Optometrist Registration key terms

The sharpness of central vision, usually measured with a Snellen or logMAR chart as the smallest letter size a person can identify at a set distance. It is recorded as a fraction comparing the patient's performance with normal vision.

A wall chart of letters arranged in rows of decreasing size, used to test distance visual acuity. The standard testing distance is 6 metres or 20 feet, and each row is labelled with the acuity it represents.

A logarithmic scale for recording visual acuity in which each line of letters represents 0.1 log unit and equal steps correspond to equal proportional changes in letter size. It allows precise statistical comparison and is common in research and low-vision work.

The clinical process of determining the lens power needed to focus light on the retina, and also the optical bending of light by the eye's surfaces. Subjective and objective methods are combined to prescribe spectacles or contact lenses.

An objective technique in which the examiner shines a streak of light into the eye and observes the movement of the reflected glow while neutralising it with trial lenses. It gives a starting refraction, especially useful when communication or cooperation is limited.

An instrument that measures the eye's refractive error by projecting a pattern into the fundus and analysing the reflected image. It provides a rapid objective starting point that is refined by subjective refraction.

Temporary paralysis of the ciliary muscle produced by anticholinergic eye drops, which also dilates the pupil. It is used to reveal latent hyperopia and to obtain an accurate refraction in children or in cases of accommodative spasm.

Dilation of the pupil, either pharmacological from a sympathomimetic or anticholinergic drop, or physiological in dim light. Dilating drops are used to examine the fundus and to relieve iris adhesions.

The increase in refractive power of the eye produced by contraction of the ciliary muscle, which relaxes the zonules and allows the crystalline lens to become more convex. It enables clear vision at near distances.

The maximum increase in optical power the eye can achieve for near vision, measured in dioptres as the difference between the far and near points. It declines steadily with age and is used to determine reading additions.

The age-related loss of accommodative amplitude that makes near work difficult, typically becoming noticeable in the early to mid-forties. It is corrected with reading spectacles, multifocal lenses, or contact lens strategies.

A refractive state in which parallel light from a distant object focuses in front of the retina, so distance vision is blurred while near vision is relatively clear. It is corrected with minus-powered lenses or surgery.

A refractive state in which parallel light from a distant object would focus behind the retina when accommodation is relaxed. Young patients may compensate by accommodating, so latent hyperopia can be missed without cycloplegia.

A condition in which the eye's optical power differs across meridians, so light is not focused at a single point. It is corrected with cylindrical lenses and described by its power and axis.

A single number representing a spherocylindrical refraction, calculated as the sphere plus half the cylinder power. It is used for comparisons, for some contact lens calculations, and for estimating refractive error in screening.

Measurement of the curvature of the anterior corneal surface, usually expressed as radii of curvature and corneal power in dioptres. It is essential for fitting rigid contact lenses and for calculating intraocular lens power.

A computerised mapping technique that measures curvature or elevation across the entire corneal surface and produces colour-coded maps. It is used to detect keratoconus, plan refractive surgery, and guide contact lens fitting.

A progressive non-inflammatory thinning and conical steepening of the cornea that causes irregular astigmatism and reduced vision. Management includes rigid contact lenses, corneal cross-linking, and in advanced cases transplantation.

A binocular microscope combined with an adjustable slit illumination system, used to examine the anterior segment and, with auxiliary lenses, the ocular fundus. It is the central instrument for anterior eye assessment.

Examination of the interior of the eye, particularly the retina, optic disc, and macula, using an ophthalmoscope or a slit lamp with a condensing lens. Pupil dilation improves the view of the peripheral retina.

The area of the retina where retinal nerve fibres converge and leave the eye, also the entry point of retinal vessels. Its appearance, including cupping and margins, is assessed for glaucoma and papilloedema.

The proportion of the optic disc occupied by the central pale cup, recorded as a decimal such as 0.3. Enlargement or asymmetry of this ratio is a structural sign associated with glaucoma.

The pressure exerted by the fluids within the eye, normally between about 10 and 21 millimetres of mercury. It is measured by tonometry and is a major modifiable risk factor for glaucoma.

The measurement of intraocular pressure, commonly by applanation or non-contact air-puff methods. Correct technique and corneal thickness influence the reading obtained.

Measurement of corneal thickness, usually by ultrasound or optical methods. Central thickness affects applanation tonometry readings and is relevant to glaucoma risk assessment and refractive surgery screening.

The total area a person can see while the eye is fixated on a central point, including central and peripheral portions. Field testing detects defects caused by glaucoma, neurological lesions, and retinal disease.

The systematic measurement of visual field sensitivity, using static or kinetic techniques and often automated instruments. Results are displayed as plots of threshold sensitivity and compared with age-matched norms.

A group of optic neuropathies characterised by progressive retinal ganglion cell loss and characteristic visual field defects, with raised intraocular pressure as a major risk factor. Diagnosis combines disc assessment, pressure measurement, and field testing.

Reduced best-corrected vision in one or both eyes caused by abnormal visual experience during development, such as strabismus, anisometropia, or deprivation. Treatment aims to correct the underlying cause and encourage use of the affected eye.

A misalignment of the eyes in which the visual axes do not point at the same object, producing a manifest deviation. It may be constant or intermittent, and can lead to amblyopia or diplopia.

A latent deviation of the eyes that is normally controlled by fusion and only becomes apparent when fusion is disrupted, for example during a cover test. It may cause asthenopic symptoms during sustained near work.

A clinical test in which one eye is briefly covered while the other fixates, and any movement of the uncovered eye is observed. It distinguishes manifest deviations from latent ones and is fundamental to binocular assessment.

The closest point at which the two eyes can maintain single binocular vision while converging on a near target. A receded near point suggests convergence insufficiency.

A condition in which the refractive error differs significantly between the two eyes. Large differences can cause aniseikonia, poor binocular vision, and amblyopia in children.

A difference in the perceived size or shape of images formed by the two eyes, often caused by unequal spectacle magnification. It can produce headaches, distortion, and difficulty with binocular fusion.

A technique in which sodium fluorescein dye is instilled and viewed under cobalt blue light to reveal corneal and conjunctival epithelial damage. It is used to assess abrasions, keratitis, and contact lens complications.

The unit of refractive power, equal to the reciprocal of the focal length in metres. Positive values indicate converging lenses and negative values diverging lenses.

The distance from a lens or optical system to the point where parallel rays converge or from which they appear to diverge. It is inversely related to lens power measured in dioptres.

The ability of a lens to converge or diverge light, expressed in dioptres and calculated from its surface curvatures and refractive index. It determines the correction a spectacle lens provides.

A dimensionless number describing how much a material slows and bends light compared with a vacuum. Higher-index lens materials allow thinner lenses for a given power.

A number expressing the dispersive power of an optical material, with higher values indicating less chromatic aberration. It guides material selection, especially for high-power lenses.

The curvature of the front surface of a spectacle lens, specified as a radius or a dioptric value. It influences lens thickness, magnification, and the optical performance of the finished lens.

A formula stating that induced prism in dioptres equals the decentration in centimetres multiplied by the lens power in dioptres. It is used to control unwanted prism when centring lenses.

The point on a lens through which the optical axis passes and where there is no induced prism. Correct placement relative to the pupil is essential for comfortable vision.

The distance between the centres of the pupils, or between each pupil and the bridge of the nose. Accurate measurement ensures correct lens centration and avoidance of unwanted prism.

An optical element that deviates light toward its base without changing the vergence of the rays. In spectacles it is used to relieve diplopia or to compensate for heterophoria.

A multifocal spectacle lens with a gradual increase in plus power from distance to near, avoiding a visible dividing line. It provides continuous vision at all distances but has peripheral distortion.

A lens with different powers in two principal meridians, used to correct astigmatism. In contact lenses the orientation of the toric surface must remain stable on the eye.

The change in retinal image size produced by a spectacle lens compared with the uncorrected eye. It depends on lens power, thickness, and vertex distance, and is relevant to aniseikonia.

The distance between the back surface of a spectacle lens and the front of the cornea. It must be accounted for when converting spectacle prescriptions to contact lens powers.

A thin optical device worn directly on the cornea or sclera to correct refractive error or for therapeutic purposes. Fitting requires assessment of corneal shape, tear film, and lid anatomy.

An optical defect in which different wavelengths of light are focused at different points because of dispersion. It causes coloured fringes and is reduced by using materials with high Abbe values.