ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials Questions and Answers 4 — Questions and Answers
Question 1: In ophthalmic lens terminology, what is the 'optical center' of a lens?
- The geometric center of the blank
- The point through which light passes without prismatic deviation (Correct answer)
- The thickest point of a minus lens
- The location of the reading addition in a bifocal
Correct answer: The point through which light passes without prismatic deviation
The optical center is the point on a lens through which a ray passes without being deviated by prism.
Question 2: Prentice's Rule states that prismatic effect equals:
- Power (D) divided by decentration (cm)
- Decentration (cm) multiplied by Power (D) (Correct answer)
- Power (D) multiplied by decentration (mm)
- Decentration (mm) divided by Power (D)
Correct answer: Decentration (cm) multiplied by Power (D)
Prentice's Rule: Prism (Δ) = Decentration (cm) × Power (D), where decentration is in centimeters.
Question 3: What distinguishes a 'fused' bifocal segment from a 'solid' bifocal?
- Fused bifocals use a different plastic for the segment; solid bifocals are ground from one piece (Correct answer)
- Fused bifocals are always round-top; solid bifocals are flat-top
- Fused bifocals are thicker overall
- Solid bifocals cannot be made in high-index materials
Correct answer: Fused bifocals use a different plastic for the segment; solid bifocals are ground from one piece
Fused bifocals embed a higher-index glass segment into the main lens blank, while solid (one-piece) bifocals are ground entirely from a single blank.
Question 4: What is the effect of increasing the pantoscopic tilt of a frame on the effective optical power experienced by the wearer?
- It decreases both sphere and cylinder power
- It increases effective sphere power and induces cylinder at the horizontal meridian (Correct answer)
- It has no effect on optical power
- It only affects the add power in progressives
Correct answer: It increases effective sphere power and induces cylinder at the horizontal meridian
Pantoscopic tilt tilts the optical axis downward, effectively increasing sphere power and inducing oblique cylinder.
Question 5: Which lens treatment absorbs UV radiation up to 400 nm and is considered full UV protection?
- UV380 coating
- UV400 coating (Correct answer)
- UV320 coating
- Standard clear CR-39
Correct answer: UV400 coating
UV400 coatings block all ultraviolet radiation up to 400 nm, encompassing both UVA and UVB wavelengths.
Question 6: A 'slab-off' grind on a lens is used to correct which problem in anisometropic patients?
- Chromatic aberration differences between lenses
- Vertical prismatic imbalance at the reading level (Correct answer)
- Difference in cosmetic thickness between lenses
- Unequal UV absorption between the two lenses
Correct answer: Vertical prismatic imbalance at the reading level
Slab-off (bicentric grinding) compensates for vertical prismatic imbalance that occurs when anisometropic patients look down to read.
Question 7: What does 'specific gravity' indicate when comparing lens materials?
- How much the material disperses light
- The material's weight relative to an equal volume of water (Correct answer)
- The material's scratch resistance rating
- The refractive index at the sodium D line
Correct answer: The material's weight relative to an equal volume of water
Specific gravity is the ratio of a material's density to that of water; lower specific gravity means a lighter lens.
ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials Questions and Answers 4
In ophthalmic lens terminology, what is the 'optical center' of a lens?