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Eye and Vision Glossary

Not everyone has a dictionary handy for tricky eye care words and terms. See below for popular terms and definitions used by both vision professionals, as well as AC Lens.

Lens Measuring & Prescription Terms

Add Power
Add Power refers to the near-distance corrective power of a bifocal lens. Add power is always a +, and may be referred to in numeric strength values, or simply as "high, medium, low".

The axis is a measurement for where additional power is placed on a Toric style lens, which is set to align with the placement of where an eye is mis-shaped due to Astigmatism.

Cylinder (Cyl) is a measurement term that refers to additional corrective power needed to fix astigmatism. Cyl is always a negative value in lens prescriptions.

Diameter refers to how wide your contact lens is. Diameter is generally within one digit of 14.x when being measured for most standard contacts.

OD refers to the right eye in lens prescriptions.

OS refers to the left eye in lens prescriptions.

OU refers to both eyes in lens prescriptions.

Power refers to the corrective value a lens has. Positive powers correct Hyperopia, while negative power values correct Myopia. Larger powers offer greater vision correction, and powers typically are measured in intervals of diopters, usually spaced .25 diopters apart.

Eye Conditions & Medical Terms

Astigmatism is a condition where the surface of the eye is mis-shaped, creating a distorted sense of vision. Astigmatism is typically corrected via toric lenses.

Cataracts are a clouding of the eye's lens, which can cause myopia in addition to clouding, and eventual vision loss. Cataracts are typically treated via surgical procedures.

Glaucoma is an eye disease that effects the optic nerve. Glaucoma is usually caused by increased intraocular pressure, and can lead to permanent blindness if left untreated.

Farsighted is medically referred to as hyperopia, and refers to an eye's inability to see close-range objects, in spite of distance vision being normal.

Hyperopia, also known as farsightedness is a condition where one can see objects in the distance clearly, but has trouble focusing on close-range objects.

Mono Vision Technique
Mono vision, or mono vision technique is a popular way to correct presbyopia (problem with near and far sight) by issuing different lens strengths for separate eyes. The technique assigns one eye to distance viewing, and the other eye to short-range vision in place of using bifocal or progressive lenses.

Myopia, more commonly referred to as nearsightedness refers to the eye condition where one doesn't have the ability to focus on objects in the distance, but can see close-range objects with clarity.

Nearsighted is another term for Myopia, where one doesn't have the ability to focus on objects in the distance, but can see close-range objects with clarity.

Presbyopia refers to a condition where the eye loses it's ability to focus on objects within a close range. Presbyopia is generally caused by a stiffening of the eye tissue, and is corrected using bifocal and multifocal lens styles.

Types of Lenses For Contacts and Glasses

Aspheric Lens
Aspheric lenses are eyeglass lenses that do not use a traditional spherical shaped lens to alter vision. Aspheric lenses are notable for giving better peripheral vision, while also using less material for a thinner lens.

Bifocal Lens
Bifocal Lenses are used to alter one's vision for short and long range viewing applications. Bifocal lenses utilize two different lens powers, with the short-distance lens typically being located at the bottom of the lens, and the distance viewing power located at the top of the lens.

Color Contacts
Colored contacts refer to contact lenses that can alter the eye's color via placing a tint or opaque color on the contact lens itself, effectively covering up or enhancing one's natural eye color.

Concentric Bifocal Lenses
Concentric bifocal contact lenses are a variation of bifocal lens styles that use rings of varying corrective power to correct long and short term vision. Concentric bifocal contacts typically will have distance viewing power in the center of the lens, and short-range viewing power in the outer edge of the lens.

Conventional Contact
Conventional contacts refer to traditional soft contact lenses that are generally designed to be worn for over three months. Conventional contacts are frequently referred to as vial contacts due to the vials they are packaged in.

Daily Wear
Daily wear refers to the frequency that a contact lens is designed to be worn for, and more specifically refers to a contact that can not be worn while sleeping.

Enhancement Tint (Enhancers)
Enhancement tinted lenses are a variation of colored contact lenses, that feature a lighter, translucent color that enhances the eye's natural color instead of fully covering up the eye.

Extended Wear
Extended wear refers to the frequency to how contacts can be worn, and more specifically, refers to a contact that can be worn while sleeping as well as awake.

High Index Lens
High index lenses are eyeglass lenses that are made from material with a high rate of refraction. By using material that has higher refractive properties, glasses can be manufactured for more severe vision problems without added bulkiness and weight. High Index lenses are often paired with Aspheric Lens styles to create a lens that uses less than 50% of the material that a normal lens would need.

High Definition Lens
High definition lenses are custom manufactured eyeglass lenses, that give greater optical clarity, reduced distortion, and reduced glare.

Multifocal Lens
Multifocal lenses refer to lens styles that feature more than two separate powers throughout the lens. Concentric, trifocal, and progressive lenses are all types of multifocal lenses.

Plano refers to a lens that has no corrective power. Plano lenses traditionally serve cosmetic purposes only.

Progressive Bifocal
Progressive bifocal lenses refer to lenses that correct short range and distance vision, but use a gradient lens curvature to adjust the focal power of a lens, instead of using definitive boundaries between areas with a higher corrective power on a lens.

Rigid Gas Permeable
Rigid Gas Permeable lenses are hard contact lenses that are manufactured for extended wear. Rigid gas permeable lenses are also known as gas permeable or RGP lenses, and are known to offer a greater vision clarity than soft contacts. Drawbacks to rigid lenses include initial discomfort when getting accustomed to wearing the lens, and

Spherical Lens
Spherical lens styles refer to the standard corrective lens, that follows a spherical curvature for vision correction.

Toric lens styles are utilized to correct astigmatism in addition to other vision problems.

Vial contacts, also known as conventional contact lenses, are standard long-term use soft contacts. Vial contacts are well known for the vial type containers that they are packaged in, and are generally though to offer a slightly higher visual clarity than disposable contacts would.

Non-Prescription Glasses
Non prescription glasses refer to any eyeglasses that do not require a prescription to purchase. Most non-prescription glasses are categorized as reading glasses, and typically offer mild, non prescription vision correction for light use.

Types of Eye Doctors and Vision Professionals

Opthalmologists are fully licensed eye doctors. Opthalmologistshave to attend medical school and residency to obtain certification. Opthalmologists prescribe lenses, diagnose disease, perform surgery, and may even perform some plastic surgery.

Opticians are eye-care professionals that can't prescribe lenses or perform eye surgery. Opticians frequently will dispense lenses and eyecare products, and are usually in charge of ordering eye care products and filling prescriptions.

Optometrists are eye care professionals who have obtained a level of certification and training so that they can prescribe lenses, diagnose eye problems, and perform tests, despite not having attended medical school to be a fully certified Opthalmologist.

Other Information

To learn about how your eyes work, eye anatomy, and the specific parts of your eye, visit our eye anatomy page.