That means the spherical power describes the most divergent meridian and the cylindrical component describes the most convergent. In the former, the cylinder power is a number of diopters more convergent than the sphere power. There are two different conventions for indicating the amount of cylinder: "plus cylinder notation" and "minus cylinder notation".
For regular toric lenses, these powers are perpendicular to each other and their location relative to vertical and horizontal are specified by the axis component. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence. The power of a toric lens can be specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power. For example, a toric lens, when rotated correctly, could focus an object to the image of a horizontal line at one focal distance while focusing a vertical line to a separate focal distance. A meridian, in this case, is a plane that is perpendicular to the optical axis. The geometry of a toric lens focuses light differently in different meridians. Patients with astigmatism need a cylindrical lens, or more generally a toric lens to see clearly. To correct for astigmatism, the "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.
Spherical lenses are adequate correction when a person has no astigmatism. A spherical lens has the same curvature in every direction perpendicular to the optical axis. Lenses derive their power from curved surfaces. The term "sphere" comes from the geometry of lenses. If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero. Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness/short-sightedness ( myopia). Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness/long-sightedness ( hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). 1 Components of a sphero-cylindrical correctionĬomponents of a sphero-cylindrical correction Sphere component Įvery corrective lens prescription includes a spherical correction in diopters.A dispensing optician will take a prescription written by an optometrist and order and/or assemble the frames and lenses to then be dispensed to the patient.
The prescription is typically determined during a refraction, using a phoropter and asking the patient which of two lenses is better, or by automated refractor, or through the technique of retinoscopy. The parameters specified on spectacle prescriptions vary, but typically include the patient's name, power of the lenses, any prism to be included, the pupillary distance, expiration date, and the prescriber's signature. If an eye examination indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam. Using a phoropter to determine a prescription for eyeglassesĪn eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.
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