Terminology

20/20 -the expression for normal eyesight (or 6/6 in countries where metric measurements are used). This notation is expressed as a fraction. The numerator (1st number) refers to the distance you were from the test chart, which is 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. See “Refraction Test”.  

anti reflective coatings -Anti-Reflective (AR) coatings consist of several layers of metal oxides applied to the front and back lens surfaces. Because of the layering effect, AR coatings sometimes have a hint of green or purple color. Each layer is scientifically calculated to block reflected light, so you'll see a reduction in glare, annoying reflections and halos around lights.

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.

cataract- a condition of the crystalline lens, in which the normally clear lens becomes clouded or yellowed, causing blurred or foggy vision. 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 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. The condition is caused by various internal and external factors. Treatment options may include prescription glasses and/or vision therapy.

cornea, corneal - 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.  

CRT contact lenses - Corneal Refractive Therapy (CRT) is a type of corneal reshaping. CRT contact lenses is a unique rigid gas permeable contact lens designed to temporarily correct myopia (nearsightedness) by gently and reversibly reshaping the cornea while sleep. You may then be able to go throughout the day without any lenses. The CRT lenses are made from an overnight contact lens material in a special design intended for this purpose. 

diopter (D) - Unit to designate the refractive power of a lens.

double vision - Perception of two images from one object; images may be horizontal, vertical or diagonal.

dry eye syndrome - Corneal and conjunctival dryness due to deficient tear production, predominantly in menopausal and post-menopausal women. Can cause foreign body sensation, burning eyes, filamentary keratitis, and erosion of conjunctival and corneal epithelium.

extraocular muscles - Six muscles that move the eyeball (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, inferior rectus).

glaucoma - Group of diseases characterized by increased intraocular pressure resulting in damage to the optic nerve and retinal nerve fibers. A common cause of preventable vision loss. May be treated by prescription drugs or surgery.

hi-index - High-index lenses can be considerably thinner and lighter, even in a strong prescription. The higher the number of the lenses, the thinner the edges.

hyperopia , farsightedness - Focusing defect in which an eye is underpowered. Thus light rays coming from a distant object strike the retina before coming to sharp focus, blurring vision. Corrected with additional optical power, which may be supplied by a plus lens (spectacle or contact) or by excessive use of the eye's own focusing ability.

keratoconus - Degenerative corneal disease affecting vision. Characterized by generalized thinning and cone-shaped protrusion of the central cornea, usually in both eyes. Hereditary.

LASIK - Acronym: LAser in SItu Keratomileusis. Type of refractive surgery in which the cornea is reshaped to change its optical power. A disc of cornea is raised as a flap, then an excimer laser is used to reshape the intrastromal bed, producing surgical flattening of the cornea. Used for correcting myopia, hyperopia, and astigmatism.

macular degeneration - is a medical condition usually of older adults which results in a loss of vision in the center of the visual field (the macula) because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness in the elderly (>50 years). Macular degeneration can make it difficult or impossible to read or recognize faces, although enough peripheral vision remains to allow other activities of daily living.

myopia,  nearsightedness - Focusing defect in which the eye is overpowered. Light rays coming from a distant object are brought to focus in front of the retina. Requires a minus lens correction to "weaken" the eye optically and permit clear distance vision.

ocular - of or relating to the eye

Orthokeratology (Ortho-K) - is a safe, non-surgical contact lens procedure to reduce or eliminate nearsightedness and astigmatism, dramatically improving natural vision.The Ortho-K procedure was developed in 1962 and has provided corrective eye care to thousands of patients through the therapeutic use of contact lenses. A series of Ortho-K lenses are fitted in progressive stages to gently reshape the eye's front curvature (the cornea) to improve visual errors.

poly carbonate lenses - Polycarbonate lenses offer the high degree of protection to keep eyes out of harm's way. The polycarbonate material used for eyeglass lenses was developed by the aerospace industry for use in helmet visors worn by astronauts. Polycarbonate can also be found in bulletproof windows, airplane windows, and riot police shields. The material is 10 times more impact-resistant than other plastics, and exceeds the FDA's resistance requirements by over 40 times.

PRK (photorefractive keratectomy) - Use of high intensity laser light (e.g., an excimer laser) to reshape the corneal curvature; for correcting refractive errors. Includes laser sculpting, LASIK.

progressive addition lens (PAL), progressive-power lens - Eyeglass lens that incorporates corrections for distance vision through midrange, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line.

retina - Light sensitive nerve tissue in the eye that converts images from the eye's optical system into electrical impulses that are sent along the optic nerve to the brain. Forms a thin membranous lining of the rear two-thirds of the globe.

retinal detachment - Separation of the retina from the underlying pigment epithelium. Disrupts visual cell structure and thus markedly disturbs vision. Almost always caused by a retinal tear; often requires immediate surgical repair.

RGP - Rigid gas permeable (RGP or GP) contact lenses generally provide sharper vision and last longer than soft contact lenses. Custom-made from a rigid oxygen-permeable material (usually a combination of silicone and fluorine), RGP lenses are more resistant to proteins and other deposits, making them easy to keep clean. Over time, however, even RGP (or GP) lenses will become scratched and dirty and will need replacement.

transition lenses - Transitions lenses are clear indoors and at night but automatically adjust their level of tint to changing light conditions outdoors.

trifocal - Eyeglass lens that incorporates three lenses of different powers. The main portion is usually focused for distance (20 ft.), the center segment for about 2 ft., and the lower segment for near (14 in.).

vitreous chamber - the space in the eyeball that contains the vitreous body; it is bounded in front by the lens and ciliary body and in back by the retina.