Fig. 9. Principle of air-puff tonometer. A puff of air of known area is generated against cornea (B). At the moment of corneal applanation a light (T), which is usually reflected from the normal cornea into space, suddenly is reflected (R) into an optical sensor (A). When the sensor is activated by the reflected light, the air generator is switched off. The level of force at which the generator stops is recorded, and a computer calculates and displays the intraocular pressure. (D) Optical detector. (Adapted from B. Grolman, American Optical Corp.)