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

Back vertex power

Answer

Overall diameter

Answer

Dk value

Question 2

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

Accepted Answer

Dk/t value

Answer

Water content percentage

Answer

Lens modulus of elasticity

Answer

Wetting angle

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

30%

Answer

45%

Answer

70%

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

Complete limbal clearance with no movement on blink

Answer

Tight fit with less than 0.1 mm lag on upgaze

Answer

Slight apical clearance with heavy edge lift

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

Group I

Answer

Group II

Answer

Group III

Question 6

Back vertex power of a contact lens is best defined as:

Accepted Answer

The vergence of light leaving the posterior surface of the lens

Answer

The power measured at the anterior surface of the lens

Answer

The base curve converted to dioptric power

Answer

The prismatic effect at the optical center

Question 7

A patient's spectacle prescription is –10.00 DS at a vertex distance of 12 mm. When converting to a contact lens power, the contact lens should be:

Accepted Answer

Less minus than the spectacle Rx because the effective power increases at the corneal plane

Answer

More minus than the spectacle Rx due to reduced vertex distance

Answer

The same power because vertex distance only matters for plus lenses

Answer

More plus than the spectacle Rx to compensate for tear lens effect