Vision Glossary

20/20 – the expression for normal eyesight (or 6/6 in countries where metric measurement are used).  This notation is expressed as a fraction.  The numerator (1st number) refers to the distance you were from the test chart, which usually 20 feet *6 meters).  The denominator (2nd number) denotes the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read.  For example, if your visual acuity is 20/100 that means that the line you correctly read at 20 feet could be read by a person with normal vision at 100 feet.  The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight).  A refraction test is used to determine the amount of correction needed for a prescription when treating refractive error such as astigmatism, myopia, or hyperopia.

Accommodation – (eye focusing) the eye’s ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power.  When the accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera.  The ciliary muscles must contract to adjust for near vision, which causes the eye’s crystalline lens, which is flexible, to be squashed.  For distant vision, the ciliary muscle must relax and the eye’s crystalline lens is stretched out.  The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia.

Acuity – clearness of eyesight.  Depends on the sharpness of images and the sensitivity of nerve elements in the retina.

Add – prescription strength of a plus lens which is used for near vision.  A plus lens can be added to another lens such as a minus lens for distance vision.

Anisometropia – the condition in which the two eyes have different refractive powers.

ANSI z87.1 – The American Nation Standards Institute’s Practice for Occupational and Educational Eye and Face Protection; eyewear that meets this standard is considered safer than eyewear that does not.

Anterior Chamber – Part of the eye behind the cornea and in front of the iris lens.

Antireflective coating (AR coating) Thin layers(s) applied to a lens to reduce the amount of reflected light and glare that reaches the eye.

Aqueous Humor – Clear fluid in the eye that both provides nutrients and determines intraocular pressure.

Aspheric – Not quite spherical. Aspheric eyeglass lenses are popular among people who have strong prescriptions because they are thin and lightweight, and reduce distortion and eye magnification.  Aspheric contact lenses can work as a multifocal, or to correct a single-vision problem like astigmatism.

Astigmatism – light rays entering the eye do not all meet at the same point (similar to a frayed string), which results in blurred or distorted vision.   An abnormally shaped cornea typically causes this condition.  Occasionally astigmatism exists in the lens of the eye.  This condition is corrected by a cylindrical (toric) eyeglass or contact lens.

Bifocal Glasses – used to correct vision at two distances, composed of two ophthalmic lenses such as a plus lens for near vision and a minus lens for distance vision.

Blurred Vision – lack of visual clarity or acuity.

Bridge- The part of eyeglasses that extends across the nose.

Cataract – a condition of the crystalline lens, in which the normally clear lens becomes clouded or yellowed, causing blurred or foggy vison.  Cataracts may be caused by aging, eye injuries, disease, heredity, or birth defects.  Surgery is a treatment option.  The affected lens is removed and is replaced with a substitute (implant) lens or with a special type of contact lens.  Generally the success rate of cataract surgery is over 90% if the eye is otherwise healthy.

Computer Vision Syndrome (CVS) the complex of the eye and vision problems related to near work that are experienced during or related to computer use.  Its symptoms include eyestrain, dry or burning eyes, blurred vision, headaches, double vision, distorted color vision, and neck and backaches.  This condition is caused by various internal and external factors.  Treatment options may include prescription glasses and/or vision therapy.

Cone – a receptor cell which is sensitive to light and is located in the retina of the eye.  It is responsible for color vision.

Conjunctiva – Mucous membrane that lines the visible part of the eye and the inner surface of the eyelid.

Conjunctivitis – an inflammation of the conjunctiva, the transparent layer covering the inner eyelid and the white portion (sclera) of the eyeball.  Conjunctivitis can be caused by a virus, bacteria, or fungus (infectious conjunctivitis, or, “pink eye”, may be contagious); by allergies to pollen, fabrics, animals, or cosmetics (allergic conjunctivitis); or by air pollution or noxious fumes such as swimming pool chlorine (chemical conjunctivitis).  Symptoms include red or watery eyes, blurred vision, inflamed inner eyelids, scratchiness in the eyes, or (with infectious conjunctivitis) a puss like or watery discharge and matted eyelids.  Conjunctivitis is usually treated with antibiotic drops and/or ointment.

Convergence – the ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.

Cornea – the transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye.  Contact lenses are fitted over the cornea.

Corneal Abrasion – Tearing or puncture of the cornea.  Usually causes pain, tearing, light sensitivity and a feeling that something is in the eye.

Corneal Ulcer – Wound in the surface of the eye caused by injury, dryness due to lack of tear production, or infection.

Cylinder Lens – an ophthalmic lens that has at least one non-spherical surface.  Used to correct astigmatism.  The values are typically from -0.75 to -1.25.  The cylinder measurement is given with a “-“ sign.  (Please note that the sign for myopia (nearsightedness) is also “-“).

Dacrostenosis – Blocked tear duct, which is characterized by a lot of tearing.

Depth Perception – the ability to judge relative distances of objects.

Diabetic Retinopathy – Leaking of retinal blood vessels in advanced or long-term diabetes, affecting the macular or retina.  Vision can be seriously distorted or blurred.

Diopter (D) – a measurement of the refractive (light bending) power of a lens or a prism (pd).  The strength of prescription glasses and contacts are measured in these unites.  For example, a lens that is 0.50 diopter (D) is very weak, whereas a lens that is 10.0 diopter (D) is very strong.

Diplopia – a single object is perceived as two rather than one; double vision.

Distance Acuity – the eye’s ability to distinguish an object’s shape and details at a far distance such as 20 feet (6 meters).

Divergence – the ability to use both eyes as a team and be able to turn the eyes out toward a far object.

Dominant Eye – the eye that “leads” its partner during eye movements. Humans also have dominant hand, foot, eye, and side of the brain (not necessarily all on the same side).

Dry Eye – Lack of sufficient lubrication and moisture in the eye.  Most dry eye complaints are temporary and easily relieved; dry eye syndrome is chronic and needs more advanced treatment by an eye care practitioner.

Emmetropia – normal vision, no correction needed.

Extraocular Muscles – the muscles attached to the outside of the eyeball which control eye movement.  Each eye has six muscles (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, and inferior rectus) that are coordinated by the brain.

Facility of Accommodation – a measure of the ease and speed of the eye(s) to change focus.

Farsighted – Also called hyperopia. To farsighted people, near objects are blurry, but far objects are in focus.

Floaters – also known as spots, are usually clouded or semi-opaque specks or particles within the eye that are seen in the field of vision.  The eyes are filled with fluid with maintains the shape of eye, supplies it with nutrition and aids in the focusing of light.  Often, particles of protein or other natural materials are left floating or suspended in this fluid when the eye is formed before birth.  If the particles are the large or close together, they cast shadows which make them visible.  This is particularly true when nearsightedness occurs or becomes more severe.  In most cases this is normal but floaters can also be caused by certain injuries, eye disease or deterioration of eye fluid or its surrounding structures.

Form Constancy – the ability to recognize two objects that have the same shape but different size or position.  This ability is needed to tell the difference between “b” and “d”, “p” and “q”, ”m” and “w”.

Glaucoma – Disease characterized by excessive fluid (aqueous humor) in the eye, high intraocular pressure and vision impairment.  Blindness can result.

Heterophoria – tendency of the eye to deviate from their normal positon for visual alignment.  This condition may be observed when one eye is covered.

High Index – Type of lens with a higher index of refraction, meaning that light travels faster through the lens of reach the eye than with traditional plastic.  It is denser, so the same amount of visual correction occurs with less material (whether glass or plastic), so the lens can be thinner.

Hyperopia – farsightedness, an individual will have difficulty SEEING CLEARLY UP CLOSE.  Light entering the eye focuses behind the retina when the eye is at rest and is corrected with a plus lens.  Vision therapy is not prescribed for hyperopia.  Children, up to about the age of 8 years, are often farsighted.

Hyperophoria – a condition in which one eye has a tendency to point higher than the other eye, causing eyestrain.  Sometimes improved by prisms in glasses.

Hypertropia – strabismus, one eye turned in an upward direction.

Hypophoria – a condition in which one eye has a tendency to point lower than the other eye.  This condition may be observed when one eye is covered.

Intraocular Pressure (IOP) Eye pressure, as determined by the amount of aqueous humor filling it.  High IOP (ocular hypertension) can be a sign of glaucoma.

Iris – the colored part of the eye located between the lens and cornea; it regulates the entrance of light.

Iritis – Inflammation of the iris.

Keratitis – Inflammation of the cornea.

Keratonoconus – condition in which the cornea develops a cone-shaped bulge that can result in major blurring and distortion

Lensometer – also called Verometer, is a device used to measure the refractive power of eyeglasses and contact lenses.

Lipid – fatlike substance that can collect on contact lenses, making tem uncomfortable.

Low Vision – Also called partial sight.  Sight that cannot be satisfactorily corrected with glasses, contacts, or surgery.  Low vision usually results from an eye disease such as glaucoma or macular degeneration.

Macula – the most sensitive part of the retina that is about the size of a pinhead and is where our most detailed vision occurs.

Macular Degeneration – a deterioration of the central portion of the retina known as the macula

Minus (-) Lens – concave lens, stimulates focusing and diverges light. The lens is thinner in the center than the edges.  It is used in glasses or contact lenses for people who are nearsighted (myopia).

Monocular Vision – only one eye having useful vision

Myopia – nearsightedness, an individual will have difficulty seeing clearly at distance.  Light entering the eye focuses in front of the retina when the eye is at rest and is corrected with a minus lens.  A condition known as high myopia occurs when myopia is greater than 6 diopters.  Typically, vision therapy is not prescribed for myopia.

Near Acuity – the eye’s ability to distinguish an object’s shape and details at a near distance such as 16 inches (40cm).

Nearsighted – Also called myopia.  Condition in which visual images come to a focus in front of the retina, resulting in defective vision of distant objects.

Nickel – Metallic element used mainly in alloys.  Many eyeglass frames are made of nickel alloy, so people who are allergic should choose a hypoallergenic substitute, such as titanium.

Occlusion – to block out light. An eye can be completely or partially blocked.  This procedure is used to promote the use of one eye or both eyes.  This therapy procedure may be used for people with amblyopia, strabismus, or closed head trauma.  It may also be used in a vision therapy program for someone with amblyopia, eye focusing (accommodation) disorder, or poor eye tracking (oculomotor) skill.  An eye patch, black contact, or another device may be used to block out light from an eye.

Oculus Dexter (OD) – right eye.

Oculus Sinister (OS) – left eye.

Oculus Uterque (OU) – both eyes.

Optic Nerve – is a bundle of nerve fiber that connects each eye to the brain and transmits images from the retina to the brain

Optician- is a professional in the field of designing, finishing, fitting and dispensing of eyeglasses and contact lenses, based on an eye doctor’s prescription. The optician may also dispense colored and specialty lenses for particular needs as well as low-vision aids and artificial eyes.

Overconvergence – Condition in which the eyes comes too far inward when focusing on a near object, resulting in blurring.

Peripheral Vision – the ability to see or be aware of what is surrounding us, our side vision.

Photochromic – Able to change lens color or darkness/density depending upon the degree of expose to light.

Photo Keratitis – “Sunburn” of the cornea; symptoms include discomfort, blurred vision, and light sensitivity.  The temporary vision loss that can result is called “snow blindness.”

Photophobia – Unusual sensitivity to light.

Plano Lens – a lens that has no prescription. No variance between the curvature of the front and back lens surfaces.  It is a flat lens.

Plus (+) Lens – convex lens (thick in the middle) relaxes focusing and converges light.  It is typically used in glasses or contact lenses for people who are farsighted (hyperopic).  Although it may also be prescribed for other visual conditions as well.

Polaroid Lens – a lens used in sunglasses and sometimes 3D glasses which consist of two glass or plastic surfaces with a plastic lamination between the two surfaces, and designed to reduce reflected glare.

Polycarbonate – Lens material that is very impact-resistant, thinner than plastic, and is used for spectacle lenses.

Presbyopia – sometimes called the fourth refractive error, is not truly a refractive error.  It is the natural process of the eye losing the ability to accommodate or change the shape of the natural crystalline lens inside the eye to see comfortably at near.  This vision defect occurs with the advancement of age; the onset usually occurs between the ages of 40 to 45.  Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life.  As the lens ages and grows it becomes harder in consistency, loses its elasticity, and therefore is resistant to changes in shape.  The result is a gradual reduction in accommodation (near eye focus), and a more dependence on reading glasses.  A plus lens or multi-focal lens, (such as a bi-focal lens) is prescribed in the form of glasses or contact lenses.

Prism – a wedge-shaped lens which is thicker on one edge than the other. This plastic lens bends light (opposite direction from its thicker end).  Prisms can be used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss.

Progressive Lenses - (also, progressive addition lenses or PALs) Multifocal lenses whose corrective powers change progressively throughout the lens.  A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work.  Each area is blended invisibly into the next, without the lines that traditional bifocals or trifocals have.

Pupil – the opening at the center of the iris of the eye.  It contracts (dilates) into the dark and when the eye is focused on a distant object.

Pupillary Distance (PD) – the distances between the pupils of the eyes, in millimeters – a necessary measurement for proper lens prescription.

Pupillometer – a device used to measure the distance between the pupils of the eyes, in millimeters, which is a necessary measurement for proper lens prescription.  It also measures the diameter of the pupil.

Refraction Test – determines the eye’s refractive error and the best corrective lenses to be prescribed.  There are several methods of performing refraction: Retinoscopy, Automated Refractor, and Subjective Refraction.

Refractive Error – defects in vision caused by the eye’s inability to bend or refract light and focus it clearly on the retina.  Astigmatism, hyperopia, and myopia are common conditions of refractive error, also called ametropia.

Refractive Power – a lens’ ability to bend parallel light rays into focus, as measured by power diopters.  In general, the greater the Curvature of a lens and the greater the difference between center thickness and edge thickness, the higher the index of refraction and the greater its refractive power.  Refractive power can also refer the strength of a person’s contact lenses or glasses.

Retina – the innermost layer of the eye, a neurological tissue, which receives light rays focused on it by the lens.  This tissue contains receptor cells (rods and cones) that send electrical impulses to the brain via the optic nerve when the light rays are present.

Retinoscopy- this technique determines the eye’s refractive error and the best corrective lenses to be prescribed.   An instrument called a retinoscope which consists of a light, lens, mirror, and handle, is used to shine light into a patient’s eye.  There are two types of retinoscope: streak and spot retiniscope.  When light is shone into the patient’s eye, the light is reflected back (‘reflex’).  If the reflection is in the same direction (‘with movement’) of the retinisocope then the refractive error is hyperopia (farsightedness) and a plus lens is prescribed.  If the reflection is in the opposite direction (‘against movement’) of the retinoscope then the refractive error is myopia (nearsightedness) and a minus lens is prescribed.  The strength of the prescription is determined when the pupil is suddenly filled with light (‘neutralized’) with the appropriate lens powers (strength).

Rigid Gas Permeable- (RGP) Type of contact lens made of breathable plastic that is custom-fit to the shape of the cornea.  RGPs are the successor to old-fashioned hard lenses, which are now virtually obsolete.

Slit Lamp (Biomicroscope) – this instrument can examine ocular tissue from the front of the cornea to the back of the lens.  A narrow “slit” beam of very bright light produced by a lamp.  This beam is focused on to the eye which is then viewed under magnification with a microscope.  A joystick control is employed to enable instrument to be moved left-right and up-down.  A chin rest, head rest and fixation target is also required.  Some slit lamps have a tilting mechanism to enable the lamp to be directed from different angles. 

Sphere – an ophthalmic lens with no cylindrical power or addition.  It has the same power in all parts of the lens.

Squint – to be unable to direct both eyes simultaneously toward a point.  Also known as strabismus (turned eye).

Strabismus – (clinical condition) turned eye (s), the eyes are misaligned.  It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury.  In strabismus, they eyes send conflicting images to the brain, and the brain cannot combine these images as it would in the other, causing a loss of depth perception.  Strabismus is more common in children, and affects four percent of all children (although it may also appear later in life).  Strabismus is also known as squint.  It may also be referred to as cross-eyes (convergent – turning inward) or wall eyes (divergent – turning outward).

Sty – A blocked gland at the edge of the lid which has become infected by bacteria.

Subconjunctival Hemorrhage – a blood clot on the eye.  It occurs when a small blood vessel under the conjunctiva (the transparent coating that covers the inner eyelid and the white of the eye) breaks and bleeds.  A common condition caused spontaneously from coughing, heavy lifting, or vomiting.  In some cases, it may develop following eye surgery or trauma.  It tends to be more common among those with diabetes, hypertension, and taking blood thinners (including aspirin).  A subconjunctival hemorrhage is essentially harmless.  The blood naturally absorbs within one to three weeks and no treatment is required.   If a mild irritation is present, artificial tear drops can be used.  You can speed up the healing process by applying cool compresses for the first two days and then warm compresses in the following days.

Subjective Refraction – the procedure in which the patient is asked to report on which lens combination provides the clearest vision.  While this is the method of choice for determining prescription in those able to understand the task and respond to the examiner, it is less reliable in children.

Titanium – a type of metal alloy that is very strong.  Eyeglasses made of titanium are lightweight, durable and often hypoallergenic.

Tonometry – an instrument that measures the pressure within the eye, which is known as intraocular pressure (IQP).

Toric – a lens design with two different optical powers at right angles to each other for the correction of astigmatism.

Ultraviolet – (UV) The invisible part of the light spectrum whose rays have wavelengths shorter than the violet end of the visible spectrum and longer than x-rays. UVA and UVB light are harmful to your eyes and skin.

Uvea – Middle layer of the eye, below the limbus, and consisting of the iris, ciliary body and choroid.

Uveitis – Inflammation of the uvea.

Vertigo – a disordered state in which the individual is dizzy or feels that the surrounding environment is whirling.

Visual Acuity – sharpness or clearness of eyesight.

Vitreous Body – A part of the eye between the lens and the retina, containing a clear jelly called a vitreous humor.

Wraparound – (wrap) Type of eyeglass frame that curves around the head, from the front to the side.  Wraparound sunglasses tend to offer extra sun protection because the lenses usually wrap as well.