NOCE Optical Instrumentation Usage 5 — Questions and Answers
Question 1: Which instrument provides a topographic map of the entire corneal surface rather than just central curvature readings?
- Keratometer
- Corneal topographer (Correct answer)
- Lensometer
- Retinoscope
Correct answer: Corneal topographer
Corneal topographers use Placido disc reflection or Scheimpflug imaging to map thousands of points across the entire corneal surface, unlike a keratometer which measures only four central points.
Question 2: When using a lensometer on a trifocal lens, the intermediate add power is calculated as:
- Half of the reading add power
- The difference between the near and distance readings
- Half of the total add, measured at the intermediate segment (Correct answer)
- The same as the near add
Correct answer: Half of the total add, measured at the intermediate segment
The intermediate segment of a trifocal is measured directly through the lensometer, and its power (typically half of the full add) is read at that specific zone.
Question 3: An auto-refractor provides an objective refractive measurement by:
- Analyzing retinal reflection patterns from projected infrared light (Correct answer)
- Measuring corneal curvature with mire rings
- Assessing the patient's subjective response to changing lenses
- Evaluating intraocular pressure with an air puff
Correct answer: Analyzing retinal reflection patterns from projected infrared light
Auto-refractors project infrared light onto the retina and analyze the reflected wavefront to objectively determine the eye's refractive error.
Question 4: The ANSI Z80.1 standard specifies that for a single-vision distance lens with a power of -5.00 D, the maximum allowable cylinder power error is:
- ±0.06 D
- ±0.13 D (Correct answer)
- ±0.25 D
- ±0.50 D
Correct answer: ±0.13 D
ANSI Z80.1 allows a cylinder tolerance of ±0.13 D for lenses with sphere powers between ±3.25 D and ±6.50 D.
Question 5: When using a lensometer to verify prism, the amount of prism is measured by:
- The number of degrees the axis wheel is rotated
- How far the target mires are displaced from the center of the reticle in prism diopters (Correct answer)
- The difference in sphere power between the two principal meridians
- The separation of the mire lines in the focal plane
Correct answer: How far the target mires are displaced from the center of the reticle in prism diopters
Prism is measured by how far the target is displaced from the optical center of the lensometer reticle, with each unit of displacement corresponding to one prism diopter.
Question 6: A lensmeter reading on a right lens shows: -2.00 -1.00 x 180. The dot is marked and the lens is flipped to measure the left lens, which reads -2.00 -1.00 x 180. This indicates the prescription is:
- Identical for both eyes with no asymmetry
- Likely an error since mirror-image axes are expected for bilateral prescriptions
- Correct — axis 180 is the same for all orientations (Correct answer)
- An oblique astigmatism in both eyes
Correct answer: Correct — axis 180 is the same for all orientations
Axis 180 is a horizontal axis and is the same value whether measured from the right or left lens; it does not mirror like oblique axes do.
Question 7: Which of the following best describes the purpose of a lens clock (Geneva lens measure) in the opticianry workflow?
- To verify finished prescription accuracy to ANSI standards
- To estimate lens surface curvature and check lens form for remake decisions (Correct answer)
- To measure pupillary distance precisely
- To determine the refractive index of an unknown lens material
Correct answer: To estimate lens surface curvature and check lens form for remake decisions
A Geneva lens measure estimates surface curvature to help identify lens form and check whether a lens matches the intended base curve, useful in remake verification and lens identification.
NOCE Optical Instrumentation Usage 5
Which instrument provides a topographic map of the entire corneal surface rather than just central curvature readings?