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A lensometer is an optometric device used to determine, or read, the prescription in an optical lens. The lensometer reads spherical power, cylindrical power, axis, and pupillary distance, and can also determine prism in a lens. The measurement images inside the lensometer are called mires, and are adjusted to determine prescription.
Adjust the eyepiece to Plano, or neutral position, by turning the drum handle on the right past zero, into plus, and then back into neutral position at Plano.
Place the eyeglasses on the platform so they are facing you, temples facing away. Lower the lens holder into place to steady the eyeglasses.
As in all optic procedures, begin with the right lens. Bring the mires into focus by turning the power drum to minus, then back toward you until the mires are solid. Mires are usually three solid lines in one direction and one or three thin lines in the opposite.
Read the mires. If all the lines come into focus at once, the lens power is spherical, with no astigmatism. If only the three thick lines or one thin line are in focus or appear broken, the lens has cylinder, or astigmatic correction.
Turn the power drum to bring the thin lines into focus and record the number as the sphere power. Adjust the axis wheel at the base of the lensometer until the thick lines come into focus; record this number as the axis of the prescription. Rotate the power drum toward you again until the three thick lines come into focus, giving you the cylinder power.
Mark the P.D., or pupillary distance, with the drop-down marker. To read bifocals, turn the eyeglasses around and read the lower part of the lens from the back side. Subtract the spherical power of the add from the distance spherical number of the lens to determine bifocal power.
Calibrate the lensometer before using to make sure power is set to zero: simply adjust mires with no lens in to make sure power reading is at zero and mires are in focus.
Check for prism in lenses if the mires are not centered inside the viewing area of the lensometer when you read the power.