According to  Troy Bedinghaus, OD, who writes for www.verywellhealth.com:

“A tonometer is used to measure the pressure of the eye. The test is used to help detect glaucoma. Numbing drops are used for the type of tonometer that actually touches the eye. Some doctors use the air-puff tonometer in which no numbing drops are needed. A tonometer measures the production of aqueous humor, the liquid found inside the eye, and the rate at which it drains into the tissue surrounding the cornea.”

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