Ophthalmic Optics and Lenses Flashcards

A patient's prescription is -5.00 D sphere. If the patient looks through the lens 4 mm below the optical center, what is the amount and direction of the induced prism?

Answer: 2.0 Δ Base Up

Prentice's Rule is used to calculate induced prism: Prism (Δ) = Power (D) x Decentration (cm). First, convert the decentration from millimeters to centimeters: 4 mm = 0.4 cm. Then, apply the formula: Δ = 5.00 D x 0.4 cm = 2.0 Δ. For a minus lens, the base of the prism is in the same direction as the decentration. Since the patient is looking below the optical center (down), the induced prism is Base Up.

Which of the following lens materials has the highest Abbe value, indicating the least amount of chromatic aberration?

Answer: CR-39

CR-39 plastic has an Abbe value of approximately 58 to 59.3, which is one of the highest among common ophthalmic lens materials. A higher Abbe value signifies less chromatic aberration (color fringing). Polycarbonate has a low Abbe value (around 30), and high-index materials also have lower Abbe values than CR-39. Trivex has a better Abbe value than polycarbonate but it is still lower than CR-39.

A patient with anisometropia requires a slab-off prism to correct for vertical imbalance at the reading level. On which lens is a conventional slab-off typically ground?

Answer: The lens with the most minus or least plus power in the vertical meridian.

A conventional slab-off is a base-up prism ground onto the back surface of the lens that has the most minus power or the least plus power in the vertical (90-degree) meridian. This counteracts the excessive base-down effect experienced when looking down to read through that lens.

What is the resulting prescription when +1.75 -2.50 x 085 is transposed?

Answer: -0.75 +2.50 x 175

To transpose a prescription: 1. Algebraically add the sphere and cylinder to get the new sphere (+1.75 + (-2.50) = -0.75). 2. Change the sign of the cylinder (-2.50 becomes +2.50). 3. Change the axis by 90 degrees (since 85 is less than 90, add 90: 85 + 90 = 175). The transposed prescription is -0.75 +2.50 x 175.

According to ANSI Z87.1 standards for basic impact safety lenses, what is the minimum thickness required for a high-index plastic lens?

Answer: 3.0 mm

The ANSI Z87.1 standard requires that for basic impact resistance, high-index plastic and glass lenses must have a minimum thickness of 3.0 mm. Polycarbonate and Trivex lenses, which are more impact-resistant, can have a minimum thickness of 2.0 mm for high-impact rating.

A primary advantage of an aspheric lens design compared to a standard spherical lens is that it:

Answer: provides a wider field of clear vision with less peripheral distortion.

Aspheric lenses have a complex surface curvature that gradually changes from the center to the edge. This design reduces optical aberrations, particularly spherical aberration and distortion, which results in a wider usable field of view and sharper peripheral vision compared to conventional spherical lenses.