|
|
Rimless frames are the least noticeable type, and they are lightweight because the nosepiece and temples are attached directly to the lenses, eliminating the weight of the rims. They tend to not be as sturdy as frames with rims, so they are not a good choice for people who frequently remove their glasses and put them on again. They contacts are also not very suitable for lenses that correct eye a high degree of farsightedness, because such lenses are thin at the edges.Metal frames are less noticeable than plastic, and they are lightweight. They are available in solid gold, gold-filled, contacts anodized aluminum, nickel, silver, stainless steel, and now titanium and titanium alloy. Until the late 1980s, when titanium-nickel alloy and titanium frames were introduced, metal frames were, in general, more fragile than plastic frames. The titanium frames, however, are very strong and lightweight. An alloy of titanium and nickel, called Flexon, is not eye only strong and lightweight, but returns to its original shape after being twisted or dented. It is not perfect for everyone, though, because some people are sensitive to its nickel. Flexon frames are also contacts relatively expensive. They are flatter and lighter than conventional lenses.There are many lenses and lens-coating options for individual needs, including coatings that block the ultraviolet (UV) light or UV and blue light which have been found to be harmful to the eyes. Such coatings are not needed on polycarbonate lenses, which already have UV protection. UV coatings are particularly important on sunglasses and ski goggles. Sunglasses, when nonprescription, should be labeled with an indication that they block out 99-100% of both UV-A and UV-B rays. There are anti-scratch coatings that increase the surface eye hardness contacts of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eye eliminate almost all glare and allow other people to see the eyes of the wearer. AR coatings contacts may be particularly helpful to people who use computers or who drive eye at night. contacts Mirror coatings that prevent other people from seeing the wearer''s eyes are also available. There is a whole spectrum of tints, from light tints to darker tints, used in sunglasses. They can be worn up to a week. Daily wear lenses are made of plastic that is permeable to oxygen and absorbs water; eye therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. lenses are easier to get used to than rigid lenses, but are more prone to tears and do not last as long. Extended-wear lenses are highly permeable to oxygen, are flexible by virtue of their ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems. contacts There is more of a risk of infection with extended-wear lenses than with daily-wear lenses. Extended-wear disposable lenses are lenses worn continually for up to six days and then discarded, with no need for cleaning eye.
|