You fit a 7.34 mm GP lens on a patient with spherical Ks of 44.00 D. What is the power of your tear layer?

• A. +2.00 D
• B. -2.00 D
• C. +1.00 D
• D. 0.00 D

Answer: (A)

The tear layer between the cornea and the back surface of a rigid GP lens acts as its own small lens. When a GP sits on the eye, this tear film is typically slightly more curved than the cornea but not drastically, forming a thin positive lens. In a standard GP fit around a normal keratometry of about 44.00 D and with a common small-diameter lens like 7.34 mm, the tear layer usually contributes roughly +2.00 diopters to the optical power. So the tear layer’s power is about +2.00 D.

This is why +2.00 D is the best choice. A zero tear-layer power would require an unrealistically perfect, tear-free interface; a negative tear layer would occur only if the lens were markedly steeper than the cornea; and +1.00 D or -2.00 D don’t match the typical tear-film behavior seen in a usual GP fit.