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A Glossary of Vision Terms

The following are terms, with their definitions, that are common to the Vision Direct site, contact lens wearers and Eye Care Professionals.

Not spherical, departs from a spherical form especially in order to correct for a spherical aberration. Aspheric eyeglass lenses are often prescribed for people who require strong prescriptions because the lenses are thin and lightweight and reduce distortion and eye magnification. In contact lenses, an aspheric lens may be used to correct a single-vision problem like astigmatism (multifocal lenses).

This is a condition in which the cornea's curvature is asymmetrical (the eye is shaped like an egg) and results in blurred vision. Astigmatism is one of the most common vision problems affecting people with farsightedness or with nearsightedness. Usually it is caused by an irregularly shaped cornea (called corneal astigmatism). But sometimes lenticular astigmatism results from an irregularly shaped lens, which is located behind the cornea.

Base Curve:
In a contact lens the base curve is the curvature of the back surface (the surface that is in contact with the eye). Base curve is often abbreviated as (BC). Typical BC values, seen in a contact lens, range from 8.0 to 10.0 mm. The base curve number is important in order to allow the contact lens to fit well to a patient's cornea for prolonged comfort, to permit tear exchange, and to enable oxygen transmission. Base curve values are prescribed to match or complement the curvature of your cornea. The lower the base curve number is the steeper the curve of your cornea.

Bifocal Lens:
A bifocal lens is a lens with one segment for near vision and one segment for far vision.  The term bifocal or bifocal lens applies to both eyeglasses and contact lenses.

This is the ability of an artificial material to coexist with living tissues without harming them. As an example, contact lenses are designed to be biocompatible with tissue inside the eye so they won't cause an allergic or immunological reaction that will harm the eye.

This is the clouding of the lens in the eye and leads to a decrease in vision.  Cataracts are usually the result of aging but may occur sooner due to other factors, such as over exposure to UV rays (sunlight), smoking, some steroid medications and diabetes. Typical symptoms include blurred vision, glare, halos around lights, colors that are less bright and a cloudy spot in your vision.

Color Tint Lenses:
A type of lens can be used to change the color of your eye. The lens has an opaque tint that will alter your true eye color completely. The tint consists of a pattern of solid colors to simulate the natural lens. Color contacts come in a wide variety of colors including, but not limited to, amber, blue, green and gray. Color tinted (colored contact) lenses are distinct from an Enhancement Tint lens.

Computer Vision Syndrome (CVS):
A variety of problems, mostly affecting a person's eyes and vision, associated with prolonged use of digital display devices. Typical symptoms include redness of the eyes, eye strain, dry eyes, blurred vision, burning sensation, sensitivity to light, headaches, pain in the shoulders, neck and back.

Concentric Bifocal Pattern Lens:
This is the most commonly used lens for bifocal and multifocal lenses. With a concentric bifocal pattern contact lens, the near correction is located in a small circle at the center of the lens and is surrounded by a larger circle that contains the distance correction. This can be flip-flopped where the distance correction is in the center and the near correction is in the outer ring.

Contact Lens Case:
A storage container used to clean and store daily wear contact lenses.  Contact lens cases come in a variety of styles and types.  The AOA (American Optometric Association) recommends the replacement of your case every three months to prevent bacterial contamination that builds up over time and which cannot be very difficult to clean.

Contact Lens Solution:
Products specifically designed for the proper maintenance of a contact lens. The use of water as a cleaning agent will not adequately disinfect the lens and can lead to contamination of the lens, and could result to harm to the eye. Some solutions must only be used with certain types of contact lenses. Contact lens solutions often contain ingredients formulated to work with specific types of lenses that do not damage the lens material.

The transparent front part of the eye that covers the iris and pupil.

Daily Disposable Lens:
A single use contact lens you dispose of at the end of the day. This type of lens does not require cleaning or disinfecting.   An ideal solution if you wear contacts infrequently or in situations not compatible with the necessity to perform the required daily maintenance of a daily wear or extended wear lens.

Daily Wear (DW):
A contact lens you wear during the day and then remove them each night for cleaning and disinfection. Daily wear lenses typically come in two types, a weekly lens (effective up to 14 days) and a monthly (effective up to 30 days).  Type availability varies by manufacturer.  The lens is disposed of after the effective period expires and is replaced with a new lens.

Disposable Contact Lens:
This is any type of contact lens that is thrown away after a short period of time. A disposable lens period of usage ranges from one day and up to 30 days (monthly).

Dry Eye:
This is a lack of sufficient lubrication and moisture in the eye. Dry eye complaints, typically associated to extended wear contact lens use, tend to be temporary and easily relieved through use of a rewetting solution.
Not the same as dry eye syndrome which is chronic and needs more advanced treatment by an eye care practitioner.

Enhancement Tint:
These contacts are designed to enrich, but not dramatically change the natural hue of your eyes. This type of lens is ideal if you want to make your original color deeper or more intense. These tinted lenses are ideal for people who have lighter hues in general and do not work well for those with dark eyes.

Enzymatic Cleaner:
A cleaner that removes protein deposits as well as other debris from contact lenses, most commonly used with rigid gas permeable (RGP) lenses.

Extended Wear (EW):
This is a type of contact lens that can be worn during sleep or for more than 24 hours. Extended wear lenses must still be removed for cleaning and disinfecting at least once a week or as directed by the manufacturer or your eye care professional.

Eye Care Practitioner:
A broad term used to describe most individuals involved in the various eye practices. Practitioners include medically trained doctors, such as an Optometrists or Ophthalmologists, but can include non-doctor (certified) eye care practitioners that include contact lens technicians and opticians.

Eye Care Professional:
A specific term used to describe medically trained eye care practitioners such as an Optometrists (ODs) and ophthalmologists (MDs).  ODs (Doctor of Optometry) examine eyes for both vision and health problems, prescribe eyeglasses, prescribe and fit contact lenses, and treat some eye conditions and diseases. An OD will attend four years of optometry school after attaining their BS or BA college degree. MDs (Doctor of Ophthalmology) are medical doctors who specialize in the eyes. They examine eyes, treat disease, perform surgery, and prescribe glasses and contacts. Like other physicians, they complete a BS or BA degree, attend four years of medical school, and complete a residency program in their practice specialty.

Common term used to describe Hyperopia. This is a condition where the length of the eye is too short, causing light rays to focus behind the retina rather than on it. Farsightedness results in blurred near vision.

FDA (Food & Drug Administration):
The U.S. government body that oversees medical devices and medications which includes contact lenses, lens solutions and eye drops. In the United States, these products must be approved by the FDA before they can be marketed.

Frequent Replacement (FW):
An alternate term sometimes used to describe contact lenses that are discarded after a month or up to 3 months of use. Some eye care practitioners use this term to distinguish between a lens used one day to two weeks ("disposable")  from that which is discarded each month or quarterly (“frequent replacement").

Handling Tint:
Lenses that say “handling tint” are slightly tinted to make them more visible for both care and insertion of the lens. The tint on these lenses has no effect on eye color.

Hard Contact Lens:
This a type of contact lens, not commonly worn now, that many people wore in the 1970s and 1980s. Compared against modern soft lenses and rigid lenses, they are less healthy to wear long-term, as the manufacturing materials used do not allow oxygen to reach the surface of the eye.

High-Index Lens:
A High-Index lens is a type of lens with a higher index of refraction. This means that unlike traditional glass or plastic, light can travel faster through the lens to reach the eye.

Hybrid Multifocal Lens:
A multifocal contact lens that is soft around the peripheral (typically a hydrogel material) with a rigid gas permeable center.

This is the medical term for farsightedness. This is a condition where the length of the eye is too short, causing light rays to focus behind the retina rather than on it. Farsightedness results in blurred near vision.

Intermediate Vision:
Term generally used to describe a person’s eyesight at approximately arm's length.  Used for tasks such as computer work and viewing a car dashboard. See also: near vision and distance vision.

A pigmented membrane that lies between the cornea and the lens, it acts as a diaphragm to widen or narrow the opening of the pupil, thereby controlling the amount of light that enters the eye.

This is the degeneration and thinning of the cornea resulting in a cone-shaped bulge (a type of irregular astigmatism) of the eye. The cause is unknown. Initial stages of this disorder are often treated with soft contact or rigid gas permeable contact lenses.

1. The nearly spherical body in the eye, located behind the cornea that focuses light rays onto the retina.
2. A device used to focus light into the eye in order to magnify or minify images, or otherwise correct visual problems. Eyeglass lenses, contact lenses, and intraocular lenses are examples.

It is an antioxidant that is found throughout the body but is concentrated in the macula, a small area of the retina responsible for central vision. Lutein is believed to help protect the eyes from free radical damage caused by the sun's harmful rays. Lutein is a natural part of human diet where fruits and vegetables are regularly consumed.  It is also available as a supplement.

The most sensitive part of the central retina, found inside the eye, it is responsible for visual acuity and color vision.

Mixed Astigmatism:
This is an abnormal curvature of the eye's surface (the cornea) that causes focusing problems with both near and distant ranges.

Is a type of spectacle lens, intraocular lens (IOL) or contact lens design that has only one area through which the eye focuses. It differs from a multifocal lens which has more than one focal area enabling sight at multiple distances.

This is a method of vision correction for people with presbyopia where one eye is corrected for near vision and one eye for far.

Multifocal Lens:
A type of spectacle lens (eyeglasses), intraocular lens (IOL) or contact lens design that includes more than one area through which the eye focuses. Examples include bifocals or trifocals. This enables sight at multiple distances, typically for people with presbyopia.

The medical term used to describe nearsightedness. A condition in which the length of the eye is too long causing light rays to focus in front of the retina rather than on it.  Myopia results in blurred distance vision. Additional symptoms can include eye strain, poor night vision and squinting.

Is a commonly used term for myopia. A condition in which the length of the eye is too long causing light rays to focus in front of the retina rather than on it.  Nearsightedness results in blurred distance vision but objects near are in focus.

Near Vision:
Eyesight used for reading and other close-up tasks, generally at a range of 12 to 16 inches from the eyes.

1. Abbreviation for Oculus Dexter, the Latin term for the right eye, used in vision correction prescriptions.
2. Doctor of Optometry.

This is a medical doctor (MD) who is a specialist in medical and surgical eye problems. Ophthalmologists perform eye exams, treat disease, prescribe medication, and perform surgery. They may also write prescriptions for eyeglasses and contact lenses.

Opticians are not doctors (United States) and generally must complete formalized training and obtain a license. In some states an optician can obtain certification to fit contact lenses. Most opticians sell and fit eyeglasses, sunglasses, and specialty eyewear made to an optometrist's or ophthalmologist's prescription.

Doctors of Optometry (ODs) examine eyes for both vision and health problems, prescribe glasses, and fit contact lenses. They can prescribe some medications and may participate in pre- and postoperative care associated to eye surgery.

Abbreviation for Oculus Sinister, the Latin term for the left eye, used in vision correction prescriptions.

Abbreviation for Oculus Uterque, the Latin term for each eye, used in vision correction prescriptions to indicate both eyes.

Plano (Lens):
A term eye care professionals use to describe lenses with no corrective power. The term is most often applied to nonprescription sunglasses or contact lenses that are worn for cosmetic purposes only.

The natural loss of close-up vision as one gets older. Presbyopia is caused by inflexibility of the eye's lens, a condition that prohibits the eye from focusing properly on objects up-close.

Prescription Lenses:
Lenses, either eyeglasses or contact lenses, that provide vision correction as prescribed by an eye care practitioner.

The round, dark center of the eye, which opens and closes to regulate the amount of light the retina receives.

Reading Glasses:
Glasses to help with close work, particularly for people who are presbyopic, also called readers.

Replacement Schedule:
The rate at which you discard and replace your contact lenses: every day, week or two weeks (daily wear); or every month, two months or calendar quarter (frequent replacement). It's important to differentiate between your replacement schedule and your wear schedule. Wear schedule is either during the daily wear (removed before sleeping) or extended wear (you may sleep with them in).

This is the light-sensitive layer of tissue lining the inner surface of the back of the eye. Cells in the retina called photoreceptors transform light energy into electrical signals that are transmitted to the brain by way of the optic nerve.

Rewetting Drops:
Eye drops designed to re-moisten and lubricate contact lenses while they are being worn, to increase comfort.

Rigid Gas Permeable (RGP):
Type of contact lens made of breathable plastic that is custom-fit to the shape of the cornea. RGP lenses are the descendant of hard lenses.  More durable than soft lens contacts, the lens can last for a year or more before requiring replacement.  RGP lenses can be as comfortable as a soft lens but typically require a short period of time for acclimation.  Usually made to order, RGP lenses can offer superior clarity over soft lenses.

This is the outer coat of the eyeball that forms the visible white of the eye and surrounds the optic nerve at the back of the eyeball.

Snellen chart:
The standard chart, with letters, numbers, or symbols printed in rows of decreasing size.  Used by eye care professionals in distance visual acuity testing. The chart is named after its inventor, the Dutch ophthalmologist Hermann Snellen.

Soft Contact Lenses:
A corrective contact lens made from a mix of gel-like plastic polymers and varying amounts of water.

The most common type of contact lens design and is in the form of a sphere. In contrast, a toric lens for astigmatism is football-shaped and less common.

Theatrical Lenses:
A contact lens that will apply a special effect or color to the eye.  Theatrical contacts typically have a unique pattern or vibrant color applied in the manufacturing process, they can come in corrective and plano powers.  This type of lens, in non-corrective (plano) powers, is considered a medical device (United States) and requires a prescription to purchase.

This is a lens design with two different optical powers, at right angles to each other, used for the correction of astigmatism.

Visibility tints:
This is usually a light (blue or green) tint which is added to a contact lens to help you better see the lens during insertion, removal or in the event you drop the lens. Since the tint is very light it will not affect your eye color.

Visual Acuity:
Your clarity of vision as measured on the standard Snellen eye chart. Baseline acuity is considered 20/20 (6/6 when used in meters) and means an uncorrected eye with is able to distinguish lines that are 1.75 mm apart.  If your vision is 20/40 the 40 value means that a person, without aid, can read the chart (from 20 feet away) as well as a normal person could read the same chart from 40 feet away.  Visual acuity values (20/20, 2040, etc.) do not relate to lens prescriptions as it does not describe the problem with the eye.

Wear Schedule:
Wear schedule is the length of time you wear your contact lenses per day – either daily wear (removed before sleeping) or extended wear (you may sleep with them in).


Information on this site is provided for informational purposes only and is not meant to substitute for the advice provided by your own eye care or other medical professional. Have your eyes examined regularly and always follow your eye care professional’s instructions for the proper use and care of your contact lenses. You should not use the information contained herein for diagnosing or treating a health problem or disease, or prescribing any medication. You should read carefully all product packaging. IF YOU ARE HAVING ANY UNEXPLAINED EYE DISCOMFORT, WATERING, VISION CHANGE OR REDNESS, REMOVE YOUR LENSES IMMEDIATELY AND CONSULT YOUR EYE CARE PROFESSIONAL BEFORE WEARING YOUR LENSES AGAIN.

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