Free National Opticianry Competency Exam (NOCE) Optical Instrumentation Use Questions and Answers — Questions and Answers
Question 1: An optician uses a lens clock to measure the front surface of a lens and gets a consistent reading of +6.00 D in all meridians. When measuring the back surface, the reading is -8.00 D in the 180° meridian and -6.00 D in the 090° meridian. Assuming the lens clock is calibrated for an index of 1.53 and the lens material is crown glass (n=1.523), what is the nominal prescription of this lens in minus cylinder form?
- +0.00 -2.00 x 180
- -2.00 -2.00 x 090
- -2.00 +2.00 x 180
- +0.00 -2.00 x 090 (Correct answer)
Correct answer: +0.00 -2.00 x 090
To find the nominal prescription, combine the front and back surface powers. The front curve (F1) is spherical at +6.00 D. The back curve (F2) is toric. To write in minus cylinder form, the sphere power is the combination of the front curve and the most plus (least minus) back curve: +6.00 D + (-6.00 D) = +0.00 D. The cylinder power is the difference between the two back curves: (-8.00 D) - (-6.00 D) = -2.00 D. The axis of the minus cylinder is the meridian of the flattest (least minus) curve, which is 090°. Therefore, the prescription is +0.00 -2.00 x 090.
Question 2: When using a corneal reflex pupillometer to take a monocular pupillary distance (PD) measurement for progressive addition lenses, which of the following is a critical step for ensuring accuracy?
- Having the patient converge their eyes as if reading.
- Ensuring the patient and dispenser are seated and at eye level with each other. (Correct answer)
- Measuring from the center of the right pupil to the temporal limbus of the left eye.
- Setting the instrument's fixation distance to infinity for all measurements.
Correct answer: Ensuring the patient and dispenser are seated and at eye level with each other.
For accurate progressive lens fitting, both the patient and the dispenser should be seated comfortably and at the same eye level. This posture ensures the patient's head is in a natural position, preventing measurement errors that can occur if the patient is looking up or down at the dispenser.
Question 3: A client's new high-plus power eyeglasses are verified to be correct in the lensometer, but the client reports blurry vision. The original refraction was performed at a vertex distance of 12 mm, but the new frames sit 16 mm from the patient's cornea. Which instrument should be used to measure this discrepancy, and what is the likely effect on the perceived power?
- Pupillometer; a decrease in perceived plus power.
- Lens Clock; an increase in perceived plus power.
- Distometer; an increase in perceived plus power. (Correct answer)
- Geneva Lens Gauge; a decrease in perceived plus power.
Correct answer: Distometer; an increase in perceived plus power.
A distometer is the specialized tool used to measure vertex distance—the distance between the back surface of the lens and the front of the cornea. For plus-powered lenses, moving the lens further away from the eye increases its effective power. This increase in perceived power is likely the cause of the patient's blurry vision.
Question 4: An optician is preparing to adjust a pair of cellulose acetate frames for a patient. Which of the following instruments is most appropriate for making the frame material pliable, and what is a key precaution to take?
- An ultrasonic cleaner; ensure the frame is fully submerged.
- A hot air frame warmer; avoid concentrating heat in one spot to prevent damage. (Correct answer)
- Axis pliers; use firm pressure to reshape the bridge.
- A salt pan; use the highest temperature for rapid softening.
Correct answer: A hot air frame warmer; avoid concentrating heat in one spot to prevent damage.
A hot air frame warmer is the recommended and safest tool for adjusting most plastic frames, including cellulose acetate. It provides controlled, even heat. It is crucial to keep the frame moving and not concentrate the heat on a single area, as this can cause the material to burn, warp, or lose its finish.
Question 5: Before neutralizing an unknown lens in a manual lensometer, what is the first and most critical step the operator must perform?
- Set the axis wheel to 90 degrees.
- Place the lens on the stage with the concave side up.
- Turn the power drum until it reads -10.00 D.
- Focus the eyepiece for their own eye. (Correct answer)
Correct answer: Focus the eyepiece for their own eye.
The first step before using a manual lensometer is to calibrate it for the user's own refractive error by focusing the eyepiece. The user should turn the eyepiece counter-clockwise until the reticle (the black target inside) is blurred, and then turn it clockwise until the reticle is in sharp, clear focus. Failure to do this will result in inaccurate power readings.
Question 6: Which of the following instruments is used to measure the base curve of a finished spectacle lens?
- Pupillometer
- Distometer
- Lens Clock (Correct answer)
- Lensometer
Correct answer: Lens Clock
A lens clock, also known as a Geneva Lens Gauge or spherometer, is used to measure the surface curvature of a lens, which is expressed as the base curve in diopters. It has three probes; the movement of the central probe relative to the two outer probes indicates the surface power.
Free National Opticianry Competency Exam (NOCE) Optical Instrumentation Use Questions and Answers
An optician uses a lens clock to measure the front surface of a lens and gets a consistent reading of +6.00 D in all meridians.
When measuring the back surface, the reading is -8.00 D in the 180° meridian and -6.00 D in the 090° meridian.
Assuming the lens clock is calibrated for an index of 1.53 and the lens material is crown glass (n=1.523), what is the nominal prescription of this lens in minus cylinder form?