Lensometry and Spectacle Lens Verification Flashcards

What instrument is used to verify the optical power of a finished spectacle lens?

Answer: Lensometer (lensmeter)

The lensometer (also called a lensmeter or focimeter) is the standard instrument used to measure and verify the refractive power of spectacle lenses after fabrication.

On a manual lensometer, focusing the sphere mire target gives you:

Answer: The sphere power

When the sphere (rotationally symmetric) mire is focused sharply, you read the sphere power directly from the power drum.

On a manual lensometer with crossed-line mires, when one set of lines focuses sharply but the other does not at the same drum setting, this indicates:

Answer: A toric (spherocylindrical) lens

When the two sets of mire lines require different drum settings to focus, the lens has cylindrical power, making it a toric or spherocylindrical lens.

By ANSI/ISO convention, the axis of a cylindrical spectacle lens is measured:

Answer: 1° to 180° counterclockwise

By ANSI/ISO convention, spectacle lens axis is measured from 1° to 180° counterclockwise when viewing the lens from the front.

Before measuring any lens on a lensometer, the first step the technician should perform is:

Answer: Zero-calibrate the lensometer eyepiece and power drum

Calibrating the lensometer by zeroing the eyepiece and confirming zero power with no lens in place ensures all subsequent measurements are accurate.

Ink dots placed on a spectacle lens during lensometry serve to mark:

Answer: The optical center and prism reference point for alignment

The lensometer inker stamps dots marking the optical center (and flanking reference points) so the lens is correctly aligned during edging and frame mounting.

When calibrating a manual lensometer with no lens in place, the power drum should read:

Answer: 0.00 D (plano)

With no lens in place and the eyepiece properly focused for the observer, the power drum must read 0.00 D (plano) to confirm the instrument is properly zeroed.