IOP measuring tonometer, including ORA and CORVIS with a contact tonometer, GAT, which is a routine instrument for IOP measurement. It was firmly evident that due to significantly low positive or negligible correlation, none of these 2 non-contact tonometers can replace the GAT. Palpation Palpation (also known as digital tonometry) is the method of estimating intraocular pressure by gently pressing the index finger against the cornea of a closed eye. This method is notoriously unreliable. Influencing factors Central corneal thickness (CCT) The thickness of the cornea affects most non-invasive methods by varying resistance to the tonometer probe. A thick cornea gives rise to a greater probability of an IOP being overestimated (and a thin cornea of an IOP being underestimated), but the extent of measurement error in individual patients cannot be ascertained from the CCT alone. The Ocular Response Analyzer and Pascal DCT Tonometers are less affected by CCT than the Goldmann tonometer. Conversely, non-contact and rebound tonometers are more affected. Corneal thickness varies among individuals as well as with age and race. It is reduced in certain disease and following LASIK surgery. Gallery References External links Tonometry - WebMD Kirstein, E. "An Update on Methods for Assessing Intraocular Pressure." Retrieved June 7, 2006. Diagnostic ophthalmology Ophthalmic equipment