a few helpful definitions


An imperfection in the image formed by the eye, due to an irregularity of eye shape (geometric aberration, spherical aberration), or an unequal refraction of different wavelengths or colours (chromatic aberration).


Ability of the eye to adjust the ocular power in order to visually perceive objects in the environment and their details at various distances.


Sterilization. All protective practices intended to stop germs from infecting a living organism or inanimate object (materials, culture medium).


An irregularity in the curvature of the eye. In astigmatism, the cornea is shaped like an oval rather than a sphere. This distorts the image and results in blurred vision for both near and far objects. Astigmatism can be present on its own or in association with another vision problem, such as myopia or hyperopia. It is therefore corrected in isolation or simultaneously with its associated problems.

Binocular vision

The ability of the two eyes to see the same point simultaneously. Vision in which the retinal images of the two eyes are fused into a single visual representation.


A common problem caused by aging of the eyes and loss of transparency of the crystalline lens, the eye’s natural lens. The crystalline lens becomes partially or completely opaque, and the person with the cataract can no longer see clearly. This problem can be corrected with microsurgery to replace the crystalline lens with an intraocular lens.


The transparent, front “window” of the eye in the shape of a dome. The cornea is a complex structure, comprised of five layers. From exterior to interior, they are: the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane and the endothelium. The cornea’s approximate thickness varies from 0.5 mm at the centre to 0.8 mm at the periphery. The cornea is the most tactile-sensitive part of the human body.

Crystalline lens

Round, biconvex and transparent lens situated behind the iris or on the inner surface of the eye ball. Its refractive power is approximately 20 dioptres. It plays a crucial role in accommodation, or the focusing of light on the retina.


Unit of measure of the refractive power of a lens, used in quantifying the loss of visual acuity. A 1 dioptre lens has a focal length of 1 m. A concave lens of -1.00 dioptre corrects a myopia of -1.00 dioptre. A convex lens of +1.00 dioptre it corrects a hyperopia of +1.00 dioptre.

Endothelial cells

Very thin lining of cells on the inner surface of the cornea, and which communicates with the aqueous humor.


Non-vascularized tissue consisting of one or more layers of cells that are stuck together, without any intercellular space or liquid. The epithelium serves to cover and protect the external surface of any living tissue or body. In the eye, the epithelium is composed of seven layers of transparent cells.


Medicated drops for the eyes.


A vision condition characterized by difficulty focusing on nearby objects and, in some cases, distant objects as well. Currently, hyperopia is treated by reshaping the periphery of the eye using an Excimer laser to increase the curvature of the eye and thereby achieve perfect vision without the need for glasses or contact lenses.

Intraocular lens

Synthetic, transparent lens that is implanted and fixed inside the eye to correct vision. It is generally used to treat cataracts as well as severe cases of myopia, hyperopia and/or astigmatism.


The pigmented structure of the eye that gives it its distinct colour and controls how much light enters the opening in the centre of the eye, called the pupil.


A thinning of the cornea, resulting in a bulging outward and deformation of the cornea. This problem, which can start in adolescence and progress until the person reaches his or her forties, inevitably causes vision loss. In recent years, two reliable approaches have been developed to slow or altogether stop the progression of keratoconus, and thus avoid the need to resort to corneal transplantation.

Local anesthesia

Reversible blocking of sensation caused by the administration of a medicated substance by way of injection or topical application.


Small, extremely thin surgical blade used to create the corneal flap during surgery.


A vision problem characterized by difficulty focusing on distant objects. Today Myopia is corrected by reshaping the surface of the eye using an Excimer laser to reduce the curvature of the eye and thus achieve perfect vision without the need for glasses or contact lenses.


Physician and surgeon with specialized training in eye care, including the prevention, diagnosis and treatment of vision problems and eye diseases.


Eye care professional with special training in assessing visual functioning in order to diagnose vision problems or screen for certain eye diseases. Optometrists can prescribe certain medications and corrective measures, such as glasses and contact lenses.

Posterior capsule of the crystalline lens

Fibrous and elastic anatomical structure in the form of an envelope that covers the lens.


A weakening of the focusing mechanism inside the eye that enables us to switch between close-up and distance vision. This problem translates into blurred vision for nearby objects, starting from approximately age 40 and often involves difficulty reading.


Circular opening, or aperture, at the centre of the iris through which light enters the eye. The iris, which is the diaphragm of the eye, contracts in the presence of light.

Refractive surgery

Any procedure intended to correct refractive errors of the eye using a medical device or equipment, such as a laser, implant or surgical instruments.


Thin, transparent membrane of the eye, which is innervated by the optic nerve, and onto which are projected the images transmitted by the crystalline lens.


Layer of the cornea that accounts for 90% of its thickness. It consists of interconnected layers that are themselves made up of parallel collagen fibrils.