An applanation tonometer including a light-conducting pressure applicator assembly having an end to be placed in contact with the cornea of a subject's eye and an opposite end, a displacer locating said pressure applicator assembly with said end against the subject's cornea thereby flattening a portion thereof, an illuminator for illuminating the subject's cornea, an imaging transducer at the opposite end of the pressure applicator assembly for receiving therethrough an optical image of at least said portion of the subject's cornea and converting said optical image of at least said portion of the subject's cornea into electrical signals representative of the optical image of at least said portion of the subject's cornea and a data processor for receiving the electrical signals representative of the optical image of at least said portion of the subject's cornea, as outputted by said imaging transducer, and utilizing this optical image as well as information indicating an amount of force applied to at least said portion of the subject's cornea for producing an output representing intraocular pressure.

FIELD OF THE INVENTION 
The present invention relates to tonometry apparatus for measuring the 
intra-ocular pressure (IOP) of a subject's eye, and particularly to an 
applanation type tonometer which produces such a measurement by flattening 
the subject's cornea. 
BACKGROUND OF THE INVENTION 
The intra-ocular pressure (IOP), i.e., the pressure within the eyeball, of 
a person is one of the most important parameters indicating the health 
status of the person's eye. Various eye diseases cause the IOP to be 
higher or lower than normal, but it is mainly elevated when the patient is 
suffering from glaucoma. Glaucoma is an extremely common condition 
afflicting about 2% of the population over 40 years of age and is one of 
the major causes of blindness in the world. This disease has no symptoms 
and is usually diagnosed by tonometry measuring the IOP of the subject. 
Tonometry is therefore a routine procedure in all eye examinations, 
especially those of adults. 
There are various types of tonometers for measuring the IOP. One type, 
called an "indentation tonometer", or "impression tonometer", measures the 
IOP by measuring a deformation produced in the subject's cornea when a 
constant force is applied. However, the more common type is the 
"applanation tonometer", which flattens the cornea and measures the force 
applied. The commonest and most reliable tonometer used at the present 
time is called the Goldmann applanation tonometer, in which a flat plate 
is pressed against the subject's cornea, and the area of applanation is 
viewed by means of a split-lamp and microscope until the diameter of 
applanation is found to be 3.06 mm. Thus, it was found by Goldmann that at 
an applanation diameter of 3.06 mm (or an applanation area of 7.35 
mm.sup.2), the force required to distort the cornea from its convex shape 
to a flat shape counterbalances the surface tension effect of the tear of 
the subject, such that when using this applanation diameter, the force in 
grams multiplied by "10" is directly converted to the IOP in mm of Hg. 
In the Goldmann tonometer, the applanation area is measured by optically 
splitting it into two halves by a biprism, one half being displaced 3.06 
mm relative to the other. A fluorescent solution is first applied to the 
eye to form a ring which is seen as two semi-circles. A dial is then 
manually rotated to apply a flattening force to the subject's cornea. When 
the two semi-circular rings touch, the position of the dial, calibrated in 
mm Hg, indicates the force required to produce an applanation diameter of 
3.06 mm. 
A problem with applanation tonometers in general, and the Goldmann 
applanation tonometer in particular, is the relatively long contact time 
between the flat plate and the subject's cornea during the measurement. 
Not only is a long contact time very unpleasant to the subject, but any 
movement of the subject's eye during the contact time may require 
restarting the procedure. This long duration of contact generally requires 
anesthetizing children, and sometimes adults, in order to make the IOP 
measurements. 
Other drawbacks in the use of the existing Goldmann applanation tonometer 
for making IOP measurements are the dependence or accuracy on the 
expertise of the person making the measurements, and the large size and 
bulkiness of the apparatus used for making such measurements. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an applanation tonometer 
having advantages in some or all of the above respects. 
According to the present invention, there is provided an applanation 
tonometer comprising: a light-conducting pressure applicator having a flat 
wall at one end to be placed in contact with the cornea of the subject's 
eye; force applying means for applying a force to the pressure applicator 
to displace it against the subject's cornea and thereby to flatten it; an 
illuminator for illuminating the subject's cornea; an image transducer at 
the opposite end of the pressure applicator for converting the optical 
image of the subject's cornea into electrical signals; an optical system 
at the opposite end of the pressure applicator for imaging the subject's 
cornea on the image transducer; and a data processor receiving the 
electrical signals outputted by the image transducer and producing an 
output corresponding to the force applied by the pressure applicator to 
the subject's cornea as determined by the flatness of the subject's 
cornea. 
According to further features in the preferred embodiments of the invention 
described below, the pressure applicator, image transducer, and optical 
system are all incorporated in a common housing which is movable towards 
the eye to flatten the cornea. Preferably, the illuminator illuminates the 
subject's cornea with light of predetermined wavelengths, and the optical 
system includes a light filter passing only the predetermined wavelength 
to the image transducer. 
Two such tonometers are described below for purposes of examples. 
In one described tonometer, the force applying means is controlled by an 
electrical control signal generated by the date processor in response to 
the electrical signals received from the image transducer to produce a 
predetermined flatness of the subject's cornea. Such tonometer is thus 
based on a closed-loop feedback system. 
A second tonometer is described wherein the force applying means applies an 
increasing force to the pressure applicator, and the data processor 
receiving the electrical signals from the image transducer produces 
outputs corresponding to the force applied, thereby enabling determining 
the force applied when a predetermined flatness in the subject's cornea is 
sensed. Such a tonometer is thus based on an open-loop non-feedback 
system. 
Applanation tonometers constructed with the foregoing features provides a 
number of important advantages over the conventional Goldmann applanation 
tonometer. Thus, the time required for making the measurement may be very 
substantially reduced. In addition, the accuracy of the measurement is not 
dependent on the expertise of the person making the measurement. In fact, 
particularly if a light filter of the illuminator bandwidth is used to 
increase the signal-to-noise ration, it may even be possible to omit the 
use of fluorescein, which is generally considered mandatory in the 
conventional Goldmann tonometry. Further, vertical alignment, which is 
also mandatory in the conventional Goldmann tonometer, is not required 
using the tonometer of the present invention. Still further, the tonometer 
of the present invention provides automatic correction for astigmatism. 
Still further, where the force applied by the pressure applicator to 
flatten the cornea is controlled by an electrical control signal generated 
by the data processor, as in the first described embodiment, the 
electrical control signal may be modulated at a frequency of many times 
(at least five times) the pulse rate of the subject, enabling a 
calculation to be made (e.g., by suing Fourier Transform analysis) of the 
dependence of the IOP on cardiac pulsation. The phase difference between 
the applied force and the measured area provides information on the tear 
viscosity, enabling a correction to be made because of tear viscosity in 
the calculation of the IOP. Also, by causing the pressure applicator to 
apply a constant force to the subject's cornea, and measuring the change 
of IOP with time, information indicating the facility of outflow of the 
eye fluid is obtainable (Tonography), which information is also useful in 
assessing the condition of the subject's eye. 
Further features and advantages of the invention will be apparent from the 
description below.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
The applanation tonometer head TH illustrated in FIG. 1 comprises a 
pressure applicator 2 having a flat wall 4 at one end to be pressed into 
contact with the cornea of the subject's eye, and thereby to flatten the 
cornea. Pressure applicator 2 may thus be a hollow or solid cylinder of 
glass or transparent plastic; the flat end wall 4 may be a glass plate, a 
prism, or biprism made of glass or other transparent material. Flat end 
wall 4 may be replaceable (for hygienic purposes) or fixed. 
The tonometer head TH further includes an illuminator at the end of the 
pressure applicator 2 opposite to that engaging the subject's cornea, for 
illuminating the subject's cornea. The illuminator includes a plurality of 
LEDs (light emitting diodes) 6, 8 at the end of the pressure applicator 2, 
and a mirror 10, 12 for each LED to reflect the light therefrom through 
the pressure applicator 2 to the subject's cornea. The light is reflected 
back from the subject's cornea through the pressure applicator to a 
collimating lens 14. Lens 14 collimates the light to a focussing lens 16, 
which focusses the light onto an image transducer 18, preferably a CCD 
(charge-coupled device) video camera. 
Focussing lens 16 thus focusses the image of the cornea, as engaged by the 
flat glass plate 4, onto the image transducer 18. The image transducer 18 
converts the optical image of the subject' cornea into electrical signals. 
The LEDs 6, 8 preferably emit short pulses of light (about one microsecond) 
in a narrow band in the infrared spectrum. A filter 19 is provided between 
the pressure applicator 2 and the CCD 18 (in this case between lens 16 and 
the CCD) to pass mainly the predetermined wavelength of the LED light. 
Such an arrangement minimizes the effects of stray light and thereby 
increases the signal-to-noise ratio. The wavelength of the light emitted 
by the LEDs 6, 8 may be selected to maximize the objects being viewed by 
the CCD. For example, if it is found that the fluorescent rings are best 
observed at one specific wavelength, while the contour of the area is best 
observed at another wavelength, the LEDs would be selected to emit the 
light including the two wavelengths, and the filter 19 would be selected 
to maximize the transmission of those wavelengths. 
The tonometer head illustrated in FIG. 1 further includes means for 
applying a force to the pressure applicator 2 for displacing it towards 
the subject's cornea SC in order to flatten the cornea to a predetermined 
diameter, namely 3.06 mm when the tonometer is used according to the 
Goldmann technique. For this purpose, the tonometer head includes an 
electrical coil 20 enclosing the pressure applicator 2 and magnetically 
coupled to a metal layer 22 on the outer surface of the pressure 
applicator to apply a displacing force to the pressure applicator 
according to the current through the coil. The tonometer head TH further 
includes a position sensor 24 for measuring the position of the pressure 
applicator 2. 
The tonometer head TH illustrated in FIG. 1 is incorporated in a common 
housing 25, e.g., resembling a pen to be applied against the subject's 
eye. Collimating lens 14 is mechanically secured to the pressure 
applicator 2 so that its distance from the end of the pressure applicator 
remains fixed. This is schematically shown in FIG. 1 by a mechanical 
connection 26 between lens 14 and the pressure applicator 2. Since the 
light exiting from collimating lens 14 is substantially parallel, the 
filter 19, focussing lens 16, and CCD 18 need not, but may be, also 
mechanically secured to the pressure applicator 2 so as to move with it 
and with the collimating lens 14. 
The electrical signals outputted from the CCD image transducer 18 in the 
tonometer head TH are processed in a video circuit 28 which may also be 
within the common housing 25 of the tonometer head. 
The electrical output of the video circuit 28 is fed to a data processor, 
generally designated 30. Data processor 30 includes CPU (central processor 
unit) 32 which receives the output of the CCD video circuit 28 via another 
video circuit 34 within the data processor 30. The CPU 32 further receives 
electrical signals from the position sensor 24 via an analog-to-digital 
converter 36. 
As a result of this inputted information, CPU 32 generates an electrical 
control signal which is applied via a digital-to-analog converter 38 to 
the coil 20 to control the current through the coil, and thereby the force 
applied by the coil to displace the pressure applicator 2. CPU 32 also 
outputs an electrical signal to a light control unit 40 which controls the 
current to the LEDs 6, 8, and thereby the pulse-duration and intensity of 
illumination applied to the subject's cornea SC. CPU 32 further outputs 
information to a display unit 42, and to a printer 44 via a printer 
circuit 46. 
Information is inputted into the CPU 32 via a keyboard 48. The power to all 
the electrical units illustrated in FIG. I is supplied by a power supply 
50. 
The tonometer illustrated in FIG. 1 may be used to measure the IOP of the 
subject's eye according to the Goldmann technique, as follows: 
The tonometer head TH is applied to the subject's eye such that one end of 
the pressure applicator 2 contacts the subject's cornea. Current is then 
fed via the CPU 32 to coil 20 to apply a force to the pressure applicator 
2, displacing it towards the subject's eye so as to flatten the subject's 
cornea SC. As this is done, the subject's cornea, illuminated by LEDs 6, 8 
and mirrors 10, 12, is imaged via lenses 14, 16 on the CCD 18, which 
outputs electrical signals via video circuits 28, 34, to the CPU 32. 
The CPU 32 continuously computes the applanated (flattened) area of the 
subject's cornea, and controls the current supplied to the coil 20 so as 
to produce an applanation area of 7.35 mm.sup.2 (corresponding to an 
applanation diameter of 3.06 mm). CPU 32 also computes the force required 
to produce the above applanation area, and outputs this value of force via 
display 42 and also via printer 44. The force value is preferably 
converted by the CPU 32 to mm of Hg. 
The system illustrated in FIG. 1 thus provides feedback control of the 
force applied by the pressure applicator 2 to the subject's cornea to 
attain and maintain the predetermined flattened area. The IOP can be 
continuously observed in the display 42, and/or recorded via the printer 
44. Fluctuations in the IOP caused by the subject's respiration and 
cardiac pulse can also be observed in a real time manner. These 
fluctuations thus change the electrical signal outputted by the CPU via 
the D/A converter 38 to the coil 20 in order to maintain the cornea in the 
predetermined flattened condition and can therefore be observed in the 
display 42. 
In addition, applying a constant force, while measuring the change in the 
IOP with time, provides a measurement of the "facility of outflow" of the 
eye fluid, which is also useful in determining the condition of the 
subject's eye. That is, with the applied external force, the IOP rises; 
this causes the aqueous humour to be driven out of the eye at a rate 
faster than normal, and consequently the IOP begins to fall. The rate of 
change depends on the resistance to aqueous outflow. 
Further, by modulating the electrical signal applied to winding 20, a 
modulated force may be applied by the pressure applicator 2 to the 
subject's cornea. When such a modulated force is applied, the dependence 
of the IOP on ocular pulsation may be calculated using, for example, 
Fourier Transform analysis. Moreover, the phase difference between the 
applied force and the measured area provides information on the tear 
viscosity. Since calculation of the IOP depends on tear viscosity, this 
information enables a correction to be made. 
FIG. 2 illustrates a tonometer similar to that described above with respect 
to FIG. 1, except that the tonometer is included in an open-loop, 
non-feedback system, rather than in a closed-loop, feedback system as 
described with respect to FIG. 1. In order to facilitate understanding, 
the same parts corresponding to those described with respect to FIG. 1 
carry the same reference numerals. 
A main difference in the tonometer illustrated in FIG. 2, is that a 
continuously-increasing force is applied to the pressure applicator 2 
tending to flatten the subject's cornea SC, and the data processor 
produces outputs corresponding to the force applied. Thus when the 
subject's cornea attains the predetermined flatness (an area of 7.35 
mm.sup.2 according to the Goldmann procedure as described above), as 
viewed by the CCD 18, the output of the CPU 32 at that time corresponds to 
the force applied to the pressure applicator at that time. 
The pressure applicator 2 may be subjected to a continuously increasing 
force in any convenient manner, either manually or by machine. For this 
purpose, the tonometer head TH is provided with a spring 72. One end of 
the spring is fixed with respect to the housing 25 of the tonometer head 
TH, whereas the other end bears against the end of the pressure applicator 
2. Thus, as the tonometer head is moved, either manually or by machine, 
towards the subject's eye, the pressure applied by the pressure applicator 
2 against the subject's cornea will be increased by the compression of 
spring 72. 
The position sensor 24 generates a signal corresponding to the position of 
the pressure applicator 2 within the housing 25. This signal is outputted 
to the CPU 32 via the analog-to-digital converter 36. Since the parameters 
of spring 72 are known, and the displacement of the pressure applicator 2 
is known by the position sensor 24, the CPU can thereby compute the force 
applied to the pressure applicator when the flattened area, as viewed by 
the CCD 18, reaches the predetermined value. 
The system illustrated in FIG. 2 is otherwise constructed, and operates, in 
substantially the same manner as described above with respect to FIG. 1. 
While the invention has been described with respect to two preferred 
embodiments, it will be appreciated that these are set forth merely for 
purposes of example, and that many variations, modifications and other 
applications of the invention may be made.