Prescription of corrective lenses:1Components of a sphero-cylindrical correction
July 27, 2008 – 11:29 pmFrom Wikipedia.com
Corrective lenses are typically prescribed by an optometrist. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller*Sphere component
Each power specification includes a spherical correction in diopters. Convergent powers are positive (ex. +4.00 D) and condense light to correct for farsightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). Divergent powers are negative (ex. -3.75 D) and spread out light to correct for nearsightedness (myopia). If neither convergence nor divergence is required in the prescription, “plano” is used to denote a refractive power of zero.*Cylinder componentIf a patient has an astigmatism, the patient needs two different correction powers in two different meridians (horizontally and vertically for example). This is specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the sphere power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence or most divergence.
Ophthalmologists record in “plus cylinder notation” where the cylinder power is a number of diopters more convergent than the sphere power. That means the sphere power describes the most divergent meridian and the cylinder component describes the most convergent. Optometrists use “minus cylinder notation” where the cylinder power is a number of diopters more divergent than the sphere component. Thus the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. (There is no difference in these forms of notation. They arise from the nature of the two professions and are easily converted between by people accustomed to working with sphero-cylindrical prescriptions. They are simply two ways to specify the same thing.)
*Axis component
The axis defines where the two powers (sphere and cylinder) are located. The sphere is almost always 90º from the cylinder. (This is regular astigmatism, which is by far more common than irregular astigmatism where separations are other than 90º). Vertical is the 90th meridian and horizontal is both zero and the 180th meridians. The axis is the meridian 90º away from the cylinder power. Since the cylinder and sphere powers almost always separated by 90º, the axis is also the location of the sphere component. If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the spherical power.
*Sample prescription
So, a prescription of -1.00 +0.25 x 180 describes a lens that has a horizontal power of -1.00 D and a vertical power of -0.75. (The same prescription written in minus cylinder notation: -0.75 - 0.25 x 090)
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One Response to “Prescription of corrective lenses:1Components of a sphero-cylindrical correction”
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By Karen on Jul 29, 2008