Lensometry and Spectacle Lens Verification Flashcards
7 cards from real CPO practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Lensometry and Spectacle Lens Verification flashcards as text
To determine the add power of a flat-top bifocal lens, a technician should:
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.
Prismatic power in a spectacle lens is measured in which unit?
Answer: Prism diopters (Δ)
Prism power is expressed in prism diopters (Δ), where 1Δ displaces light by 1 cm at 1 meter.
When the lensometer target mire appears displaced from the center of the reticle, this indicates:
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.
For a progressive addition lens (PAL), where should the lensometer be positioned to measure the distance power?
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.
A lens prescription reads -1.00 -0.50 x 180. When transposed to plus cylinder form, it becomes:
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.
The base direction of prism in a spectacle lens refers to the direction:
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.
When measuring a high-powered lens (e.g., +14.00 D aphakic correction) on a standard lensometer, the technician may find that:
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.