CPO Lensometry and Spectacle Lens Verification 2 — Questions and Answers
Question 1: To determine the add power of a flat-top bifocal lens, a technician should:
- Subtract the distance power reading from the near segment power reading (Correct answer)
- Read the add directly from the segment without measuring the distance zone
- Add the sphere and cylinder powers together
- Measure the upper half of the lens only
Correct answer: Subtract the distance power reading from the near segment power reading
Add power is calculated by measuring the power through the near segment and subtracting the previously measured distance power; the difference equals the addition.
Question 2: Prismatic power in a spectacle lens is measured in which unit?
- Diopters (D)
- Prism diopters (Δ) (Correct answer)
- Millimeters of displacement
- Degrees of angular deviation
Correct answer: Prism diopters (Δ)
Prism power is expressed in prism diopters (Δ), where 1Δ displaces light by 1 cm at 1 meter.
Question 3: When the lensometer target mire appears displaced from the center of the reticle, this indicates:
- The cylinder axis is misaligned
- Prismatic power is present in the lens (Correct answer)
- The add power is incorrect
- The sphere power is out of range
Correct answer: Prismatic power is present in the lens
Displacement of the mire target from the optical center of the lensometer reticle indicates prismatic power; the direction and amount of displacement identify the prism base and magnitude.
Question 4: For a progressive addition lens (PAL), where should the lensometer be positioned to measure the distance power?
- Through the near zone at the bottom of the lens
- Through the intermediate corridor
- Through the distance zone in the upper portion of the lens (Correct answer)
- Through the widest part of the lens
Correct answer: Through the distance zone in the upper portion of the lens
The distance power of a PAL must be measured through the distance optical zone located in the upper portion of the lens.
Question 5: A lens prescription reads -1.00 -0.50 x 180. When transposed to plus cylinder form, it becomes:
- -1.50 +0.50 x 090 (Correct answer)
- -0.50 +0.50 x 090
- -1.50 +0.50 x 180
- -0.50 -0.50 x 090
Correct answer: -1.50 +0.50 x 090
Transposing -1.00 -0.50 x 180: new sphere = -1.00 + (-0.50) = -1.50; new cylinder = +0.50; new axis = 180 − 90 = 090. Result: -1.50 +0.50 x 090.
Question 6: The base direction of prism in a spectacle lens refers to the direction:
- The apex (thinnest edge) of the prism points
- The base (thickest edge) of the prism points (Correct answer)
- Light is deviated by the prism
- The patient's eye turns in response to the prism
Correct answer: The base (thickest edge) of the prism points
The base direction is defined as the orientation of the thickest part (base) of the prism — for example, base-in means the thick edge faces nasally.
Question 7: When measuring a high-powered lens (e.g., +14.00 D aphakic correction) on a standard lensometer, the technician may find that:
- The lens absorbs too much light to allow measurement
- The mire appears brighter than normal
- The power exceeds the lensometer's measurement range (Correct answer)
- The ink dots will not adhere to the lens surface
Correct answer: The power exceeds the lensometer's measurement range
Many standard lensometers have a measurement range of approximately ±20 D, but very high-powered lenses may still be difficult to focus or may fall outside the instrument's usable range.
CPO Lensometry and Spectacle Lens Verification 2
To determine the add power of a flat-top bifocal lens, a technician should: