ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 4 — Questions and Answers
Question 1: Which anti-reflective coating residual color is most commonly seen on AR-coated lenses?
- Red/orange
- Blue/green or purple/green (Correct answer)
- Yellow
- Brown
Correct answer: Blue/green or purple/green
Most AR coatings are designed to have a blue-green or purple-green residual reflex, which is the color of light reflected off the coated surface when viewed at an angle.
Question 2: What does 'index of refraction' measure in an ophthalmic lens material?
- The material's resistance to impact
- The ratio of light speed in a vacuum to light speed in the material (Correct answer)
- The material's ability to absorb UV radiation
- The material's scratch resistance
Correct answer: The ratio of light speed in a vacuum to light speed in the material
Index of refraction (n) is the ratio of the speed of light in a vacuum to its speed in the material; a higher index means light slows more, allowing the lens to be made thinner.
Question 3: A patient is prescribed a +2.00 add in their progressive lenses. Where does the corridor (channel) connect the distance and near zones?
- Along the top edge of the lens blank
- Through the intermediate zone between the distance and near reference points (Correct answer)
- Directly through the optical center of the lens
- Along the temporal edge of the lens
Correct answer: Through the intermediate zone between the distance and near reference points
The corridor or channel of a PAL is a narrow zone of increasing power connecting the distance zone (top) to the near zone (bottom), passing through the intermediate zone for arm's-length vision.
Question 4: Which lens form (design) provides the best optical performance by minimizing off-axis aberrations?
- Flat form
- Plano (zero power front surface)
- Best form (corrected curve or Tscherning ellipse curve) (Correct answer)
- Minus cylinder form only
Correct answer: Best form (corrected curve or Tscherning ellipse curve)
Best form lenses use the Tscherning ellipse to select front curves that minimize oblique astigmatic error for a given prescription power, providing superior peripheral optical performance.
Question 5: When verifying a progressive addition lens on a lensometer (focimeter), what is the standard method for locating the distance reference point?
- Place the dot-marked fitting cross under the lensometer stop and read the power there (Correct answer)
- Find the clearest distance power by scanning the upper portion of the lens until the mires are sharpest
- Always read power at the geometric center of the lens
- Use the segment top as the distance reference point
Correct answer: Place the dot-marked fitting cross under the lensometer stop and read the power there
The manufacturer's markings (temporary dots, engravings, or fitting cross) on a PAL identify the distance reference point; placing the fitting cross under the lensometer stop gives the distance prescription reading.
Question 6: What is the purpose of a 'lenticular' lens design?
- To provide a built-in prism for strabismus correction
- To reduce weight and thickness in very high-power prescriptions by limiting the full-power zone to a central optical cap (Correct answer)
- To provide a multifocal correction without a visible segment line
- To maximize the field of view by using an aspheric periphery
Correct answer: To reduce weight and thickness in very high-power prescriptions by limiting the full-power zone to a central optical cap
Lenticular lenses have a central optical portion (the lenticular bowl or cap) containing the full prescription power, surrounded by a carrier of reduced power, dramatically reducing weight and thickness for very high prescriptions.
Question 7: Which statement BEST describes the relationship between lens power and center thickness for a minus lens?
- Higher minus power requires greater center thickness to maintain edge strength
- Higher minus power allows thinner center thickness but produces thicker edges (Correct answer)
- Center thickness is unrelated to prescription power
- Higher minus power requires thicker center AND thicker edges
Correct answer: Higher minus power allows thinner center thickness but produces thicker edges
Minus lenses are thinnest at the center and thickest at the edges; as minus power increases, the center can be made thinner but the edges become progressively thicker.
ABO NOCE Basic Opticianry Ophthalmic Lenses and Materials 4
Which anti-reflective coating residual color is most commonly seen on AR-coated lenses?