NOCE Optical Instrumentation Usage 3 — Questions and Answers
Question 1: When calibrating a lensometer, a technician uses a standard lens of known power. If the reading differs from the known value, the next step is to:
- Replace the lensometer
- Adjust the eyepiece diopter compensation first, then recalibrate (Correct answer)
- Use an offset correction factor for all future readings
- Clean the lens aperture and retest
Correct answer: Adjust the eyepiece diopter compensation first, then recalibrate
Before calibrating the instrument, the eyepiece must be properly focused for the examiner's eye to eliminate measurement error from accommodation.
Question 2: An optical coherence tomographer (OCT) used in an optical setting primarily helps the optician by:
- Measuring corneal curvature for contact lens fitting
- Detecting macular pathology that may affect vision correction outcomes (Correct answer)
- Calculating pupillary distance automatically
- Verifying spectacle lens power
Correct answer: Detecting macular pathology that may affect vision correction outcomes
OCT imaging can detect retinal pathology such as macular degeneration that may explain reduced visual acuity not correctable with spectacles.
Question 3: A lensometer with an automated printout system marks a dot on the lens. This dot represents the:
- Optical center of the lens (Correct answer)
- Prism reference point
- Near vision point
- Segment optical center
Correct answer: Optical center of the lens
The lensometer marks the optical center (or reference point) of the lens so the optician can verify correct placement relative to the patient's pupil.
Question 4: Which of the following correctly describes the function of the axis wheel on a manual lensometer?
- It adjusts the total power of the lens
- It rotates the target to align the cylinder axis of the lens (Correct answer)
- It changes the add power reading
- It adjusts the eyepiece for the examiner
Correct answer: It rotates the target to align the cylinder axis of the lens
The axis wheel rotates the lensometer target to align with the cylinder axis of the lens being measured, allowing the cylinder meridians to be properly read.
Question 5: When measuring a progressive addition lens on a lensometer, the reference point for the distance power should be:
- The geometric center of the lens
- The prism reference point (fitting cross) (Correct answer)
- The near vision zone
- The segment top
Correct answer: The prism reference point (fitting cross)
For progressive lenses, distance power is measured at the prism reference point (fitting cross), which is marked by the manufacturer.
Question 6: A radiuscope is primarily used in contact lens practice to measure:
- The refractive index of contact lens material
- The base curve radius of rigid contact lenses (Correct answer)
- The diameter of soft contact lenses
- The water content of hydrogel lenses
Correct answer: The base curve radius of rigid contact lenses
A radiuscope (or optical spherometer) measures the base curve radius of curvature of rigid gas-permeable and hard contact lenses.
Question 7: In automated lensometry, the instrument determines the cylinder axis by:
- Requiring the technician to manually rotate until the mires align
- Using a rotating detector array to sense the principal meridians electronically (Correct answer)
- Comparing front and back surface curvatures
- Measuring the polarization angle of transmitted light
Correct answer: Using a rotating detector array to sense the principal meridians electronically
Automated lensometers use electronic sensors or rotating detector arrays to automatically identify the principal meridians and determine cylinder axis without manual alignment.
NOCE Optical Instrumentation Usage 3
When calibrating a lensometer, a technician uses a standard lens of known power.
If the reading differs from the known value, the next step is to: