Tonometry is a test used to evaluate a patient for glaucoma and to monitor the progression of glaucoma. It is performed with an instrument called a Tono-Pen, a hand held instrument used to measure fluid pressure inside the eye, or intraocular pressure (IOP). By gently tapping on the cornea three measurements are taken. The unit then displays an average of the readings.

The fluid inside the eye is called the aqueous humor. The aqueous humor is produced and drains from the eye at approximately the same rate, resulting in a stable pressure inside the eye of 15 to 20 mmHg (millimeters of mercury). Glaucoma occurs as a consequence of a defect in the drainage angle resulting in inadequate outflow of aqueous humor and a subsequent buildup of pressure inside the eye. The resulting high pressure damages the optic nerve and results in blindness.