JCAHPO Corrective Lenses 3 — Questions and Answers
Question 1: What does 'base curve' refer to in the context of ophthalmic lenses?
- The front surface power of the lens (Correct answer)
- The cylinder axis of the lens
- The add power of a bifocal
- The index of refraction of the material
Correct answer: The front surface power of the lens
The base curve is the curvature of the front (anterior) surface of an ophthalmic lens, measured in diopters.
Question 2: A patient with +4.00 D of hyperopia wears contact lenses. After fitting, the over-refraction shows -0.50 D. What is the revised contact lens power?
- +3.50 D (Correct answer)
- +4.50 D
- +4.00 D
- +3.00 D
Correct answer: +3.50 D
Over-refraction result is added algebraically: +4.00 + (-0.50) = +3.50 D.
Question 3: What is the primary advantage of high-index lens materials over standard CR-39 plastic?
- Greater impact resistance
- Thinner and lighter lenses for high prescriptions (Correct answer)
- Better UV blocking without coatings
- Lower cost
Correct answer: Thinner and lighter lenses for high prescriptions
High-index materials bend light more efficiently, producing thinner and lighter lenses for patients with high refractive errors.
Question 4: Which axis orientation describes a 'with-the-rule' astigmatism in spectacle correction?
- Axis near 90° (vertical) (Correct answer)
- Axis near 180° (horizontal)
- Axis near 45°
- Axis near 135°
Correct answer: Axis near 90° (vertical)
With-the-rule astigmatism has the steepest corneal meridian vertical, so the correcting minus cylinder axis is near 180°; however, the plus cylinder axis (and thus the steeper meridian) is near 90°.
Question 5: Anti-reflective (AR) coating on spectacle lenses primarily works by:
- Absorbing light energy at the lens surface
- Destructive interference canceling reflected light waves (Correct answer)
- Increasing the index of refraction
- Hardening the lens surface to reduce scratches
Correct answer: Destructive interference canceling reflected light waves
AR coatings use thin film layers that create destructive interference, canceling reflected light and improving light transmission.
Question 6: A patient reports that their new progressive lenses cause 'swim' or peripheral distortion. What is the most likely cause?
- Incorrect pupillary distance
- Inherent peripheral aberrations in progressive lens design (Correct answer)
- Wrong anti-reflective coating
- Incorrect segment height
Correct answer: Inherent peripheral aberrations in progressive lens design
Progressive lenses have unavoidable lateral aberration zones on either side of the progressive corridor that cause peripheral distortion.
Question 7: The 'add power' in a bifocal or progressive lens represents:
- The total near power of the lens
- The additional plus power needed beyond distance correction for near vision (Correct answer)
- The prism added to the reading segment
- The difference between left and right eye powers
Correct answer: The additional plus power needed beyond distance correction for near vision
Add power is the additional plus power placed in the near zone above the distance correction to compensate for reduced accommodation in presbyopia.
JCAHPO Corrective Lenses 3
What does 'base curve' refer to in the context of ophthalmic lenses?