A tonometry test measures the pressure inside your eye, which is called intraocular pressure (IOP). This test is used to check for glaucoma, an eye disease that can damage the optic nerve at the back of the eye. Damage to the optic nerve may be caused by a build-up of fluid that raises the pressure within the eye.
Non-contact tonometry measures IOP indirectly by measuring the force required to flatten a certain area of the cornea using a puff of air. This is part of every standard eye examination for those over the age of 40 or who are considered to be at risk of developing glaucoma.
Contact (Goldmann) tonometry uses a small probe to gently flatten part of your cornea to measure eye pressure. The pressure in your eye is measured by how much force is needed to flatten your cornea. This type of tonometry is very accurate and is often used to measure IOP after a simple screening test (such as air-puff tonometry) finds an increased IOP.