Aberrations: a deviation in power of the optical components of the eye, (either individually or in combination), from the ideal system that would produce a perfect point image on the retina. The aberrations induced by refractive surgery are best described as irregular but some studies tend to refer to them as being most similar to spherical aberration and coma.
Accommodation: the ability of the crystalline lens to increase its power and therefore focus for near targets. Accommodation reduces with age.
Astigmatism: a difference in power between the two principle meridians of the eye, usually perpendicular to each other (regular astigmatism). The resulting retinal image is elongated. Astigmatism can be associated with myopia or hypermetropia
Contact lens associated papillary conjunctivitis: inflammation of the conjunctiva that lines the inside of the upper eyelid. Redness and significant surface roughness (papillae) prevent successful contact lens wear. Cause: mechanical factors, immune response to lens deposits and lack of oxygen to the eye.
Contrast sensitivity: the lowest contrast at which a particular spatial frequency can be resolved. The peak of the average contrast sensitivity function falls between 2 and 5 cycles per degree.
Corneal oedema: swelling of the cornea resulting in clouding of the tissue, increased light scatter and reduced contrast sensitivity. Usually caused by lack of oxygen to the cornea as a result of prolonged contact lens wear.
Extended wear contact lenses: contact lenses designed to be worn throughout the day and night, usually for a continuous period of 7-30 days before being replaced.
Hypermetropia: long sight, which may manifest itself as poor near vision. The retinal image is blurred because the eyeball is too short or the power of the refracting elements is too weak. Young hypermetropes can accommodate to increase the positive power of the eye and overcome some or all of their hypermetropia. Older hypermetropes may require a refractive correction at both distance and near.
Intraocular light scatter: the scattering of light by particles within the ocular media. Light scattered towards the retina reduces the contrast of the retinal image and hence reduces visual performance.
Keratectasia: bulging of the cornea as a result of corneal thinning. Can be associated with a condition known as keratoconus or occasionally refractive surgery (LASIK).
LASEK: an epithelial flap is created using alcohol and the underlying anterior stroma is ablated with an excimer laser before the flap is replaced.
LASIK: a thin flap of corneal tissue is cut using a microkeratome and reflected back. The underlying stroma is ablated using an excimer laser to treat refractive error and the flap is repositioned.
LogMAR: logarithm of the minimum angle of resolution
Monovision: modification of the refractive correction to focus one eye for distance and one eye for near vision. This is usually achieved with contact lenses and allows presbyopes to function without extra glasses under the majority of conditions
Myopia: or short sight. Distance vision is blurred because the retinal image is focused in front of the retina. The eyeball is too long or its refractive components are too strong.
Neovascularisation: the growth of new vessels around the edge of the cornea (limbus), usually in response to excessive soft contact lens wear. In severe cases, the vessels extend towards the centre of the cornea and are at risk of leaking and causing corneal opacification.
Presbyopia: an individual is said to be presbyopic when their accommodation is no longer sufficient to focus for objects at their habitual near working distance. This generally occurs when the accommodation is 3D or less, around the age of 45 years.
PRK: following removal of the corneal epithelium, the underlying stroma is ablated using an excimer laser to reshape the surface to treat refractive error. The epithelium regrows across the treated zone within 5-7 days.
Rhegmatogenous retinal detachment: detachment of the neural layers of the retina from the retinal pigment epithelium due to the entry of intraocular fluid in to the subretinal space through a break in the retina.
Unaided vision: the measurement of vision without correction of any refractive error
Visual acuity: the measurement of vision corrected with spectacles or contact lenses