An eye prescription is a formal document issued by a registered UK optometrist or ophthalmic medical practitioner following a comprehensive sight test. It summarizes the precise optical correction required for clear vision. Understanding the terms and numerical values on this slip allows individuals to correctly purchase glasses or contact lenses. This guide breaks down the common terminology found on a UK eye prescription slip.
Understanding Sphere and Eye Identification
Identifying which eye the measurements correspond to is the first step in reading any prescription. UK prescriptions often use Latin abbreviations: “OD” (Oculus Dexter) denotes the right eye, and “OS” (Oculus Sinister) refers to the left eye. Simpler abbreviations like “R” and “L” may also be used. These labels ensure the correct lens power is placed in the corresponding side of the spectacle frame.
The primary measurement of lens strength is found under the heading “Sphere,” often abbreviated as SPH. This value represents the main corrective power needed to focus light precisely onto the retina. Sphere measurements are always expressed in units called Diopters (D), which indicate the lens’s optical power. A higher number, regardless of the sign, signifies a greater degree of vision correction is required.
The number in the SPH column will carry either a positive (+) or a negative (-) sign, which denotes the type of refractive error being corrected. A negative sign indicates myopia, commonly known as nearsightedness, where distant objects appear blurry. Conversely, a positive sign signifies hyperopia, or farsightedness, meaning the eye needs help focusing on near objects.
Decoding Astigmatism Measurements
Vision issues caused by astigmatism, where the cornea or lens is imperfectly curved, are addressed by the “Cylinder” column (CYL). This measurement indicates the additional lens power necessary to correct the irregular curvature, which otherwise causes blurred or distorted vision. If a person does not have astigmatism, the CYL column may be left blank or marked with “Plano” or “PL,” indicating zero corrective power is needed.
The Cylinder power must be applied at a specific orientation, determined by the “Axis” measurement. The Axis value is a number between 1 and 180 degrees. This number specifies the exact angle, or meridian, at which the cylindrical correction must be placed within the lens to counteract the eye’s irregular shape.
The Cylinder and Axis measurements must always be considered together, as one is meaningless without the other. The Cylinder indicates how much power to add, while the Axis specifies where to orient that power. These two values work in tandem to create a lens surface that compensates for the uneven focusing of the eye.
Interpreting Supplementary Powers
Beyond the distance correction, some prescriptions include an “Addition” power, labelled as ADD. This figure represents the extra magnifying power required for close-up tasks, such as reading or working on a computer. The ADD measurement is typically included only for patients who require multifocal lenses, such as bifocals, trifocals, or progressive lenses, to correct presbyopia.
The addition power is always a positive value and is typically the same for both eyes. Optometrists calculate this fixed amount of extra power needed to bring near objects into clear focus. This single ADD value is then combined with the SPH and CYL measurements to create the specific near-vision segment of the lens.
A less common but important supplementary power is the “Prism,” which is used to correct specific eye alignment issues like strabismus or diplopia (double vision). Prism lenses shift the image seen by the eye, forcing the eyes to work together more comfortably. If Prism is present, it will also include a “Base” direction, indicating the thickest edge of the lens where the light is bent.
The Role of Pupillary Distance and Prescription Validity
An important physical measurement on the prescription is the Pupillary Distance, or PD. This value is the distance, measured in millimeters (mm), between the centers of the pupils of the two eyes. The PD is crucial for the accurate manufacture of spectacles because it ensures the optical center of each lens is perfectly aligned with the wearer’s pupil. Inaccurate centering, especially with high-power prescriptions, can cause eye strain and visual distortion.
Under the Opticians Act 1989, UK optometrists are legally required to provide the patient with a copy of their spectacle prescription immediately after the sight test, without charging an additional fee. This ensures consumers can purchase corrective eyewear from any provider. The prescription must clearly state the date of the examination and the professional’s details.
UK spectacle prescriptions are typically valid for two years from the date of examination. Prescriptions specifically for contact lenses usually have a shorter validity period, often only one year, due to the direct contact with the eye and the need for regular health checks. It is recommended to have eyes re-examined before the prescription expires to ensure the correction remains appropriate and to check for underlying eye health changes.