CPO Lensometry and Spectacle Lens Verification 1 — Questions and Answers
Question 1: What instrument is used to verify the optical power of a finished spectacle lens?
- Keratometer
- Lensometer (lensmeter) (Correct answer)
- Autorefractor
- Phoropter
Correct 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.
Question 2: On a manual lensometer, focusing the sphere mire target gives you:
- The cylinder power
- The axis of the lens
- The sphere power (Correct answer)
- The prism amount
Correct answer: The sphere power
When the sphere (rotationally symmetric) mire is focused sharply, you read the sphere power directly from the power drum.
Question 3: 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:
- A spherical lens with no cylinder
- A toric (spherocylindrical) lens (Correct answer)
- A lens with prism only
- A defective lensometer
Correct 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.
Question 4: By ANSI/ISO convention, the axis of a cylindrical spectacle lens is measured:
- 0° to 360° clockwise
- 1° to 180° counterclockwise (Correct answer)
- 0° to 180° clockwise
- 1° to 360° counterclockwise
Correct 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.
Question 5: Before measuring any lens on a lensometer, the first step the technician should perform is:
- Apply ink dots to the lens surface
- Zero-calibrate the lensometer eyepiece and power drum (Correct answer)
- Measure the bifocal add power first
- Remove the lens from the spectacle frame
Correct 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.
Question 6: Ink dots placed on a spectacle lens during lensometry serve to mark:
- The cylinder axis location only
- The optical center and prism reference point for alignment (Correct answer)
- The bifocal segment top edge
- The location of the anti-reflective coating
Correct 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.
Question 7: When calibrating a manual lensometer with no lens in place, the power drum should read:
- +0.25 D to account for instrument error
- 0.00 D (plano) (Correct answer)
- -0.25 D as a standard offset
- Whatever the last measured lens read
Correct 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.
CPO Lensometry and Spectacle Lens Verification 1
What instrument is used to verify the optical power of a finished spectacle lens?