bifocal lens


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lens

 [lenz]
1. a piece of glass or other transparent material so shaped as to converge or scatter light rays. See also glasses.
2. the transparent, biconvex body separating the posterior chamber and the vitreous body of the eye; it refracts (bends) light rays so that they are focused on the retina. Called also crystalline lens. In order for the eye to see objects close at hand, light rays from the objects must be bent more sharply to bring them to focus on the retina; light rays from distant objects require much less refraction. It is the function of the lens to do accommodation, making of adjustments for viewing both near objects and more distant ones. To accomplish this it must be highly elastic so that its shape can be changed and made more or less convex. The more convex the lens, the greater the refraction. Small ciliary muscles create tension on the lens, making it less convex; as the tension is relaxed the lens becomes more spherical in shape and hence more convex.

With increasing age the lenses lose their elasticity; thus their ability to focus light rays in the retina becomes impaired. This condition is called presbyopia. In farsightedness (hyperopia) the image is focused behind the retina because the refractive power of the lens is too weak or the eyeball axis is too short. Nearsightedness (myopia) occurs when the refractive power of the lens is too strong or the eyeball is too long, so that the image is focused in front of the retina.
The biconvex lens of the eye. From Frazier et al., 1996.
achromatic lens one corrected for chromatic (color) aberration.
apochromatic lens one corrected for chromatic (color) and spheric aberration.
biconcave lens one concave on both faces.
biconvex lens one convex on both faces.
bifocal lens one having two segments with different refracting power, the upper for far vision and the lower for near vision. See also bifocal glasses.
concave lens one curved like a section of the interior of a hollow sphere; it disperses light rays. Called also diverging lens.
contact l's corrective lenses that fit directly over the cornea of the eye; see also contact lenses.
converging lens (convex lens) one curved like the exterior of a hollow sphere; it brings light to a focus.
convexoconcave lens one that has one convex and one concave face.
crystalline lens lens (def. 2).
cylindrical lens one with at least one nonspherical surface, used to correct astigmatism.
diverging lens concave lens.
honeybee lens a magnifying eyeglass lens designed to resemble the multifaceted eye of the honeybee. It consists of three or six small telescopes mounted in the upper portion, directed toward the center and right and left visual fields. Prisms are included to provide a continuous, unbroken magnified field of view.
omnifocal lens one whose power increases continuously and regularly in a downward direction, avoiding the discontinuity in field and power inherent in bifocal and trifocal lenses.
orthoscopic lens one that gives a flat and undistorted field of vision, especially at the periphery.
planoconcave lens a lens with one plane and one concave side.
planoconvex lens a lens with one plane and one convex side.
Stokes's l's an apparatus used in the diagnosis of astigmatism.
trial l's ones used in testing the vision.
trifocal lens one having three segments of different refracting powers, the upper for distant, the middle for intermediate, and the lower for near vision.

bi·fo·cal lens

a lens used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general; the reading addition may be cemented to the lens, fused to the front surface, or ground in one-piece form; other bifocal lenses are the flat-top Franklin type, or blended invisible.

bi·fo·cal lens

(bī-fō'kăl lenz)
A lens used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general.

lens

(lenz) [L. lens, lentil]
1. A transparent refracting medium, usually made of plastic.
2. The crystalline lens of the eye.

accommodating intraocular lens

A flexible intraocular lens inserted into the eye during cataract surgery. When tugged upon by the ciliary muscle, this lens can alter its shape to focus on objects that are near, far, or middle distances from the eye.

achromatic lens

A lens that transmits light without separating it into the colors of the visual spectrum.

anterior chamber intraocular lens

Abbreviation: ACIOL
An artificial lens placed in the anterior chamber on top of the iris after natural lens has been removed. The lens may be fixated to the iris or positioned in the anterior chamber angle.
See: intraocular lens

aplanatic lens

A lens that corrects spherical aberrations.

apochromatic lens

A lens that corrects both spherical and chromatic aberrations.

bandage lens

A lens placed on the cornea to protect it while it heals, e.g., after a corneal abrasion or keratoplasty.

biconcave lens

A lens that has a concave surface on each side.
See: biconcave for illus

biconvex lens

A lens that has a convex surface on each side.
See: biconcave for illus

bifocal lens

See: bifocal eyeglasses

bifocal contact lens

A contact lens that contains two corrections in the same lens.

bitoric lens

A lens that has a toric shape on both sides, used to correct astigmatism.

concave spherical lens

A lens formed of prisms with their apices together (thin at the center and thick at the edge), used for correcting myopia.

contact lens

A lens made of various materials, either rigid or flexible, that fits over the cornea or part of the cornea to supplement or alter the refractive ability of the cornea or the lens of the eye. Contact lenses of any type require special care with respect to storage when they are not being worn, directions for insertion and removal, and the length of time they can be worn. The manufacturer's or dispensing health care worker's instructions should be read and followed. Failure to do this may result in serious eye diseases. Wearing contact lenses while swimming is inadvisable.

convergent lens

Plus lens.

convexo-concave lens

A lens that has a convex surface on one side and a concave surface on the opposite side.

convex spherical lens

A lens formed of prisms with their bases together (thick at the center and thin at the edge), used for correcting hyperopia.

corneal contact lens

A type of contact lens that adheres to and covers only the cornea.

crystalline lens

A transparent colorless biconvex structure in the eye, enclosed in a capsule, and held in place just behind the pupil by the suspensory ligament. It consists principally of lens fibers that at the periphery are soft, forming the cortex lentis, and in the center of harder consistency, forming the nucleus lentis. Beneath the capsule on the anterior surface is a thin layer of cells, the lens epithelium. The shape is changed by the ciliary muscle to focus light rays on the retina.

cylindrical lens

A segment of a cylinder parallel to its axis, used in correcting astigmatism.

disposable contact lens

A soft contact lens worn for a week or two and then discarded.

divergent lens

Minus lens.

extended wear contact lens

A contact lens made of materials that permit permeation of gas (such as oxygen) so that there is less chance for corneal irritation.

gas-permeable lens

Abbreviation: GP lens
A contact lens that allows oxygen to pass through it, enhancing eye health, lens durability, and comfort. Gas permeability derives from the incorporation of silicone in lens plastic. GP lenses are used to manage visual conditions such as astigmatism, keratoconus, and presbyopia. Synonym: rigid gas-permeable lens

gonio lens

Gonioscopy lens.

gonioscopy lens

A lens with one or more tilted mirrors that is applied to the cornea for use in visualizing the anterior chamber of the eye during gonioscopy.
Synonym: gonio lens

hard contact lens

A contact lens made of rigid translucent materials.

high-index lens

A lens material that refracts more light than standard lens materials do. It is used to minimize lens thickness and weight.

hydrophilic lens

Soft contact lens

implantable collamer lens

Abbreviation: ICL
An artificial lens that can be inserted between the cornea and a patient's own lens, used to correct severe nearsightedness.

implanted lens

Intraocular lens.

intraocular lens

Abbreviation: IOL
An artificial lens made of acrylic, polymethymethacrylate, or silicone. The lens may be placed posterior to the iris (PCIOL) or anterior to the iris (ACIOL). Posterior chamber lens may be monofocal or multifocal and can also correct an astigmatic error. A lens is removed because of abnormalities such as cataracts. If the original lens capsule is present and an IOL is placed inside it, the surgical procedure is called posterior chamber IOL implantation. If the capsule has been removed in a previous surgical procedure, the IOL may be placed in front of the iris, directly adjacent to the cornea. This is called anterior chamber IOL implantation. In another procedure, the IOL is implanted behind the iris. Which method of IOL implantation produces the best results is being investigated.
Synonym: implanted lens See: cataract

minus lens

A concave lens used to improve visual acuity in myopic patients.
Synonym: divergent lens

monofocal intraocular lens

An intraocular lens inserted by the ophthalmologist during cataract surgery that allows the wearer to see clearly at a single distance (close to the eye, far from it, or in intermediate focal points). Monofocal lenses were the only lenses used in the first few decades of cataract surgery.
See: multifocal intraocular lens

multifocal intraocular lens

An intraocular lens inserted into the eye during cataract surgery that gives the wearer clear vision at near, intermediate, and far focal points.
See: monofocal intraocular lens

multifocal lens

Progressive lens.

oil immersion lens

A special lens with oil placed between the lens and the object being visualized. This eliminates a layer of air between the microscope slide and the lens, producing a clearer image than if the oil were not used.

orthoscopic lens

A lens that produces no distortion of the periphery of the image.

piggyback lens

A combination of a hard contact lens placed over a soft contact lens, used to treat two different ocular conditions.

plus lens

A convex lens used to improve visual acuity in hyperopic patients.
Synonym: convergent lens

rigid gas-permeable lens

Gas-permeable lens.

posterior chamber intraocular lens

Abbreviation: PCIOL
See: intraocular lens

progressive lens

An eyeglass lens, used to treat presbyopia, that gradually changes prescription strength from the top of the lens, for distance viewing, to the bottom of the lens, for seeing objects close-up. Progressive lenses enable the eyes to adjust from one distance to another (as when one looks up from a book) without the image jump associated with bifocals. Synonym: multifocal lens

silicone hydrogel contact lens

Abbreviation: SH lens
A soft, extended-wear contact lens designed to improve the delivery of oxygen to the corneal epithelium. Depending on their design specifications, such lenses may be worn 6 to 30 days and nights consecutively.

soft contact lens

A contact lens made of flexible, translucent materials. Such lenses are more comfortable, can be worn longer, and are harder to displace than hard lenses, but there are disadvantages. They may not provide the same degree of visual acuity as hard lenses, and they require more cleaning and disinfection. Production of tears may be decreased, esp. in older patients. The soft lenses may need to be replaced every 6 to 18 months. Corneal infections can prevent further use of soft lenses and also cause permanent loss of vision. Synonym: hydrophilic lens

spherical lens

A lens in which all surfaces are spherical.

toric contact lens

A contact lens with two separate curvatures, used to correct astigmatism and distance vision simultaneously.

trial lens

A lens used in testing the vision.

trifocal lens

A corrective eyeglass lens containing three segments for near, intermediate, and distant vision.

zoom lens

A type of lens that can be adjusted to focus on near or distant objects.
References in periodicals archive ?
19 What was the surname of Benjamin, the US scientist, author and philanthropiust who invented the bifocal lens in 1784?
introduced a bifocal lens with five concentric rings of different optical powers.
Incomnet markets long-distance service, manufactures and markets bifocal lens, and operates a network that allows automobile dismantlers to buy, sell and trade auto parts.