Introduction to Lensometry
Susan Brummett, COMT
Little Rock, Arkansas
- 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.
Sphere –lens with optical power being the same in all meridians (expressed in diopters).
Cylinder –lens that has different refractive/optical power in each meridian. It is used to correct astigmatism.
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.
- 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.