ABO NOCE Basic Opticianry Dispensing Instrumentation 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 +6.50 D in all meridians. What does this measurement represent?
- The spherical equivalent of the lens
- The total power of the lens
- The base curve of the lens (Correct answer)
- The cylindrical power of the back surface
Correct answer: The base curve of the lens
A lens clock measures the surface power of a lens. When the front surface is spherical (has the same power in all meridians), this reading is defined as the base curve. The total power would require measuring the back surface as well, and the spherical equivalent is a calculation involving both sphere and cylinder powers.
Question 2: A patient's prescription is OD -8.50 D sphere. The original glasses were fit at a vertex distance of 14 mm, but the new, larger frame sits at 10 mm from the cornea. To ensure the patient experiences the correct prescribed power, the compensated lens power should be:
- More minus (e.g., -8.87 D)
- Less minus (e.g., -8.25 D) (Correct answer)
- The same (-8.50 D)
- More plus (e.g., -8.12 D)
Correct answer: Less minus (e.g., -8.25 D)
When a minus lens is moved closer to the eye, its effective power becomes less minus. Therefore, to compensate and provide the patient with the intended correction, the ordered power must be slightly less minus. Conversely, moving a minus lens away from the eye makes it effectively stronger (more minus).
Question 3: When neutralizing a lens on a manual lensometer, you first focus the thin, single mires at +2.00 D with the axis wheel at 090. You then turn the power drum further into the plus to focus the thick, triple mires, which become clear at +3.50 D. What is the correct prescription in plus cylinder form?
- +3.50 +1.50 x 180
- +2.00 -1.50 x 090
- +3.50 -1.50 x 180
- +2.00 +1.50 x 090 (Correct answer)
Correct answer: +2.00 +1.50 x 090
In plus cylinder lensometry, the sphere power is the first reading where the single (sphere) lines are in focus (+2.00 D). The cylinder power is the difference between the second reading (triple lines) and the first reading (+3.50 - +2.00 = +1.50 D). The axis is the setting on the axis wheel when the sphere lines were focused (090).
Question 4: Which of the following instruments is specifically designed to measure the distance from the posterior surface of a spectacle lens to the apex of the cornea?
- Pupilometer
- Lens Clock
- Caliper
- Distometer (Correct answer)
Correct answer: Distometer
A distometer, also known as a vertexometer, is the specialized tool used to measure vertex distance. A pupilometer measures the distance between pupils, a lens clock measures surface curvature, and a caliper measures thickness or diameter.
Question 5: An optician needs to make a significant adjustment to a thick zyl (cellulose acetate) frame temple. Which tool provides the most uniform and safest heat to make the material pliable without causing localized overheating or bubbling?
- A salt pan with glass beads (Correct answer)
- A handheld butane torch
- A hot water bath
- A high-velocity hot air blower
Correct answer: A salt pan with glass beads
A salt pan, typically filled with glass beads or fine salt, provides gentle and even heat across the entire portion of the frame being adjusted. This uniformity is crucial for thick acetate frames to prevent damage like bubbling or warping, which can occur with the concentrated heat from a hot air blower or other methods.
Question 6: While verifying a pair of uncut lenses with a caliper, an optician is primarily measuring the:
- Base curve
- Prismatic effect
- Lens thickness (Correct answer)
- Refractive index
Correct answer: Lens thickness
Calipers are precision tools used in optics to measure linear dimensions with high accuracy, most commonly the center thickness and edge thickness of a lens. This is essential for calculating final lens weight, ensuring proper fit in a frame, and verifying that a high-index lens has been surfaced correctly to be thin.
ABO NOCE Basic Opticianry Dispensing Instrumentation Questions and Answers
An optician uses a lens clock to measure the front surface of a lens and gets a consistent reading of +6.50 D in all meridians.
What does this measurement represent?