A tonometer is an instrument that is used to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometer is calibrated to measure pressure in millimetres of mercury (mmHg). Tonometers measure the internal pressure of the eye and tonometry is one of the principal tests for glaucoma, but until relatively recently their use in the eye examination was far from routine.
