ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers 4 — Questions and Answers
Question 1: Which instrument is used to measure the vertex distance between the back surface of a spectacle lens and the front of the cornea?
- Distometer (vertex distance gauge) (Correct answer)
- Pupillometer
- Lensometer
- Corneal topographer
Correct answer: Distometer (vertex distance gauge)
A distometer (vertex distance gauge) measures the distance from the posterior lens surface to the anterior cornea, important for high-power prescriptions.
Question 2: A lensometer target appears as two sets of lines oriented 90 degrees apart. The lines running in one direction clear at +2.00 and those running perpendicular clear at +3.50. What is the prescription?
- +2.00 –1.50 × axis of blurred lines
- +3.50 –1.50 × axis of first clear lines
- +2.00 +1.50 × axis of first clear lines (Correct answer)
- +3.50 +1.50 × axis of second clear lines
Correct answer: +2.00 +1.50 × axis of first clear lines
Starting with the lower power meridian (+2.00) as the sphere, the cylinder is the algebraic difference (+1.50), and the axis aligns with the first set of lines to clear.
Question 3: What does a seg height measurement represent in opticianry?
- The distance from the bottom of the frame to the top of the bifocal segment
- The distance from the lowest point of the frame to the optical center
- The distance from the pupil center to the top of the bifocal segment
- The distance from the bottom of the frame to the pupil center (Correct answer)
Correct answer: The distance from the bottom of the frame to the pupil center
Seg height is measured from the lowest point of the finished frame to the center of the patient's pupil, determining where the bifocal segment should be placed.
Question 4: When measuring a plano lens on a lensometer, the target should be:
- Displaced from center and blurred
- Centered and clear when the drum is at zero (Correct answer)
- Centered only when the drum is at +1.00
- Completely invisible
Correct answer: Centered and clear when the drum is at zero
A plano (no power) lens does not alter the light path, so the target remains centered and clear when the power drum is set to zero.
Question 5: The 'mire' in a keratometer refers to:
- The corneal tear film measurement
- The illuminated target pattern projected onto the cornea (Correct answer)
- The instrument's calibration scale
- The patient's fixation point
Correct answer: The illuminated target pattern projected onto the cornea
Mires are the specific illuminated rings or crosses projected onto the cornea whose reflected images are measured to calculate corneal curvature.
Question 6: Which of the following would indicate that a lensometer needs recalibration?
- The target moves when the patient lens is repositioned
- A known plano lens reads a power other than zero (Correct answer)
- The cylinder axis changes when rotating the lens
- The target appears slightly yellow
Correct answer: A known plano lens reads a power other than zero
If a verified plano lens does not read zero on the lensometer, the instrument is out of calibration and must be adjusted.
Question 7: When measuring a lens with the lensometer, moving the lens to the right causes the target to move to the left. This relationship tells you that:
- The lens has plus power
- The lens has minus power (Correct answer)
- The lens has cylinder power
- The lens has no power
Correct answer: The lens has minus power
When moving a lens causes the target to move in the opposite direction (against motion), the lens has minus (concave) power.
ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers 4
Which instrument is used to measure the vertex distance between the back surface of a spectacle lens and the front of the cornea?