Corrective Lenses Flashcards

What does 'base curve' refer to in the context of ophthalmic lenses?

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.

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?

Answer: +3.50 D

Over-refraction result is added algebraically: +4.00 + (-0.50) = +3.50 D.

What is the primary advantage of high-index lens materials over standard CR-39 plastic?

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.

Which axis orientation describes a 'with-the-rule' astigmatism in spectacle correction?

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°.

Anti-reflective (AR) coating on spectacle lenses primarily works by:

Answer: Destructive interference canceling reflected light waves

AR coatings use thin film layers that create destructive interference, canceling reflected light and improving light transmission.

A patient reports that their new progressive lenses cause 'swim' or peripheral distortion. What is the most likely cause?

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.

The 'add power' in a bifocal or progressive lens represents:

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.