NOCE Optical Instrumentation Usage 2 — Questions and Answers
Question 1: When using a lensometer to measure a bifocal lens, where should the segment be positioned relative to the lens stop?
- Centered on the lens stop
- Below the lens stop (Correct answer)
- Above the lens stop
- To the right of the lens stop
Correct answer: Below the lens stop
The bifocal segment should be positioned below the lens stop so the distance portion is measured first through the aperture.
Question 2: A keratometer reading of 44.00 @ 90 / 46.00 @ 180 indicates which type of corneal astigmatism?
- Against-the-rule astigmatism (Correct answer)
- With-the-rule astigmatism
- Oblique astigmatism
- Irregular astigmatism
Correct answer: Against-the-rule astigmatism
Against-the-rule astigmatism occurs when the flattest meridian is vertical (90°) and the steepest is horizontal (180°).
Question 3: Which instrument is specifically used to measure the back vertex distance of a spectacle lens?
- Distometer (Correct answer)
- Pupillometer
- Focimeter
- Geneva lens measure
Correct answer: Distometer
A distometer (also called a vertex distance gauge) measures the distance from the back surface of the lens to the cornea.
Question 4: On a manual lensometer, what does a clear, sharp image of the target mires indicate?
- The lens is properly centered
- The sphere power has been neutralized (Correct answer)
- The cylinder axis is aligned
- The add power is correct
Correct answer: The sphere power has been neutralized
When the target mires appear clear and sharp in the lensometer eyepiece, the sphere power of the lens has been neutralized.
Question 5: A Geneva lens measure is used on a plus lens and reads +3.00 on the front surface and -1.00 on the back surface. What is the approximate lens power?
- +4.00 D
- +2.00 D (Correct answer)
- -2.00 D
- +3.00 D
Correct answer: +2.00 D
Lens power approximation by lens measure is calculated by adding front and back surface powers: +3.00 + (-1.00) = +2.00 D.
Question 6: When using a pupillometer for a monocular PD measurement, the patient should fixate on:
- A distant target straight ahead
- The examiner's left eye while the right eye is measured
- The light source inside the pupillometer corresponding to each eye (Correct answer)
- Any comfortable target at near distance
Correct answer: The light source inside the pupillometer corresponding to each eye
For monocular PD, the patient fixates the light source within the pupillometer that corresponds to the eye being measured to simulate the visual axis.
Question 7: Which lensometer target pattern indicates that a cylindrical correction is present in the lens being measured?
- All mire lines are clear simultaneously
- The mire lines appear doubled
- One set of mire lines clears at a different power than the other set (Correct answer)
- The target rotates continuously
Correct answer: One set of mire lines clears at a different power than the other set
Cylinder is present when the two sets of mire lines (sphere and cylinder lines) focus at different power settings, requiring two separate power readings.
NOCE Optical Instrumentation Usage 2
When using a lensometer to measure a bifocal lens, where should the segment be positioned relative to the lens stop?