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Definitions of key terms related to the study of the eye and visual brain

In vision health research, we use a specialized vocabulary not always easily understood by everyone. To facilitate the understanding of the studies undertaken by the members of our network, here is a glossary (i.e. an alphabetical list of terms commonly used in vision healthcare and their explanation) you can consult.

Accommodation refers to the changes of the lens’ radius of curvature to ensure sharp images for different viewing distances.

Age-related Macular Degeneration (AMD)

Age-related Macular Degeneration is a disease caused by a progressive degeneration of the macula, the central part of the retina. AMD can first appear at the age of 50, although its onset occurs most frequently after the age of 65, causing a significant reduction in visual abilities, without annihilating them completely. The precise causes of this disease are unknown and no cure has been found to date. Existing treatments only slow its progression. Patients suffering from AMD have a diminished capacity to distinguish colours and a distorted perception of lines.



Partial loss of visual acuity, also called « lazy eye » in children. It is usually caused by anisometropia, a large refractive error and/or strabismus.



Blurred vision due to a malfunction of the optical system, which includes the cornea, lens and axial length of the eye.



Astigmatism is an abnormality of the curvature of the cornea, which is oval (football ball) rather than spherical (soccer ball). Depending on  their source relative to one of these two axes, the light rays focus mainly on two different points, either behind or in the front (i.e. on either side) of the retina.


Bifocal lens 

A lens that corrects two vision conditions at the same time, such as myopia (nearsightedness) and presbyopia (farsightedness). A segment of the lower part of the glass allows the wearer to better see near objects (for example, while reading); the rest of the glass improves visualization of distant objects. The lens has two optical powers: the upper region is intended to correct far vision, while the lower region corrects near vision.



Blepharitis is an inflammation of the skin covering the eyelids, often of microbial aetiology or as a result of an allergic condition. It frequently occurs in the context of dermatological illnesses.



Blindness is a total loss of vision, which can be caused by a large number of eye diseases.



A cataract is the partial or total opacification of the cristalline lens, which is a converging transparent structure located inside the eye. This opacification causes progressive vision loss, accompanied initially by discomfort during exposure to light.


Color blindness

Color blindness is an anomaly caused by a deficiency in one or more of the three types of retinal cones responsible for color perception. 



Conjunctivitis is an inflammation of the conjunctiva caused by viruses (viral conjunctivitis), bacteria (bacterial conjunctivitis), allergies (allergic conjunctivitis) or irritation. Conjunctivitis can be very contagious. It is characterized by ocular redness and irritation, itching, a burning sensation and even purulent ocular secretions.



The cornea is the transparent anterior portion of the eye, shaped like a slightly projecting and a spherical dome. The cornea, like the lens, plays an important role in the focus of the image on the retina.


Corrective lens

A corrective lens is an optical lens worn in front of the eye, mainly to correct ametropias (i.e. refractive errors): myopia, hyperopia, astigmatism or presbyopia.



A diopter is the unit of measure of visual defects in patients. A myopic patient uses divergent, negative diopter glasses, while a hyperopic patient uses convergent, positive diopter glasses.


Eye dryness

Eye dryness results in a burning sensation and irritation of the eyes. It is usually associated with a reduction in the quality of tears.



An emmetropic (or normal) eye is an eye without visual defects.


Extraocular muscles

Extraocular muscles are the six muscles responsible for eye movements in the orbit.



Light-detecting visual organ shaped as a sphere of about 25 mm in diameter when the eye is emmetropic (i.e. without visual defects). It consists of three envelopes – the sclera, the uvea and the retina – and contains the aqueous humour, the lens and the vitreous humour.



The eyebrows are hairs that grow on the supraorbital ridges, located above the eyes. Their function is to protect the eyes from the sun, rain, sweat and external aggressions in general, such as dust or sand.



The eyelashes are hairs that grow on the free edge of the eyelids and that prevent perspiration and foreign bodies, such as dust or liquid, from falling into the eye.



Floaters are filaments, more or less opaque, that cross the visual field and become especially apparent when the affected person stares on a light background. The floaters result from the presence of abnormal translucent cell residues suspended in the vitreous humor.



The fovea, central area of the macula, is the area of the retina where vision of details is most accurate. It is located in the extension of the optical axis of the eye.



Glaucoma is a degenerative disease of the optic nerve that causes progressive vision loss; initially, it affects the peripheral visual field; gradually, the central visual field becomes affected, too. Glaucoma is often associated with elevated intraocular pressure (IOP) .



From a pure optical point of view, hyperopia is the opposite of myopia: in the resting hyperopic eye, distant objects are focused behind the retina. In case of low hyperopia, one sees correctly from far compensating his farsightedness by accommodation.


Intraocular tension

Intraocular tension, also called intraocular pressure (IOP) is the pressure within the eye.



The iris is a sphincter around the pupil of the eye. It acts as a diaphragm which contracts depending on the intensity of the light passing through the eye; thus, it determines the size of the pupil.



Keratitis is an inflammation of the cornea; its most common aetiology is infectious. Keratitis is often associated with conjunctivitis. It is characterized by reduced corneal transparency and by thin corneal vascularization.



The eye has two natural converging lenses: the cornea, located at the entrance of the eye, and the cristalline lens, a bi-convex structure located behind the iris. Both lenses focus the light rays on the retina, which acts as a screen at the back of the eye.



The macula lutea, or yellow spot, is the area of the retina with the maximal concentration of cones. Located at the back of the eye, in line with the pupil, the macula has a diameter of about 2 mm.


Mineral lenses

Glasses made of silica and a mixture of melted oxides at high temperatures. They are heavy and breakable, but have the advantage of being resistant to scratches.



Myopia is a disorder affecting distant vision. The eye is too long, causing the light rays to meet in front of the retina. A myopic person has a good near vision. The higher the myopia, the closer the text must be brought to the eyes to be read by the person. 



Ophthalmology is the branch of medicine responsible for the diagnosis and treatment of diseases of the eye and of its adnexa. It is both a medical and surgical specialty.


Optic nerve

Nerve originating at the retina, more precisely at the optic disc, where the optical fibers of the retinal nerve cells come together.


Optical add

Power added to the prescription for distance vision to help see in near vision.


The optician has a DEC in the field of health, more precisely a visual orthotic technical degree. This training allows him,  upon presentation of a prescription, to make a full analysis of the oculo-visual needs of patients in order to properly advise them on equipment that corrects visual deficiencies. He can adjust contact lenses and glasses according to the needs of patients. Among other things, he assembles spectacles and fit corrective lenses inside eyeglass frames based on the individual measurements of each patient.



The optometrist holds a Doctor of Optometry degree. The optometrist performs eye and visual function examinations, diagnoses eye diseases and treats some of them. They can fit glasses and contact lenses to patients’ needs, provide orthoptic treatment and prescribe magnifiers if required.



Bony cavity of the skull inside which the eye and its appendages are located.


Organic glasses

Glasses made of polymerized resin. They are resistant to shock and much lighter than glass lenses. They must be treated to be resistant to scratches.



The study of oculomotor and binocular vision disorders.


Peripheral vision

Peripheral vision is an important part of human vision. Unlike central vision, which allows the observation of details in high-resolution, peripheral vision delivers compressed and distorted impressions of the total visual field.


Photochromic lenses

The photochromic lens is a corrective lens that has the ability to change its color depending on the amount of ultraviolet (UV) light to which it is exposed. When UV exposure ends, these optical lenses gradually regain their clear state.



Photophobia is a discomfort or a painful sensation produced by exposure to light in certain diseases such as conjunctivitis and keratitis.



Condition characterized by blurred vision while performing a task that requires near vision (for example, reading). Presbyopia is a normal aging process of the eye, namely of the lens (which hardens and scleroses with time), rather than a disease. This aging process begins at birth, but its impact typically becomes apparent at the age of 40 to 45.


Progressive lens

A lens used to correct presbyopia and whose power varies continuously from the top of the lens (for distance vision) and the bottom of the lens (for near vision). 



Pterygium is a corneal invasion by the limbal conjunctiva. It is a benign lesion of the conjunctiva usually related to sun exposure. Surgery is only performed if the pterygium obstructs the visual axis.



The pupil is the hole in the middle of the iris. It appears black, because the majority of the light entering the inside of the eye is absorbed by the tissues, especially the retina. Its diameter varies according to the brightness of the entering light.



The retina is the sensitive organ of vision where images are formed to be further transmitted to the brain where they are interpreted. It is composed of photoreceptors: about 5 million cones (diurnal and color vision) and ~ 120 million rods (twilight and night vision in black and white).


Retinitis pigmentosa

Retinal disease that begins with a loss of night vision followed by a narrowing of the visual field. The loss of central vision arrives late in the course of the disease.



The sclera is a very tough, white and opaque layer that forms the « white » of the eye.


Snellen Scale

A scale in which the first letter is a big « E » and which used to determine visual acuity or the ability to distinguish details during a visual exam.



The ability to see objects in three dimensions. The binocular neurons in the cerebral cortex are responsible for this ability.



Strabismus, commonly referred to as « crossed eyes », is a defect in the parallelism of the visual axes. The different types of strabismus are named based on the direction of the deviation of the visual axes: convergent (inward), divergent (outward) or vertical.



The tear fluid is a salty fluid secreted by the lacrimal glands. It cleans and protects the eye by facilitating removal of foreign bodies (dust, insects) that could lodge on the surface of the eye.


Ultraviolet (UV)

Ultraviolet (UV) radiation is electromagnetic radiation of a shorter wavelength than that of the visible light. The ultraviolet rays are the ones that produce suntan, but high doses are harmful to human health and can cause skin cancers (such as melanomas), premature aging of the skin (wrinkles, sunburns) and cataracts.



The uvea is the intermediate pigmented part of the eye. It is a vascular layer that includes the choroid, the ciliary body and the iris.



Uveitis is an inflammation of the uvea. It can impair vision and cause an increase in the intraocular pressure.


Visual acuity

The visual acuity is the ability to discern a small object (an optotype) located as far as possible, i.e. the ability to see an optotype at a fixed distance (usually five meters) under the smallest possible angle.


Visual field

The visual field is the portion of space seen by a still eye looking straight ahead. It extends over 60 degrees up, 70 degrees down, and 90 degrees laterally.


Vitreous humor

The vitreous humor is a transparent gelatinous substance that fills the eye cavity behind the lens.