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Vision Correction in India

About Vision Correction ?

A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but recently for purely refractive purposes. Myopia (near-sightedness) requires a divergent lens, whereas hyperopia (far-sightedness) requires convergent lens.

Prescription of corrective lenses

Corrective lenses are typically prescribed by an ophthalmologist or 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 increments (e.g., eighth-diopter steps / 0.125 D).

Axis component

The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a one line around an axis. The curve of that cylinder is 90° from that axis of rotation. When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertcal. The "3 o'clock" position is defined as zero, and the 90th meridian is a vertical line. A horizontal line passes through both zero and the 180th meridians. By convention, a horizontal axis is recorded as 180.

Other considerations

Single vision lenses correct for only distance or near vision. Patients with presbyopia or other disorders of accommodation often benefit from corrections for both distance and near vision (see Lens Types below). Infrequently, prism and base curve values may also be specified to correct for binocular vision disorders.

Vision Correction

  • correction power is measured in diopters.
  • by convention, an axis of 90° is vertical, 0° or 180° are horizontal.
  • if the cylinder power is positive, the lens is most convergent 90° from the axis.
  • if the cylinder power is negative, the lens is most divergent 90° from the axis.
  • if the cylinder power is zero (D.S.), the lens is spherical and has the same power in every meridian.
  • Advantages of higher indices
  • Thinner, sometimes lighter lenses.
  • Improved UV protection over CR-39 and glass lenses.
  • Disadvantages

    Lower Abbe number meaning, amongst other things, increased chromatic aberration.

    Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection equation) increasing importance of anti-reflective coating.

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