push against and indent the cornea. For very high levels of IOP, extra weights can be added to make the plunger push harder. The movement of the plunger is measured using a calibrated scale. The Schiøtz tonometer is the most common device to use this principle. Non-corneal and transpalpebral tonometry Transpalpebral tonometry refers to methods of measuring intraocular pressure through the eyelid. The Diaton non-corneal tonometer calculates pressure by measuring the response of a free-falling rod, as it rebounds against the tarsal plate of the eyelid through the sclera. The patient is positioned so that the tip of the device and lid are overlying sclera. Non-corneal and transpalpebral tonometry does not involve contact with the cornea and does not require topical anesthetic during routine use. Transpalpebral tonometry may be useful for measuring postsurgery IOP after myopic LASIK ablation because this technique is not influenced by the treatment. The Diaton tonometer still requires further evaluation and is not a substitute or alternative for more established methods. The Diaton tonometer has a large margin of error compared with commonly used tonometers (e.g., GAT) in most patients (including those with ocular hypertension, glaucoma, and glaucoma tube shunts). Non-contact tonometry Non-contact tonometry (or air-puff tonometry) is different from pneumatonometry and was invented by Bernard Grolman of Reichert, Inc (formerly American Optical). It uses a rapid air pulse to applanate (flatten) the cornea. Corneal applanation is detected via an electro-optical system. Intraocular pressure is estimated by detecting the force of the air jet at the instance of applanation. Historically, non-contact tonometers were not considered to be an accurate way to measure IOP but instead a fast and simple way to screen for high IOP. However, modern non-contact tonometers have been shown to correlate well with Goldmann tonometry measurements and are particularly useful for measuring IOP in children and other non-compliant patient groups. Ocular response analyzer The ocular response analyser (ORA) is a non-contact (air puff) tonometer that does not require topical anaesthesia and provides additional information on the biomechanical properties of the cornea. It uses an air pulse to deform the cornea into a slight concavity. The difference between the pressures at which the cornea flattens inward and outward is measured by the machine and termed corneal hysteresis (CH). The machine uses this value to correct for the effects of the cornea on measurement. In a population based study in healthy children that compared non-contact