  •  Stainless Steel: it does not cause an allergy and it is lighter than nickel, it may cost a little more
  •  Titanium: it is the best material for a lens, it is hypoallergenic and very lightweight, it is also very durable and more expensive
  •  Aluminum: it is rarely used in a frame, but it is very lightweight and corrosion resistant
  •  Memory Metal: it is a new metal alloy has been developed that can bend, but it will remember its original shape; it is very lightweight, hypo-allergenic and corrosion resistant

Zyl is a plastic material that many frames are made of. It is corrosion resistant and hypoallergenic. It can be made in many shapes and forms. It is typically pretty inexpensive.

Nylon frames are similar to plastic frames, but they are more durable.

Frame Designs

Full Eye Wire

A full eye wire frame has a rim 360 degrees around the lens. This is the most common type of frame. In patients with a high Rx, this frame type with a small lens size is the best cosmetic and functional option.

Semi-Rimless

A semi-rimless frame has a rim usually halfway around the lens and then a plastic wire around the other 180 degrees. To some it is more

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