A non-contact tonometer measures an intraocular pressure precisely based on a deformation amount of a cornea when a pressurized pulse of air is injected to the cornea. The intraocular pressure is measured on the basis of the maximum value of a correlation function curve, instead of detecting a peak of a light changing curve.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a non-contact tonometer. More 
particularly, the present invention relates to a non-contact tonometer 
which measures intraocular pressure based on deformation amount of a 
cornea when a pressurized pulse of air is injected to the cornea from a 
nozzle. 
2. Background of the Invention 
A non-contact tonometer is conventionally known. Such a conventional 
non-contact tonometer includes an air injector for deforming a cornea by 
injecting air to a subject eye, a cornea deforming detector for detecting 
a deformation amount of the cornea based on a change of reflected light 
received from the cornea that is being projected by a light, a pressure 
measurer for measuring an air pressure of the air injected by the air 
injector, with the pressure measurer being located in the air injector. 
Non-contact tonometers are described in detail in "Tonometer Utilization, 
Accuracy, and Calibration Under Field Conditions", published in Arch. 
Ophthalmology, Vol. 108, pages 1709-1712, December 1990; "Intraocular 
Pressure Measurement With the Tono-Pen Through Soft Contact Lenses", 
published in American Journal of Ophthalmology 109, pages 62-65, January, 
1990; "Glaucoma Screening in Primary Care: The Role of Noncontact 
Tonometry", published in The Journal of Family Practice, Vol. 34, No.1, 
pages 73-77 (1992); and "Microaerosol Formation in Noncontact `Air-Puff` 
Tonometry", published in Arch. Ophthalmology, Vol. 109, pages 225-228, 
February 1991. The above-listed articles are incorporated herein by 
reference. 
In the conventional non-contact tonometer, the air injector injects air to 
the cornea C from the nozzle by energy obtained from a piston working 
together with a rotary solenoid. The cornea C is deformed according to a 
change of air pressure, as shown in FIGS. 8 (a)-(e). FIG. 9 shows an 
intensity of reflected light that changes depending on the amount of 
deformation of the cornea C. 
FIG. 8 (a) shows a deformation of the cornea C as air starts to be injected 
to it (corresponding to time period t1 in FIG. 9). FIG. 8 (b) shows a 
deformation of the cornea C corresponding to the time period t2 in FIG. 9. 
The cornea C is deformed to a flat, planar surface at the end of time 
period t2, due to the air pressure being increased after a desired time 
from start of the air injection to the cornea C. 
By increasing the pressure of the injected air, the cornea C is made to a 
planar surface, or a flat surface, at time t0 in FIG. 9 (corresponding to 
FIG. 8 (c)). Moreover, by increasing the pressure of the injected air, the 
cornea C is made concave during time periods t3 and t4 in FIG. 9 
(corresponding respectively to FIGS. 8 (d) and 8 (e)). 
The intensity of the reflected light from the cornea C increases according 
to a profile of the cornea C being deformed from a convex shape to a flat 
shape, as seen in FIG. 9. When the profile of the cornea C becomes flat, 
the intensity of the reflected light is maximum. The intensity of the 
reflected light is decreased according to the profile of the cornea C 
being deformed from the flat shape to a concave shape. The intensity of 
the reflected light is changed, as shown by the curve Da, from the time t0 
to the time t4 in FIG. 9. 
The pressure detected by a pressure detecting sensor over a period of time 
is shown by a pressure changing curve P in FIG. 10. It is possible to 
obtain an intraocular pressure Iop based on a value of pressure P0 in the 
air injector at the time when the curve D1 becomes maximum. This is due to 
a known relationship between a pressure value in the air injector and an 
intraocular pressure of the subject eye E. 
However, the peak of the light intensity curve D is not always readily 
apparent when a profile of the cornea C becomes a flat surface. For 
example, if some cilia (e.g., eyelashes) are accidentally in an optical 
path of the cornea deformation detecting optical system before the cornea 
C attains a flat shape, the intensity of the reflected light is decreased, 
as shown by the light changing curve Db in FIG. 11, during a time period 
t5 which includes the time when the cornea C attains a flat shape. In this 
case, there are two peaks Db1, Db2 at respective times before and after 
the time when the cornea C becomes flat due to the air pressure applied to 
it, as shown in FIG. 11. 
The intraocular pressure is determined on the basis of either one peak or 
the other one. It is hard to get accurate intraocular pressure of the 
subject eye E in this case, because neither the peak Db1 nor the peak Db2 
corresponds to the flatness of the cornea C. This results in an 
uncertainty of an eye measurement, and can lead to a faulty output. A 
light changing curve of the shape Dc, Dd as shown FIG. 12 and FIG. 13 
sometimes occurs due to tearing in the subject eye E, elasticity of a 
cornea C and/or lack of uniformity of the air being applied to the cornea 
C. 
A light changing curve of the reflected light as shown FIG. 12 increases 
gradually. In this case, it is hard to find a peak of the light changing 
curve and to determine an appropiate measurement timing. As a result, an 
uncertainty of an eye measurement is likely to occur, and an eye 
measurement error may occur as a result. 
A light changing curve of the reflected light as shown FIG. 13 includes 
several slight peaks. In this case as well, it is hard to find a peak of 
the light changing curve and to determine a measurement timing 
corresponding to a flat surface condition of the cornea C. Like the other 
cases described above, an eye measurement error may occur as a result. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an apparatus and a method 
for measuring accurate intraocular pressure from a signal with various 
noises in the environment. 
A further object of the present invention is to provide an apparatus and a 
method for measuring accurate intraocular pressure on the basis of a 
calculated correlation. 
A further object of the present invention is to provide an apparatus and a 
method for measuring accurate intraocular pressure on the basis of a 
calculated correlation between a standard light value and a light changing 
value. 
A further object of the present invention is to provide an apparatus and a 
method for measuring accurate intraocular pressure on the basis of a 
calculated correlation function between a standard light value and a light 
changing value. 
A further object of the present invention is to provide an apparatus and a 
method for measuring accurate intraocular pressure on the basis of a peak 
of a calculated correlation function between a standard light value and a 
light changing value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A noncontact tonometer of this invention will be described in detail with 
reference to the accompanying drawings. 
FIG. 1 and FIG. 2 show a non-contact tonometer that includes a fixation 
target projecting optical system 10 for projecting a fixation target to a 
subject eye E, an anterior part of the eye observing optical system 20, an 
alignment light projecting optical system 30 for projecting alignment flux 
to the subject eye E, an alignment light receiving optical system 40 for 
detecting an alignment distance away from the subject eye E, and a cornea 
deformation detecting optical system 50 for detecting the deformation of 
the cornea C by using an optical factor. 
The fixation target projecting optical system 10 includes a light emitted 
diode (LED) 11, a pinhole 12, a dichroic mirror 13 having characteristics 
of passing visual light and reflecting near infrared light, a collimator 
lens 14, a half mirror 15, a chamber window glass 16, and a nozzle 17. 
As seen in FIG. 3, the chamber window glass 16 forms a part of a cylinder 
75 of an air injecting device 70 which outputs an air pulse to the nozzle 
17. Referring back to FIG. 1, visual light from LED 11 for the fixation 
target passes through the pinhole 12 and transmits through the dichroic 
mirror 13, then the transmitted light is collimated by the collimator lens 
14 to obtain parallel light. 
After the collimated light is reflected by the half mirror 15, the 
reflected light forms an image in the cornea C of the subject eye E, 
passing through the chamber window glass 16 and the nozzle 17. 
The anterior part of the eye observing optical system 20 comprises plural 
LEDs 21 which emit infrared light for illuminating the subject eye E 
directly from a right side and a left side, a cover glass 22 fixed at one 
end of the nozzle 17, a supporting glass 23 for supporting the other end 
of the nozzle 17, the chamber window glass 16, the half mirror 15, an 
objective lens 24, a half mirror 25, an image forming lens 26, and a CCD 
camera 27. 
The infrared light reflected at the subject eye E is collimated by the 
objective lens 24 in order to form parallel light, after passing through 
the cover glass 22 and the supporting glass 23. After the collimated light 
passes through the half mirror 25, the passed light forms an image of the 
anterior portion of the subject eye E on an imaging device 27a of the CCD 
camera 27. 
In FIG. 1, the anterior portion of the subject eye E is displayed with an 
alignment area 28a on the imaging device 27a. This alignment area 28a is 
displayed with an image forming circuit (not shown). The alignment light 
projecting optical system 30 includes an LED 31 for alignment and for 
detecting cornea deformation, condenser lenses 32, 33, an aperture 34, a 
pin hole 35 for forming the image projected to the cornea C, the dichroic 
mirror 13, the collimator lens 14, the half mirror 15, the chamber window 
glass 16, and the nozzle 17. The pin hole 35 is conjugated with a back 
focal point of the condenser lens 14. Infrared light emitted from the LED 
31 passes through the condenser lenses 32, 33, the aperture 34 and the pin 
hole 35, and is then reflected by the dichroic mirror 13. 
The reflected light is collimated by the collimator lens 14 to obtain 
parallel light. After the collimated light is reflected by the half mirror 
15, the reflected light is projected to the cornea C of the subject eye E, 
and the light passes through the chamber window glass 16 and into the 
nozzle 17. 
The projected light is reflected at a surface of the cornea C. The 
reflected light from the cornea C passes through and is collimated by the 
cover glass 22, the supporting glass 23 and the half mirror 15. Then, the 
passed light is collimated by the objective lens 24. 
After a part of the collimated light passes through the half mirror 25, 
that part of the light is formed as a target image 28b by the image 
forming lens 26. The target image 28b is then displayed on the display 28. 
An examiner (i.e., ophthalmologist) moves a main body of the noncontact 
tonometer either left or right, up or down, or front or back, for placing 
the target 28b into the alignment area 28a. In case of no alignment of an 
optical axis 01 of the apparatus with an optical axis 02 of the subject 
eye E, the target image is moved either up or down, right or left, or in 
both directions in order to obtain proper alignment. 
In case the working distance is too close or too far, the target image is 
out of focus and unclear, and a size of the target is either too big or 
too small. Referring to the characteristics of the target image, the 
examiner can arrange an alignment of the optical axes and a rough working 
distance of the noncontact tonometer can be obtained. 
The alignment light receiving optical system 40 includes an image forming 
lens 41, a reflecting mirror 42, a half mirror 44, apertures 47, 48, 
receiving sensors 45, 46, and co-elements from the cover glass 22 through 
half mirror 25 for the anterior part of the eye observing optical system 
20. 
The other part of the reflected light from the cornea C is reflected by the 
half mirror 25 towards the image forming lens 41. The passing light 
through the image forming lens 41 is reflected by the reflecting mirror 42 
towards the half mirror 44. One part of the light from the reflecting 
mirror 42 passes through the half mirror 44, and the other part of the 
light is reflected by the half mirror 44. The one part corresponding to 
the light passing through the half mirror 44 is projected to the receiving 
sensor 45 through the aperture 48. The other part corresponding to the 
reflected light that is reflected by the half mirror 44 is projected to 
the receiving sensor 46 through the aperture 47. 
The receiving sensors 45 and 46 are located at a same distance from an 
image forming position Pn in a front and back direction, respectively. It 
is possible to have a system such that the receiving sensors 45, 46 and 
apertures 47, 48 are a same type of device. 
The working distance in the Z direction (to the optical axis) is calculated 
by a calculation device (not shown) on the basis of a ratio of an 
intensity of the light received by receiving sensor 45 to an intensity of 
the light received by the receiving sensor 46. 
Suppose a level .alpha. denotes the intensity of the light received by the 
receiving sensor 45, and a level .beta. denotes the intensity of the light 
received by the receiving sensor 46. Then, it is possible to calculate the 
working distance on the basis of the intensity ratio 
.gamma.=(.beta.-.alpha.)/(.beta.+.alpha.). 
When the intensity ratio .gamma. is equal to zero (.alpha.=.beta.), the 
appropriate working distance is found. When the intensity ratio .gamma. is 
greater than zero (.gamma.&gt;0), the working distance is shorter than the 
appropriate working distance. In other words, the body of the non-contact 
tonometer is too close to the subject eye E. When the intensity ratio 
.gamma. is smaller than zero (.gamma.&lt;0), the working distance is longer 
than the appropriate working distance. In other words, the body of the 
non-contact tonometer is too far from the subject eye E. 
With the system according to the invention, it is possible to detect an 
alignment condition in an up and down direction, and in a left and right 
direction (i.e., X direction, Y direction). It is also possible to detect 
an alignment condition in a Z direction by determining whether both the 
ratio of the intensities of the receiving sensors 45, 46 are greater than 
or less than a desired level. 
The cornea deformation detecting optical system 50 includes a reflecting 
mirror 51, an aperture 53, a receiving sensor 54, and co-elements from the 
cover glass 22 through the half mirror 25 for the anterior part of the eye 
observing optical system 20. 
In FIG. 2, light for detecting projects to the subject eye E at a same time 
when the air pulse is injected to the subject eye E. This detecting light 
passes to the cornea C through the condenser lenses 32, 33, the aperture 
34, the pin hole 35, the dichroic mirror 13, the collimator lens 14, the 
half mirror 15, the chamber window glass 16, and the nozzle 17. Then, the 
detecting light is reflected at the cornea C of the subject eye E. The 
reflected light from the cornea C is reflected at the half mirror 25, and 
the reflecting mirror 51 reflects light through the aperture 53 and 
towards the receiving sensor 54. 
Referring now to FIG. 3, the air injecting device 70 includes a cylinder 
75, a piston 76 being moved into the cylinder 75 reciprocally, and a 
rotary solenoid 72 for moving the piston 76 into the cylinder 75 
reciprocally. An axis of the rotary solenoid 72 is connected to the piston 
76 with a clank arm 73 and a connecting arm 74. The piston 76 is moved 
into the cylinder 75 reciprocally as driven by the rotary solenoid 72. 
FIG. 4 shows a block diagram of a control system of the non-contact 
tonometer 1. A pressure detecting sensor 78 for detecting pressure in the 
cylinder 75 is located at a upper portion of the cylinder 75 in FIG. 3. A 
memory 81 stores outputs of receiving sensor 54 at periodic instants in 
time. The memory 81 stores the intensities of the received light signal, 
such as that given by a light changing curve R shown FIG. 6 (e.g., a light 
intensity at every 0.1 second). 
A memory 82 stores pressure values detected by the pressure detecting 
sensor 78. In other words, the memory 82 stores the pressure values, such 
as that given by the pressure changing curve P shown in FIG. 6 (e.g., a 
pressure value at every 0.1 second). 
A memory 83 stores a standard curve F (see FIG. 5) showing an ideal light 
changing curve of the receiving sensor 54 on the basis of the deformation 
of the cornea C. 
One of ordinary skill in the art will recognize that it is possible to 
choose several different ways to determine the standard curve on the basis 
of calculated values with simulation or real detected values. 
A printer 84 prints out an output of measured intraocular pressure. A 
recorder 85 records measurement data including the measured intraocular 
pressure. An arithmetic control unit 87 includes a CPU, which controls the 
LEDs 11, 21, 31, the printer 84, and the recorder 85 on the basis of 
orders received from a control panel 88. 
The arithmetic control unit 87 obtains the light changing curve R on the 
basis of the data stored in the memory 83, then it obtains a cross 
correlation function on the basis of a comparison between the calculated 
light changing curve R and the standard curve F stored in the memory 83. 
Moreover, the arithmetic control unit 87 obtains a correlation curve S as 
shown FIG. 6 on the basis of the cross correlation function. 
The cross correlation function is defined by the following equation: 
##EQU1## 
wherein f(x) and g(x) are functions of parameter x. 
In the system according to the invention, a value of the correlation 
function S(.DELTA.t) is obtained on the basis of n samples of a receiving 
signal value and a standard value from a value of a standard curve, 
(R.sub.t1, F.sub.t1+.DELTA.t), (R.sub.t2, F.sub.t2+.DELTA.t) . . . 
(R.sub.tn, F.sub.tn+.DELTA.t). 
##EQU2## 
In the above equation, F is a mean value of F.sub.tk, and R is a mean value 
of R.sub.tk. Then, the correlation function curve S is computed on the 
basis of S(.DELTA.t) by changing the factor .DELTA.t gradually. The 
arithmetic control circuit 87 obtains the intraocular pressure on the 
basis of a pressure of the pressure changing curve P corresponding to the 
maximum of the correlation function curve S. The arithmetic control 
circuit 87 also detects alignment information on the basis of the signals 
received from the receiving sensors 45, 46. 
The operation of the system according to the invention will be explained 
hereinbelow. After the LEDs 11, 21, 31 turn on, the anterior part of the 
subject eye and the alignment area 28a are displayed on a display 28. The 
examiner performs a rough alignment by moving the body of the non-contact 
tonometer, for moving the target image 28b by LED 31 into the alignment 
area 28a on the display 28. 
In case where the target image 28b enters in the circle mark, the 
arithmetic control circuit 87 determines that the X-Y directional (up and 
down, left and right) alignment is correct. 
The examiner then performs a Z direction (optical axis) alignment by moving 
the body of the non-contact tonometer along the Z direction (optical 
axis). When the non-contact tonometer is located at the appropriate 
working distance, the arithmetic control circuit 87 determines the 
appropriate Z direction alignment on the basis of the light levels 
received by the receiving sensors 45 and 46 being nearly the same (or 
exactly the same). 
After X, Y, Z directional alignments have been done, the examiner operates 
a measurement start button (not shown). The arithmetic control circuit 87 
controls the rotary solenoid 72 when the correct X, Y, Z directional 
alignment is made. The piston 76 moves upwardly into the cylinder 75 
according to this control. 
An air pulse is injected from the nozzle 17 to the cornea C in accordance 
with the movement of the piston 76. The cornea C is deformed toward a 
concave direction due to the injected air pulse. The anterior part 
deformed by the air pulse is displayed on the display 28 through the 
anterior part of the eye observing optical system 20. 
Reflected light (parallel flux) based on a cornea deformation is conducted 
to receiving sensor. The reflected light reaches the receiving sensor 54 
after reflecting off of the cornea C into the nozzle 17, through the 
chamber window 16 and the half mirror 15. The light is then converged by 
the objective lens 24, reflected at the half mirror 25 and the reflecting 
mirror 51, and an image is formed on the aperture 53 (shown by the solid 
lines in FIG. 2). A received light level of the receiving sensor 54 varies 
in accordance with the amount of deformation of the cornea C. The 
receiving sensor 54 outputs a signal corresponding to the deformation of 
the cornea C. 
The arithmetic control circuit 87 includes the memory 81 which stores the 
value of the light received by the receiving sensor 54. The intensity of 
the reflected light of the receiving sensor 54 is shown as the light 
changing curve R (see FIG. 6) in accordance with the amount of deformation 
of the cornea C caused by the injected air pulse. 
The memory 81 stores a receiving signal value indicative of the light 
changing curve R. The pressure sensor 78 detects the air pressure in the 
cylinder 75, in other words, the pressure of the air pulse injected from 
the nozzle 17. A pressure in the cylinder 75, as shown by the pressure 
changing curve P in FIG. 6, is changed in accordance with a movement of 
the piston 76. The memory 82 stores a pressure signal value indicative of 
the pressure changing curve P. 
After injecting of the air pulse, the arithmetic control circuit 87 obtains 
the value S(.DELTA.t), on the basis of the standard curve F stored in the 
memory 81 and the light changing curve R. Then, the arithmetic control 
circuit 87 obtains the correlation function curve W as shown FIG. 6 on the 
basis of the value S(.DELTA.t), and obtains the maximum value M of the 
correlation function curve W. 
It is possible to obtain the maximum M of the correlation function curve W 
after performing a spline interpolation to the correlation function curve 
W. 
It is also possible to obtain the maximum M of the correlation function 
curve W after approximating the correlation function curve W as a Gaussian 
curve by a least-squares estimation method. 
The arithmetic control circuit 87 obtains a time tm corresponding to the 
maximum value M, and obtains a pressure P1 at the time tm on the basis of 
the pressure changing curve P corresponding to the data stored in the 
memory 82. Then, the arithmetic control circuit 87 obtains an intraocular 
pressure of the subject eye E on the basis of the pressure P1 at time tm. 
In case of elasticity of the subject eye E, non-uniformity of the air, 
tearing in the subject eye E, blockage caused by cilia of the subject eye 
E, an alignment miss etc., a real peak sometimes does not appear in the 
output signal of the light receiving sensor. In this case, it is possible 
to find the real peak of the detected signal on the basis of the maximum 
value M of the correlation function curve W according to the present 
invention. 
In that case, it is possible to make sure of the reliability of measurement 
on the basis of the maximum value M of the correlation function curve W 
according to the present invention. 
In the present invention, it is possible to measure the intraocular 
pressure accurately, because the intraocular pressure is measured on the 
basis of the maximum value M of the correlation function curve S, instead 
of detecting a peak of the light changing curve R. 
As a result, it is possible to avoid unwanted (e.g., noisy) influences due 
to elasticity of the cornea C, non-uniformity of the air, tearing in the 
subject eye E, blockage caused by cilia of the subject eye E, an alignment 
miss, etc. 
In cases where it is hard to detect the real peak from the detected signal 
using conventional non-contact tonometers, it is possible to find the real 
peak of the signal which corresponds to the cornea C having a planar (flat 
surface) in accordance with the present invention. Then, it is possible to 
obtain a precise intraocular pressure, and a measurement result with high 
accuracy. 
In the system described above, the pressure value detected by the pressure 
sensor 78 is used for obtaining the intraocular pressure. However, it is 
also possible to use the time corresponding to the peak of the correlation 
function curve W to obtain the intraocular pressure, in a condition with 
the air injecting device of which the air-injecting time is predetermined. 
It is also possible to select a standard curve from a plurality of standard 
curves in accordance with a measurement condition, etc. For example, two 
standard curves for high pressure and low pressure, respectively, can be 
utilized in the system according to the invention. With this example, in a 
case where the intraocular pressure is higher than 30 mmHg, the standard 
curve corresponding to the high pressure is selected for measuring. In a 
case where the intraocular pressure is less than 30 mmHg, the standard 
curve corresponding to the low pressure is selected for measuring. 
While a preferred embodiment of the invention has been described herein, 
modification of the described embodiment may become apparent to those of 
ordinary skill in the art, following the teachings of the invention, 
without departing from the scope of the invention as set forth in the 
appended claims.