Resource Centre - Glossary

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Aberration - A distortion seen through a lens. Non-convergence of rays of light

Accommodation - Ability of eye to automatically change focus - see at one distance and then at another.

Aftercare - A never-ending period of time needed to check eyes and lenses after they are fitted.

Allergy - Abnormal sensitivity reaction of the body to substances that are usually harmless.

Amblyopia - Also called lazy eye; diminished sight in one eye.

Anterior chamber - Area behind the cornea and in front the iris and lens filled with fluid called aqueous humour. Usually about 3mm deep.

Antioxidant - Substance: inhibits oxidation and can guard the body from the damaging effects of free radicals. Molecules with one or more unpaired electrons, free radicals can destroy cells and play a role in many diseases. Antioxidants may help prevent macular degeneration and other serious eye diseases.

Aqueous humour - Clear fluid in the eye - provides nutrients and determines intraocular pressure.

Aphakia - Condition of eye where crystalline lens has been removed.

Astigmatism - Normal condition where the eyeball is not spherical.

Bifocal - A lens that focuses for distance and reading.

Blank - A singular piece of lens material that is yet to be trimmed to specification- a raw cast button.

Calcium - A constituent of tears that is readily deposited on areas of a lens that has lipid contamination. It becomes crusty and irritating often attracting more proteins as a result of foreign body sensation.

Capsular bag - Sack positioned behind the iris that contains the crystalline lens.

Cataract - Name given to crystalline lens when it becomes opaque, resulting in impaired vision.

Choroid - Layer of blood vessels and pigments (usually brown or blue) - lies beneath the sclera (the white of the eye). Often mistakenly called the iris.

Cones - Colour-sensitive nerve endings in retina.

Conjunctiva - Transparent skin on the front of eye covering the white visible part of the eye and underside of eyelids.

Conjunctivitis - Inflammation of conjunctiva.

Convergence - Ability of eye to turn inward - causes eyestrain, blurred vision, with near tasks such as reading.

Cornea - Clear part of the eye covering the iris and pupil - lets light into the eye, permitting sight.

Corneal implants - Devices (such as rings or strips) placed in the eye, usually to correct vision.

Corneal lens - Contact lens that sits within the boundaries of the cornea -usually a rigid lens.

Corneal topography - Process of using a camera/computer system to map the cornea for refractive surgery, contact lens fitting and corneal disease management.

Corneal ulcer - Wound in the surface of the eye caused by injury, dryness due to lack of tear production, or infection.

Cosmetic lens - A lens used to cover an unsightly eye or to change the appearance of the eye.

Crystalline lens - Variable lens inside the eye behind the iris responsible for one third of the focusing of the eye and changing the focus from distance to reading.

Daily wear - Soft contact lenses worn every day for six months up to a couple of years. They require daily cleaning and disinfecting, as well as a periodic enzymatic soak (usually once a week).

Diameter - Overall diameter of a lens. Although it may refer to one of the zones or optic sizes.

Diabetic retinopathy - Leaking of retinal blood vessels in advanced or long-term diabetes, affecting the macula or retina. Vision can be seriously distorted or blurred.

Dioptre - Unit that measures the refractive (light-bending) power of a lens; eyecare practitioners use it in eyeglass and contact lens prescriptions. A negative number refers to nearsightedness, while a positive number refers to farsightedness. For example, someone with -8.00 dioptre lenses is very nearsighted, while someone with +0.75 dioptre lenses is only slightly farsighted.

Dispensing Optician - An optician who makes up spectacles or fits contact lenses to a written prescription.

Disposable lenses - Contact lenses with a limited life span (daily, one week, two weeks).

Dk - Permeability value of a given contact lens material - low Dk is around the 2- 30 mark, Mid Dk is between the 30- 50 mark & High Dk is above these.

Dry eye - Lack of sufficient lubrication and moisture in the eye. Most dry eye complaints are temporary and easily relieved; dry eye syndrome, also called keratoconjunctivitis sicca, is chronic and needs more advanced treatment by an eyecare practitioner.

Enzymatic cleaner - A cleaner that removes protein deposits and other debris from contact lenses. It's recommended for use either daily, weekly, or monthly.

Extended wear lenses - Lenses designed to be kept in for longer periods.

FDA - Food & Drug Administration - A United States government body - oversees medical devices, including contact and Intraocular lenses. The FDA must review these products before they can be marketed in the US.

Fenestration - Hole drilled through contact lens to increase tear flow.

Fovea - A depression in the retina that contains only cones (not rods), and that provides acute eyesight.

Gas Permeability - Amount of air that can go through a lens.

Glaucoma - Condition where internal pressure of the eye increases causing damage to the retina and possibly blindness.

HAPTIC - Means by which the optic portion of a lens is held in position - generally used in scleral lenses.

H.E.M.A - Hydroxy ethyl methacrylate - The backbone of many hydrophilic contact lenses. HEMA has a nominal water uptake of 38% giving an expansion factor of approx 1.20. This material was one of the first to be used in hydrophilic contact lenses and by adding other monomers, such as VynilPyrrolidone(VP) ,by cross linking , the water content can be raised . For example, CF42 = Hema / VP. Hema is a very stable & safe material once polymerised.

Hybrid - Hybrid of materials - a mixture of RGP & Hydrophilic monomers.

Hydrophilic - Have an affinity for water. Soft contact lenses are hydrophilic.

Hypermetropia - Long sight normal eye condition where the eye is too small and eyes have to over focus to see clearly.

Hyperopia - Long or farsightedness - near objects are blurry, but far objects are in focus.

Implant - Plastic lens inserted into the eye during surgery to replace crystalline lens that has been removed.

Intraocular lens (IOL) - Artificial lens that an opthalmic surgeon places in a patient's eye after removing the eye's natural lens. Like a contact lens, it has a built-in refractive power tailored specifically to the patient's visual condition.

Iris - Coloured part of the eye that is in fact a diaphragm opening and shutting to let in more or less light into the eye. Also opens under other influences (eg. Fear).

Keratoconic - A design of contact lens that tends to have a very steep/small back optic radius that is used in the fitting of keratoconic corneas. A keratoconic cornea is usually as a result of the thinning of the cornea and the resultant intraocular pressure distends the corneal tissue. In severe cases the cornea can rupture.

Keratoconus - Condition in which the cornea develops a cone-shaped bulge that can result in major blurring and distortion.

Keratometer - Instrument used to measure the curves on the cornea. (Ophthalmometer)

Keratitis - Inflammation of the cornea.

Keratoconjunctivitis - Inflammation of the cornea and conjunctiva.

Lens - Nearly spherical body in eye - focuses light rays onto retina.

Lipid - Lipids are fatty acids present in tears. They are very attracted to areas of the surface of contact lenses that are dry or hydrophobic. In hydrophilic lenses they penetrate the matrix and become sites for deposition of calcium and phosphates.

Liquid lens - Lens made of tears trapped between the contact lens and the cornea.

Macula - Curved spot at the centre of the retina where 'cones' are concentrated to give very accurate sight and colour vision.

Monovision - Vision correction method for those with presbyopia in which one eye is corrected for near vision and the other for far, either through contact lenses or refractive surgery. Eliminates the need for reading glasses, but decreases depth perception.

Moulded lens - Lens that is made by casting between two pre-made moulds. A very efficient way of making lenses but initial setup cost is high and not easily variable on parameters.

Myopia - Nearsightedness - visual images come to a focus in front of the retina, resulting in defective vision of distant objects.

Myopic creep - Increase in myopia induced by contact lenses

Multifocal - Type of spectacle or contact lens design - includes more than one focal area, such as bifocals or trifocals.

Non ionic - Refers to the surface charge of a material. A material is classed as non ionic if it contains less than 0.2% ionisable material. All of the Contamac hydrophilic materials are classed as non ionic and they repel a larger proportion of deposits.

Ophthalmic Optician - Name given to opticians who examine eyes. Now almost obsolete as the name optometrist is used.

Ophthalmic Medical Practitioner - Doctor who examines eyes.

Ophthalmologist - Doctor who specialises in treating eye disease.

Ophthalmic Surgeon - An ophthalmologist who also carries out eye surgery.

Ophthalmoscope - An instrument used to examine the inside of the eye through the pupil.

Optic Nerve - Second cranial nerve that carries information from the retina to the brain

Optometrist - An optician who specialises in examining eyes and prescribing optical appliances such as spectacles and contact lenses.

Orthokeratology - Procedure where doctor fits special gas permeable contact lenses to reshape cornea and correct errors like nearsightedness. Often, patients wear the lenses just at night. (ortho-k).

PEG - Poly Ethylene Glycol is a soap like substance used in the polishing mix when tumble polishing PMMA IOLs. It acts as a cleaner and lubricant.

Perspex - Trade name (ICI) for acrylic resin.

Phakic IOLs - High plus power Intraocular lenses to correct aphakia (Severe long sight as a result of no natural or artificial intraocular lens present in the eye).

Phosphate - Phosphates are present in tears and combine with fatty acids and calcium to make hard, rounded deposits on the surface of hydrophilc lenses. These are commonly known as "calculii".

PMMA - Old-fashioned hard contact lenses were made of PMMA, (polymethyl methacrylate) and non-oxygen permeable. Now rigid (Rigid Gas Permeable) lenses contain other polymers that allow oxygen to reach the eye.

Posterior chamber - Part of the eye behind the iris and in front of the lens.

Presbyopia - Inability of the eye lens to focus incoming light because of age-related stiffening of the lens tissue - result is blurred vision at reading distance and eyestrain.

Prism - Wedge shape to a lens that has the optical effect of displacing the image by a given amount in dioptres. Prism is used in special lenses such as a toric to provide a ballast effect to prevent the lens from rotating.

Prism ballast - Specifically making a lens with a thin top and a thick bottom so that it is heavier at the bottom.

Protein - Proteins are naturally present in animal tears and become deposited on contact lens surfaces during wear (and storage if not removed by cleaning). Initially they make the lens surface wettable and "comfortable" but soon become mineral in character - causing discomfort and cloudy vision. The eye deposits more protein to counter this and a cycle of deposition is started. Enzyme tablets are used to remove the deposits.

Pupil - Round, dark centre of the eye - opens and closes to regulate the amount of light the retina receives.

Reduced optic - Process by which a lens is made thinner by making the centre smaller.

Registered Optician - Term used in the UK to describe a practise where both optometrists and dispensing opticians work together.

Retina - Layer at the back of the eye that has millions of light-sensitive nerve cells connecting to the brain via the optic nerve.

Retinal blood - vessels Transparent blood vessels that supply the retina that are seen with the ophthalmoscope during an eye examination.

Retinoscope - Instrument used to reflect light off the retina so that the prescription can be gauged objectively.

Rigid lens - A hard (rigid gas permeable) lens rather than a soft lens.

Rods Light - sensitive nerve cells in the retina that tell light from dark and are responsible for night vision.

Sclera - White outer coat of the eye that surrounds the optic nerve.

Soft contact lenses - Hydrated hydrophilic contacts made of gel-like plastic/ polymers containing varying amounts of water.

Spectacle blur - Reduced vision noticed particularly with spectacles when contact lenses are removed.

Stress - Uneven tension across a mass of material - usually due to poor polymerisation and annealing or can be induced by machining or heat. Instability and warpage can be detected using polarised light

Suction holder - A rubber sucker used to remove contact lenses.

Surfactant - Cleaner used to clean the surface of the contact lens.

Swell factor - Value that represents the amount a hydrophilic material expands by when immersed in saline or water. Linear swell describes the increase in diameter and radial swell describes the amount a base curve changes. For example, CF38 tends to have a linear sell of around 1.20.A lens made with a dry diameter of 11.80mm will expand to 14.16 mm when fully hydrated in saline.

Tensile strength - Measurement of how strong a material is. Tested by stretching so see the force needed to stretch a material. Elongation to break value expressed as a percentage and the modulus of elasticity.

Tint - Generally refers to handling tint in contact lenses. Usually it is a pale blue overall colour to assist in lens manipulation. Normally it is a chemical dye but can be a pigment.

Tonometer - Instrument used to measure internal eye pressure.

Toric lens - Contact lens specially designed with two different optical powers at right angles to each other to correct astigmatism.

Truncation - Special edge shaping to stop lens rotating on the eye.

Ultra-Violet - Harmful end of spectrum light, which causes ageing of the eye.

Vitreous humour - Transparent jelly filling globe between crystalline lens and retina.

Water content - Amount of water in a soft lens (hydrated hydrophilic) material presented as a percentage. Generally the higher the value the softer and less strong the material will be but will have a higher Dk value.

Wettability - The surface wetting of a lens material is normally expressed as an angle ie 240 - poor wettability is indicated by a high value > 40.

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