Glossary
	Definitions for some of the words which you may encounter whilst at the opticians, or in these pages.
	• astigmatism. Occurs when the front surface of the eye or its internal lens is elliptical (rugby ball shaped) instead of spherical (football shaped). It can be associated with long (hypermetropia) or short sightedness (myopia).
	• bifocal. A lens with two areas, one of which is more powerful than the other. Usually one area focuses in the distance, the other at the near.
	• cataract. The name given to the misting of the natural lens inside the eye which can reduce vision. If this causes difficulty with every day tasks it can usually be treated surgically.
	• complex prescriptions. For National Health Service purposes this is defined as a lens with a power in one meridian of 10 dioptres.
	• contact lens. Either a thin rigid plastic shell (hard or gas permeable lens), shaped like the front of the eye, or a soft lens material which drapes over the front of the eye. These lenses are used to correct vision as an alternative to spectacles,as a bandage in some painful eye conditions or for cosmetic reasons.
	• dispensing optician. A professional qualified to interpret the spectacle or contact lens prescription , advise on appropriate frames and lenses and take accurate measurements. Some also fit contact lenses.
	• fundus. The inner aspect of the posterior (back) of the eye. A fundus examination includes a view of the retina and optic nerve as it leaves the eye and the retinal arteries and veins.
	• glaucoma. The name for a group of eye conditions in which the optic nerve is irreversibly damaged at the point where it leaves the eye resulting in blind areas on the retina. It is commonly associated with increased pressure in the eye but not always.
	• high index glass. A type of glass with a refractive index greater than the 1.523 of normal glass making the lens thinner and lighter in weight.
	• high index plastic. Plastics material with a refractive index greater than the 1.498 of normal plastic lenses making the lens thinner and lighter in weight.
	• hypermetropia. Long or farsightedness. A person with long sightedness will find that their distance vision is relatively better than their near vision. Rays of light focus behind the retina.
	• hyperopia. Another term for hypermetropia.
	• myopia. Short or near sightedness. A person who is short sighted has near vision that is relatively better than distance vision. Rays of light focus in front of the retina.
	• optometrist. Practitioner who is qualified to examine eyes, to recognise any ocular abnormality and refer for medical treatment if necessary, to test vision and prescribe spectacles or contact lenses. Also trained to measure and fit spectacles.
	• polycarbonate. The toughest lens material available. Flexes in response to impact . It is 20 times more impact resistant than glass and three times lighter.
	• presbyopia. Is the technical term for reduction in accomodation of the eye (focusing power) due to the natural ageing process and affects everyone in time.
	• retina. The innermost layer of the eye. Its cells convert light into nerve impulses that are sent to the brain to be converted into visual images.
	• toric lens. A curved lens on which one of the surfaces has two powers worked in order to correct astigmatism.
	• varifocal. A lens having more than two focal powers.
	• visual field (field of vision). That portion of space in which objects are visible at the same time without movement of the eye.
	Schematic cross section of the eye

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