Introduction to Lensometry
Susan Brummett, COMT
Little Rock, Arkansas
January 2007
AROUND THE OFFICE
Terminology
- Lensmeter – instrument designed to measure the prescription of an optical lens.
- Also defined as an instrument to determine the spherical power, cylinder power with axis, and optical center of a lens.
- Mires –measurement images viewed by technician.•Reticle –eyepiece or ocular adjusted for examiner’s focusing point.•Instrument consists of an ocular for viewing the mires, a flat stage or table for supporting the spectacle frame, a power dial, and an axis wheel.
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Sphere –lens with optical power being the same in all meridians (expressed in diopters).
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Cylinder –lens that has different refractive/optical power in each meridian. It is used to correct astigmatism.
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Axis –meridian of cylinder with the minimum power perpendicular to maximum power meridian. Expressed in units of degrees.
Steps to Neutralizing Spectacles
- Adjust the eyepiece
- Adjustment is critical for obtaining accurate measurement
- Turn power drum to plano
- Turn ocular to most PLUS position (counterclockwise)
- Slowly turn ocular toward minus until the numbers and circles appear sharp and focused.
- STOP!!!!
- Check calibration of lensmeter
- Rotate the diopterdial until all mires come into focus without a lens on the frame table.
- If the diopterdial reads “0”once all mires are in focus, the lensmeteris properly calibrated.
- Position spectacles
- Place glasses on platform so that both lenses are resting on stage with the temples facing away from you (for distance RX)
- This prevents movement of the lenses and inaccurate reading of the axis
- Locate optical center of lens
- Beginning with the right lens (as with most ophthalmic procedures) center the lens on the platform and lock in place with the lens holder.
- Upon viewing the lens through the ocular, the mires should be centered. If not, possibly have ground-in prism present.