NOCE Optical Instrumentation Use 5 — Questions and Answers
Question 1: When a lensometer target shows two distinct sets of lines perpendicular to each other that cannot both be brought into focus simultaneously, this indicates the lens has:
- Spherical power only
- Cylinder (astigmatic) power (Correct answer)
- Prism
- An error in the instrument
Correct answer: Cylinder (astigmatic) power
Inability to focus two perpendicular line sets simultaneously in a lensometer indicates cylindrical (astigmatic) power in the lens.
Question 2: On a Bausch & Lomb keratometer (one-position instrument), mire doubling is achieved by which mechanism?
- Rotating the instrument barrel
- Moving the eyepiece lens
- Adjusting internal prisms or drums (Correct answer)
- Changing the fixation target distance
Correct answer: Adjusting internal prisms or drums
The B&L keratometer uses internal prisms (drums) to double the mire images, allowing measurement in a single instrument position.
Question 3: When verifying the optical center placement of a finished spectacle lens using a lensometer, the ink dots should be placed while the lens is:
- Held at arm's length
- Resting against the lens stop with the front surface facing the examiner
- Placed with the front surface facing the instrument (Correct answer)
- Tilted at 15 degrees
Correct answer: Placed with the front surface facing the instrument
The lens is placed in the lensometer with the front surface facing the instrument (convex side toward the lens stop) for standard measurement.
Question 4: A contact lens projection magnifier (shadowgraph) is used to measure which contact lens parameter by projecting an enlarged image?
- Base curve radius
- Center thickness
- Diameter and surface quality (Correct answer)
- Oxygen permeability
Correct answer: Diameter and surface quality
A shadowgraph projects an enlarged shadow image of a contact lens to measure its overall diameter and inspect surface quality or edge defects.
Question 5: When using a PD ruler to measure monocular PDs, the examiner should close which of their own eyes to eliminate parallax error?
- The left eye
- The right eye
- The eye opposite to the patient's eye being measured (Correct answer)
- Either eye equally
Correct answer: The eye opposite to the patient's eye being measured
To eliminate parallax, the examiner closes the eye opposite to the patient's eye being measured, so their open eye is aligned over the measurement point.
Question 6: Which lensometer reading sequence is the correct method for measuring a lens in minus cylinder form?
- Read sphere first at the clearest line target, then rotate to clear the dot target for add power
- Focus the line target for the most plus power, record as sphere; rotate axis to focus lines for least plus power, record cylinder as the difference (Correct answer)
- Focus dots first for prism, then lines for sphere, then rotate for cylinder
- Adjust power wheel to most minus setting then advance toward plus until target is clear
Correct answer: Focus the line target for the most plus power, record as sphere; rotate axis to focus lines for least plus power, record cylinder as the difference
In minus cylinder form, start with the most plus (least minus) power meridian for the sphere, then rotate 90° to the other meridian; the difference between the two readings is the cylinder.
Question 7: An optical coherence tomographer (OCT) used in an ophthalmic setting produces cross-sectional images using which principle?
- X-ray absorption
- Infrared light interferometry (Correct answer)
- Ultrasound echoes
- Fluorescent dye imaging
Correct answer: Infrared light interferometry
OCT uses low-coherence infrared light interferometry to produce high-resolution cross-sectional images of ocular structures without contact.
NOCE Optical Instrumentation Use 5
When a lensometer target shows two distinct sets of lines perpendicular to each other that cannot both be brought into focus simultaneously, this indicates the lens has: