NBEO Contact Lenses & Fitting

Question 1

Which parameter describes the radius of curvature of the posterior surface of a contact lens?

Accepted Answer

Base curve radius

Answer

The base curve radius defines the curvature of the posterior surface of the contact lens that interfaces with the tear film and cornea.

Question 2

Which parameter best describes the oxygen transmissibility of a contact lens material of a given thickness?

Accepted Answer

Dk/t value

Answer

Dk/t combines the oxygen permeability (Dk) of the material and lens thickness (t), making it the best indicator of how much oxygen reaches the cornea.

Question 3

According to the FDA classification system, a soft contact lens with water content at or above what percentage is classified as 'high water content'?

Accepted Answer

50%

Answer

The FDA classifies soft contact lens materials into Groups I–IV; lenses with water content ≥50% are classified as high water content.

Question 4

When evaluating a soft contact lens fit, which finding best indicates an optimal fit?

Accepted Answer

Full corneal coverage, good centration, and 0.5–1 mm movement with blink

Answer

An ideal soft lens fit shows complete corneal coverage, good centration, and 0.5–1 mm of movement with each blink to allow adequate tear exchange.

Question 5

Which FDA contact lens group is characterized by both high water content and ionic properties, making it most susceptible to protein deposits?

Accepted Answer

Group IV

Answer

FDA Group IV lenses have high water content (≥50%) and are ionic, which promotes protein binding and deposit formation on the lens surface.

NBEO - National Board of Examiners in Optometry Practice Test

NBEO - National Board of Examiners in Optometry Practice Test

NBEO Contact Lenses & Fitting