The eye’s ability to clearly see objects at close distances thanks to a change of the radii of the curvature of the crystalline lens.


Amblyopia, or lazy eye, is a fall in visual acuity that cannot be improved by wearing eyeglasses. Amblyopia is synonymous with low vision.


Age-related macular degeneration. AMD is a disease that damages the macula, the central part of the retina, leading to a loss of central vision and leaving only the peripheral or lateral vision intact. AMD sufferers begin to have trouble distinguishing colors and see straight lines as if they were deformed.


Characterizes all eyesight disorders that prevent a clear image from forming on the retina, with the exception of age-related presbyopia. Myopia, hyperopia and astigmatism are all forms of ametropia.


Condition in which the refractive state of a pair of eyes differs.

Aqueous Humor

Transparent liquid responsible, together with the vitreous humor, for maintaining the pressure within the ocular globe and hence the shape of the eyeball.


Anomaly most often due to an irregularity in the curvature of the cornea, resulting in a distorted view of objects.


Bifocal Lens

Lens with two points of focus.

Bifocal Lenses

Corrective lenses designed to relieve presbyopia. The lower part of the lens allows near vision; the rest of the lens is designed for far vision. Bifocal lenses therefore comprise two focus points.


State of a person who is blind, i.e. deprived of all visual sensation.



Opacification of the crystalline lens, resolved by the extraction of the lens and its replacement, most often by an intra-ocular lens (implant). Cataracts may be hereditary or the result of a trauma.

Coated Surface

Optical surface of the lens coated with one or more thin transparent layers designed to protect the lens and reduce loss of vision through reflection and parasite images.


Several types of coating can be applied to corrective lenses after surfacing. The techniques used differ according to the purpose of the coating: scratch-resistant, anti-reflective, polarizing, coloring, antistatic, anti-smudge.

Cone Cells

Cone-shaped photoreceptive neurons located in the retina used for central vision and color perception.

Contact Lenses

Special optically calculated plastic lenses adapted to the eyes of each wearer and floated on the tear film over the cornea to correct eyesight. There are two categories of contact lens: soft lenses and rigid lenses.

Convergence / Divergence

Movement of the eyes together turning inward / outwards so that they are both “aimed” towards the object being viewed.


Transparent front part of the ocular globe shaped like a spherical, or slightly domed, cap. In combination with the crystalline lens the cornea plays an important role in focusing images on the retina.

Corrective Lenses

Corrective lenses are designed to correct eyesight disorders. The corrective lens is a combination of material, optical surface and coatings.


Crizal is the brand name of a range of optical lens coatings manufactured by Essilor International. Its products were created to allow greater transparency in lenses and an optimal clarity of vision, with 5 main benefits: no glare, smudge & scratch resistance, dust & water repellence.

Crystalline Lens

Transparent biconvex optical lens located behind the pupil. The crystalline lens refracts light to focus images on the retina. The aging of the crystalline lens is at the origin of presbyopia.



Optical system with a focal distance of 1 m. Refraction defects are measured in diopters (visual insufficiency).


The condition in which a single object is seen as two rather than one.

Distance Vision

Vision of objects situated either at infinity or more usually at some 5 or 6 meters.


Deformation of an optical system resulting in an image which does not conform in shape with the object.



Clear and comfortable eyesight in both far and near vision. Emmetropia is the opposite of ametropia.


Finished Lenses and Semi-finished Lenses

Prescription laboratories transform semi-finished lenses into finished lenses by processes of surfacing, coloring, coating and edging / assembly. In all cases the front face is finished in the plant and the rear face is surfaced on demand.

Fitting Height

The Distance from the pupil centration to the bottom of the choosen frame.


Small depression in the central part of the macula measuring 2mm in diameter and located close to the optical axis of the eye. The fovea contains about 50,000 cone cells.



Increase in intra-ocular pressure resulting, if left untreated, in an irreversible deterioration of the optical nerve and of the retina, as well as an alteration of the visual field, i.e. a reduction in visual performance, often accompanied by headaches and aching eyes.


Half-eye Lenses

Lens for near vision only shaped like the lower half of a normal lens.


Hyperopia, or far-sightedness, is an eyesight defect due to an eye that is too short and/or insufficiently powerful. The image forms behind the retina, which explains why the hyperopic subject has better eyesight in far vision than in near vision. In cases of mild hyperopia, the subject sees correctly in far vision by compensating the hyperopia through accommodation. In cases of severe hyperopia, the eye can no longer compensate in this way. Hyperopia is the opposite of myopia.


Intermediate Vision

Vision of objects situated beyond 40cm from the eye but closer than, say, 1,5 meter .

Intraocular Pressure

Fluid pressure exerted inside the eyeball (ocular globe), which keeps the wall taut.


Circular membrane that delimits the pupil. The iris acts as a diaphragm that contracts according to the intensity of light. The pigmentation of the iris determines the color of the eyes.


Janus Prize

Official prize for design sponsored by the French Minister for Foreign Trade and Industry. The prize is recognized by eye-care professionals and is a benchmark in the world of design. There are several categories of Janus Prizes: the Health Janus, Student Janus, Trade Janus and the Community Janus.



Central part of the retina. The macula is composed uniquely of cone cells directly exposed to rays of light and enables the precise vision required for reading or for the recognition of details. This is the area with the maximum activity in the eye.


Corrective lenses are most often manufactured from either organic or mineral glass. Organic lenses are divided into two categories: thermo-hardened and thermoplastic (polycarbonate). The properties of these materials are: a high refractive index procuring thinness, transparency, lightness, protection against ultraviolet rays and resistance to shocks.

Mid-distance Lenses

Mid-distance lenses are intended for presbyopic subjects; these are lenses designed for near vision but with an extended depth of field.

Mineral Lenses

Mineral lenses are made of silicon and a combination of different oxides fused at high temperature. They are scratch-resistant, and may be ""photochromic"". They are relatively heavy and breakable but have the advantage of being resistant to abrasion.


Basic material used for the production of organic lenses.


Myopia (near-sightedness) is an eyesight disorder caused by an eye that is too powerful, or too long. The image forms in front of the retina; a person with myopia thus sees badly in far vision but well in near vision.


Near Vision

Vision of objects situated 25-50 cm from either the eye or more commonly the spectacle plane. 40 or 33 cm are a good compromise for reading zone measurements of progressive lenses.


A repetitive unvoluntary movement of the eye whose direction, amplitude and frequency is invariable.


Ocular globe

Spherical organ (the eyeball) that receives vision, about 25 mm in diameter when the eye is emmetropic (with no eyesight defect). It consists of three layers, the sclera, the uvea and the retina and their content: vitreous humor, crystalline lens and aqueous humor.


Physician specialized in the treatment of eye diseases and conditions, and in the correction of eyesight disorders. Ophthalmologists may also carry out corrective surgery.

Optical Aberration

Defect in the way an optical system forms images, with the result that rays of light emanating from an object-point fail to form a perfect image-point. The lateral parts of progressive lenses all present undesirable deviations of this kind.

Optical Addition

The optical power (of a lens) required for near vision, in addition to that required for far vision.

Optical correction

Combination of the curvatures of the front and rear faces of a man-made lens. Correction is measured in diopters.

Optical Nerve

Cylindrical cord measuring 5 mm in diameter and 35 to 55 mm-long linking the retina of each eye to the brain, to which it transmits images by means of a nervous influx.

Optical Power

Optical power defines the ability of a lens or contact lens to correct a visual defect. Optical power is measured in diopters.

Optical surface (or design)

Surface on which either a reflection or refraction of the light is produced. The optical surface gives the material its ability for optical correction. The number of optical surfaces is almost infinite: they may be single-vision, bifocal or progressive.


Eyecare professional who designs and adapts eyeglasses in accordance with measurements specific to each wearer.


In many English-speaking countries, eyecare professional who dispenses refractive examinations. Optometrists do not deal with pathologies or surgery.


Branch of ophthalmology treating eyesight disorders by means of re-education and eye-training sessions. Orthoptics is a paramedical profession exercised by a medical aide, the orthoptician.


Peripherical Vision

Vision resulting from stimulation of the retina outside the fovea or macula .


Quality of an organic or mineral lens that can darken or lighten depending on the light level.

Polarized Lenses

A polarized lens is a quality sun lens that not only reduces bright light from the sun, as regular tinted sun lenses do, but also eliminates dazzling polarized light thanks to a very thin polarizing film inserted inside the lens, playing the role of a Venetian store. This results in a better clarity of vision, a truer color perception, and a greater visual comfort.

Polarized Light

Polarized light is an intense bothersome reflected glare that causes discomfort for the eye, and which appears when the sun’s rays hit a smooth, horizontal surface. This is particularly common in areas where reflected light is intense (sea, mountains, road surfaces etc...).


Material characterized by its exceptional lightness and resistance to shocks. Its high refractive index enables the manufacture of extremely light, thin lenses. Polycarbonate cuts out 100% of UV rays and is scratch-resistant thanks to its hardened coating. This material is a by-product of the Compact Disc (CD) industry.


Eyesight disorder caused by the aging of the crystalline lens, which with time thickens and loses its suppleness. As the crystalline lens becomes more rigid, it changes shape less easily and the subject sees less and less well in near vision. Everybody over the age of forty suffers from presbyopia.

Prescription laboratories

Production units that transform semi-finished lenses into finished lenses with the precise characteristics of the order. The custom work carried out by the laboratories enables us to provide the very large number of optical combinations required by wearers, especially as regards the correction of presbyopia. The laboratories are responsible for surfacing (grinding and polishing) and coating (coloring, anti-scratch, anti-reflective, anti-smudge etc.) the lenses.

Prismatic Effect

The change in direction imposed on a ray of light induced by an ophthalmic lens when the eyes look in various directions of gaze (except through the optical centre). In the case of a progressive lens, such deviation characteristics are more complex since the lens power is not constant. The prismatic effect at each point on the lens has an impact, in the central vision area, on the wearer's oculomotor strategy.

Production plants

The plants mass-produce finished lenses (mainly single-vision lenses) and semi-finished lenses in great numbers.

Progressive lenses

Progressive lenses are designed to correct presbyopia by varying optical power progressively from an upper part that is designed for far vision to a lower part designed for near vision. There is no optical break point.


Central opening of the iris through which rays of light enter the eye. The diameter of the pupil varies according to the ambient light level.

Pupillary Distance (PD)

The distance between the centres of the pupils of the eyes. PD measurement is used to ensure proper lens placement monoculary. It is a major measurement in dispensing progressive lenses.



Change in the direction of propagation of a ray, determined by variations in the speed of propagation. Term used to qualify the optical examination.

Refractive index

Used to characterize the capacity of a transparent optical material to refract light and produce an optimal correction. The higher the index is for the same correction, the thinner the lens.


Device used to automatically measure the refraction of the eye.


Light-sensitive membrane at the back of the eye on which object images are formed and which transmits information to the brain. This hypersensitive membrane plays an essential role in the perception of light, colors, details, shape and movement.


Thin, cylinder-shaped cells located in the retina that react to light but which are incapable of distinguishing colors. Rods are highly sensitive and ensure vision in very low light conditions. Retinal rods are external extensions of the rod cell, which are neurons located in the retina.



Major international optics and eyewear exhibition held once a year in Paris in the fall. During this event, awards are given for the best products in 5 categories including corrective lenses and optical instruments.

Single vision lenses

Single vision lenses are used to correct ametropia. They may also be used for the correction of presbyopia but, although they permit near vision, far vision will be blurred. The power is the same over the entire surface of the lens.


Eyesight disorder related to a defect in the parallelism of the visual axes. Early detection in children is vital in order to avoid any risk of amblyopia. There are three forms of strabismus: * convergent (esotropy), also called ’cross-eyes’ * divergent or exotropy * vertical, which is characterized by a defect in the alignment of the eyes.

Swim Effect

Many people have a difficult time adjusting to the different lens powers in progressive lenses. If wearers are not used to multiple changes in lens power, progressive lenses can make them nauseous and dizzy at first. Another disadvantage is that peripheral vision can be slightly altered by the changes that occur at the edge of progressive lenses. This distortion in viewing is often referred to as a “swim effect.”



Varilux is a brand name belonging to Essilor International. It is used to designate the first progressive lens to correct presbyopia, which was invented by Bernard Maitenaz. The Varilux lens is characterized by correcting near, intermediate and far vision. The first version of the lens was released in 1959

Visual acuity

The eye’s ability to discern the details of an object. A normal average eye is capable of discerning an angle of one minute of arc (1/60th of a degree).

Visual fatigue

Visual fatigue is characterized by a smarting or acute irritation of the eyes, blurred vision and headaches, most often at the end of the day.

Vitreous humor

Transparent and vitreous gel contained between the pupil and the retina, located behind the crystalline lens.



Xperio® is registered trademark qualifying polarized sun lenses that rigorously respect very strict manufacturing quality norms that Essilor requires from its lenses. Xperio® polarized lenses eliminate dazzling reflected glare, which results from the light reflection on a smooth and horizontal plane, and called polarized light. Xperio polarized lenses offer a better clarity of vision, a truer color perception, and a visual comfort adapted to all outdoor activities. All Xperio® lenses offer 100% UVA/UVB protection.