How to Check the Prism in a Pair of Progressive Glasses

Prism is used in spectacle lenses to help bring eyes into alignment if one or both eyes deviate, or tend to look more in or out. Prism helps both eyes see images together. Progressive lenses provide an additional challenge for testing prism because the power of the bifocal is blended and more difficult to differentiate between distance, near and prism power.

Things You'll Need

• Lens marker
• Lensometer

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Instructions

• 1

  Make a dot on the lens directly in front of the center of the pupil of both eyes as the patient looks straight ahead.

• 2

  Place the glasses, right lens first, on the lensometer, and center the dot you made on the lens directly under the lensometer port, the part of the lensometer that looks like a microscope.

• 3

  Put down the ring that holds the lenses steady and look through the eyepiece.

• 4

  Adjust and focus until you see the power cross, also called the mires, and refocus until they are clear. Work with just the spherical correction in the solid lines of the mires, and disregard astigmatic correction in the lenses.

• 5

  Note where the cross comes into focus. If the cross is displaced from the center of the bull's-eye target, there is prism in the lenses. Make sure you are looking through just the spherical distance power and not into the bifocal by keeping the dot centered in the eyepiece view.

• 6

  Determine the amount of prism by where the power cross comes into focus along the lines of the bull's-eye. Each ring is one degree or diopter of power of prism.