Eyeglasses have optical properties that include the sphere, cylinder, axis and add power. The sphere measurement is the general strength of the lens. The cylindrical measurement is the correction of the lens for astigmatism. The axis measurement provides the rotation of the lens for astigmatism. The add power is the additional power of a bifocal lens. A lensometer is a device that an optometrist or ophthalmologist uses to measure these optical properties.
Instructions
things you’ll need:
Lensometer
1 Mount the eyeglasses so that both lenses are flush against the bottom of the lensometer’s platform. Center the viewing lens on the optical center of one of the lenses, and fix it in place with the brace.
2 Turn the axis knob to make the narrow lines in the viewfinder parallel, and turn the focus knob to bring these lines into focus. Record the measurement on the focus knob to the nearest quarter diopter, including the sign. This measurement is the spherical value of the lens.
3 Read the measurement on the axis knob. This measurement is the axis value of the lens.
4 Rotate the focus knob to bring the thick lines in the viewfinder into focus. Subtract this measurement from the spherical value of the lens in Step 2. This difference is the cylinder value of the lens.
5 Release the brace and center the lensometer’s viewing lens on the bifocal lens of the eyeglasses. Lock the brace, and turn the focus knob to focus the thick lines. Subtract the focus knob measurement in Step 4 from the focus knob’s current reading. This difference is the add value for the bifocal lens.
6 Repeat Steps 1 through 5 with the other lens in the eyeglass.