NOCE key terms

The clear, dome-shaped front surface of the eye that provides most of the eye's focusing power. It protects the inner eye and, unlike the crystalline lens, cannot change shape to focus.

The process by which the crystalline lens inside the eye changes shape to increase its focusing power so that near objects appear clear. This ability declines steadily with age.

The age-related loss of the eye's ability to focus on near objects, caused by stiffening of the crystalline lens. It usually becomes noticeable in the early to mid-forties and is corrected with reading or multifocal lenses.

A refractive condition in which distant objects focus in front of the retina, so far vision is blurry while near vision stays relatively clear. It is corrected with minus, or concave, lenses.

A condition in which the eye's refractive power differs between meridians, so light focuses at two separate points instead of one. Vision is blurred at all distances and is corrected with cylindrical lenses.

A clouding of the eye's crystalline lens that reduces light transmission and causes gradual blurring, glare, and poor contrast. It is treated surgically by removing the cloudy lens and implanting an intraocular lens.

A group of eye diseases, usually associated with elevated internal eye pressure, that damage the optic nerve and can cause permanent peripheral vision loss. It often produces no early symptoms and requires regular monitoring.

A progressive thinning and outward bulging of the cornea into a cone shape, producing irregular astigmatism and distorted vision. Rigid or specialty contact lenses are often required to restore acceptable vision.

A condition in which the two eyes have significantly different refractive powers. Large differences create unequal image sizes at the eyes and may not be well tolerated in spectacle lenses.

Reduced vision in one or both eyes caused by abnormal visual development rather than by eye disease, often from strabismus or unequal refractive error in childhood. It may become permanent if not treated early.

A number describing how much a material slows and bends light compared with air. Higher values allow a given lens power to be produced with a thinner, lighter lens.

The unit of lens power, equal to the reciprocal of the focal length measured in meters. A one-diopter lens brings parallel light rays to a focus at one meter.

A unit of prism power that displaces a light ray by one centimeter at a distance of one meter. It is used to write prism in prescriptions and to calculate unwanted prism from lens decentration.

A formula stating that induced prism, in prism diopters, equals the lens power in diopters multiplied by the distance in centimeters from the optical center. It is used to predict prism from decentration.

The point on a lens where a light ray passes through without being deviated, so no prism effect occurs. Positioning this point correctly in front of the pupil prevents unwanted prism.

A lens defect in which rays passing through the outer zones focus at a different point than rays near the center, softening the image. Aspheric front surfaces are used to reduce it.

The distance from the back surface of a spectacle lens to the front of the cornea. Changes in this distance alter the effective power at the eye, most noticeably in strong prescriptions.

A number indicating how much a lens material disperses light into its component colors. Higher values mean less chromatic aberration, which matters most in stronger powers and off-center viewing.

A tough, lightweight lens material with a refractive index of about 1.59 and very high impact resistance. It is a common choice for children's eyewear and safety or sports frames.

A lightweight, impact-resistant lens material with a refractive index near 1.53 and better optical clarity than polycarbonate. It works well in drilled rimless frames and for active wearers.

A lens made from material with a refractive index above roughly 1.60, allowing thinner and lighter lenses in stronger prescriptions. These materials generally have lower Abbe values than standard plastic.

A lens whose plus power increases smoothly from the distance zone through a corridor to the near zone, with no visible dividing line. It gives clear vision at all distances but produces peripheral distortion.

A thin multilayer coating applied to lens surfaces that reduces reflections, increases light transmission, and lessens glare and halos at night. It also makes the wearer's eyes more visible to others.

A lens that darkens when exposed to ultraviolet light and clears again indoors, serving as both clear eyewear and sunglasses. Darkening and clearing speed varies with temperature and lens chemistry.

A lens whose front surface flattens toward the edge instead of following a simple sphere. This yields a flatter profile, less edge magnification, and reduced aberration compared with a spherical design.

The curvature of the front surface of a lens, expressed in diopters. The laboratory selects it to control aberrations, and it is measured when verifying lens form and thickness.

A measuring method that encloses each lens shape in a rectangle tangent to its farthest points, providing the A dimension, B dimension, and effective diameter used when ordering lenses.

The distance between the centers of the two pupils, normally recorded in millimeters. It determines where the optical centers of the finished lenses should sit relative to the frame.

The vertical distance from the lowest point of the lens shape to the top of a multifocal segment or to the fitting cross of a progressive lens. It controls where the reading zone sits.

The downward angle of the frame front at its bottom edge, typically about 8 to 12 degrees, which aligns the lens plane closer to the eye's natural line of sight when looking down.

The horizontal angle formed as a frame front wraps around the face, measured between the planes of the two lenses. Excessive wrap can introduce unwanted prism and affect binocular comfort.

The main lens power, in diopters, that corrects a uniform refractive error across all meridians. It is written first in a prescription and may be either plus or minus.

The extra power in a lens that corrects astigmatism, acting only along one meridian while leaving the meridian at right angles unaffected. It is always written together with an axis.

The direction, stated in degrees from 1 to 180, of the meridian in a sphero-cylindrical lens that carries no cylinder power. It sets the orientation of the astigmatism correction.

An instrument that measures the sphere power, cylinder power, axis, prism, and optical center of a finished lens. It is the primary tool for verifying that a prescription was made correctly.

The United States standard that sets acceptable tolerances for prescription spectacle lens power, axis, prism, and impact resistance. Lenses falling outside these limits are considered unacceptable and must be remade.

The smallest uncut lens diameter that will yield the required finished size and decentration without cutting into the needed optical zone. It is calculated from frame size, pupillary distance, and decentration.