Ophthalmic Lenses and Materials Flashcards
7 cards from real ABO NOCE Basic Opticianry practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Ophthalmic Lenses and Materials flashcards as text
What is the primary reason glass lenses are rarely dispensed today compared to plastic (organic) lenses?
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.
A patient complains that their new progressive lenses cause 'swim' or motion distortion when they move their head. This is MOST likely due to:
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.
Which UV protection statement about ophthalmic lens materials is 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.
In a flat-top (FT-28) bifocal lens, the '28' refers to:
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.
What is 'prism thinning' (slab-off prism) used for in progressive or bifocal lens dispensing?
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.
Which characteristic makes CR-39 plastic the most popular lens material for patients with low to moderate prescriptions?
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.
When a patient's prescription includes both sphere and cylinder power, the lens has what type of surface on the back?
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.