JCAHPO Corrective Lenses 5 — Questions and Answers
Question 1: When fitting a patient's progressive lenses, the segment height should be set so that the fitting cross aligns with:
- The lower lid margin
- The center of the pupil (Correct answer)
- The upper lid margin
- The lower limbus
Correct answer: The center of the pupil
The fitting cross (or pupil center dot) of a progressive lens should align with the center of the pupil to position the distance zone correctly.
Question 2: Which lens coating reduces glare from horizontal surfaces like water and roads by blocking horizontally polarized light?
- Anti-reflective coating
- Mirror coating
- Polarized coating (Correct answer)
- Photochromic coating
Correct answer: Polarized coating
Polarized lenses contain a filter that blocks horizontally polarized light, which is the primary component of glare from flat surfaces.
Question 3: A lensometer reading of 2Δ base-down is found in the left lens. To correct for unwanted prism, the optician should:
- Raise the optical center of the left lens (Correct answer)
- Lower the optical center of the left lens
- Move the optical center nasally
- Move the optical center temporally
Correct answer: Raise the optical center of the left lens
Base-down prism is induced when the optical center is below the pupil; raising the optical center eliminates this unwanted prism.
Question 4: Which of the following best describes the purpose of the 'boxing system' in opticianry?
- A method for calculating prism in bifocals
- A standardized frame measurement system using the smallest enclosing rectangle (Correct answer)
- A lens thickness measurement technique
- A pupillary distance calculation method
Correct answer: A standardized frame measurement system using the smallest enclosing rectangle
The boxing system defines frame dimensions (A, B, DBL, ED) using the smallest rectangle that encloses each lens opening.
Question 5: A patient's prescription is -6.00 D sphere. Compared to a standard plastic (CR-39) lens, a 1.74 high-index lens would be:
- The same thickness but lighter
- Thinner and lighter (Correct answer)
- The same weight but clearer
- Thicker but more durable
Correct answer: Thinner and lighter
A higher refractive index (1.74 vs. 1.50 for CR-39) allows for a thinner lens and, combined with less material, results in a lighter lens.
Question 6: What does the term 'optical center' of a lens refer to?
- The geometric center of the lens blank
- The point on the lens through which light passes without prismatic deviation (Correct answer)
- The center of the segment in a bifocal
- The thickest point of the lens
Correct answer: The point on the lens through which light passes without prismatic deviation
The optical center is the specific point on a lens where incident light passes straight through without being deviated by prism.
Question 7: For a patient with anisometropia (significantly different prescriptions between the two eyes), which lens type helps minimize differential prismatic effect at near?
- Flat-top bifocal with matched segment heights
- Slab-off (bicentric grinding) lens (Correct answer)
- Executive bifocal
- Single-vision reading glasses only
Correct answer: Slab-off (bicentric grinding) lens
Slab-off (bicentric grinding) creates base-up prism on the more minus (or less plus) lens to compensate for differential vertical prismatic imbalance at near.
JCAHPO Corrective Lenses 5
When fitting a patient's progressive lenses, the segment height should be set so that the fitting cross aligns with: