Terms to Know

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.

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.

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).

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.