Glossary of contact lens related words and phrases
Accommodation- changing the focusing power of the eye by altering the shape of the eye’s crystalline lens to focus light from near objects onto the retina to give clear vision at that distance.
Amblyopia- diminished vision in one or both eyes without pathology of ocular structures, and despite best spectacle or contact lens correction. Is a lack of development, mentally, in the part of the brain that corresponds to a particular eye.
Aphakia- absence of the crystalline lens of the eye.
Astigmatism- a defect in an optical system in which rays from a point source fail to come to a point focus; usually due to an irregular optical surface such as the front surface of the cornea. The cornea has a shape like the back of a spoon rather than a ball.
Bandage lens- a contact lens used to speed healing or to protect the cornea.
Biocompatible- not causing injury, toxic or immunologic reaction to living tissue. Exists in harmony with the body because the body's immune system recognizes the biocompatible object as "self" rather than "foreign" in nature.
Cataract- a clouding of the internal natural lens of the eye or its surrounding transparent membrane, obstructing the passage of light and causing a reduction in vision.
CE Mark- a mark on contact lens and lens care product packaging showing that the products were designed, developed and manufactured using systems approved by the Regulatory Authorities of the European Union.
Contact lens- a thin plastic lens designed to fit over the cornea, usually for the correction of refractive error.
Conventional wear- mode of contact lens wear involving use of the lens for as long as the lens is functional, as opposed to disposable wear.
Cornea- the transparent circular 'window' on the front of the eye that covers the iris and pupil; the initial optical element of the eye.
Corneal abrasion- damage or defect of the superficial layer of epithelium of the cornea. Usually caused by scraping of the eye’s surface.
Corneal distortion- an irregularity of the normal corneal curvature. May be due to injury, degeneration, or the wearing of improperly fit hard contact lenses; tends to cause blurred vision which is not completely correctable with spectacles.
Corneal edema- swelling of the cornea, due to fluid accumulation typically from a lack of oxygen (hypoxia).
Cosmetic wear- contact lens wear that is for the correction of refractive error and/or eye color enhancement only; not for the treatment of a medical condition such as aphakia or keratoconus.
Crystalline lens- the natural lens of the eye, a transparent structure suspended behind the iris; focuses light rays on the retina and changes shape to change the focus of the eye for different distances.
Daily wear- wearing contact lenses during waking hours only. Healthier than extended wear.
Deposits- accumulations of substances, usually tear film components such as protein and fat, onto the contact lens surface and/or in the lens material.
Depth perception- the ability to judge the relative distance of objects and the spatial relationship of objects at different distances.
Disinfection- process whereby microorganisms are killed, but not their spores; re-growth of organisms from the spores is possible.
Disinfecting Solution—The solution your lenses are stored in overnight. Kills most types of germs on the lens surface.
Disposable contact lens- contact lens intended to be used for a short period of time--anywhere from a day, to a week, or a month, depending on the particular brand.
Disposable wear- wearing a contact lens on a disposable basis; when lens is removed from eye, it is discarded; allows benefits of decreased problems from lens deposits and spoilage.
Edema- an abnormal excess accumulation of fluid in a tissue.
Emmetropia- the condition of the normal eye where light rays from distant objects are focused on the retina so that vision is sharp and clear (usually measured as 20/20).
Endothelium- the innermost corneal layer of flat, hexagonal cells which function to pump fluid out of the cornea.
Enhancing tint- a transparent tint on a contact lens which blends with the underlying iris color to achieve a new, enhanced iris color effect.
Extended wear- wearing a contact lens for any period that includes sleep, up to a maximum of seven days continuously. Less safe than daily wear.
Eyelid- the moveable folds of skin and muscle that can be closed over the eyeball; also called palpebrae.
FDA- abbreviation for the Food and Drug Administration; regulates contact lenses and lens solutions as medical devices.
Field of view- the area that is visible through the lens of an optical system.
Flare- starburst effect observed when light rays are reflected from the edge of a lens or optic zone.
Fungi- microorganisms including molds and mildews; capable of causing ocular disease.
Gas Permeable Contact Lens -see RGP.
Glaucoma- a condition in which the pressure inside the eye is higher than the eye can handle. This damages the optic nerve and causes a loss of peripheral vision.
Hard lens- contact lens made from a material which is stiff and rigid; lens will flex only a small amount, and will break if excessive force is applied.
HEMA- abbreviation for hydroxyethyl methacrylate, a hydrophilic material containing 38% water commonly used in the manufacture of soft contact lenses.
Hydration- step during the manufacturing process for soft contact lenses where the dry lenses are immersed in solution in order to absorb as much water as their formulation allows.
Hydrogel- polymer network in which a liquid (water) has been absorbed; also known as hydrophilic plastic.
Hydrophilic- "water-loving" i.e., absorbs water; term used for soft lenses; technically, any plastic with 4% or greater water content by weight.
Hydrophobic- "water-fearing," i.e., repels water; technically any plastic with less than 4% water content by weight.
Hydroxyethyl methacrylate (HEMA)- the first soft lens material; composed of monomers of hydroxyethyl methacrylate which form crosslinked chains; 38% water content, non-ionic polymer.
Hypermetropia- also known as hyperopia or farsightedness; refractive error in which parallel light is focused behind the retina.
Hypoxia- relative lack of oxygen; causes corneal physiological problems (e.g., corneal edema).
Intraocular lens- abbreviated to IOL; plastic lens implanted in place of the crystalline lens during cataract surgery.
IOL- abbreviation for intraocular lens.
Ionic- possessing either an overall positive or negative electric charge; such lens polymers are more reactive and tend to be more prone to deposit formation.
Iris- the pigmented tissue behind the cornea which gives eye color and dilates and contracts to control pupil size.
Keratoconus- pathological cone-shaped protrusion of the cornea; contact lenses provide clearer vision than spectacles.
LASIK (laser assisted in-situ keratomilieusis)—Laser vision correction in which part of the cornea is ‘evaporated away’ so that light focuses better within the eye. Clickhere for more information.
Lathe cutting- contact lens manufacturing method in which dry buttons of the lens material are cut with lathes; most labor intensive and least reproducible method of contact lens manufacture, but useful for production of small quantities or custom parameters.
Lens deposits- see Deposits.
Lens intolerance- discomfort during contact lens wear; can involve foreign body sensation, light sensitivity, blurred vision; often due to overwear of contact lenses, especially PMMA lenses.
Lens vial- small glass container in which many types of soft contact lenses are shipped.
Light transmission- the amount of incident light transmitted through a contact lens material; also known as transparency.
Limbus- anatomical location where the cornea and sclera meet; contains many blood vessels and is a critical area in contact lens fitting for indicating the success of a fit.
Low vision- impaired vision with a significant reduction in visual function which is not fully correctable with conventional vision correction devices (i.e., spectacles or contact lenses).
Microbial contamination- soiling, staining, or infection by contact or association with microorganisms.
Microorganism- an organism of microscopic size; usually refers to bacteria, virus, yeast and fungi.
Monovision- a correction therapy for presbyopia using contact lenses; one eye is fitted for distance, the other for near vision.
Myopia- also known as near or shortsightedness; condition in which rays of light from distant objects are focused in front of the retina, causing blurred vision.
Nonionic- the absence or lack of an overall electric charge; such lens polymers are relatively inert.
Opaque tint- tint applied to a contact lens which blocks all light passage; allows total eye color change (e.g., from brown to blue eyes).
Oxygen permeability (Dk)- the amount of oxygen diffusing through a given amount of lens material in a given amount of time, under specified testing conditions.
Oxygen transmissibility (Dk/L)- the amount of oxygen expected to pass through an actual lens; calculated by dividing the oxygen permeability (Dk) by the center thickness of the lens (L).
Oxygen transmission- general term referring to the passage of oxygen through a contact lens or lens material.
Palpebral conjunctiva- the mucus membrane lining the inside of the eyelids; major source of oxygen to the cornea during sleep.
Peripheral vision- the outer part of the field of vision. Opposed to central vision.
Physiological- pertaining to the functions and activities of living matter (organs, tissues, cells).
Planned replacement- the scheduled replacement of contact lenses at periodic intervals as determined by the contact lens practitioner; also called programmed replacement. Usually 3 or 6 months.
Plano- having no optical power; light passes through without being deviated from its original path or focused.
PMMA- abbreviation for polymethylmethacrylate. The original hard lens material. Problems developed in patients wearing these lenses due to a lack of oxygen transmission.
Presbyopia- gradual loss of the ability to focus clearly for near vision (accommodation) with onset in middle age.
Prosthetic- pertaining to an artificial device to replace a missing part of the body.
Pupil- the round, black contractile aperture of the eye; surrounded and defined by the iris; located in the anterior chamber behind the cornea.
Refractive error- condition where parallel light entering the eye is not clearly focused on the retina, resulting in blurred vision.
Refractive index- the ratio of the speed of light in air to the speed of light in the specified material. Lens materials with a higher index can be made thinner yet provide the same prescription.
Retina- sensory membrane that lines the posterior chamber of the eye; converts light to electrochemical impulses for transmission to the brain to complete the visual process.
Rigid Gas Permeable lens (RGP)- a rigid lens that allows the passage of oxygen molecules. Safer than original PMMA lens.
Soft contact lens- contact lens made from a hydrogel (hydrophilic) material; such lenses are soft and flexible, and offer the best initial comfort and shortest adaptation times.
Spectacle blur- blurred vision with spectacles after removal of contact lenses; usually due to corneal edema or improper fit of rigid/hard contact lens.
Spin casting- method of contact lens manufacturing in which liquid polymer is poured into a spinning mold; offers benefits of high reproducibility and high-volume production.
Sterilization- process of killing all vegetative organisms and spores by boiling water, steam, or dry heat; no regrowth of organisms can occur.
Strabismus- inability of one eye to obtain binocular vision with the fellow eye; usually due to imbalance of the muscles of the eyeball. Commonly called crossed eyes.
Tear exchange- the action of fresh tears being pumped behind a contact lens during blinking; estimated to be from 14% to 20% with hard lenses and 1% to 5% with soft lenses.
Tear film- the clear saline fluid and oils covering the front surface of the eye and secreted by the lacrimal and accessory glands.
Tear pump- system by which fresh tears flow under and behind a contact lens when blinking. Pressure from the eyelids and the movement of the contact lens create pressure which pushes old tears from behind the lens and pulls fresh tears under the lens to flush away any debris that might accumulate there.
Therapeutic lens- see Bandage Lens.
Tinted Lenses- contact lenses which have either a transparent or an opaque color applied to the lens for the purpose of visibility, eye color enhancement, or eye color change.
Topical medication- a medication (pharmaceutical) that is applied externally.
Toric lens- a lens with two different optical powers at right angles to each other for the correction of astigmatism.
Transparency- the clearness (clarity) of a material; denoted as a percentage of incident light passing through a sample of the material.
Water content- the percentage volume of water in a lens material by weight; also known as hydration.
Wettability- the degree to which water or other liquid will spread evenly across a surface.