The ophthalmotonometer of the invention comprises a frequency-output transducer of linear motions which correspond to intraocular pressure; a reference-frequency generator; a frequency comparator whose inputs are connected to the outputs of the frequency-output transducer and of the reference-frequency generator, a difference-frequency divider connected to outputs of the frequency comparator; a circuit for measurement of the cycle of difference-frequency oscillations connected to outputs of the reference frequency generator and the frequency divider, said cycle corresponding to intraocular pressure in millimeters of mercury; and a measurement results registration circuit connected to outputs of the cycle measurement circuit.

FIELD OF THE INVENTION 
This invention relates generally to medical apparatus and more specifically 
it concerns ophthalmotonometers. 
The invention is applicable in ophthalmology for diagnosis and control over 
the treatment coursing of eye diseases under clinical and outpatient 
conditions. 
BACKGROUND OF THE INVENTION 
Known in the present state of the art are impression tonometers and 
tonographs that measure intraocular pressure in conventional units, one 
such unit corresponding to a diaplacement of 50 .mu.m. The readings taken 
in conventional units are translated into millimeters of mercury by means 
of special conversion tables or through the use of a computing device, 
which is more convenient in medical practice (cf. SU A 135,583, SU A 
294,608, SU A 1,044,272). However, additional computation extends the 
patient's servicing time, while incorporation of computing devices 
sophisticates construction of such tonometers. 
In addition, the aforesaid conversion and computation involves errors due 
to stepwise arranged numerical data in special conversion tables so that 
the data increments substantially increase within the domain of low values 
of the measured pressure expressed in conventional units. 
The technical solution closest to the present invention is a high-frequency 
tonometer for measuring and recording the intraocular pressure, comprising 
a frequency-output linear-motion transducer, said motions corresponding to 
intraocular pressure expressed in conventional units, and a recording 
instrument. The transducer incorporates a plunger which is traversable 
with respect to the casing so as to change its position relative to the 
inductors accommodated in the transducer casing. The transducer output 
frequency is proportional to the length of the plunger travel with respect 
to the transducer casing and is converted into a d.c. signal which is 
taken down by the recording instrument. Then the pressure measured in the 
conventional units is represented in millimeters of mercury by the 
conversion technique described above (cf. SU A 119,651). 
However, making use of a special conversion table involves interpolation of 
the tabulated numerical data, while application of a special computing 
unit complicates substantially the entire tonometer. Moreover, the initial 
section, according to which intraocular pressure is as a rule estimated, 
is featured in a majority of cases by the widely variable displacement 
values, which are to be averaged. The data averaging procedure is usually 
performed by the operator in the course of data processing, which affects 
adversely the effectiveness and accuracy of measurements. 
The invention is aimed at the provision of an ophthalmotonometer which 
would be capable of measuring intraocular pressure directly in millimeters 
of mercury. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an ophthalmotonometer which 
would be capable of measuring intraocular pressure directly in millimeters 
of mercury. 
It is another object of the invention to render an operator's work more 
efficient. 
It is one more object of the invention to provide higher measurement 
accuracy. 
The objects mentioned above are accomplished by an ophthalmotonometer, 
comprising a frequency-output linear-motion transducer, said linear 
motions corresponding to intraocular pressure, and a circuit for 
registration of the measurement results, said circuit being interconnected 
with the frequency-output linear-motion transducer according to the 
invention, the ophthalmotonometer also comprises a reference-frequency 
generator; a frequency comparator to one of whose inputs is connected the 
output of the reference-frequency generator, while connected to the other 
input thereof is the output of the frequency-output linear-motion 
transducer; a difference-frequency divider connected to the output of the 
frequency comparator; and a circuit for measuring the cycle of the 
difference-frequency oscillations, there being connected to one of the 
inputs of said circuit the ouput of the difference-frequency divider, 
while connected to the other input thereof is the output of the 
reference-frequency generator. The signal appearing at the output of the 
aforesaid circuit and corresponding to intraocular pressure in millimeters 
of mercury is then applied to the input of the measurement result 
registering circuit. 
The ophthalmotonometer proposed herein makes it possible to dispense with 
the use of special conversion tables for determining intraocular pressure, 
is simple in construction and provides for an adequate accuracy of 
intraocular pressure measurement.

DETAILED DESCRIPTION OF THE INVENTION 
The construction and operation of the herein-disclosed ophthalmotonometer 
is based on the heretobefore-known intraocular pressure measurement 
technique, the pressure measured being expressed in conventional units 
(the so-called Schiotz' units). 
FIG. 1 represents the curves characteristic of the intraocular pressure 
value P.sub.o in millimeters of mercury versus the length R.sub.o of the 
transducer plunger displacement. The curve 1 corresponds to the plunger 
weight G of 5.5 g, the curve 2 corresponds to the weight of 7.5 g, and the 
curve 3 correpsonds to the weight of 10 g. 
The values of P.sub.o, as well as design values of the coefficient 
EQU A.sub.i =R.sub.o P.sub.o (1) 
are tabulated in Table I. 
Table 1 contains the tabulated values of P.sub.o and the design values of 
the coefficient A.sub.i =P.sub.o R.sub.o. Found therien are also design 
values of intraocular pressure P.sub.o1 obtained from the following 
relation: 
EQU P.sub.o1 =A.sub.o R.sub.o, (2) 
where 
##EQU1## 
The aforesaid Table contains design values of intraocular pressure 
measurement error .delta. by Eq (2) 
##EQU2## 
FIG. 2 represents graphic charts 4, 5, 6 showing changes in the value 
.delta. of the measurement error versus the intraocular pressure P.sub.o 
being measured, expressed in conventional units for the weight G equal to 
5.5, 7.5, and 10 g. The same FIGURE illustrates also a 10-percent margin 7 
of error, which is adopted as such to suit a required intraocular pressure 
measurement accuracy high enough for clinical studies. 
The graphic charts 4, 5 and 6 are so replotted as to correspond to the 
value of the coefficient A.sub.2 that satisfies the condition, wherein an 
extreme (mathematical) value .delta. of error equals 10 percent. As a 
result, new reconstructed graphic charts 4.sup.1, 5.sup.1, 6.sup.1 are 
obtained. Table 1 gives design values of P.sub.o2 calculated by the 
formula 
EQU P.sub.o2 =A.sub.2 /R.sub.o (2.sup.I) 
and the corresponding error values of .delta..sub.2, which is calculated by 
the formula 
##EQU3## 
As it is evident from FIG. 2, the values of R.sub.o from 3 to 10 
conventional units fall within the 10-percent margin 7 of error. 
TABLE I 
__________________________________________________________________________ 
Conventional 
No. 
R.sub.o 
units 1 2 3 4 5 6 7 8 9 10 11 
1 2 3 4 5 6 7 8 9 10 11 12 13 14 
__________________________________________________________________________ 
1 G = 5.5 g 
P.sub.o, mm Hg 
35 29 25 21 17 14.5 
12 10 8.5 7 6 
2 A.sub.i = P.sub.o R.sub.o 
35 58 75 84 85 87 84 80 76.5 
70 66 
3 P.sub.o1, mm Hg 
72.8 
36.4 
24.25 
18.2 
14.55 
12.13 
10.4 9.09 
8.08 
7.28 6.6 
4 .delta., % 
108 25.5 
3 8.6 14.4 
16.3 
13.3 9.1 4.9 4 10 
5 P.sub.o2, mm Hg 
78.3 
39.15 
26.1 19.6 
15.66 
13.1 
11.2 9.79 
8.7 7.83 7.118 
6 .delta..sub.1, % 
123.7 
35 4.4 6.79 
7.88 
10 6.78 2.12 
2.35 
11.8 18.6 
7 G = 7.5 g 
P.sub.o, mm Hg 
50 45 36 30 26 22 18 15.5 
13 11 10 
8 A.sub.i = P.sub.o R.sub.o 
50 84 108 120 130 132 126 124 117 110 110 
9 P.sub.o1, mm Hg 
110.1 
55 36.7 27.5 
22.0 
18.35 
15.3 13.76 
12.23 
11.01 
10.01 
10 .delta., % 
120.2 
31 1.94 8.33 
15.3 
16.5 
12.6 11.2 
5.9 0.09 0.1 
11 P.sub.o2, mm Hg 
118.8 
59.4 
39.6 29.7 
23.7 
19.8 
17.0 14.85 
13.2 
11.9 10.8 
12 .delta..sub.1, % 
137.6 
41.4 
10 1.0 8.6 10 5.7 4.2 1.54 
8 8 
13 G = 10 g 
P.sub.o, mm Hg 
69 59 51 43 37 32 27 23 19.5 
16 14 
14 A.sub.i = P.sub.o R.sub.o 
69 118 153 172 185 192 189 184 175.5 
160 154 
15 P.sub.o1, mm Hg 
159.2 
79.6 
53.07 
39.8 
31.8 
26.5 
22.7 19.9 
17.7 
15.9 14.47 
16 .delta., % 
13.07 
34.9 
4.05 7.46 
14.05 
17.2 
15.9 13.4 
4.2 0.5 3.36 
17 P.sub.o2, mm Hg 
172.8 
86.4 
57.6 43.2 
34.6 
28.8 
24.7 21.6 
19.2 
17.3 15.7 
18 .delta..sub.1, % 
150.4 
46.4 
12.94 
0.46 
6.59 
10 8.5 6.09 
1.54 
8 12.2 
__________________________________________________________________________ 
FIG. 3 represents historgrams obtained from processing six thousand 
experimental tonographic findings taken from out patients. The aforesaid 
findings are obtained using the standard tonographic precedure and 
standard tonometer. It has been found that tonograms taken with the 
plunger weight of 10 g occur about ten times less frequently than those 
with the plunger weight of 5.5 g, whereas no tonograms with the plunger 
weight of 7.5 g are encountered altogether. It can be inferred, on the 
grounds of the data represented in FIGS. 2 and 3, that 96 percent of the 
tonometric procedures feature the measurement accuracy within 10 percent, 
the approximation of the pressure relationship being expressed as follows: 
EQU P.sub.o =A.sub.2 /R.sub.o (2.sup.II) 
The studies carried out also give evidence that the coefficient A.sub.o 
selected from the root-mean-square relationship gives the similar results. 
In addition, reduction of the margin 8 of error to 5 percent (FIG. 2) 
demonstrates that 91 percent of measurement results in case of out-patient 
examinations fall within that margin of error. It is by appropriately 
varying the mass of additional load-weights that one seeks for a 
100-percent pressure measurement in the margin 7 or 8. 
Similar results are obtained from examination of glaucoma patients, but are 
not herein considered for the sake of simplification of the present 
disclosure. 
Thus, it may be considered to be proven that the value of P.sub.o of 
intraocular pressure in millimeters of mercury is inversely proportional, 
within an allowable error, to the length of travel of the plunger of the 
transducer measuring linear motions R.sub.o, that is 
EQU P.sub.o =A.sub.2 /R.sub.o (2.sup.II) 
Therefore when use is made of a frequency-output linear-motion transducer 
featuring a proportional relationship of the frequency `f` at its output, 
the length of travel R.sub.o corresponding to intraocular pressure in 
conventional units is proportional to the measured frequency `f`, whence 
EQU R.sub.o =kf, (4) 
where k is the proportionality factor, while the pressure value of P in 
millimeters of mercury is proportional to the period T of oscillations at 
the output of the frequency-output transducer, that is, 
EQU R.sub.o =A.sub.2 /R.sub.o =A.sub.2 /kf=(A.sub.2 k)T (5) 
The mathematical expression (5) points to the direct proportionality of the 
intraocular pressure measured in millimeters of mercury to the period of 
oscillations at the output of the frequency-output transducer employed for 
the pressure measurement, which makes it possible to provide an 
ophthalmotonometer featuring such a functional diagram that enables 
intraocular pressure to be measured directly in millimeters of mercury, 
such a functional diagram being represented in FIG. 4. 
The ophthalmotonometer of the invention comprises a frequency-output 
linear-motion transducer 9, said motions corresponding to intraocular 
pressure in conventional units, a reference-frequency generator 10, and a 
frequency comparator 11 to whose inputs 12 and 13 are connected the 
outputs of the frequency-output transducer 9 and of the generator 10. To 
the output of the frequency comparator 11 is connected an input 14 of a 
difference-frequency divider 15. 
The tonometer comprises also a circuit 16 for measuring the cycle of the 
difference-frequency oscillations connected to whose input 17 is the 
output of the difference-frequency divider 15, while connected to an input 
18 thereof is the output of the generator 10. 
A signal corresponding to intraocular pressure in millimeters of mercury is 
applied to the measurement results registering circuit. Used as such a 
circuit in the herein-considered tonometer is a digital display unit 20 to 
whose inputs the output of the circuit 16 is connected. The same signal is 
impressed upon inputs 21 of a digital-to-analogue converter 22 at whose 
output a signal is registered, corresponding to time-referenced 
intraocular pressure variation. 
Used as the digital display unit 20 and the converter 22 may be any 
heretofore-known similar device. 
FIG. 5 illustrates an elementary electric diagram of the measurement 
circuit of the ophthalmotonometer under consideration. 
The frequency-output transducer 9 comprises an inductor 23 whose inductance 
varies with the travel of a core 24 associated with a plunger which is 
adapted to interact with the patient's cornea (omitted in the Drawing) by 
a widely adopted technique. 
The inductor 23 makes part of the oscillatory circuit of a self-excited 
oscillator 25 built around transistors 26, 27, capacitors 28, 29, 30, 31, 
32, and resistors 33, 34, 35, 36, 37, 38 using a heretofore-known pattern, 
e.g., such illustrated in FIG. 5. 
The reference-frequency generator 10 is assembled according to the same 
electric circuitry as the self-excited oscillator 25 of the 
frequency-output transducer 9; it comprises an inductor 39, a trimmer 40, 
transistors 41, 42, capacitors 43, 44, 45, 46, 47, and resistors 48, 49, 
50, 51, 52, 53. 
The frequency comparator 11 comprises diodes 54, 55 connected to a junction 
point 56 of which through resistors 57 and 58 are the outputs of the 
transducer 9 and of the generator 10. 
There is provided at the output of the comparator 11 a filter 59 adapted to 
discriminate the difference frequency and built around a reactor 60, 
capacitors 61, 62 and a resistor 63. 
Connected to the output of the filter is an amplifier 64 based on 
transistors 65, 66, capacitors 67, 68, 69, 70, 71 and resistors 72, 73, 
74, 75, 76, 77, 78. 
The difference-frequency divider 15 is made up according to any of the 
heretofore-known and widely adopted techniques, e.g., can be based on 
flip-flops 78, while the cycle measuring circuit 16 is made as any counter 
of pulses of the reference frequency produced by the generator 10. The 
counting time corresponds to the cycle being measured. 
The division ratio `m` of the frequency divider 15 is calculated according 
to the following relation: 
EQU m=(A.sub.2 .multidot.n)/f.sub.o, (6) 
where 
n is the conversion ratio of the transducer 9 expressed in Hz/conventional 
unit; and 
f.sub.o is the frequency of the generator 10. 
The ophthalmotonometer operates as follows. 
The plunger of the transducer 9 placed on the patient's cornea is urged to 
assume a certain position with respect to the casing under its own mass 
and the mass of the additional load weight, the sum of which equals the 
value of G. The plunger motion is converted into a proportional change of 
the frequency `f` of the self-excited oscillator 25. The frequency `f` is 
the compared with the frequency f.sub.o of the generator 10 in the 
comparator 11. A signal next appears at the output of the comparator 11, 
proportional to the difference frequency .DELTA.f=f.sub.o -f (7), which 
is divided, with the division ratio `m`, by the frequency divider 15. 
Thereupon the cycle T of the difference frequency oscillations is measured 
in the circuit 16 by counting the number of pulses delivered by the 
reference-frequency generator 10 for a full oscillation cycle of a signal 
coming from the output of the divider 15. The measurement result 
represented in the digital form is displayed by the digital display unit 
20 on a light panel. The same result converted into a d.c. signal by the 
converter 22 is written down by a known technique on any of the 
heretofore-known recording devices. 
The ophthalmotonometer proposed herein measures true intraocular pressure 
in millimeters of mercury. 
Tonometric procedure has been carried out in 25 patients (41 eyes) 
suffering from glaucoma, in persons suspected of glaucoma, in 
postoperative patients and in children. The patients were aged from 3 to 
78. Intraocular pressure in the patients ranged within 5 to 35 mm Hg. All 
the patients noted good toleration of the examination carried out. 
The examination findings are tabulated in Table II. 
The ophthalmotonometer of the invention satisfies fully the requirements 
imposed by medical practice, is simple in handling and reliable in 
operation, cuts down the examination time considerably, and is 
economically efficient. In addition, the instrument is free from 
contraindications for use and is indispensable in carrying out preventive 
population screening. 
The present invention makes it possible to obtain the results of 
intraocular pressure measurements directly in millimeters of mercury. 
Moreover, it is instrumental in attaining higher reading accuracy of the 
pressure measured due to an averaged initial measurement section, as well 
as in higher accuracy of measuring some other parameters, such as 
hydrodynamic coefficients, since their calculation involves greater 
differences between the initial and final pressure values due to the fact 
that the values are expressed in milimeters of mercury rather than in 
conventional units. 
TABLE II 
__________________________________________________________________________ 
No. 
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 
__________________________________________________________________________ 
1. Prior-art tonometer- 
15.2 
23.4 
21.1 
22.7 
16.5 
15.7 
17.4 
25.3 
12.7 
16.9 
17.3 
21.6 
24.8 
19.3 
tonograph, mm Hg 
2. Tonometer of the here- 
14.2 
23.6 
20.8 
21.9 
15.8 
15.0 
16.9 
26.1 
12.1 
16.1 
16.2 
20.1 
23.1 
18.6 
in-proposed construc- 
tion, mm Hg 
3. Prior-art tonometer-to- 
26.4 
16.3 
31.3 
15.6 
19.3 
14.7 
17.5 
35.9 
20.2 
17.3 
15.8 
23.4 
21.2 
17.9 
nograph, mm Hg 
4. Tonometer of the here- 
27.0 
15.4 
32.5 
14.7 
18.6 
13.6 
17.1 
33.8 
18.8 
16.7 
14.9 
24.0 
22.1 
16.2 
in-proposed construc- 
tion, mm Hg 
5. Prior-art tonometer-to- 
11.2 
17.4 
16.1 
15.9 
17.6 
20.4 
18.3 
22.3 
15.9 
17.4 
16.3 
14.2 
13.6 
-- 
nograph, mm Hg 
6. Tonometer of the here- 
12.1 
17.1 
15.2 
16.2 
16.9 
21.0 
17.6 
22.0 
15.1 
18.2 
17.0 
12.9 
12.2 
-- 
in-proposed construc- 
tion, mm Hg 
__________________________________________________________________________