Table of Contents
Definition of Corrective Lenses and Function
Eyeglasses or contact lenses that alter the focal point of the lightwaves entering the eye to correct refractive errors of vision, including hyperopia (farsightedness), myopia (nearsightedness), astigmatism (blurred or distorted vision), and presbyopia (age-related hyperopia).
The eye’s natural focusing structures, the cornea and the lens, gather lightwaves and refract (bend) them toward their centers. The cornea refracts the lightwaves first. The lens, which can thicken or flatten to refine its focal efforts, refracts the somewhat focused lightwaves that come to it from the cornea. In normal vision, this sequence results in the focal point of the lightwaves striking the retina.
When refractive errors exist the focal point falls in front of or behind the retina, resulting in blurred images. Corrective lenses add a third level of refraction to compensate for the error, bending the lightwaves before they enter the cornea to realign their focal point.
The direction of refraction depends on the refractive error:
- In myopia, the focal point falls short of the retina. A lens that corrects for myopia bends the lightwaves inward, narrowing the span of light as it enters the cornea to lengthen the focal point. Such a lens is thicker at the edges than in the middle (concave); it is a minus spherical correction.
- In hyperopia, the focal point extends beyond the retina. A lens that corrects for hyperopia bends the lightwaves outward, broadening the span of light as it enters the cornea to shorten the focal point. Such a lens is thicker in the center than at the edges (convex); it is a plus spherical correction.
- In astigmatism, irregularities in the surface of the lens cause a second focal point. A lens that corrects for astigmatism refracts along a specific axis, realigning the lightwaves. This is a cylinder correction.
Corrective lenses can, and often do, combine spherical and cylindrical corrections. A multifocal lens further incorporates a correction for presbyopia in the form of a bifocal, trifocal, or progressive lens. The bottom of the lens is a plus section, added to the corrective prescription, that accommodates the limited ability of the lens to focus on near objects (such as when reading).
Eyeglasses are plastic resin or polycarbon, and less commonly glass, lenses ground to the thicknesses and shapes necessary to achieve the desired refractive specifications. Because eyeglasses are external to the eye, they can correct for a broad range of refractive errors and are the most common means of refractive correction.
Eyeglasses also can contain tints and dyes that change their color; some have additives that provide protection from ultraviolet light. About 85 percent of people who have refractive errors of vision wear eyeglasses to correct them.
Bifocal and trifocal eyeglasses have a clear shift (sometimes visible as a line on the lens) to the presbyopic correction; a progressive lens transitions to the presbyopic correction. Reading glasses such as those available without an eye care practitioner’s prescription, are magnifying lenses that enlarge close objects, requiring the lens to make less of an adjustment to bring them into focus.
How well reading glasses work depends on whether there are refractive errors that remain uncorrected. With aging, most people develop at least a small degree of astigmatism, which can result in blurred or distorted images not related to presbyopia.
The primary risk of wearing eyeglasses is traumatic injury due to a blow that strikes the glasses. The energy of such a blow concentrates initially at the contact points on the NOSE. The frame may break, causing lacerations to the face. Of more significant consequence is a blow that breaks the lens, which can result in vision-threatening injury to the eye.
Polycarbonate lenses have the highest inherent shatter resistance; plastic resin and glass lenses should have shatter-resistant coatings or additives. People who engage in physical activities such as ball sports should wear polycarbonate eyeglasses or custom protective eyewear.
Contact lenses fit directly onto the eye, covering the cornea. There are two basic kinds of contact lenses in use today: gas permeable (hard) and hydrophilic (soft). Gas-permeable contact lenses float on a layer of tears over the center of the cornea and often are the contact lens of choice to correct for moderate to significant astigmatism as well as keratoconus, a condition in which the cornea’s center bulges outward.
Gas-permeable lenses also can correct for mild to moderate myopic and hyperopic refractive errors. Made of rigid polymers of fluorocarbon and polymethyl methacrylate, gas-permeable lenses allow oxygen molecules to pass through but do not absorb moisture from the eye.
Hydrophilic contact lenses cover the entire cornea and can correct for mild to moderate myopia and hyperopia. Soft and flexible, hydrophilic lenses contain a high percentage of water and draw additional moisture from the tears to remain hydrated. A special kind of hydrophilic lens, the toric lens, is necessary to correct for astigmatism.
A toric lens has varying thicknesses that compensate for corneal irregularities to correct refraction.
Contact lenses can incorporate correction for moderate presbyopia, though this tends to be a less satisfactory approach than eyeglasses. There are two methods for accommodating presbyopia with contact lenses: progressive or bifocal lenses and monovision. Progressive or bifocal contacts function much the same as progressive or bifocal eyeglasses, with the lower portion of the lens containing the presbyopic correction.
Because contact lenses shift position on the eye with blinking and when the wearer alters the angle of the head (such as when lying down), the presbyopic correction may not remain in an effective position. Monovision takes the approach of modifying the brain’s interpretation of visual signals. One eye, usually the dominant eye, wears a contact lens with the refractive correction.
The other eye wears a contact lens with the presbyopic correction. The brain learns to distinguish which signals to interpret, accepting those from the dominant eye during normal visual activities and those from the other eye when reading or doing close-focus work.
The primary risks of wearing contact lenses are damage to the cornea and infection. even hydrophilic lenses can irritate the cornea and cause corneal abrasions, particularly in dusty, windy, or dry environmental conditions. Contact lenses tend to accumulate protein deposits that cause irritation.
Most hydrophilic lenses are disposable, so frequent replacement helps minimize this as a problem. The optician may need to clean or gently grind the surface of gas-permeable lenses to clear away deposits. Contact lens hygiene, including diligent hand washing before handling lenses and storing lenses in the appropriate disinfectant solution, is essential.
Reading a Corrective Lens Prescription
Optometrists and ophthalmologists measure refractive errors in diopters, a representational scale of the distance in front of or behind the eye’s lens the focal point of lightwaves entering the eye must shift to allow the light waves to clearly focus on the retina.
The larger the diopter number, the more the lens refracts, or bends, the light. A corrective lens prescription represents the diopter as minus or plus, according to the direction the correction shifts the focal point.
For example, the following prescription corrects for myopia and astigmatism:
|OD||– 5.75||+ 0.50||164|
This prescription denotes different refractive corrections for the right eye (OD) and left eye (OS). The minus diopter is the spherical correction for the myopia; the plus diopter is the cylindrical correction for the astigmatism, and the last number is the axis position for the cylindrical correction.
A lens with a strong correction may also include an adjustment that tilts the lens to alter its optical center, the prism, allowing a thinner lens to deliver the same corrective power or to accommodate a significant difference in the refractive correction for each eye (anisometropia).