SSC logo Scottish Sensory Centre
university of edinburgh

Adapting Video for VI Learners

Glossary of terms associated with Visual Impairment

  • A B C D E F G H I K L M N O P R S T U V



    The adjustment of the eye for seeing at different distances by changing the shape of the lens through the action of the ciliary muscle. Adjustment of the dioptric power of the eye. The process is involuntary.
    The difference in power between the distance and near corrections. Often abbreviated to "Add". The power of the add is related to the working distance from the eye that the print is held and the amplitude of accommodation. In the case of low vision patients, the power of the add is usually greater than +4.00 dioptres, and hence the working distance is less than 25 centimetres.
    Congenital anomaly characterised by an absence of pigment in the skin, hair, in some cases iris, retina and choroid. The iris is usually a pale blue-grey in colour and the visual acuity is poor. There may be nystagmus present (dependent on the type of albinism) and there is usually photophobia.
    Loss of visual acuity without any apparent disease of the eye.
    Amsler charts
    A series of charts used to detect abnormalities in the central visual field. It consists of a regular grid of 5 mm squares with a central black spot. The patient fixates the central spot and any abnormality of the visual field is demonstrated by the distorted or irregular grid lines.
    Congenital absence of the iris.
    anterior chamber
    Absence of the lens of the eye.
    anti-reflection coating
    A thin film of transparent material, usually a metallic fluoride, deposited on the front and back surfaces of lenses which increases the transmission of the lens as well as reducing surface reflection.
    Space in the front of the eye, bounded in front by the cornea, and behind by the iris; filled with aqueous. An ocular condition where the crystalline lens is absent. It may be congenital, but is usually due to surgical removal of a cataract. As a result the eye is left very hypermetropic and has no accommodating power.
    A person who has aphakia.
    Clear, watery fluid which fills the anterior and posterior chambers within the front part of the eye.
    Refractive error due mainly to curvature of the cornea which prevents light rays from coming to a single focus on the same object.


    bifocal lens
    A lens having two portions of differing power. Normally the upper portion of the lens is reserved for distance vision the lower portion for near vision, although in certain cases this can be reversed. They are custom made to a specific prescription normally and require a prescription from a registered optometrist.
    binocular vision
    The ability to use the two eyes simultaneously to focus on the same object.
    In the United States, the legal definition of blindness is: central visual acuity of 20/200 or less in the better eye after correction; or visual acuity of more than 20/200 if there is a field defect in which the widest diameter of the visual field subtends an angle distance no greater than 20 degrees. Some states include up to 30 degrees. This precise measurement of acuity, however, is not a regulation in the English definition of blindness which is referred to in terms of a subject's inability to learn or to work by the use of sighted methods.
    Infantile glaucoma characterised by a large eyeball.

    C (Cum Correction)
    With correction wearing prescribed lenses (found on medical records).
    canal of Schlemm
    A circular canal situated at the point where the sclera and cornea meet and through which the aqueous is excreted.
    The enclosing membrane of the eye.
    Partial, or complete loss of transparency of the crystalline lens of the eye. Cataract can occur as a result of age, trauma, systemic disease (e.g. diabetes), ocular disease (e.g. anterior uveitis high myopia), long term steroid therapy, exposure to high levels of ultra-violet light and heredity.
    Inflammation of the choroid and retina.
    ciliary body
    Portion of the vascular coat between the iris and the choroid consisting of the ciliary processes and the ciliary muscle.
    colour deficiency
    Diminished ability to perceive differences in colour, usually red or green but occasionally blue or yellow.
    concave lens
    Lens having the power to diverge rays of light; also known as diverging, myopic or minus lens. Denoted by a negative sign
    A photoreceptor of the retina involved in the perception of colour and high visual acuity. There are around 6 million cones in the retina with the greatest concentration in the macular area.
    cone dysfunction
    A very rare disorder in which the cones at the macular area have impaired function. Because the cones are predominantly concerned with daytime and vision in lit situations, patients who have cone dystrophy often see better in dim illumination. (Remember cones function in higher light levels and rods in lower light levels.)
    Cells of the retina concerned with visual acuity and colour discrimination.
    Present at birth.
    Mucous membrane lines the eyelids and covers the front part of the eyeball. Inflammation of the conjunctiva is called conjunctivitis or 'red eye'.
    contact lens
    Lenses that fit directly onto the eyeball.
    convergent squint
    The eye turn in towards the nasal side.
    convex lens
    Lens having the power to converge rays of light; also known as converging, hypermetropic or plus lens. Denoted by a positive sign +.
    The clear, transparent part of the outer coat of the eyeball.
    cortical visual impairment -
    a temporary or permanent visual loss caused by a disturbance of the posterior visual pathways and/or occipital lobes of the brain.
    crystalline lens
    A transparent, colourless body suspended in front of the eyeball, between the aqueous and the vitreous. Its function is to bring the rays of light to a focus on the retina.


    degenerative or pathological myopia
    Myopia attributed to any degenerative changes in the choroid or retina. The level of myopia is usually greater than -6.00 Dioptres and continues to worsen during adolescence and increases further during adult life. The acuity is usually subnormal, even after correction.
    depth perception
    The ability to perceive the solidity of objects and their relative position in space.
    diabetic retinopathy
    Retinal changes occurring in long standing cases of diabetes mellitus. In general the amount of the retinopathy is related to the duration of the disease rather than the severity of the disease. Both eyes are involved but usually to differing degrees. Visual acuity is unaffected unless the central macular area is involved.
    Unit of measurement of strength or refractive power of a lens. It is equal to the product of the refractive index of the image space and the reciprocal of the focal length of a lens in meters. E.g. A 1 dioptre lens in air has a focal length of 1 metre and a 2 dioptre lens a focal length of 50 centimetres (0.5m).
    divergent squint
    The eye turns outward to the temporal side.


    eccentric fixation
    Fixation, where the object of regard is not imaged on the fovea. In this condition the patient feels he is looking straight ahead at the object, but in fact the image is stimulating an area just outside the fovea.
    Complete surgical removal of the eyeball.
    Mongolian fold at the inner comer of the eye.


    field of vision
    The entire area that can be seen without shifting the gaze.
    The act of directing the eye to a given object so that its image is formed on the fovea.
    The point to which the rays of light are converged after passing through a lens.
    A small depression in the retina at the back of the eye; the part of the macula adapted for the most acute vision.
    The back of the eye which can be seen with an ophthalmoscope.


    Galilean telescope
    A simple optical system which allows the magnification of distance objects with low magnification and without image inversion. It consists of a convex lens acting as the objective and a concave lens as the eyepiece. Magnification levels rarely exceed 5X. The optical system exists in simple opera glasses.
    Increased pressure within the eye. An eye disease characterised by an elevated or unstable intraocular pressure, which cannot be sustained without damage to the the eyes internal structure or impairment to its visual function. The increased pressure causes damage to the optic nerve, causing characteristic field losses. It is usually divided into open angle and closed angle types. If the cause of the glaucoma is is a recognised ocular disease or injury, then it is called secondary, whereas if the cause is hereditary or unknown it is called primary.


    A refractive error in which the rays of light come to a focus behind the retina.
    A refractive condition of the eye in which distance objects are focused behind the retina when the accommodation is fully relaxed.


    The coloured circular part of the eye which regulates the amount of light entering the eye by changing the size of the pupil.


    Kay pictures
    A method of measuring vision and visual acuity in Snellen form using the aid of pictures of various sizes at 3 metres. This technique is particularly useful with children where they are unable to read letters or cope with Sheridan Gardiner.
    Keplerian or astronomical telescope
    An instrument allowing for magnification of distance objects by means of 2 convex lenses as the eyepiece and objective.


    light perception
    The ability to distinguish light from dark. (LP or PL or LPO)
    A magnifing aid, monocular or binocular held in the hand or mounted in front of the eye, for viewing objects at close range. The image produced is real and upright.
    Low vision aid, for example a magnifier.


    Strictly speaking called the macula lutea, this is the area of the retina concerned with our most sensitive detailed vision. The most central area of the macula is called the fovea and it is this area which contains the majority of cones. The foveal area contains no rods.
    macula degeneration
    Affection of the most sensitive part of the retina, the macula. A central degenerative condition of the retina in which the photoreceptors (the rods and cones) in the macular area lose their function, due to the disruption of the underlying pigment epithelium. The disease is usually age related but may occur in younger age groups (e.g. Best's disease).
    Increase in the image size produced by an optical system, relative to the object. By convention in the U.K. the magnification of a device is taken to be the power of of the lens in dioptres divided by 4. Thus an 8 dioptre lens has a magnification factor of 25.
    monocular vision
    Vision in one eye only.
    A refractive condition of the eye in which distance objects are focused in front of the retina, making them blurred. The amount of blur depends on the degree of myopia. When the myope removes their spectacles, near objects remain in focus at a distance related to the amount of myopia. If the reciprocal of the degree of myopia in dioptres is taken, then this will give the distance at which close print will be clear. This point is called the "far point".


    'N' Print scale
    A test for near visual acuity.
    night blindness
    A condition in which the vision is good by day but extremely poor at night and in faint light.
    An involuntary, rapid movement of the eyeball. A regular, repetitive, involuntary movement of the eyes whose direction, frequency and amplitude is variable. The movements may be horizontal or vertical although vertical nystagmus is rare. The condition may be congenital or of unknown aetiology.


    An optical system or lens used to produce a real image of an object. The image is then viewed through an eyepiece.
    Pertaining to the eye.
    An instrument for examining the interior of the eye.
    optic atrophy
    Degeneration of the optic nerve fibres characterised by the pallor of the optic disc, which appears a yellowish grey colour or white. The condition leads to a progressive loss of visual acuity and changes to the visual field. There are two types, primary and secondary. In secondary disease there is usually some evidence of previous optic nerve inflammation.
    optic disc
    Head of the optic nerve as it leaves the retina.
    A person registered in the U.K.with the British College of Optometrists and the General Optical Council who is qualified to practise Optometry.
    The profession which includes the services and care involved in the:-
    • determination and evaluation of the reactive status of the eye and other physiological functions.
    • recognition of ocular abnormalities.
    • determination of optically related corrective measures.
    • selection, design, provision and adaptation of optical aids.
    • preservation, maintenance, protection, improvement and enhancement of visual performance.


    peripheral vision
    Ability to perceive the presence, motion, or colour of objects outside the direct line of vision.
    Abnormal sensitivity to, and discomfort from light.
    Plano lens
    An optical quality lens which has no dioptric power.
    pupillary distance
    The distance between the centre of the pupils when the eyes are looking into the distance.


    The process of measuring and correcting the refractive state of the eyes and correction by glasses.
    refractive error
    A defect in the eye that prevents light rays from being brought to a single focus exactly on the retina.
    Innermost coat of the eye.
    retinal detachment
    A separation of the retina from the choroid.
    retinitis pigmentosa
    Hereditary affection of the retina characterised by pigment changes. A primary pigmentary dystrophy of the retina followed by migration of pigment. It is an inherited disease characterised by night blindness and restricted visual fields. The condition is bilaterally symmetrical. It is the rod system which is predominantly damaged but the cones are affected later in the disease to a certain extent. The disease begins in adolescence with night blindness followed by a ring type visual field defect that extends peripherally and then centrally, until only a small contracted central field remains.
    A cancerous tumour of the retina in infants.
    An instrument for determining the refractive state of the eye objectively. It consists of a light source, a condensing lens and a mirror. The mirror has a small hole in it, through which the optometrist can view the patient's eye along the retinoscope`s beam of light. The beam is reflected off the retina and produces a red reflex, which moves in conjunction with the movement of the retinoscope. The neutralisation of this movement using lenses gives the optometrist an accurate assessment of the refraction, without requiring the patients cooperation.
    The determination of the refractive state of the eye using a retinoscope.
    A photoreceptor of the retina involved in vision in lower light levels. There are approximately 130 million rod cells throughout the retina, but there are no rod cells in the macular area. Retinal cells which are concerned with motion and vision in low levels of illumination.


    S. sc (sine correction)
    Without correction, that is, not wearing glasses.
    The white part of the eye.
    A blind spot in the field of vision.
    Sheridan Gardiner
    A method of measuring vision and visual acuity in Snellen form using a comparison of shapes or letters.
    Snellen chart
    A chart used to measure visual acuity.
    Snellen fraction
    A form of representing acuity in fraction form, where the numerator is the testing distance in metres and the denominator the distance at which the smallest Snellen letter read by the eye has an angular subtense of 5 minutes.
    strabismus (squint)
    Failure of the two eyes to direct their gaze simultaneously at the same object.


    tunnel vision
    Contraction of the visual field to such and extent that only a small area of central visual acuity remains.


    The entire vascular coat of the eyeball consisting of the iris, ciliary body and choroid.


    Visual acuity without any spectacle or contact lens correction.
    visual acuity
    Capacity for seeing objects distinctly and in detail. In the U.K. it is represented by the Snellen fraction.
    visual purple
    The pigment in the outer layers of the retina.
    The transparent jelly which occupies the interior of the globe behind the lens.