Intraocular pressure is measured with a(n):

Intraocular pressure (IOP) is a critical measurement in ophthalmology, especially for diagnosing and managing conditions like glaucoma. The instrument specifically used to measure this pressure is called a tonometer. Tonometers work by assessing the resistance of the eye to a force applied, which allows for the calculation of the intraocular pressure.

The other instruments listed serve different purposes. An ophthalmometer is used to measure the curvature of the cornea, which is essential for determining the proper fitting of contact lenses. A keratometer, similar to the ophthalmometer, specifically measures the corneal curvature. A lensmeter, on the other hand, is used to measure the prescription of glasses or contact lenses by assessing the optical power of a lens. Each of these tools plays an important role in eye care, but the tonometer is the only device designed for measuring intraocular pressure directly.