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Spherical Equivalent Formula:
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The Spherical Equivalent (SE) is a calculation used in optometry to convert a sphero-cylindrical prescription into a single spherical value. It's particularly useful for contact lens fitting and certain refractive procedures.
The calculator uses the spherical equivalent formula:
Where:
Explanation: The formula accounts for the fact that cylindrical correction has its greatest effect in one meridian and no effect in the perpendicular meridian.
Details: SE is crucial for contact lens fitting, as most soft contact lenses don't correct for astigmatism (cylinder). It's also used in refractive surgery planning and when simplifying prescriptions for certain visual needs.
Tips: Enter both sphere and cylinder values from your glasses prescription in diopters. The calculator will compute the spherical equivalent which may be used for contact lens prescriptions in some cases.
Q1: When is spherical equivalent used?
A: Primarily for contact lens fitting (especially soft lenses), refractive surgery planning, and when simplifying prescriptions for patients with low astigmatism.
Q2: Does spherical equivalent work for all prescriptions?
A: No, it's most appropriate for low to moderate astigmatism (typically ≤1.50D cylinder). Higher cylinder corrections usually need toric lenses.
Q3: How accurate is spherical equivalent?
A: It provides a good approximation but doesn't fully correct astigmatism. Visual acuity may be slightly reduced compared to full sphero-cylindrical correction.
Q4: Can I use this for my contact lens prescription?
A: Only under professional guidance. Contact lens fitting requires additional parameters and professional assessment.
Q5: What about the axis in my prescription?
A: The spherical equivalent calculation ignores axis as it converts the prescription to a purely spherical form.