ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 5 — Questions and Answers
Question 1: What is the primary reason glass lenses are rarely dispensed today compared to plastic (organic) lenses?
- Glass lenses cannot be tinted
- Glass lenses are heavier and have greater risk of shattering upon impact (Correct answer)
- Glass lenses have a lower refractive index than all plastics
- Glass lenses cannot have anti-reflective coatings applied
Correct answer: Glass lenses are heavier and have greater risk of shattering upon impact
Glass lenses are significantly heavier than plastic alternatives and pose a serious safety hazard because they can shatter into sharp fragments upon impact, leading to eye injury.
Question 2: A patient complains that their new progressive lenses cause 'swim' or motion distortion when they move their head. This is MOST likely due to:
- An error in the sphere power of the distance zone
- The inherent peripheral distortion zones (blur zones) on either side of the PAL corridor (Correct answer)
- A coating defect on the lens surface
- Incorrect pupillary distance measurement causing prism imbalance
Correct answer: The inherent peripheral distortion zones (blur zones) on either side of the PAL corridor
PALs have unavoidable peripheral blur/distortion zones on the nasal and temporal sides of the corridor where astigmatic error increases rapidly; these cause the 'swim' sensation that patients must adapt to.
Question 3: Which UV protection statement about ophthalmic lens materials is CORRECT?
- CR-39 fully absorbs UV to 400nm without any additional treatment
- Polycarbonate and Trivex inherently block 100% of UV to 400nm (Correct answer)
- All lens materials block UV equally regardless of composition
- Glass lenses inherently provide complete UV-400 protection
Correct answer: Polycarbonate and Trivex inherently block 100% of UV to 400nm
Polycarbonate and Trivex inherently absorb virtually all UV radiation up to 400nm due to their chemical composition, without requiring additional UV-blocking coatings or treatments.
Question 4: In a flat-top (FT-28) bifocal lens, the '28' refers to:
- The total lens diameter in millimeters
- The width of the near segment at its widest point in millimeters (Correct answer)
- The depth of the near segment in millimeters
- The distance from the segment top to the optical center in millimeters
Correct answer: The width of the near segment at its widest point in millimeters
In bifocal nomenclature, the number following the segment type designation refers to the width of the near segment at its widest point; FT-28 has a flat-top segment 28mm wide.
Question 5: What is 'prism thinning' (slab-off prism) used for in progressive or bifocal lens dispensing?
- To increase the prismatic effect for patients with strabismus
- To compensate for vertical imbalance caused by anisometropia when patients look through the reading zone (Correct answer)
- To reduce the center thickness of high plus lenses
- To add base-in prism for convergence insufficiency treatment
Correct answer: To compensate for vertical imbalance caused by anisometropia when patients look through the reading zone
Slab-off (prism thinning) introduces base-up prism in the near zone of one lens to neutralize the vertical prismatic imbalance that occurs in anisometropic patients when they read through bifocal or progressive lenses.
Question 6: Which characteristic makes CR-39 plastic the most popular lens material for patients with low to moderate prescriptions?
- Its extreme impact resistance exceeding polycarbonate
- Its combination of low cost, excellent optical clarity (high Abbe value), ease of tinting, and light weight (Correct answer)
- Its ultra-thin profile due to the highest refractive index
- Its complete inherent UV-400 absorption without coatings
Correct answer: Its combination of low cost, excellent optical clarity (high Abbe value), ease of tinting, and light weight
CR-39 remains popular because it offers excellent optical quality (Abbe ~58), accepts tints readily, is cost-effective to manufacture, and is approximately half the weight of glass.
Question 7: When a patient's prescription includes both sphere and cylinder power, the lens has what type of surface on the back?
- Spherical surface (single radius of curvature)
- Toric surface (two different radii of curvature at 90° to each other) (Correct answer)
- Plano (flat) surface
- Aspherical surface with no axis orientation
Correct answer: Toric surface (two different radii of curvature at 90° to each other)
A toric surface has two principal meridians with different radii of curvature oriented 90° apart, allowing it to correct both spherical and cylindrical (astigmatic) refractive errors simultaneously.
ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 5
What is the primary reason glass lenses are rarely dispensed today compared to plastic (organic) lenses?