Next
Home
Back
4
1. Introduction
1.3 How the intraocular pressure is measured
The cornea is flattened by an acrylic measuring prism on a
ring support at the end of the Tonometer sensor arm assembly.
It is flat with smooth or rounded margins to avoid any damage
to the cornea.
The measuring prism is brought into contact with the patient’s
eye by moving the slit lamp forward. The measurement drum is
then turned to increase the pressure on the eye until a continuous,
uniform applanated surface 3.06 mm in diameter (7,354 mm² area)
is obtained. The doubling prism divides the image and presents
the two opposing semicircular halves at 3.06mm (see section 7.4.2
Measurement procedure for further details).
1.4 Advantages of using a Goldmann Type Tonometer
• Intraocular pressure can be measured during routine
examination with the Slit Lamp.
• The standard deviation among single measurements is
approximately <_ 0,5 mmHg.
• The value is expressed in mmHg and is read directly on the
instrument.
• Scleral rigidity need not be taken into consideration because
the small volume moved (0,56 mm
3
) increases intra-ocular
pressure by only about 2.5%.
• There are no difficulties as regard standardisation and
calibration.
Position of the
measurement drum
Force
mN
Pressure
kPa
mmHg
1
9.81
1.33
10
2
19.62
2.66
20
3
29.43
39.9
30
4
39.24
53.2
40
5
49.05
66.5
50
6
58.86
79.8
60
7
68.67
93.1
70
8
78.48
10.64
80
Relationship between the pressure of the
measurement drum and the force and
pressure on the applanated surface.
The intra-ocular pressure, expressed in
mmHg, is calculated by multiplying the
drum measurement by ten (for conversion
from one unit to another).
