Eye Glasses

Definition

Eye glasses are devices that correct refractive errors in vision. Eye-glass lenses are mounted in frames that position the lenses in front of the eyes.

Purpose

Eye glasses are used to correct or improve the vision of patients with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism. They are also utilized to correct refractive errors after cataract surgery.

Precautions

Ophthalmic assistants, technicians, and nurses take a careful patient history to determine patient sensitivities to certain frame materials. Patients allergic to certain plastics should avoid frames or lenses manufactured from that type of plastic. Patients allergic to nickel should not wear Flexon frames. Ophthalmic personnel also address the patient's lifestyle. People at risk for accidents due to professions, sports, or hobbies are advised to choose plastic lenses, preferably polycarbonate. Also, people at risk of receiving electric shocks should avoid metal frames.

Description

Eyes are examined by optometrists (O.D.s) or ophthalmologists (M.D.s), with assistance from ophthalmic assistants, technicians, or nurses. If necessary, prescriptions are given to patients for glasses. An optician generally makes the glasses. Eye glasses can be created in an in-office laboratory or an off-site manufacturing lab.

Patients whose eyes have refractive errors do not see clearly without glasses. This is due to the fact that the light emitted by the objects they see does not come into focus on their retinas. For farsighted (hyperopic) patients, images come into focus behind the retina; for nearsighted (myopic) patients, images come into focus in front of the retina.

Lenses

Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of light that passes through it, and the greater the correction.

Lenses can be unifocal, with one correction for all distances, or they can correct for more than one distance (multifocal). One type of multifocal lens, the bifocal, has an area of the lens (usually at the bottom) that corrects for near objects (about 14 in [35.5 cm] from the eyes); the remainder of the lens corrects for distant objects (about 20 ft [6 m] from the eyes). Another type of multifocal lens, a trifocal, has an area in-between that allows correct viewing of intermediate objects (usually about 28 in [71 cm]), such as computer screens or automobile dashboards.

The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). Plastic is lightweight and more impactresistant than glass. Glass is scratch-resistant and provides the best visual acuity. In recent years, however, glass that is thinner and more impact-resistant has been developed.

A plastic called CR-39, introduced in the 1960s, is the plastic of choice of most opticians. Today, eyeglass wearers can also choose between polycarbonate, which is the most impact-resistant material available for eyewear, and polyurethane, which has exceptional optical qualities and an index of refraction of up to 1.66, much higher than the conventional plastics. Polycarbonate is the most easily scratched of the plastics used for lenses, so an anti-scratch coating is always applied to the lenses. In addition, an improvement in the polycarbonate manufacturing process now produces clear lenses—previously all polycarbonate lenses had a bluish cast.

Patients with high prescriptions should consider high index materials. Aspheric lenses also are useful for high prescriptions. They are flatter and lighter than conventional lenses. These lenses make it possible for patients with higher prescriptions and thick lenses to wear metal and titanium frames, when formerly they could wear only plastic frames.

There are many lenses and lens-coating options for individual needs, including coatings that block harmful ultraviolet (UV) light or UV and blue light. Such coatings are not needed on polycarbonate lenses, which already have UV protection.

There are anti-scratch coatings that increase the surface hardness of lenses and anti-reflective (AR) coatings that eliminate almost all glare. AR coatings may be particularly helpful to people who use computers or drive at night. Polarized lenses that block reflected light also allow better vision in sunny weather. Photosensitive (photochromic) lenses that darken in bright light are handy for people who do not want to carry an extra pair of sunglasses. Photochromic lenses are available in glass, plastic, and polycarbonate.