Extending the Range of Your Keratometer

by Michael A. Johnson, FCLSA
Director of Consultation Services

In consulting with practitioners during the course of a day, I typically deal with several keratoconus fittings. In some instances the steep K is beyond 52.00D, which exceeds the instrument’s ability to read the steeper meridian. You can extend the range by placing a +1.25 hand-held trial ring lens over the keratometer’s central aperture (patient’s side of instrument). This trial ring can be taped into place by taping the ring handle to the instrument. Tape the handle at the 10 or 2 o’clock position to make sure that you do not occlude the superior minus (-) mire. After you take the readings, multiply your findings by 116.59% to determine the extended K. Example, K readings with +1.25 trial ring in place; 48.00×51.00 multiply each K by 116.59% = 56.00×59.50. Make sure that you remove the trial ring lens from instrument after each use to avoid your next patient’s K’s being off. A handy conversion chart is also available for converting your readings to the extended range. Call Art Optical Contact Lens, Inc. and we will be happy to send you a copy of this chart.

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