Microprocessor controlled tonometry.

Microprocessors have enabled new techniques to be applied to the design of tonometers. Three new microprocessor controlled tonometers, one operating to the principle of Mackay-Marg (Tono-Pen) and two non-contact airstream tonometers (Pulsair and CT-10) have been examined using as a reference Goldmann and the Hand Held Applanation Tonometers (Draeger-HAT) which used the Goldmann applanation principle. The non-contact tonometers produce a conically formed airstream with top speeds of 90 m/s (CT-10) and 70 m/s (Pulsair) which at the moment of pressure measurement applanates the cornea exactly with a diameter of 4 mm. The readings of the microprocessor controlled tonometers correlate well with the readings of HAT (Tono-Pen r = 0.8414 and s = 3.783, CT-10 r = 0.8606 and s = 4.5477, Pulsair r = 0.8968 and s = 3.2641, where r is the correlation coefficient and s is the standard deviation in mmHg). Repeated measurements with one tonometer give the following standard deviations (HAT 1.6617, Tono-Pen 3.4969, CT-10 2.9179, Pulsair 2.9434). Although the three new instruments are able to measure the intraocular pressure in a wide range they do not work with the same precision as the classical applanation tonometer.