NOCE Optical Instrumentation Usage 4 — Questions and Answers
Question 1: When using a Burton lamp (UV lamp) to inspect contact lenses with fluorescein, a dark (black) area under a rigid contact lens indicates:
- Good central alignment
- Bearing or touch between the lens and cornea (Correct answer)
- Excessive edge lift
- Proper fluorescein pooling
Correct answer: Bearing or touch between the lens and cornea
A dark area under a rigid lens during fluorescein evaluation indicates lens-to-cornea touch or bearing, where fluorescein is displaced by contact pressure.
Question 2: A pachymeter instrument is used to measure:
- Intraocular pressure
- Corneal thickness (Correct answer)
- Axial length of the eye
- Lens thickness of spectacle lenses
Correct answer: Corneal thickness
A pachymeter measures corneal thickness, which is important for contact lens fitting and monitoring corneal health.
Question 3: When using a Geneva lens measure on a minus lens, the gauge pointer deflects:
- In the positive direction
- In the negative direction (Correct answer)
- It does not deflect
- In either direction depending on lens index
Correct answer: In the negative direction
A minus (concave) lens surface causes the center pin of the Geneva lens measure to be pushed back, resulting in a negative (minus) reading.
Question 4: A prismatic effect found at the optical center of a lens during lensometry most likely indicates:
- Incorrectly prescribed cylinder
- A manufacturing defect or incorrect prism in the prescription (Correct answer)
- Normal variation acceptable within ANSI tolerances
- The lens is a high-plus design
Correct answer: A manufacturing defect or incorrect prism in the prescription
Prism measured at the optical center of the lens indicates either intentional prescribed prism or a manufacturing error, since a properly made non-prism lens should have zero prism at its optical center.
Question 5: The slit lamp biomicroscope is used in optical practice primarily to:
- Measure intraocular pressure using applanation
- Examine the fit and position of contact lenses on the eye (Correct answer)
- Perform automated refraction
- Measure corneal curvature for spectacle fitting
Correct answer: Examine the fit and position of contact lenses on the eye
The slit lamp allows the optician or practitioner to examine contact lens centration, movement, edge lift, and fit characteristics under high magnification.
Question 6: What does the term 'vertex power' refer to when measured by a lensometer?
- The power measured at the front surface of the lens
- The back vertex power, which is the reciprocal of the back focal length (Correct answer)
- The average of front and back surface powers
- The prismatic power at the lens periphery
Correct answer: The back vertex power, which is the reciprocal of the back focal length
Back vertex power, as measured by a lensometer, is the reciprocal of the distance from the back surface of the lens to its secondary focal point.
Question 7: When a spectacle prescription exceeds ±4.00 D in sphere power, back vertex distance becomes clinically significant because:
- The lens becomes too heavy to fit comfortably
- Small changes in vertex distance produce meaningful changes in effective lens power (Correct answer)
- The optical center shifts with vertex distance changes
- Chromatic aberration increases at this power level
Correct answer: Small changes in vertex distance produce meaningful changes in effective lens power
At higher powers, the effective power of the lens at the corneal plane changes significantly with small shifts in vertex distance, requiring compensation during dispensing.
NOCE Optical Instrumentation Usage 4
When using a Burton lamp (UV lamp) to inspect contact lenses with fluorescein, a dark (black) area under a rigid contact lens indicates: