ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 2 — Questions and Answers
Question 1: Which lens material has the highest Abbe value among commonly used ophthalmic plastics?
- CR-39 (Allyl diglycol carbonate) (Correct answer)
- Polycarbonate
- Trivex
- High-index 1.74
Correct answer: CR-39 (Allyl diglycol carbonate)
CR-39 has an Abbe value of approximately 58, the highest among common ophthalmic plastic materials, meaning it produces the least chromatic aberration.
Question 2: A patient's prescription is -8.00 DS. Which lens material would BEST minimize edge thickness?
- CR-39 (n=1.50)
- Polycarbonate (n=1.586)
- High-index 1.67
- High-index 1.74 (Correct answer)
Correct answer: High-index 1.74
High-index 1.74 has the highest index of refraction among common ophthalmic materials, bending light most efficiently and producing the thinnest lenses for high minus prescriptions.
Question 3: What is the primary optical disadvantage of polycarbonate lenses compared to CR-39?
- Lower impact resistance
- Lower refractive index
- Lower Abbe value causing chromatic aberration (Correct answer)
- Higher specific gravity
Correct answer: Lower Abbe value causing chromatic aberration
Polycarbonate has an Abbe value of about 30, significantly lower than CR-39's 58, meaning it disperses light into its spectral components more, causing chromatic aberration (color fringing).
Question 4: An aspheric lens design primarily reduces which aberration compared to a spheric lens?
- Chromatic aberration
- Marginal astigmatic error (oblique astigmatism) (Correct answer)
- Petzval field curvature only
- Prismatic effect at the optical center
Correct answer: Marginal astigmatic error (oblique astigmatism)
Aspheric lens designs flatten the peripheral curves to reduce oblique astigmatic error (marginal astigmatic error), improving peripheral vision quality.
Question 5: What does the term 'specific gravity' indicate about a lens material?
- Its resistance to scratching
- Its density relative to water, affecting lens weight (Correct answer)
- Its ultraviolet light absorption capability
- Its ability to be tinted
Correct answer: Its density relative to water, affecting lens weight
Specific gravity is the ratio of a material's density to that of water; materials with lower specific gravity produce lighter lenses for greater patient comfort.
Question 6: Which coating is applied FIRST directly to the lens substrate during the manufacturing process?
- Anti-reflective coating
- Hard coat (scratch-resistant coating) (Correct answer)
- Mirror coating
- Anti-fog coating
Correct answer: Hard coat (scratch-resistant coating)
Hard coat (scratch-resistant coating) is applied directly to the lens substrate first, providing a durable base layer before any anti-reflective or other coatings are applied.
Question 7: Which lens type would you recommend for a patient who needs both distance and near correction but refuses bifocals for cosmetic reasons?
- Single vision distance lenses with reading glasses
- Progressive addition lenses (PALs) (Correct answer)
- Flat-top bifocal with invisible segment
- Lenticular lens design
Correct answer: Progressive addition lenses (PALs)
Progressive addition lenses (PALs) provide a seamless transition from distance to near correction without visible lines, making them the cosmetically preferred alternative to bifocals.
ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 2
Which lens material has the highest Abbe value among commonly used ophthalmic plastics?