It is used to measure the pressure of the fluid in the eye (intraocular pressure), the normal value of which is 10-21 mmHg. Attaching the test head containing the small prism to the surface of the cornea under local anesthesia, the examiner reads the calibrated intraocular pressure using a small scale. Intraocular pressure can be elevated for example in glaucoma and in some cases of intraocular inflammation.