Applanation tonometers are used to measure the Intraocular Pressure, or IOP, of the eye. IOP is often measured in millimeters of Mercury, (mmHg) and is determined by the amount of force that is needed to flatten, or Applegate, a consistent area of the cornea. A topical anesthetic is often used because of the contact required.
The most widely accepted method is Goldmann Tonometry. A disinfected prism is placed on the tonometer and then placed against the cornea. With the use of a cobalt blue filter and a variable tension spring, IOP can be roughly measured with this less-invasive approach. Another method is Perkins tonometry, which is useful for children and anesthetized patients required to lay prone. Due to the concern of cross-contamination between patients, some vendors are now offering one-time use prisms to rule out this risk.