Understanding a spectacle prescription
Dr Anitha Arvind
Every spectacle prescription includes numbers and abbreviations. This article helps to understand the components of any spectacle prescription.
(Image of Prescription to be added here)
The spectacle prescription consists of rows and columns to include a prescription for the right and left eye. In most cases, the right and the left prescriptions wouldn’t match, so it is common to notice a difference in prescription between the right and left eye.
The right eye prescription is abbreviated as RE or sometimes mentioned as OD Latin for Oculus Dexter, meaning right eye and left eye prescription is abbreviated as LE or sometimes mentioned as OS Latin for Oculus Sinister, meaning left eye.
The prescription could also include a column stating BE meaning ‘Both Eyes’ or sometimes mentioned as OU Latin for Oculus Uterque, meaning both eyes. The following figure is an image for a spectacle prescription (Figure to include a picture of a spectacle prescription)
The components of a spectacle prescription are as follows:
a) Spherical component:
Refractive errors such as myopia (short-sightedness) or hyperopia (long-sightedness) are corrected with spherical powered lenses. Myopia is corrected with minus (concave) lenses, while hyperopia is corrected with plus (convex) lenses.
The spherical lenses’ power or strength for correction of the refractive error is indicated in numbers. The higher the numerical number, the higher the refractive error. The prescription’s spherical component is abbreviated as DS (Dioptre Sphere) or ‘D sph’ after the numerical component.
Spherical powers up to plus or minus 3.00DS are low refractive errors, while those ranging between plus or minus 3.00DS to 6.00DS are medium refractive errors while those above plus or minus 6.00DS are high refractive errors.
b) Cylindrical component:
The refractive error called astigmatism is corrected by cylindrical lenses in plus or minus denomination. The cylindrical component can be prescribed alone without spherical correction or with spherical correction.
Cylindrical correction is abbreviated as DC (Dioptre Cylinder) or ‘D cyl’ after the numerical component. A prescription can be written in either plus cylinder or minus cylinder form. However, minus cyl form is the correct convention.
Every cylindrical prescription is followed by an axis, which denotes the lenses’ placement in the frame. The spherical power has the same power on any part of the lens, while a cylindrical lens has its maximum strength only along a particular axis as required by the eye.
Improper cylindrical axis placement in the frame could result in problems to the wearer ranging from blurred vision, discomfort, and headache, sometimes nausea and double vision.
Presbyopia (near power that one gets around the age of 40 years) is a condition resulting in a gradual loss of the ability of the eyes to focus on near objects. This is a part of normal ageing as the crystalline lens hardens/stiffens with age.
The presbyopic correction for an individual is mentioned as ‘addition’ or ‘add’ in the prescription. Presbyopia can occur in isolation or along with other refractive errors (if present) like myopia (short-sightedness), hyperopia (farsightedness) and astigmatism (cylindrical correction).
For hyperopes who wear plus lenses for distance, the presbyopic correction would require them to wear more plus power for near work, while myopes who wear minus lenses for the distance the presbyopic correction would require them to wear less minus power (due to the addition of plus lenses for near) for near.
d) Prism correction:
Some prescriptions could also incorporate prism correction. The prism prescription is written in numerical followed by a triangle(Δ) and the prism’s base direction. The numerical indicates the prism’s strength while the triangle (Δ) symbolises the prism.
The base direction of the prism could be mentioned as Base Up (BU), Base Down (BD), Base In (BI) or Base Out (BO). Sometimes more than one prism could be incorporated into the prescription depending upon the requirement of the condition. The prisms may be combined and denoted by the axis meridian (e.g. 4 Δ @ 270 deg)
Prisms are incorporated in the prescription to correct any muscular imbalance in the eye causing double vision. A muscle palsy could also lead to double vision requiring prismatic correction. Prism prescriptions are not very common.
e) Inter-Pupillary Distance (IPD):
The Inter-Pupillary Distance or IPD is the distance between the pupillary centre of one eye to the pupillary centre of the other eye. It is measured in millimetres (mm).
This measurement is essential for centring the lenses correctly within the frame and in front of the eyes. Improper lens centring could affect the wearer, causing problems ranging from blurred vision and discomfort to even headache and double vision.
Optical centres of the lenses are generally placed on the pupillary centres, though exceptions may be there.
f) Vertex distance:
Vertex distance is the distance from the front of the cornea (the transparent front layer of the eye) to the back of the spectacle lens and is measured in millimetres (mm).
This measurement is important in higher prescriptions, especially prescriptions above or equal to plus or minus 5.00DS or 5.00DC, as the strength of the lens varies as it is taken farther or closer to the eye. The vertex distance indicates the distance at which lenses are placed on the eyes.
The prescription also includes recommendations on the type and design of the lens. For example, it could mention if the lenses are made as single vision, bifocal or progressive lenses. Information about the usage, whether for contact use or only for near, may be specified.
Recommendations of any tint or coating like Anti Reflection Coating (ARC) could also be specified. An optician or optometrist could further help recommend the best spectacle lenses based on the wearer’s prescription, lifestyle, and preferences.