Re-evaluation of the Schiotz tonometer calibration.

The calibration scale for the Schi0tz tonometers toas tested by comparing Goldmann applanation and Schi0tz measurements on the eyes of 906 patients. The study differed from several previous studies in that both the applanation and Schi0tz measurements were made with the subjects lying on their backs, thus eliminating errors due to change in intraocular pressure as the subject changed from one position to another. This direct calibration approach is quite different from the approach used for the currently accepted 1955 scale, which was an indirect calibration based on calculations from theoretical considerations and measurements on enucleated eyes. In this study, the average intraocidar pressure (measured by applanation) for any given Schi0tz scale reading was higher than predicted by the 1955 scale and is closer to the 1948 calibration scale. Analysis of the data also points out that calibration scales give only an average value for intraocular pressure, and that because of varying ocular rigidities, the Schi0tz reading indicates only that the intraocular pressure is ivithin a certain range. The table giving the results of this study indicates a range of values with 95 per cent confidence in addition to an average value.