Definition of the most frequently used terms in the field of optometry.
The eye’s ability to clearly see objects at close distances by modifying the radius of curvature of the crystalline lens.
In distance vision, the eye’s power to discern a small object as far away as possible, i.e. to see an object at the smallest angle possible at a set distance (typically five metres). Visual acuity is measured using optotypes (drawings, letters, etc.) at maximum contrast, making it a macular function test.
A lens’s optical power required for near vision that is added to that required for distance vision.
Describes all vision defects preventing a sharp image from forming on the retina, except age-related presbyopia. Myopia, hyperopia and astigmatism are ametropias.
Partial or relative loss of visual acuity. Also known as “lazy eye” in children, this loss of vision in one eye results from poor transmission between the eye and the brain.
An abnormality of the curvature of the cornea, resulting in a shape that is irregular and oval instead of round. In astigmatism, light rays are focused on different points in front of and behind the retina, creating a distorted image. For example, astigmatics will only see horizontal or vertical lines clearly.
Thin, cylindrical cells in the retina that react to light but are incapable of distinguishing colours. Highly sensitive, they enable us to see in dim light. Retinal rods are external extensions of rod cells, which are neurons in the retina.
Chronic or recurring inflammation of the skin on the eyelids, usually limited to their outer rim.
A clouding of the eye’s crystalline lens that compromises vision. This clouding is responsible for gradual vision loss, initially accompanied by irritation with light. In a normal eye, the crystalline lens is nearly transparent and can change shape to focus objects at different distances from the eye. When the crystalline lens loses its flexibility and becomes opaque, we call it a cataract.
The state of a person deprived of sight. Blindness is an eye disease. This absence of vision in one or both eyes has a variety of causes.
This is the peripheral visual field as seen by the eye. It normally extends 60° upward, 70° downward and approximately 90° laterally, corresponding to a wide-angle photographic objective of 180 °. When visual field is altered, areas of the field are less sensitive and even blind.
The part of the brain where the two optic nerves (the group of neurons that carry nerve impulses from the retina to the occipital lobe) cross in the brain.
Fringe of hair rimming the eyelid that protects the eye from external stress.
Inflammation of the conjunctiva, or thin membrane covering the white of the eye and producing mucous that coats and lubricates the eye surface. Close observation reveals a membrane criss-crossed by fine blood vessels. When the conjunctiva is irritated, these vessels dilate and the eye appears red. There are some 20 different types of conjunctivitis, from fairly common strains that usually pose no long-term danger to you or your child’s vision, to types that are resistant to antibiotics.
Reflex faculty that enables the eyes to focus on a single point in near vision.
The front transparent part of the eyeball, shaped like a spherical or slightly protruding cap. Together with the crystalline lens, the cornea plays an important role in focusing images on the retina.
Refers to a small speck moving in your field of vision. It might be what’s called a floater: a tiny clump of gel or cells in the vitreous, the clear jelly-like fluid inside your eye. Age, eye injury, and breakdown of the vitreous humor are the main causes of floaters.
Combination of curvatures of the front and rear surfaces of a lens. Correction is measured in diopters.
Organ located behind the pupil that focuses light rays on the retina. The crystalline lens is responsible for focusing and converging images on the retina. Its aging is the cause of presbyopia.
An abnormality whereby one or more of the three types of cone in the ocular retina, responsible for colour perception, are deficient. The most common form results in an inability to distinguish red and green.
A disease that damages the macula, the central part of the retina, causing loss of central vision and leaving only the peripheral or lateral vision intact. People suffering from AMD begin to have trouble distinguishing colours, and see straight lines as deformed. It is the principal cause of vision loss and blindness in people 65 and over.
Visual fatigue is characterized by smarting or acute irritation of the eyes, blurred vision and headaches, most often at the end of the day.
Small depression in the centre of the macula measuring 2mm in diameter and located close to the optical axis of the eye. It is responsible for clear vision. The fovea contains approximately 50,000 cones.
Mass of nerve cells in the retina that transmits messages from the rods and cones to the nerve fibres, and through them, to the brain.
The lacrimal glands secrete tears that flow out of the eye via tear ducts (and then into the nostrils), so that the eyes stay constantly moist. The drops of salty water known as tears clean and protect the eye, especially when it is dirty (if dust is in it, for example).
An increase in intra-ocular pressure leading, if untreated, to irreversible deterioration of the optic nerve and retina, and an alteration of the visual field, i.e. a reduction in visual performance, often accompanied by headaches and aching eyes.
Receptor organ of vision in the shape of a sphere approximately 25 mm in diameter when eye is emmetropic (with no vision defect). It consists of three envelopes, the sclera, uvea and retina, and content: the vitreous humor, crystalline lens and aqueous humor.
Commonly called “night blindness” because it describes impaired vision in dim light or darkness.
Also known as hypermetropia or farsightedness. Vision defect due to an eye that is too short and/or insufficiently powerful. The image forms behind the retina, which explains why hyperopes have better eyesight in far vision than in near. In cases of mild hyperopia, the subject sees correctly in far vision by compensating their hyperopia through accommodation. In cases of severe hypermetropia, the eye can no longer compensate this way. Hyperopia is the opposite of myopia.
Used to characterize the ability of a transparent optical material to refract light and produce an optical correction. The higher the index is for the same correction, the thinner the lens.
Circular membrane rimming the pupil. The iris acts as a diaphragm that contracts according to light intensity, controlling the size of the pupil. The iris’s pigmentation determines eye colour.
Inflammation of the cornea caused by a germ, virus, allergy or neuroparalytic source. Keratitis is manifested by a disappearance of transparency, and the appearance of fine corneal vascularization. In the case of chronic keratitis, the cornea is gradually covered over with a brownish pigmentation.
Watery saline solution produced by the lacrimal glands.
An enzyme present in tears and able to dissolve certain germs. It is a globular protein formed from amino acids that is found in a number of secretions (tears, saliva, human milk, mucus, etc.).
Small muscle attached to the crystalline lens that controls accommodation.
Muscles controlling the movements of the eyeball.
Vision disorder due to an eye that is too powerful or too long. In myopes, the image of a distant object is focused on a spot in front of the retina, and distance vision is blurred (near vision, on the other hand, is sharp). Myopes can read without glasses. The more severe the myopia, the closer the text must be brought to the eye.
Nerve that begins on the retina, which is the membrane lining the inner surface of the eye and containing the cells that capture light rays. The latter are then transformed into nerve impulses that travel to the brain. At this point on the retina, known as the optic disk, the optic fibres from the retina’s nerve cells converge.
Physician specialized in the diagnosis and treatment of diseases and abnormalities of the eye and its appendages. They are qualified to make a complete assessment of the visual function and eye’s condition and to treat the problems they have identified, either through a therapeutic approach or a surgical one.
Eyecare professional and member of a multidisciplinary team responsible for assessing ocular visual needs. Thanks to their training, they are experts in visual prosthetics. They design and adapt pairs of glasses according to each wearer’s measurements.
Vision professional who examines the eyes, analyzes their function and assesses and treats visual impairments by one or more therapeutic means at their disposal, e.g. recommendations, orthotics (visual re-education), glasses or contact lenses. An optometrist can assess and treat ocular pathologies, such as conjunctivitis, or deal with minor emergencies such as foreign bodies by treating and prescribing the appropriate medication.
Bony cavity of the skull containing the eye and its appendages: the optic nerve, ophthalmic blood vessels, muscles and oculomotor nerves. There are two eye sockets in the facial bones.
Branch of ophthalmology that treats vision disorders by means of re-education and eye training sessions. Orthotics is a paramedical profession exercised by a medical aide, the orthoptist.
Pressure maintained inside the eyeball. When increased, it is a sign of glaucoma.
Without it, the sun’s dangerous rays can damage your eyes. Make sure to wear glasses that block 100 percent of UVA and UVB rays.
Thickening on the eye surface usually linked to exposure to the sun. Dry, dusty conditions can also be a cause. Protecting the eyes from UV rays is a critical preventive measure.
Defines the faculty of a lens or contact lenses to correct a vision defect. It is measured in diopters.
The hole in the middle of the iris. It appears black because most of the light entering the eye is absorbed by tissues, particularly the retina. The pupil’s diameter varies according to the ambient light, and changes in size to control the quantity of light penetrating the eye.
Refers to the light-dark adaptation of the eye.
Commonly known as “U.V. rays,” ultraviolet radiation consists of UVA and UVB rays from the sun. UV rays can cause various conditions and ocular damage if the eyes are not protected.
Device used to automatically measure the refraction of the eye.
Photosensitive membrane at the back of the eye on which images of objects are formed. This ultrasensitive membrane transmits information to the brain and plays an essential role in the perception of light, colour, details, shapes and motion.
Hereditary disease that gradually attacks the photoreceptor cells of the retina, often little by little leading to blindness.
Dryness of the eyes, characterized by itching, burning and irritation of the eyes, is one of the most common problems treated by vision professionals. It is usually caused by the quality of the tears that lubricate our eyes. As we age, our bodies produce less and less oil to seal the eye’s watery layer. Hot and arid climates, air conditioning, certain medications, and irritants such as cigarette smoke can all increase dryness. Your eyecare professional might prescribe “artificial tears” or another type of eye drops to help alleviate the problem.
The white part of the eye that is composed of fibrous tissues that protects the inner workings of the eye.
An unmoving gap in the visual field (its size is perceived by holding the open eye still) due to an absence of perception in the retinal area.
A condition also known as “deviated eye” manifested in young children and characterized by an absence of coordination between the eyes so that one or both look in different directions. Early detection in children is vital in order to avoid any risk of amblyopia. There are three forms of strabismus:
Refers to the part of the retina in which the optic tract (optic nerve), which relays nerve impulses from the internal plexiform layer to the cerebral cortex, is inserted, along with the blood vessels going to and from the eye. In practice, it is a small portion of the retina deprived of photoreceptors and therefore completely blind.
Pressure exerted inside the eyeball that keeps its walls taut.
A variety of contagious conjunctivitis caused by Chlamydia trachomatis that is the leading cause of blindness in the world. Infection is spread by direct contact or indirectly, i.e. by clothing, hands and soiled toiletry items, among others. The germs that cause trachoma, which only affects humans, can also be carried on dust and sand by the wind.
Ultraviolet (U.V.) radiation is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than that of X-rays. Ultraviolet (U.V.) rays emitted by the sun are invisible and dangerous to the eyes, causing cell aging in the long term.
Inflammation of the uvea, often infectious, that can greatly alter vision.
The pigmented middle part of the eye. The uvea, also known as the vascular tunic, is made up of the choroid, ciliary body and iris. This membrane is separated from the sclera by the suprachoroidal space, and from the retina by the pigment epithelium.
Lenses that correct two vision problems at once, such as myopia and difficulty reading. A segment of the lens’s lower part is used for near vision, the rest of the lens for far.
A corrective lens corrects vision defects. It results in a combination of materials, optical surfaces and treatments.
Additional plastic lens materials fall into this category, in numbers ranging from 1.56 to 1.74 (the higher the number, the thinner the lens). They are used when thinner and/or stronger materials are needed.
Made from silicon and a combination of different oxides fused at high temperatures. They are scratch-resistant and can be “photochromic.” They are heavy and breakable, but have the advantage of being resistant to abrasion.
Bi- and multifocal lenses that correct more than one vision problem at time, such as myopia and difficulty reading.
Organic lenses are manufactured from “polymerized" resin and are of very high optical quality. Twice as light as mineral lenses, they are shock-resistant and can be coloured and photochromic. They are, however, more vulnerable to scratching and must be treated.
Lenses that automatically change from clear to dark in the presence of ultraviolet (UV) radiation.
Bifocal or multifocal lenses with no visible lines. Lenses designed to correct presbyopia whose powers change progressively between the upper portion, intended for distance vision, and the lower, intended for near. There is no optical discontinuity.
Single-focus lenses used to correct ametropia. They can also be used to correct presbyopia, but will only make near vision clearer; distance vision will be blurred. The power is the same at each point on the lens surface.
Ability to see objects on the periphery, without looking at them directly. A number of problems can lead to loss of peripheral vision, including glaucoma, brain seizures, optic nerve damage (ischemic optic neuropathy) and pituitary tumours.
Ability to see objects in three dimensions. Stereoscopic vision is largely due to the binocular neurons located in the part of the cerebral cortex that processes visual information (V1). These binocular neurons are the only neurons to receive the nerve impulse from two neurons of the same name via the neuronal chain.