How to check the calibration of the B&L keratometer
The B&L keratometer should be checked for accuracy periodically (perhaps once a year), but it rarely needs recalibration. All ophthalmic assistants/technicians are capable of checking the accuracy of the instrument.
Although the steps necessary for recalibration are relatively simple, they must be performed with care to avoid mistakenly adjusting the keratometer out of proper calibration. A professional ophthalmic maintenance person should be the one who performs recalibration if the instrument is found to be out of adjustment.
In order to check the accuracy of the keratometer, you will need a set of standard spheres. These are usually highly polished steel balls that have a know curvature. Some kits have only one sphere, some have three. There is some type of mounting device included with the kit. The kit pictured here includes a magnetized mount that attaches to the headrest of the keratometer. The steel spheres are magnetically held onto the end of the mount. The steel spheres include curvatures of 40.25, 42.25, and 44.25 diopters.
1. Set the eyepiece.
This step is absolutely critical. If you do not adjust the eyepiece, you will be in danger of finding your keratometer to be out of adjustment when it may not be. To set the eyepiece, dial it in the plus direction until the crosshair is out of focus. Now slowly turn the eyepiece in the minus direction until the crosshair just come into focus. Do not move the the eyepiece back and forth in the plus and minus directions, only approach the point of focus from the plus direction.
2. Mount the test sphere.
Clamp the sphere mount to the headrest of the keratometer. If the kit has removable spheres, place one (it doesn’t matter which one you start with) on the mount. If the spheres are steel they may have a dull finish if they have not been used in a while. If so, use a soft cotton cloth to polish the surface.
3. Measure the test sphere.
Measure the curvature of the sphere just as you would a cornea. You will find the sphere to be much more cooperative than an actual patient. If the reading on the horizontal and vertical measuring drums matches the designated diopter value of the sphere (plus or minus an eighth of a diopter), then you are finished! You have checked the calibration of the keratometer and it has been found to be accurate. If you have other spheres, you can use one or more to confirm the calibration.
As discussed, if you find your keratometer to be out of calibration, it is best to have the instrument re-calibrated by a professional ophthalmic service person.