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7. Measurement procedure
7.4.2 Taking the measurement
• Move the slit lamp forward to bring the measuring
prism into contact with the centre of the cornea in the area
above the pupil. The limbus will be illuminated with a bluish
light. The examiner will be able to better directly observe this
phenomenon from the opposite side.
• As soon as the corneal limbus is correctly illuminated,
immediately stop all forward movement of the slit lamp.
• After contact is established, observe the cornea through
the microscope. With the measurement drum set to position
1, the two semicircular fluorescein rings (which will vary in
size according to ocular pressure) will pulse rhythmically
when the Tonometer is in the correct position for
measurement.
• Use the Slit Lamp joystick control to make any corrections
needed until the applanated surface is observed as two
semicircular surfaces of equal area at the Centre of the field
of vision. (Figure 1). Small adjustments downward made with
the joystick will have no effect on the sizes of the semicircular
images.
• Increase applanation pressure by rotating the Tonometer
measurement drum until the margins of the fluorescein rings
touch and the cross as the eye pulses (Figure 2). The width of
the fluorescein rings around the contact position of the
measuring prism should be equal to about 1/10 of the
diameter of the applanation surface (0,3 mm).
• To read the scale, multiply the readings by a factor of ten
(10). The result is the ocular pressure expressed in mmHg.
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Figure 1: Semicircular images at the
Centre of the field of vision.
Figure 2: Correct Final Position
