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.
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.
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.
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 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.
Frames can be made from metal or plastic, and they can be rimless. There is an almost unlimited variety of shapes, colors, and sizes. The type and degree of refractive correction in the lens determine to some extent the type of frame most suitable. Some lenses are too thick to fit into metal rims, and some large-correction prescriptions are best suited to frames with small-area lenses, since a smaller lens area minimizes the thickness of a lens with a large correction for myopia.
Rimless frames are the least noticeable type, and they are lightweight because they have only an upper rim to which the nosepiece and temples are attached. The lenses are held in place in the frames by nylon string that encircles the bottom of the lenses and attaches to the upper rim.
Metal frames are less noticeable than plastic and are lightweight. They are available in solid gold, gold-filled, anodized aluminum, nickel, silver, stainless steel, 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 strong and lightweight and returns to its original shape after being twisted or dented. It is not perfect for everyone, though, because some patients are sensitive to nickel.
Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are also offered in a variety of plastics (including acrylic, epoxy, cellulose acetate, cellulose propionate, polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. Epoxy frames are resilient and return to their original shape after being deformed, so they do not need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.
Before eye glasses are prescribed, an optometrist or an ophthalmologist examines the patient's eyes. The exam begins with the physician or ophthalmic assistant taking a detailed medical history from the patient. Then the physician, or in some cases a highly trained ophthalmic assistant, begins the ocular examination by measuring visual acuity and refracting.
During the exam, the physician or ophthalmic assistant also determines ocular motility and alignment, nearpoint of convergence, near fusional vergence amplitudes, relative accommodation measurements, and accommodative amplitude and facility. Corrective lens prescriptions, if necessary, are then given to patients.
To be correctly fitted for eye glasses, the distance between the patient's eyes (PD) is measured, so that the optical centers of the lenses correlate with the pupillary axis. Bifocal heights also are measured with the chosen frame in place and adjusted on the patient. If not positioned correctly, the patient may experience eyestrain or will not have the optimal visual results.
After the eye glasses are ready for the patient, the dispensing optician, physician, or ophthalmic assistant ascertains the proper fit to ensure the best vision and comfort.
Patients may sometimes need up to two weeks to adjust to a new prescription. However, if problems persist the glasses should be rechecked.
Presbyopic patients may choose over-the-counter reading glasses instead of professionally prescribed eye glasses. These patients should be advised that the distance between the lenses in these glasses is for a "standard" person. If a patient is sensitive or has more closely set eyes, for example, blurred vision and headaches may result from the use of these glasses. In addition, the prescriptions are equal for both eyes, these glasses do not correct for astigmatism, and they are made with cheaper lens and frame materials. As a result, these over-thecounter reading glasses may not be a patient's best option.
Patients are advised to return to the physician's office if they experience headaches or blurred vision. If these symptoms do not occur, they are instructed to return to the office for a regular eye examination in one or two years.
Matching an individual with the correct frames and lenses usually avoids complications. Ensuring that the lenses are positioned properly will greatly reduce the likelihood of headaches and blurred vision. A thorough patient history minimizes the chance that the patient will select a frame that causes sensitivities or lenses that could harm his or her eyes.
It is normally expected that people will achieve 20/20 vision while wearing corrective lenses.
Health care team roles
Nursing and allied health professionals assist with eyeglass examination and fitting, and with patient education . With advances in refracting technology, technicians now have duties that previously were performed only by optometrists or ophthalmologists.
Ophthalmic assistants and technicians facilitate the fitting process by recording the pertinent patient history and measuring the eye for the proper lens fit. Advancedand intermediate-level ophthalmic technicians perform refractions and determine the patient's depth perception. These professionals also may perform corneal topography (mapping). Some of these professionals seek certification through the American Board of Opticianry and the National Contact Lens Examiners or other organizations. These organizations offer seminars and testing that help professionals keep current with technological advances in refracting and eyeglass manufacturing.
Patients are told that it may take up to two weeks to adjust to new eye glasses. They also are advised to return to the physician's office if they experience blurry vision and headaches.
AR-coating and other specialized lens coatings need special care to maintain the coat and prevent scratching. Patients are told to carefully clean their lenses with special cloths and cleaners obtained from the physician or optician, never to use paper towels or harsh soap.
Optometrists and ophthalmologists receive training in optometry or during a medical residency for these procedures. In some practices, ophthalmic personnel complete part of the eye exam. Ophthalmic assistants, technicians, and nurses can complete specialized training from certified programs to learn how to refract and complete other parts of the eye exam. Opticians receive training in creating lenses.
Astigmatism —A vision condition that occurs when the cornea is slightly irregular in shape. This irregularity prevents light from focusing properly on the retina.
Cornea —The clear outer covering of the front of the eye.
Index of refraction —A constant number for any material that is an indicator of the degree of the bending of the light caused by that material.
Lens —A device that bends light waves.
Polycarbonate —A very strong type of plastic often used in safety glasses, sport glasses, and children's eye glasses. Polycarbonate lenses have approximately 50 times the impact resistance of glass lenses.
Presbyopia —A condition affecting people over the age of 40 where the system of accommodation that allows focusing of near objects fails to work because of age-related hardening of the lens of the eye.
Retina —The inner, light-sensitive layer of the eye containing rods and cones; it transforms the image it receives into electrical messages sent to the brain via the optic nerve.
Ultraviolet (UV) light —Part of the electromagnetic spectrum with a wavelength just below that of visible light. It is damaging to living material, especially eyes and DNA.
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