NOCE / ABO Basic key terms

A refractive condition in which the eye's focusing power is too strong for its length, so images of distant objects focus in front of the retina and look blurred, while near objects usually appear clear.

A refractive condition in which the eye's focusing power is too weak for its length, so light from distant objects would focus behind the retina, and the eye must work to see clearly at any distance.

A condition in which the cornea or crystalline lens curves unequally in different directions, creating two focal lines instead of a single point and causing blur or distortion at all distances.

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

The process by which the eye increases its focusing power for near objects by contracting the ciliary muscle, allowing the crystalline lens to become rounder and thicker.

The unit of lens power equal to the reciprocal of the focal length measured in meters; a 1.00 diopter lens focuses parallel light at one meter, and stronger lenses carry higher diopter values.

The unit of prism power in which a ray is displaced one centimeter at a distance of one meter; it describes prism used to correct binocular or gaze problems and prism created by decentration.

A number comparing the speed of light in a vacuum with its speed inside a material; higher values bend light more, which lets a lens be made thinner for the same power.

The distance in millimeters from the back surface of a lens to the front of the cornea; changing it alters the effective power of strong lenses at the eye.

Rewriting a lens prescription into an equivalent form by adding the sphere and cylinder, changing the cylinder sign, and rotating the axis 90 degrees, without changing how the lens performs.

A formula stating that prism in prism diopters equals lens power in diopters multiplied by decentration in centimeters; it is used to predict unwanted prism or to create prescribed prism.

One power that approximates a spherocylindrical lens, found by adding half the cylinder power to the sphere; it is used in contact lens fitting and quick power comparisons.

A measure of how strongly a lens material spreads light into separate colors; lower values cause more color fringing, so stronger lenses favor materials with higher values.

A lightweight, highly impact-resistant lens material with a high index of refraction; it blocks most ultraviolet light but produces more color fringing than materials with higher Abbe values.

A thin multilayer coating applied to lens surfaces that reduces reflections, lets more light pass through, and improves appearance and vision in low light by cutting glare.

A lens whose plus power increases smoothly from the distance area down to the reading area, giving clear vision at all distances without the visible line of a bifocal.

A lens property in which special molecules darken when exposed to ultraviolet light and fade indoors, so one pair of glasses adjusts automatically between bright and dim conditions.

The standard way to measure a frame front by drawing a box around each lens shape, touching its highest, lowest, and widest points, which gives lens width and height.

The horizontal width of one lens shape in millimeters, taken at its widest point using the boxing system; it is the first number in a frame's size marking.

The distance in millimeters between the two lenses, measured at the narrowest point across the bridge using the boxing system; it is the second number in a frame's size marking.

The overall length of a temple arm in millimeters, measured from the hinge to the end of the tip; it is the third number in a frame size marking and affects fit behind the ear.

The downward angle of the frame front relative to the temples, commonly about 8 to 12 degrees, which lines the lens up with the wearer's line of sight for better optics and comfort.

An instrument that reads the spherical, cylindrical, and prism power of a lens and finds its optical center, used to check finished eyewear against the prescription.

A device with three contact prongs that measures the curvature of a lens surface in diopters, used to identify the base curve and confirm a lens was surfaced as ordered.

The smallest uncut lens diameter that can still yield a finished lens at the needed decentration, figured from the frame's longest dimension plus the amount of decentration required.

Moving a lens's optical center away from the center of the frame shape, usually so it sits in front of the pupil; it is measured in millimeters and creates prism.

A fit check in which both nose pads and both temple tips rest evenly on the wearer, showing the frame is balanced rather than rocking or pressing on one side.

A small pad, usually silicone or plastic, that rests on the sides of the nose and supports the frame; its angle, spread, and height are adjusted for comfort and alignment.

A temple style that bends downward behind the ear to hold the frame securely; the bend is shaped to follow the ear without pinching or slipping.

The U.S. standard for prescription ophthalmic lenses that sets tolerances for power, thickness, prism, and impact resistance, used to judge whether finished eyewear passes inspection.

The U.S. standard for protective eyewear, covering impact resistance, side coverage, and product labeling for safety glasses and goggles worn at work or in labs.

An impact test in which a steel ball is dropped from a set height onto a lens; passing shows the lens meets federal impact-resistance requirements for prescription eyewear.

A federal rule requiring eyecare providers to hand patients their contact lens prescription so they may buy lenses elsewhere, and barring them from withholding it to force a purchase.

A contact lens value showing how much oxygen passes through a given thickness of lens material; higher numbers mean more oxygen reaches the cornea, which supports eye health.

The percentage of a soft contact lens's weight that is water; higher-content lenses feel wetter and more comfortable for some wearers but tend to lose moisture faster.

Measurement of the central cornea's curvature in millimeters and diopters along two meridians, used to select contact lens base curves and to detect corneal astigmatism.

A contact lens made of firm plastic that keeps its shape, lets oxygen reach the eye, and often gives crisp vision; it needs a careful fit and a period of adaptation.

An insurance or benefit program that pays part of the cost of eye exams and eyewear; staff must verify eligibility, covered services, and allowed charges before ordering or dispensing.

An organized way of contacting patients when an exam or eyewear replacement is due, using mail, phone, or text reminders to support continuing care and steady practice traffic.

The maker's or seller's written policy stating how long and under what conditions a frame will be repaired or replaced for manufacturing defects; it usually excludes accidental damage.