How Do I Measure Base Curve of the Eye?

The base curve of the eye is the curve of the cornea over the iris and pupil. This curve matches the concave surface of the contact lens. The measurement is key for fitting hard contact lenses. Soft contact lenses do not require the same measurements since the lens material flexes and forms to the eye. Measuring the base curve of the eye requires special training and equipment. In most cases, your optometrist measures the base curve during your vision care visit. The wrong base curve will distort your vision and may injure your eye.

Things You'll Need

• Chair
• Keratometer
• Alcohol swabs

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Instructions

• 1

  Sit in a chair on one side of the keratometer. The keratometer has a chin and forehead rest in front of a machine with a lens. The vision care professional will sit on the other side of the machine.

• 2

  Wipe the forehead and chin rest with an alcohol swab. The vision care professional often will do this for you.

• 3

  Keep your eye open as the vision care professional focuses the lens on either the right or left eye. Focus on the image you see in the lens. Different machines have different images, from geometric shapes to landscapes. The machine makes a noise as the picture goes in and out of focus. The machine shines a light on your eye and measures the reflection of that light off your retina. It measures the refraction of the light as it passes through your eye's lens and cornea. Then it calculates the base curve of that eye.

• 4

  The vision care professional moves the lens to the other eye and repeats.