Sphere
The spherical component of a lens prescription, expressed in dioptres (D).
Explanation & example →THE CLEAR VISION GUIDE
Eyewear glossary: understand the words
Learn prescription terms, eye anatomy, lens technology and contact-lens care through plain explanations, examples and common mistakes.
Reading a prescription
Cylinder
The power component used to describe astigmatic correction, in dioptres.
Explanation & example →Cylinder axis
The orientation of cylinder correction, recorded as an angle up to 180°.
Explanation & example →Pupillary distance
A distance in millimetres used to help position lenses relative to the pupils.
Explanation & example →Near addition
Additional plus power relative to the distance prescription for a near task.
Explanation & example →Right eye, left eye, both eyes
Common abbreviations: OD is the right eye, OS the left, and OU both eyes.
Explanation & example →Spherical equivalent
A summary value calculated as sphere plus half the cylinder.
Explanation & example →Dioptre
A unit of optical power, equal to inverse focal length in metres in a simple optical model.
Explanation & example →Fitting height
A vertical fitting measurement taken with the chosen frame positioned on the wearer.
Explanation & example →Prism and base direction
Prescribed prism redirects light and is recorded with both an amount and a base direction.
Explanation & example →Eyes and vision
Visual acuity
Performance at recognising detail under specified test conditions, often recorded with a chart.
Explanation & example →Astigmatism
Unequal focusing in different directions through the eye can cause blur or distortion.
Explanation & example →Myopia
With accommodation relaxed, distant light focuses in front of the retina rather than on it.
Explanation & example →Hyperopia
With accommodation relaxed, the eye tends to focus distant light behind the retina.
Explanation & example →Presbyopia
Age-related reduction in the ability to change focus for near tasks.
Explanation & example →Accommodation
The eye’s adjustment of focusing power for different viewing distances.
Explanation & example →Cycloplegic refraction
Refraction performed while medication temporarily reduces the eye’s accommodation.
Explanation & example →Axial length
The front-to-back length of the eye, usually measured in millimetres.
Explanation & example →Defocus
An optical image focused away from the intended retinal position.
Explanation & example →Tear film
The thin fluid layer covering the eye surface, supporting lubrication and a smooth optical interface.
Explanation & example →Cornea
The transparent front tissue of the eye, which contributes substantially to focusing incoming light.
Explanation & example →Retina
Light-sensitive tissue at the back of the eye that helps turn light into visual signals.
Explanation & example →Vitreous
The clear gel-like material filling much of the inside of the eye.
Explanation & example →Double vision
Seeing two images of one object.
Explanation & example →Undercorrection
Correction that leaves some of the intended refractive error uncorrected.
Explanation & example →Lens design and materials
Spherical surface design
A spherical surface is shaped as part of a sphere with a constant radius.
Explanation & example →Aspheric and double-aspheric design
A surface whose curvature is not described by one spherical radius throughout.
Explanation & example →Freeform lens technology
A broad term for flexible surface design and digitally controlled lens machining.
Explanation & example →Single-vision lens
A lens prescribed for one main viewing task, such as distance or reading.
Explanation & example →Progressive lens
A lens with a gradual change in power between viewing areas, commonly from distance to near.
Explanation & example →Refractive index
A material property describing light propagation relative to a reference, conventionally vacuum.
Explanation & example →Abbe value
A material dispersion measure; a higher value generally means less colour separation under the same convention.
Explanation & example →Anti-reflective coating
Optical surface layers designed to reduce selected reflections from a lens.
Explanation & example →Water-repellent and easy-clean treatments
Surface treatments that reduce how readily water or certain contaminants spread or adhere.
Explanation & example →UV protection labelling
A label commonly used for protection extending toward the ultraviolet boundary around 400 nm.
Explanation & example →Contact lenses and care
Base curve
A contact-lens curvature specification, usually expressed in millimetres.
Explanation & example →Contact-lens diameter
The overall lens diameter, normally specified in millimetres.
Explanation & example →Orthokeratology
Specially fitted rigid lenses that temporarily change corneal shape to alter focusing.
Explanation & example →Oxygen permeability and transmissibility
Dk describes a material’s oxygen permeability; Dk/t also accounts for lens thickness.
Explanation & example →Contact-lens water content
The proportion of water in a lens material under the stated measurement conditions.
Explanation & example →Rigid gas-permeable lens
A contact-lens category with a relatively stable shape and a material that permits oxygen passage.
Explanation & example →Contact-lens cleaning and disinfection
Cleaning removes deposits; disinfection uses a defined process to reduce harmful microorganisms.
Explanation & example →Peroxide neutralisation
The required step that removes active peroxide before lenses return to the eyes.
Explanation & example →Lens replacement schedule
The instructed timing for discarding and replacing a contact lens.
Explanation & example →