Glossary

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	Add Powers In multifocal lenses, the part of the lens that corrects for near vision.
	Astigmatism A common disorder in which one or more of the eye's refractive surfaces (i.e., cornea, lens) is not symmetrical. Astigmatism causes a perceived image to become distorted or blurred.
	Autorefractor A computerized screening device designed to detect abnormalities of vision (i.e., nearsightedness, farsightedness, astigmatism).
	Axis In correcting for astigmatism, the cylinder power determines how much correction and the axis determines where the correction goes.
	Base Curve The front curve on a lens.
	Biomicroscope Known as a slit lamp, this device is used to closely examine external parts of the eye to detect degenerative changes in the cornea and is also useful in routine contact lens evaluation.
	Bridge The area of the frame-front that rests on the wearer's nose.
	Cataract A condition characterized by clouding in the natural lens of the eye. The cause is usually unknown but can result from exposure to ultraviolet light, injury, heredity, disease and advancing age. Some medical treatments include surgical replacement of the natural lens with an artificial lens. Cataracts are usually treated on an out-patient basis.
	Computerized Visual Fields Test This test is used to detect deficiencies in central or peripheral vision.
	Cornea The clear outer covering of the eye. Known as the window of the eye, it plays an important role in focusing images.
	Cylinder Power In correcting for astigmatism, the cylinder power determines how much correction and the axis determines where the correction goes.
	Demo Lenses Plastic lenses inserted into frames for display purposes only.
	Depth Perception The appearance of depth is created when both eyes focus on the same plane simultaneously.
	Eyewire The section of the frame enclosing each lens.
	Fitting Triangle The three contact points on an eyeglass wearer's head: right ear, left ear and nose.
	Frame Width The size of the frames from left side to right side. Frame width should equal the width of the wearer's head.
	Glaucoma An eye disease characterized by an increase in pressure inside the eye capable of causing damage to the optic nerve. Left untreated, this condition can lead to blindness. Glaucoma is usually treated with eye drops and medications. Severe cases may require surgical correction.
	Hyperopia Farsightedness.
	Keratometer An instrument used to measure the curvature of the cornea. This measurement enables the doctor to accurately fit contact lenses.
	Lens Blank An unfinished lens which is then ground to fit a customer's prescription.
	Lensometer An instrument used to measure the power of an individual's current prescription eyeglasses.
	Multifocal Lens A lens with more than one optical center (i.e., a bifocal with two, a trifocal with three, a progressive with many).
	Myopia Nearsightedness.
	O.D. Doctor of Optometry, or Optometrist.
	Ophthalmologist A doctor specializing in the eye who is trained to examine, diagnose, treat and manage diseases of the visual system, including all types of surgical procedures. Requires completion of a four-year undergraduate program, four years of medical school and a minimum of three years in ophthalmology training.
	Ophthalmoloscope This hand-held, lighted instrument helps detect a wide range of systemic and vision problems by giving the doctor a close-up view of the retina and other important parts of the eye.
	Optical Center The spot on the eyeglass lens where light passes through without bending (refracting). This point is normally placed in front of the pupil.
	Optician A professional trained to fit and adjust eyewear based on the specifications provided by an optometrist or ophthalmologist. Training requirements vary by state.
	Optometrist A primary health care professional who can diagnose, manage and treat conditions and diseases of the human eye and visual system, as required by state law.
	Peripheral Vision The area of vision lying just outside the line of sight to either side of the face. For activities such as driving and sports, it is important not to obstruct peripheral vision.
	Phoroptor Instrument used by the doctor to determine the final lens prescription and alignment of the eye.
	Photo Refractive Keratotomy (PRK surgery) A non-invasive, laser surgical procedure reshaping the cornea, which is the major refractive surface of the eye, to correct a refractive error. The only approved process right now is for nearsightedness (myopia).
	Polarized Lenses Tinted lenses specifically designed to reduce glare. Those who work or drive in bright conditions and those who play snow or water sports would benefit from polarized lenses.
	Polycarbonate A synthetic, light material used to create impact-resistant lenses.
	Presbyopia A condition given to "elder eyes" whereby the crystalline lenses in the eye lose elasticity and do not change shape as easily as they should. Presbyopia causes people to have increased difficulty with near vision and reading, and is corrected with multifocals.
	Prescribed Prism An effect produced by de-centering a lens so that the optical center is not in front of the wearer's pupils. Less than five percent of all eyeglass wearers receive this prescription.
	Progressives Multifocal lenses with no telltale lines.
	Pupil In the eye, a round hole formed by the iris that regulates what light passes into the eye. Light must pass through the pupil to get to the retina.
	Pupillary Distance The distance, in millimeters, from one pupil center to the other. PD measurement is used to ensure proper lens placement.
	Pupilometer A machine which measures the precise pupillary distance.
	Refractions Eye examinations conducted to determine appropriate eyewear prescriptions.
	Refractive Index (RI) The ability of the lens to bend light. Materials that bend the most light have the highest refractive index.
	Retina Part of the eye that picks up images and transmits them to the brain via the optic nerve.
	Segment Segments of the eyeglass lenses that are used for near vision bifocals. The surrounding larger portion of the lens is used for distance vision.
	Seg Height The distance from the lowest portion of the eyewire to the top horizontal line of the Seg.
	Sphere Power The first number in a prescription for corrective lenses which indicates the type and amount of correction.
	Tonometry Computerized measure of internal eye pressure-glaucoma screening. A tonometer detects glaucoma by measuring the internal pressure of the eye with a puff of air.
	Transition Zone The area of progressive eyeglass lenses where the distance vision curve gradually changes into the near-vision curve.
	Trifocals Eyeglass lenses having three areas of viewing, each with its own focal power. Usually, the upper portion is for distance viewing, the lower for near, and the middle for the distance in-between.
	UV Protection The degree to which a lens or its coating protects the wearer's eye from the sun's harmful rays.