[Comparative studies of the calibration of new electronic automatic tonometers]

Over the last years the number of newly developed automatic microprocessor controlled tonometers has been increasing. These devices follow totally different measurement principles. Therefore the physical methods for calibration of these tonometers differ widely. Described is the possibility of calibration of new microprocessor-controlled tonometers. First the Tonopen, which follows the Mackay-Marg principle was tested. Its accuracy in measuring the intraocular pressure and its independency of position was examined. Moreover, the non-contact tonometers were tested. Especially the airstream of non-contact tonometers (CT 10, Pulsair) was measured with a Constant Temperature Anemometer (CTA). The topspeed of the airstream was 335 km/h for the CT 10 and 249 km/h for the Pulsair tonometer. The profile of the airstream was paraboloid. The base measured about 5 mm in diameter. For detection of the effect of the airstream to the cornea, a camera was connected over a special interface with the CT 10 and triggered with a flashlight generator. The airstream produced at the moment of pressure measurement an exact applanation of the cornea with a diameter of about 4 mm. The results demonstrate the possibility of physical calibration, but this does not substitute biometrical measurements.