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The laser eye surgery glossary

Laser eye surgery London

If you are looking for a laser eye clinic in London, then the London Vision Clinic should be on your shortlist.  

That may seem like a strong statement, but the London Vision Clinic emphasises that, in expert hands, laser eye surgery can be an extremely safe, life enhancing experience. More than 100,000 laser eye surgery procedures will be done in the UK this year. Read the guide to Laser Eye Surgery to help make the right choice for your eyesight.

Contact London Vision Clinic




The removal of organic material from the cornea by an excimer laser in laser eye surgery.



The automatic adjustment of the eye for seeing at different distances effected chiefly by changes

in the convexity of the crystalline lens.



The sharpness or clarity of vision. The most common measure of visual acuity is the Snellen acuity

chart used by optometrists and ophthalmologists. Normal acuity is 20/20 (6/6 metric version).



A trade name for a very-high frequency ultrasound (VHFU) manufactured by UltraLink LLC, that measures corneal thickness to the accuracy of 1 micron. It produces a 3D image, which means it detects the thinnest point of the cornea with great accuracy, and displays a profile of the depth of the cornea.



A condition in which the surface of the cornea is not spherical, but shaped like a rugby ball. An astigmatic cornea focuses incoming images on two separate points in the eye, creating a distorted image. The second number on your glasses prescription refers to your degree of astigmatism.



BCVA or Best Corrected Visual Acuity

A measure of your sight whilst wearing corrective lenses, such as glasses or contact lenses.



Corrective lenses that have two powers of correction. Typically, most of the lens is used for distance

vision while a smaller area is for near vision. Bifocals and trifocals are normally prescribed for individuals with Presbyopia.


Binocular vision

The blending of images seen individually by each eye into a single image.


Blended vision

A laser eye surgery technique for Presbyopia in which one eye is treated to view objects mainly at distance, but a little up close, and the other is treated to view objects mainly up close, but a little at distance. The brain puts the two images together and enables the individual to see distance and near without effort. In most cases, the brain is able to compensate and you will experience a depth of focus and overall visual acuity, without the need to wear glasses or contact lenses.



A beam of excimer laser energy applied across the entire ablation zone of the cornea at one time. See also variable beam and flying spot.



Central Ablation Zone

See Optical Ablation Zone.


Clear Lens Exchange (CLE)

A surgical procedure, similar to cataract surgery, in which a surgeon removes the crystalline lens from the eye and replaces it with a clear plastic intraocular lens (IOL). The IOL corrects the

refractive error.


Conductive Keratoplasty (CK)

A surgical procedure for hyperopia and mild long-sightedness in which a surgeon uses a probe to apply high frequency radio waves to the corneal tissue, causing it to shrink by heating. This controlled shrinkage reshapes the cornea to correct its refractive error.


Contrast sensitivity

The ability to perceive differences between an object and its background, i.e. the ability to distinguish a grey object on a grey background.



The transparent anterior part of the external coat of the eye covering the iris and the pupil and

continuous with the sclera. The dome-like cornea provides approximately two-thirds of the optical power of the eye. Light passes into the eye through the cornea, and is reflected out of the eye, making the iris and pupil of the eye visible.


Corneal abrasion

A scratch or similar trauma to the outer surface of the cornea.


Corneal haze

A condition in which the cornea develops hazy scarring that can reduce Contrast Sensitivity and

visual acuity.


Corneal topography

The process of mapping the surface shape of the cornea with a camera or computer system.


Crystalline lens

The natural lens of the eye, located behind the iris, helps rays of light to focus on the retina. The lens is transparent, but with age, it can become cloudy (a cataract). The lens has the ability to vary its power to focus light from objects at different distances.


Custom cornea

A trade name for the use of wavefront-guided laser eye surgery, using the LADARVision™ excimer

laser built by Alcon.



A trade name for the use of wavefront-guided laser eye surgery with the VISX S4 excimer laser.™




A potential complication of laser eye surgery. In perfect centration the centre of the corneal ablation exactly coincides with the centre of the visual axis. This is like looking through the very centre of your spectacle lens, allowing for sharp, in focus vision. If you look through the periphery of your spectacle lens the optics are distorted. Decentration can cause various symptoms, including edge glare or even monocular double vision. Other factors, such as the size of the pupil, whether it is dark outside (so your pupil will enlarge), or the size of the ablation zone will affect the severity or presence of symptoms.


Diffuse Lamellar Keratitis (DLK)

DLK is an inflammation under the LASIK flap of the cornea, which is caused by a response to the presence of sterile inflammation in the flap interface. The complication occurs in the early post-operative period. Vision is hazy and at times the eye is painful and teary. Sufferers may also

experience intolerance to bright light.



The process by which the pupil enlarges, usually in low light conditions.


Dioptre (D)

A unit of measurement of the refractive power of a lens. A one-dioptre lens will focus parallel light rays one metre from the lens and a two-dioptre lens will focus half a metre from the lens.


Double image

Also called ghosting. If you look at a clock and some of the numbers have a lighter image just off to the side, this is a typical double image problem.


Dry eye

The term dry eye is used to describe a variety of disorders with similar symptoms: discomfort, a feeling of dryness, burning or stinging, grittiness, foreign body sensation, and photophobia.




An outward bulging and thinning of the cornea due to raised internal eye pressure and/or a weakened cornea.



The inner layer of cells on the inside surface of the cornea.



A secondary refractive surgery treatment, or retreatment, made to refine or improve the original visual result. Higher corrections and wider optical zones sometimes result in under correction or over correction. Enhancement treatment is usually a small correction and generally is very accurate.



See Laser Assisted Epithelial Keratomileusis.



The outer surface layer of the cornea, like the epidermis or outer layer of the skin.


Epithelial ingrowth

A potential complication of LASIK in which epithelial cells under the corneal flap begin to grow and multiply. The most common treatment is lifting the corneal flap, removing the cells, irrigating the interface, and repositioning the flap. Most cases, if managed appropriately, result in a good outcome.


Excimer laser

An argon-fluoride laser that emits ultraviolet light in pulses, at a wavelength of 193nm. The term excimer comes from the concept of ‘an energized molecule with two identical components’. Each pulse of this ‘cool’ laser removes 1/4000th of a millimetre of tissue from the targeted surface, by breaking the bonds between molecules of collagen. It would take about 200 pulses from an excimer laser to cut a human hair in half.




Common term for hyperopia or long-sightedness, i.e. being unable to see nearby objects clearly.


Femtosecond laser

A femtosecond laser is a laser that emits optical pulses with a duration that is one quadrillionth of a second. The Intralase laser uses femtosecond laser technology to create a corneal flap in LASIK eye surgery.


Flying spot

A very small spot of excimer laser energy applied in rapid succession at different locations across the ablation area of the cornea.


Focusing power

The cornea is responsible for about two thirds of the focusing power of the eye. As light enters the eye, it is focused by the cornea. Then, as it passes through the pupil, the lens adjusts the focus, depending on the distance of the object being viewed. Nearby objects, such as a book or computer screen, require more power than distant ones, such as traffic signs.




A common term for seeing double images. If you look at a clock and some of the numbers have a lighter image just off to the side, this is ghosting.




Images from light sources look blurred, with circles radiating outward from the centre. Halos can appear as a complication of refractive surgery but they also occur naturally.



The cornea becomes cloudy, or opaque, rather than clear.



Also known as farsightedness or long-sightedness. Hyperopia occurs when the eyeball is too short from front to back, or the focusing mechanism is too weak. This causes light rays to be focused behind the retina, rather than on the retina. People with hyperopia have difficulty seeing close objects.



Implantable Contact Lens (ICL)

The trade name for the STAAR Myopic Implantable Contact Lens (ICL). It is used for the correction of high refractive error, and is worn behind the iris in the posterior chamber of the eye. Also known as a Phakic Intraocular Lens (P-IOL), the design of the ICL is very similar to that of intraocular lenses used for cataract surgery. The lens material, known as Collamer, has a history of safe use in approved-standard posterior chamber intraocular lenses.



The trade name for Intrastromal Corneal Ring Segments.



A femtosecond laser that uses amplified light energy to cause tiny bubbles to form at a predetermined

depth to create a corneal flap in LASIK. Thousands of these bubbles next to each other create an

incision. The other technique for making an incision is by using a microkeratome.



An invented word referring to LASIK where the corneal flap is created by a femtosecond, or

IntraLase laser.



Inside the eye.


Intraocular Lens (IOL)

An artificial silicone, acrylic or plastic lens used to replace the natural crystalline lens of the eye.

See ICL, Phakic Intraocular Lens (P-IOL), and Clear Lens Exchange.


Intrastromal Corneal Ring Segments (ICRS)

Small slivers of plastic implanted at the outer edges of the cornea to flatten the centre and reduce

myopia and keratoconus. Trade name Intacs.™



Intraocular Pressure – pressure inside the eye.



Surgical removal of corneal tissue.



A disorder that causes thinning and asymmetry of the cornea. The normally symmetrical shape of the cornea becomes distorted. A cone-shaped bulge develops, resulting in significant visual impairment.



A refractive surgical technique in which a thin, circular flap of the cornea is removed, frozen, reshaped on a lathe, and replaced upon the cornea.



A surgical incision of the cornea as in Radial Keratotomy.




See Laser Assisted sub-Epithelium Keratomileusis.



Light Amplification by the Stimulated Emission of Radiation. Laser light is composed of one colour (wavelength), travelling in one direction, and each light wave is in step with the next, making the laser light millions of times more powerful than ordinary daylight.


Laser Assisted In-Situ Keratomileusis (LASIK)

A surgical procedure to reshape the central cornea, decreasing or eliminating myopia, hyperopia, and astigmatism. The refractive surgeon uses a microkeratome device to cut a thin layer from the cornea. This flap is then lifted like a hinged door and the exposed eye surface is reshaped using the excimer laser. After altering the corneal curvature, the flap is replaced. It adheres quickly, without stitches.


Laser Assisted Sub-Epithelium Keratomileusis (LASEK)

A surgical procedure to reshape the cornea by detaching the epithelium with an alcohol solution that softens it and allows it to be rolled back into a flap. After excimer ablation to correct the vision, the flap of epithelium is repositioned over the cornea.


Laser Thermal Keratoplasty (LTK)

A surgical procedure where an instrument applies rings of laser energy to the mid-periphery of the cornea. Each ring gently heats collagen in the cornea to change the corneal shape. Performed

as a non contact procedure.



A transparent double convex (outward curve on both sides) structure between the iris and the

vitreous humour of the eye. The human lens provides focusing power. In people under 45 years the lens is able to adjust its power, allowing the eye to switch between nearby and distant objects while seeing clearly.



Micron (m)

A unit of length equal to one-millionth of a metre.



A surgical device for creating a flap of corneal tissue, used in LASIK and some transplant techniques.

It is fixed to the eye with a vacuum ring, and flattens, then cuts the cornea. The mechanical

microkeratome uses a very sharp, thin metal blade that oscillates at 1000 rpm. The femtosecond laser microkeratome uses amplified light energy to cause tiny bubbles to form at a predetermined depth. Thousands of these bubbles next to each other create an incision. (See IntraLASIK.)



A contact lens technique to overcome the effects of Presbyopia by correcting one eye for near vision and the other for far vision.



Also known as nearsightedness or short-sightedness. Myopia occurs when the eyeball is too long from front to back, or the eye’s focusing mechanism is too strong. This causes light rays to be focused in front of, rather than on, the retina. People with myopia have difficulty seeing distant objects. This refractive abnormality is corrected with a minus (negative or concave) lens.




Common term for myopia.




To do with the eye.



A medical doctor who specialises in the diagnosis and medical or surgical treatment of eye diseases. Ophthalmologists have medical degrees and further specialist training. Ophthalmologists are traditionally surgeons but some choose not to perform surgery and work as medical ophthalmologists. An ophthalmologist may also prescribe glasses and contact lenses.


Optic nerve

The optic nerve is a bundle of nerve fibres, about the diameter of pencil, which passes through the back of the eyeball and connects the retina to the brain. The optic nerve carries visual messages from the photoreceptors of the retina to the brain.


Optical zone

The area of the eye through which light passes to the retina. To reach the retina light must pass

through the cornea, the aqueous humour, the pupil, the crystalline lens, and the vitreous gel. The

optical ablation zone is the area where a laser has created full refractive error correction.



An expert in the art and science of making and fitting glasses. An optician may also be qualified to dispense and/or fit contact lenses.



An optometrist is a non-medical eye health provider, who specializes in the examination, diagnosis,

treatment, management, and prevention of diseases and disorders of the visual system. In the UK, optometrists complete a three-year degree. Many optometrists dispense glasses and contact lenses. Optometrists may not prescribe medicine, as they are not medical doctors.



An outcome of refractive surgery in which the expected amount of correction is more than

desired. Overcorrection occurs most frequently when healing does not occur as predicted. It is

easily treated by an enhancement procedure.




A device that measures the thickness of the cornea. Ultrasound pachymeters have a probe that is placed gently onto the anaesthetised corneal surface. It emits an ultrasound probe that measures the thickness of the cornea.


Peripheral vision

The ability to see objects and movement outside the direct line of vision.


Phakic Intraocular Lens (P-IOL)

This is a tiny lens that is placed inside the eye, in front of the natural crystalline lens, to provide

additional refractive change. It is placed either immediately behind or in front of the iris. It is generally reserved for cases where there is an extreme refractive error.



The ‘cold’ process of tissue removal using excimer laser radiation at a 193nm wavelength. This far-ultraviolet wavelength possesses light photons so powerful that the molecular bonds of the target corneal tissue break down and fly off the surface. Microscopic pictures show incredibly precise cuts with no evidence of tissue damage in the remaining tissue.


Photorefractive Keratectomy (PRK)

A surgical procedure using an excimer laser to reshape the central cornea to a flattened shape for

people who are myopic, or a more curved surface for people who are hyperopic. Photorefractive

Keratectomy techniques may also be used to correct astigmatism.



No refractive error. Normal vision without the need for glasses or contact lenses.



Part of the normal process of ageing. As we become older, the crystalline lens begins to lose its ability to zoom from distance to near vision. To compensate for this, people wear reading glasses such as Bifocals. Mild myopia effectively counteracts presbyopia. Refractive surgery does not cure presbyopia but it can now help with the vision problems that result.


Presbyopic Lens Exchange (PRELEX)

The term Presbyopic Lens Exchange (PRELEX) was created to describe a Clear Lens Exchange with an intraocular lens designed to accommodate and alleviate presbyopia.



The small black circular space in the centre of the iris. The pupil changes its diameter in response

to different light levels, it become bigger in the dark and smaller in bright light. The pupil varies the amount of light reaching the retina and the depth of focus of the eye.



A diagnostic test to measure the size of your pupils, the windows that let light into your eye.




A test to determine the best glasses or contact lenses to correct a refractive error (myopia, hyperopia, or astigmatism). The bending of light by the use of a lens or other material.


Refractive Error

A measurement of visual imperfection. The degree to which images received by the eyes are not

focused on the retina (causing myopia, hyperopia, or astigmatism), measured in dioptres.


Refractive Lens Exchange

Also called Clear Lens Exchange (CLE). The process of removing the natural crystalline lens of

the eye and replacing it with a plastic or silicon intraocular lens, to correct refractive error. It is similar to cataract surgery. It carries more risk than laser eye surgery as it involves entering the eye. It is generally performed when laser eye surgery is not possible.


Refractive surgery

Any surgical procedure that alters the focusing power of the eye. Refractive surgery may include

corneal surgery such as LASIK, LASEK, PRK, Intacs, CK, RK, LTK or lens surgery such as CLE

or P-IOL.



A return towards the original refractive state. Usually corrected by an enhancement procedure.



The light sensitive layer of cells (rods and cones) on the inner, back surface of the eye that converts

light images into nerve impulses. These are then sent along the optic nerve for transmission to the brain. Akin to the film in a camera.



See enhancement.



Snellen Visual Acuity Test

The Snellen Test is one of many tests used to determine visual acuity. It uses a white chart with the big black letter E at the top and lines of letters that become increasingly smaller. The test gives a result such as 20/40, which means that the person can see an item 20 feet away with the same clarity as a normally sighted person can see at 40 feet.



A complication of refractive surgery. Images from light sources blur, with spikes radiating out from the centre. Starbursts may also occur naturally.


Stromal tissue

This is the layer directly under the epithelium of the cornea. Because it undergoes very minimal

regeneration, this is the tissue that allows the excimer laser to make permanent changes in the

shape of your eye.




A laser is applied in variable spot sizes across the ablation area. See ‘Broadbeam’ and ‘flying spot’ for a definition of other techniques.


Visual acuity

Seeing clearly. The ability to distinguish the details and shapes of objects; also called central vision.


Visual field

The extent of an area seen by the eye in a given position of the gaze. The central visual field is directly in front of the object at which we are looking. The peripheral visual field is ‘side vision’. The fields of each eye partly overlap.




See Wavefront Supported Customised Ablation.



A technology used to determine and measure high order aberrations. These aberrations affect the quality of vision. Conventional eye examinations can detect two types of error on the cornea: spherical (myopia and hyperopia), and cylindrical (astigmatism). Wavefront diagnostics can detect an infinite set of ocular aberrations. When the laser beam of the wavefront sensor enters the eye, it has a flat wavefront. This flat wavefront is distorted by imperfections as it travels through the eye. Using this information to correct these small irregularities in the optics of an eye can lead to better vision without glasses than previously with glasses.


Wavefront Supported Customised Ablation

A trade name of the Carl Zeiss Meditec WASCA Aberrometer and the MEL 80 excimer laser system, when used for wavefront-guided excimer laser treatment.




The brand name for wavefront guided custom ablation on the Bausch & Lomb Technolas excimer laser.

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