Accommodation

The adjustment of the total refractive power of the eye to allow seeing at various distances. Accommodation is effected mainly by changes in the convexity of the lens.

Artificial tears

Topical eyedrops, often formulated to match the pH of human tears. Artificial tears are used to soothe the eyes, moisturise dry spots on the eyes, supplement the eye's own tears, and protect the eye from irritation.

Astigmatism

A defect of an ocular structure (most often the cornea), causing parallel rays of light to be refracted without meeting at a single focal point. An imperfect blurred or smeared image is formed on the retina.

Cyclopentolate

An anticholinergic topical eyedrop used, especially in the form of its hydrochloride C17H25NO3·HCl, to dilate the pupil, and relax the ciliary body of the eye for ophthalmological examination and measurement of the refractive error of the eye in the absence of accommodation.

Emmetropia

The absence of refractive errors of the eye with its accommodation relaxed. In emmetropia, parallel rays of light are all brought accurately to a focus upon the retina of the unaccommodating eye.

Epi-LASIK

Epi-LASIK includes mechanical separation of the corneal epithelial layer, as a sheet, from the underlying stroma. After excimer laser ablation, the epithelial sheet is repositioned over the treatment zone.

Excimer laser

Excimer lasers are ultraviolet gas lasers. The term excimer is a contraction of exited dimer. At first, intermediary structure for the excited state of these laser heads was thought to include the formation of dimers, two atoms of the same element. Although the gas halide is now recognised as the activated intermediary, instead of a dimer, the term excimer is broadly accepted.

Femtosecond laser

Femtosecond lasers are neodymium glass lasers able to generate femtosecond pulses with high output and short exposure time. The pulses are focused to preset depths below the corneal surface. Blunt separation of the photodisrupted corneal stroma creates the LASIK flap.

Flying spot

Flying spot ablation uses a computer-controlled small fixed diameter beam that allows for peripheral ablation or customised ablation pattern. This technique needs prolonged ablation time, and, not uncommonly, it is coupled with eye-tracking systems.

Hyperopia

A condition in which visual images come to a focus behind the retina of the eye. Accommodation often improves distance vision, but might not be sufficient to improve near vision.

Keratectomy

Surgical excision of part of the cornea.

Keratoconus

Progressive thinning of the central cornea, resulting in corneal bulging and formation of a rounded cone. Keratoconus produces moderate to severe corneal distortions and is often associated with myopia and irregular astigmatism.

Keratomileusis

Keratoplasty in which the anterior lamellae of the cornea are removed, frozen, reshaped, and reinserted to correct refractive error.

Laser in-situ keratomileusis

LASIK is a refractive surgical procedure that permanently changes the shape of the cornea with an excimer laser and a microkeratome or femtosecond laser. The microkeratome or femtosecond laser produces a hinged lamellar flap in the cornea. The flap is folded back, exposing the corneal stroma. Pulses from a computer-controlled excimer laser vapourise a portion of the stroma before the flap is replaced.

Myopia

A condition in which parallel rays of light are bent to a focus in front of the retina of the eye. In myopia, the cornea might be too steep or the axial length of the eyeball might be too long, resulting in defective vision of distant objects.

Presbyopia

A visual condition that generally becomes apparent in middle age and in which loss of elasticity of the lens of the eye causes defective accommodation and inability to focus sharply for near vision.

Photorefractive keratectomy

The use of laser ablation to reshape the stromal surface of the cornea for the correction of myopic or hyperopic refractive errors, or both.

Phototherapeutic keratectomy

A method to treat corneal pathology using ablative laser energy.

Radial keratotomy

A surgical operation on the cornea to correct myopia. This operation includes making a series of incisions in a radial pattern, resembling the spokes of a wheel, to flatten the cornea.

Visual acuity

The relative ability of the visual system to resolve detail, usually expressed as the reciprocal of the minimum angular separation, in minutes of arc, of two lines just resolvable as separate and that forms in the average human eye an angle of one minute of arc; often measured by Snellen test, a test presenting letters of graduated sizes to establish the smallest size that can be read at a standard distance (a 20/20 letter located 20 ft away from an eye subtends an angle of 5 minutes of arc at the eye). The normal acuity in the human eye is 20/20, although some eyes are capable of 20/15 or even 20/10 acuity.