Glossary of Optical Terms

Sep 29, 2026

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Glossary of optical terms defining key concepts used in eye care and lens optics. Entries cover vision performance and testing methods, including normal eyesight (20/20) and the Snellen chart, as well as refractive testing for errors such as astigmatism, myopia, and hyperopia. It also explains optical measurements and lens-related phenomena, from lens dispersion and chromatic aberration to optical aberrations, absorption, accommodation, and related vergence and acuity terms.

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GLOSSARY OF OPTICAL TERMS

Key vision testing and refractive concepts

Lens optics, measurements, and related visual function

GLOSSARY OF OPTICAL TERMS 20/20 – The expression for normal eyesight (6/6 in metric measurements) . Expressed as a fraction, the numerator refers to the distance the patient is from the test chart – the standard distance is 20 feet (6 meters) . The denominator denotes the distance at which a person with normal eyesight could read the same line. For example, if the patient’s visual acuity is 20/100 then the smallest line they correctly read at 20 feet could be read by a person with normal vision at 100 feet. The Snellen chart – consisting of letters, numbers, or symbols – is used to test visual acuity or the 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. A “A” Box Measurement – The longest horizontal measurement of a lens; from the furthest nasal point to the furthest temporal point of a shape. Abbe Value – A measure of the degree to which light is dispersed when entering a lens. The average range is 20 to 60. A lower Abbe value causes greater dispersion of light resulting in chromatic aberration (color fringing) when viewed through the periphery of the lens. A higher Abbe value results in less chromatic aberration. Aberration – Any flaw introduced during the manufacturing or grinding of a spectacle lens that can result in image blur. It may be inherent in the lens design or may result from an error in processing. Abrasive – A substance used in friction process, such as grinding or polishing. Absorption – Occurs when light is not reflected, refracted, or transmitted. The light energy is transformed into another form, such as heat. Absorptive Lens – The ability of a lens to intercept or block general or specific wavelengths of radiant energy. Designed to provide eye comfort against glare. Accommodation – Automatic increase in the eye’s focusing power enabling the eye to see clearly at different distances. This process is achieved by the lens of the eye changing its shape. Without the ability to accommodate, the image of an object would blur. Accommodative Vergence – A convergence response (to turn the eyes inward) which occurs as a direct result of accommodation (eye focusing) . Acuity – The ability to respond to faint sense impressions or to distinguish light differences between stimuli. Adaptation/Adaptive process – The maneuvers (conscious or unconscious, automatic or voluntary) by which the individual attempts to make organismic decisions as comfortable and gratifying as possible within the demands of that individual’s environment. Add Power (addition)–The difference in spherical power between the distance and near corrections found in bifocals, trifocals, and progressive addition lenses to assist the eye’s accommodation at a given distance. This power is needed for near vision. Also referred to as add power or seg power. Addition – The optical power (of a lens) required for near vision, in addition to that required for far vision. Adjustable Nose Pads – Type of bridge that consists of small pads made of plastic or silicone to help prevent slippage and aid in evenly distributing weight of glasses on bridge of the nose. GLOSSARY OF OPTICAL TERMS Against Motion – Objects sighted through convex (plus) lenses move in an opposite (against) direction as the convex lenses are moved from side to side. Air Tempering – The strengthening of a lens as a result of cooling a heated lens. Albinism – Pigmentation is deficient or absent. May occur in skin, hair, and eyes. Ocular albinism is a pigmentation deficiency occurring mainly in the eyes. Individuals with albinism including ocular albinism commonly have decreased visual acuity (20/70 – 20/200), strabismus, photophobia, and nystagmus. There is no known treatment. Individuals may benefit from low-vision aids. Treatment options for strabismusand nystagmus does apply to these individuals. Alignment – The placing

FAQs

What does 20/20 mean in visual acuity measurements?

20/20 expresses normal eyesight, where the numerator is the test distance (20 feet / 6 meters) and the denominator is the distance at which a person with normal vision can read the same line.

How is a Snellen chart used?

A Snellen chart, made of letters, numbers, or symbols, is used to test visual acuity or the sharpness of eyesight.

What is accommodation in the eye?

Accommodation is the automatic increase in the eye’s focusing power, achieved by changing the eye lens shape, allowing clear vision at different distances.

Sep 29, 2026

Upload by dbqoiu