Eye Information : A-Z of Terms Explained

ABERRATION

Any defect or a distortion in any optical system.

ACCOMMODATION

The process by which the eye increases its power to focus on a specific object.

ACUITY (V.A. or visual acuity)

Describes the clarity of vision.

AFOCAL

A lens or optical system with zero power.

AMBLYOPIA

(Also known as having a lazy eye) often occurs when the eye does not develop fully during early childhood and is not usually correctable via opticial means.

AMETROPIA

A defective refractive ability of the eye e.g short-sightedness or long-sightedness.

ANISOMETROPIA

Unequal refractice power in each eye, typically greater then 1.00D.

ANTIMETROPIA

A condition in which one eye is myopic and the other is hypermetropic.

APHAKIA

The ocular condition in which the lens of the eye is absent, or has been surgically removed.

AQUEOUS HUMOUR

Fluid that fills and separates the anterior (front) and posterior (back) of the human eye. It is a transparent gelatinous fluid.

ASPHERICAL

Not spherical, aspherical lenses have a curved surface.

ASTIGMATISM

When the cornea at the front of the eye is unevenly curved, often resulting in a rugby ball shaped eye, causing blurred vision.

For more information on astigmatism please click here.

BIFOCAL

A lens with two focal lengths, usually for distance/near vision.

For more information on bifocals please click here.

BINOCULAR

Relating to both eyes.

BIOMICROSCOPE

An instrument designed for detailed examination of the eye, used particularly in contact lens practice. (Often referred to as a slit-lamp).

BLEPHARITIS

A chronic inflammation of the eyelid margins.

For more information on blepharitis please click here.

CANTHUS

Either corner of the eye where the eyelids meet.

CATARACT

Is when the clear lens inside your eye becomes cloudy or misty.

For more information on cataracts please click here.

CONCAVE

A surface shaped like the inside of a sphere.

CONVERGENCE

Movement of the eyes turning inwards, i.e. towards each other.

CONVEX

Is when a lens is shaped outward.

CORNEA

The transparent anterior portion of the eye.

CRYSTALLINE LENS

The lens of the eye, which focusses light on to the retina.

DECENTRATION

The displacement, horizontal and/or vertical, of the centration point of a spectacle lens from the standard optical centre position.

DIOPTRE

The unit of measurement of refractive power of a lens.

DIPLOPIA

(Double vision) A condition where objects are seen 'double'.

DISC (OPTIC)

The region of the retina, where the optic nerve joins the eyeball.

DISTORTION

Defect of an optical system resulting in an alteration of an objects original shape.

DIVERGENCE

Movement of the eyes, turning away from each other.

EMMETROPIA

An eye which gives perfect vision so that no glasses are required. Occurs when light from an object at infinity is sharply focussed on the retina.

FIXATION

Maintenance of visual gaze on a single location. The object is sharply formed on the retina.

FOCIMETER

An instrument that measures the power of a spectacle lens, or contact lens.

FUNDUS

The back of the eye. The fundus is viewed using an instrument called an ophthalmoscope.

GLAUCOMA

An eye disease characterised by increased intra-ocular fluid pressure which causes a restriction in field of vision.

For more information on glaucoma please click here.

GRAFT

Surgical procedure to replace damaged structures (e.g.cornea) with donor tissue.

HYDROGEL

A type of plastics material that contains water, commonly used in the manufacture of soft contact lenses.

HYPERMETROPIA (HYPEROPIA)

Often referred to long-sightedness. When people are long sighted, they can see distance objects well but have difficulty focussing on objects that are close. This vision problem occurs when light rays entering the eye focuses behind the retina, rather than directly on it.

INJECTION (e.g. conjunctival )

Redness of the eye caused by the dilation (expanding) of superficial blood vessels in the sclera (white of the eye).

INTER-PUPILLARY DISTANCE

The horizontal distance measured between the pupil centres. Usually abbreviated to PD. Essential when making up spectacles.

IRIS

The coloured part of the eye surrounding the pupil.

IRITIS

A condition causing inflammation of the iris.

KERATITIS

An inflammation of the cornea.

KERATOMETER

An instrument used to measure the curvature of the front surface of the cornea. Often used in contact lens fitting.

LACRIMAL

Relating to the tears.

MEIBOMIAN GLANDS

Small glands in the eyelids known as meibomian glands are responsible for secreting oil that covers and protects the surface of the eye.

MIOSIS

Contraction of the pupil. (A drug that causes this is a miotic)

MONOCLE

A single lens, with or without a frame, worn by holding between the brow and the cheek.

MONOCULAR

Relating to one eye (or lens).

MULTIFOCAL

A lens with multiple powers eg a trifocal or varifocal  lens.

MYDRIASIS

Dilation (enlarging) of the pupil. (A drug that causes this is a mydriatic)

MYOPIA

Often referred to short-sightedness. When people are short sighted, they can see near objects well but have difficulty focussing on objects that are far away. This vision problem occurs when light rays entering the eye focuses infront the retina, rather than directly on it.

NEAR VISION

The ability to read/carry out close work, usually at a distance of 33 - 40 cm.

NEUTRALIZATION

The process of determining the power of an unknown lens, using trial lenses of known power.

OCCLUDER

A device placed in front of an eye to effectively block vision.

OPACITY

Condition of a tissue or structure that is not transparent.

OPHTHALMOSCOPE

A hand held instrument for viewing the eye in particular the retina.

OPTICAL CENTRE

The point on a lens through which a ray of light will pass undeviated.

ORBIT

The bony socket of the skull where the eye is situated.

ORTHOPTICS

Is the diagnosis and non-medical management of  abnormalities of binocular vision. This includes strabismus (squint), amblyopia (lazy eye).

PERMEABILITY

The ability of a material to allow the passage of a gas or fluid.

PHOTOCHROMIC

A lens made of a material that changes colour as a result of exposure to UV light and heat. For more information on photochromatic lenses please click here.

PRESBYOPIA

When the lens inside the eye loses some of its flexibility so that it becomes difficult to focus on close objects. Usually occurs with age.

For more information on presbyopia please click here.

PRISM

Is used in spectacles to correct a muscle imbalance.

PROSTHESIS

An artificial eye, or implant.

PUPIL

The opening within the centre of the iris. Thsi determines the amount of light which enters the eye.

PUPILLARY DISTANCE

This is the distance between the centre of the the pupils in each eye (measured in millimeters). Important in positioning the lenses of the glasses correctly in relation to the pupils so as to obtain the optimum vision.

REFLEX

Reaction of certain parts of the eye to a stimulus, e.g. the pupil reflex being the reaction of the pupil to a light stimulus.

RETINA

The light sensitive layer of cells lining the back of the eye. Often liked to the film of a camera.

RETINOSCOPE

Hand held instrument used to obtain an objective assessment of a patient's refractive condition (their prescription)

SALINE

A sterile solution of sodium chloride (salt) in water. Often used to rinse contact lenses

SCLERA

The white of the eye. A tough opaque fibrous tissue which serves as the eyes protective coat.

SCOTOMA

An area of partial or complete loss of vision surrounded by a  normal field of vision.

SLIT - LAMP (SLIT-LAMP BIOMICROSCOPE)

An instrument used to examine the eye under high magnification. Also used in contact lens practice.

STRABISMUS

A misalignment of the eyes so that the eyes are not directed towards the same point, when the patient is fixating. As images are formed at different points this often results in  'double vision'.

SURFACING

The process of generating, smoothing and polishing a spectacle lens surface to a given curvature.

TONOMETER

An instrument used for measuring the fluid pressure inside the eyeball (the intra ocular pressure (IOP)

TRIAL CASE

A case containing both spherical and cylindrical lenses which are either positive or negative in power. Used in testing eye sight.

TRIAL FRAME

An adjustable spectacle frame which can hold several lenses and is used in testing eye sight.

UNAIDED VISION

Vision without any form of correction i.e spectacles or contact lenses.

UNIOCULAR

Another expression for monocular, i.e. referring to one eye.

VERTEX DISTANCE

The distance (in mm) from the surface of the cornea, to the posterior surface of the spectacle lens or trial lens.

VISUAL ACUITY

The ability of the eye for seeing distinctly the details of an object at a specific distance.

VITREOUS HUMOUR

The clear gelly that fills the space between the lens and the retina of the eyeball.