ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 3 — Questions and Answers
Question 1: When dispensing lenses with anti-reflective (AR) coating, why must a hard coat be applied beneath the AR coating?
- To increase the refractive index of the lens
- To provide a smooth, durable surface for the AR layers to adhere to (Correct answer)
- To block UV radiation from reaching the AR layers
- To reduce the weight of the finished lens
Correct answer: To provide a smooth, durable surface for the AR layers to adhere to
Hard coat creates a smooth, durable surface that allows the thin vacuum-deposited AR layers to adhere properly and reduces microabrasions that would compromise coating performance.
Question 2: A photochromic lens darkens when exposed to UV radiation because of which mechanism?
- A chemical reaction with visible light photons that changes dye molecular structure
- UV radiation activates silver halide or organic molecules embedded in the lens, causing a color change (Correct answer)
- The lens temperature increases, causing thermal darkening
- An electronic sensor detects light intensity and adjusts tint electronically
Correct answer: UV radiation activates silver halide or organic molecules embedded in the lens, causing a color change
Photochromic lenses contain silver halide crystals or organic photochromic molecules that undergo a reversible chemical reaction when exposed to UV radiation, causing the lens to darken.
Question 3: What is the main advantage of Trivex over polycarbonate as an impact-resistant lens material?
- Trivex is significantly less expensive
- Trivex has a higher refractive index (1.74 vs 1.586)
- Trivex has a higher Abbe value providing better optical clarity (Correct answer)
- Trivex is available in a wider range of tints
Correct answer: Trivex has a higher Abbe value providing better optical clarity
Trivex has an Abbe value of approximately 45 compared to polycarbonate's 30, giving Trivex significantly better optical clarity and less chromatic aberration while maintaining excellent impact resistance.
Question 4: In a progressive addition lens, where is the distance optical power measured?
- At the segment top
- In the fitting cross or distance reference point (DRP) (Correct answer)
- At the geometric center of the lens blank
- At the prism reference point (PRP)
Correct answer: In the fitting cross or distance reference point (DRP)
The distance power of a PAL is measured at the distance reference point (DRP) or fitting cross, which is positioned in front of the patient's pupil for distance viewing.
Question 5: Which type of lens segment shape in a bifocal provides the widest near field of view?
- Round segment (22mm)
- Flat-top (D-segment) 28mm
- Executive (E-style) full-width segment (Correct answer)
- Curved-top (C-segment)
Correct answer: Executive (E-style) full-width segment
The Executive (E-style or Franklin-style) bifocal extends the near segment across the full width of the lens, providing the widest possible near field of view.
Question 6: What is 'base curve' in ophthalmic lens terminology?
- The curvature of the frame front
- The front surface curve of the lens, which determines the lens form (Correct answer)
- The curve of the back surface that provides the prescription power
- The curve used to measure the patient's corneal curvature
Correct answer: The front surface curve of the lens, which determines the lens form
Base curve refers to the front (convex) surface curvature of an ophthalmic lens; it determines the lens form (flat, steep, or best form) and affects peripheral optics.
Question 7: A patient's lens prescription is +5.00 DS. Where is the thickest point of this finished lens?
- At the edge, equally around the periphery
- At the optical center (Correct answer)
- At the segment midpoint
- At the nasal edge only due to prismatic decentration
Correct answer: At the optical center
Positive (plus) lenses are thickest at the optical center and taper toward the edges — opposite of minus lenses which are thinnest at the center.
ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 3
When dispensing lenses with anti-reflective (AR) coating, why must a hard coat be applied beneath the AR coating?