[Evaluation of Goldmann applanation tonometer with thin membrane water manometer].

In order to evaluate the accuracy of applanation tonometry, we prepared a thin membrane water manometer with a 0.07 mm thick HEMA soft contact lens and measured the internal pressure using a Goldmann type applanation tonometer (KOWA HA-1). The results were as follows. When the inner rims of the fluorescein ring were in contact with each other, the measured pressure value was lower than the theoretical value by about 5-15 mmHg; the higher the internal pressure, the larger the difference from the theoretical value. When the applanated condition was observed carefully, a dark ring about 0.2 mm in width could be seen inside the fluorescein ring. By applanating until the inner rims of this dark ring contacted each other, the theoretical value and the measured value matched approximately. Observing the contact surface by means of specular reflection, the applanated surface was located inside the dark ring, maintaining a circular shape with distinct boundary and this plane showed concentric circles or map-like color interference fringes. Newton's rings appeared on the boundary between this light interference surface and the dark ring. So we concluded that the true applanated area existed inside the dark ring and the inner rim of the fluorescein ring did not constitute an indicator of the boundary of applanated area.