A portion of the cornea or similar flexible membrane is flattened by urging against the cornea a footplate with a measurable force and the magnitude of the applanated area of the cornea is sensed and compared to a threshold which corresponds to an area equal to a selected reference area of the planar footplate contacting the cornea. When the selected threshold is reached the force urging the footplate against the cornea is processed for scaling by another signal proportional to the reference area. In another embodiment signals proportional to both the applied force and the applanated area are digitized for averaging and/or storage and the two parameter functional dependence of area on the force or a selected region thereof is displayed and recorded for analysis.

FIELD OF THE INVENTION 
The invention is in the field of tonometry and particularly relates to 
automated apparatus for opthamalogical measurements suitable for use 
without anaesthetic. 
CROSS-REFERENCE TO RELATED APPLICATION 
Copending U.S. Ser. No. 599,967, commonly filed herewith, describes another 
tonometer closely related to the present invention. 
BACKGROUND OF THE INVENTION 
The measurement of the intraocular hydrostatic pressure is a major 
diagnostic tool for the identification of eye disorders, especially 
glaucoma, and as such, apparatus capable of accurate and reliable 
measurement is greatly to be desired. Tonometry is used routinely for 
screening the population at large as well as following individuals with 
known pathology: consequently, risk, inconvenience and trauma must be 
minimized. Moreover, of those tested, the overwhelming majority will not 
exhibit intraocular pressure pathology. An accurate and reliable 
measurement is essential to assure that the screening does indeed identify 
incidence of abnormal intraocular pressure while not burdening the health 
care system with erroneous positive identification of pathology in normal 
individuals. Other desiderata include safety, non-invasiveness and 
practice of the method without the necessity of topical anaesthetic. 
In the prior art, the Goldman tonometer has been a standard for 
opthamalogical measurement for many years. In this approach the 
intraocular pressure is obtained by flattening a standard area of the 
cornea to conform to a planar surface placed in contact with the cornea. 
Applanation tonometry, as the method is known, employs a circular 
transparent plane surface of precisely known diameter which is urged 
against the anaesthetized cornea while the observer confirms the 
applanation condition by observing the cornea through the transparent 
plane surface or footplate with the aid of a small amount of flourescein 
in the lacrimal fluid and a slit lamp or similar light source. The optical 
source is preferably rich in the blue portion of the spectrum to excite 
the fluorescein and thereby provide enhanced contrast. The observer 
adjusts the pressure applied to the foot plate until the cornea just 
conforms to a circular region marked on the transparent foot plate, at 
which point the force urging the foot plate against the cornea is 
recorded. 
Measurements of this type suffer from error in establishing the applanation 
condition. This determination is subjective and prone to error arising 
from, among other effects, the presence of a meniscus of tear fluid at the 
periphery of the foot plate and the resistance of the cornea to bending. A 
significant source of error and difficulty arises with the length of time 
required to adjust the device and to observe the flattened area. This may 
require an interval ranging from a few seconds to a minute or more. It is 
difficult for the subject to maintain the eye in a fixed position for that 
period without blinking. Moreover, such protracted contact of the 
instrument with the cornea increases the risk of a scratch or other trauma 
because of the prolonged contact and the possibility of gross eye 
movements. Clearly, the prolonged contact also requires application of an 
anaesthetic to the eye. 
A significant improvement in applanation tonometry apparatus due to Mackay 
and Marg (see Marg et al, Archiv. Opthalm., v. 4, no. 1, pp 67-74 (1961) 
and references therein) utilizes electronic means to measure the force 
required to produce the applanation condition between the foot plate and 
the cornea, deriving a signal proportional to the applied force. The force 
is necessarily applied as a function of time and the resulting 
displacement of a plunger linking the cornea with the central region of 
the foot plate is monitored on a trace recorder. In the Mackay-Marg 
instrument the trace characteristically rises to a first relative maximum 
as the central plunger responds to the full corneal resistance and the 
intraocular pressure. As the cornea is applanated against the footplate 
region surrounding the central plunger, the corneal resistance is 
distributed over the annular region and the signal derived from the 
plunger displacement drops to a relative minimum, thereafter rising 
monotonically as the cornea continues to yield in response to the 
increasing pressure. A second maximum will be recorded when the area of 
the cornea applanated by the probe reaches its maximum. As the probe is 
withdrawn, the sequence is reversed and a near mirror image of the trace 
is generated. For this type instrument it has been found that the 
significant indicia for establishing the magnitude of the intraocular 
pressure is the relative amplitude of the aforementioned relative minimum 
or trough with respect to the baseline of the trace. The force measurement 
is derived from the displacement of the plunger. The displacement is quite 
small, of the order of a micron. The Mackay-Marg tonometer is (at least in 
principle) free of the need for an anaesthetic because the entire 
measurement is obtained in an interval of the order of 10's of 
milliseconds. An interpretation of the complex trace is still required for 
this instrument to extract the critical intraocular pressure parameter. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved tonometer 
especially exhibiting improved consistency in measurement. 
It is another object to simplify tonometrical instrumentation and to reduce 
the influence of subjective judgement in both acquisition and 
interpretation of the data. 
In one feature of the invention a controlably variable force is applied to 
a planar probe in contact with the cornea, where the magnitude of the 
force applied is electrically cognizable. 
In another feature of the invention means are provided to electronically 
monitor the magnitude of the applanation area. 
In yet another feature of the invention comparator means are provided to 
trigger the measurement of the applied force upon sensing equality to, or 
an excess thereof over a selected threshold value of the applanation area. 
In still yet another feature of the invention scaling means are provided to 
compute and display the pressure obtained from the measured force and 
measured area. 
In again another feature of the invention, the applanation area is 
continuously sensed by measurement of the capacitive reactance from the 
planar probe to ground potential as referenced from the cornea. 
In yet again another feature of the invention, both the area proportional 
signal and force proportional signal are sampled during the application 
and removal of the applanation force whereby the transient response is 
obtained, processed and recorded to yield a still more accurate and 
reliable measure of intraocular pressure and other opthamalogic 
information. 
In one further feature of another embodiment of the invention, an 
applanation probe is designed for support directly on the surface of the 
cornea applanating same by the weight of the probe thereon, and from an 
applanation area proportional signal developed as described above, there 
is developed a signal representative of the internal hydrostatic pressure 
of said cornea. 
In one simple embodiment a linear variable differential transformer (LVDT) 
is disposed to sense the applied force through the force-displacement 
relationship of the springs which support a shaft coupling the LVDT to the 
footplate of the instrument. Such devices are well known and have been 
employed in similar fashion in the art. The foot plate of the tonometer 
transmits the applied force through the shaft from the cornea to the 
armature of the LVDT. An insulative sheet of known dielectric constant and 
geometry provides D.C. isolation between the cornea and the foot plate to 
which an A.C. signal is applied. The A.C. current to ground (the cornea) 
is measured to ascertain the capacity which in turn is determined by the 
applanation area. The A.C. current for a constant voltage (or the A.C. 
voltage for constant current) is sampled and presented to a comparator for 
a threshold determination which may be set to correspond to a given area 
magnitude. A further embodiment records the continuous response, analyzing 
same to extract the above and other information discoverable from the 
shape of the response function. 
In another embodiment, the force and the applanation area are independently 
sensed and the resulting two parameter function space may be constrained 
to exhibit desired information. 
The foregoing objectives, features and advantages of the present invention 
will appear from the following more particular description as illustrated 
in the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
One simple embodiment is shown schematically in FIG. 1a. A shaft 30 
supports a foot plate 32 which bears against the cornea 34 for an internal 
pressure measurement. The foot plate is electrically isolated from cornea 
34 by dielectric 36 and the shaft is isolated from ground by appropriate 
means. The force exerted by the cornea against the foot plate is balanced 
by springs 40. The relative displacement of the shaft 30 is ascertainable 
by means of a linear displacement sensor 37 from which a signal is taken 
and which is calibrated to yield a force proportional signal in accord 
with 
EQU F=-k x 
where k is the spring constant for the springs 40 and x is the relative 
displacement of the shaft 30. 
An AC signal 103 is presented to a high gain operational amplifier 102 
which is connected in such manner that the signal applied to footplate 32 
exactly duplicates the AC signal 103. The voltage appearing on the 
footplate is returned to the inverting input 202 of the operational 
amplifier 102. A difference appearing between the excitation signal 103 
and the footplate signal will be greatly amplified and will appear at the 
output. The amplified output is, in turn, connected to the footplate 
through series impedance 101 of magnitude Z. Provided that certain 
stability requirements are met and that the gain of operational amplifier 
102 is sufficiently large (of the order of 10.sup.3 to 10.sup.5 ), the 
footplate potential will be a very close reproduction of the excitation 
signal 103. Any current flowing from the operational amplifier output to 
footplate 32 will necessarily flow through series impedance 101 thereby 
imposing a potential difference between the inputs of the differential 
amplifier 104. The voltage difference will be proportional to the current 
flowing to the footplate 32 and proportional to the impedance Z. 
The footplate 32, shaft 30, springs 40, and associated items electrically 
connected to the footplate are guarded by a shield 107 driven to the same 
potential as the footplate. Current flowing to/from the footplate results 
from the capacitative coupling to the cornea. The capacitive coupling is 
clearly proportional to the contact area between the cornea and the 
footplate. The surface of the cornea is a relatively good conductor and is 
maintained at ground potential through the electrical contact of the 
patient with his environment or by the relatively large capacitance to 
ground presented by the human body even without direct ohmic contact to 
ground. If the impedance 101 is capacitative, the voltage developed across 
it will be in phase with the excitation 103 because the footplate current 
will lead the phase of excitation signal 103 by 90.degree.. This current 
will inturn produce a drop across the capacitor that lags the phase of the 
current by 90.degree., thereby producing a resulting voltage signal at the 
differential amplifier that is in phase with the excitation signal 103. It 
is apparent that the impedance 101 need not be a capacitative reactance: 
it is only necessary to note that the voltage developed across it and the 
phase relationship for that voltage are selectable by the designer. The 
detector 106 is preferably a phase sensitive circuit in order to exploit 
synchronous properties and enhance noise rejection. It is important to 
note that phase sensitive detector is not essential for this application 
and a peak detector, envelope detector or similar means for producing a dc 
signal from the ac signal across impedance 101 would be suitable (although 
somewhat less satisfactory) for the purpose. 
One suitable mechanical suspension for support of shaft 30 is illustrated 
in FIG. 1b which is a schematic perspective illustration of a flat spring 
suspension structure. Flat springs 40 are formed by etching a metal foil. 
Arc segment perforations, as shown are found to provide enhanced radial 
compliance and a wide dynamic range. Spring carriers 58 are secured to 
probe housing 107 and the springs 40 are held in the respective carrier, 
against the end plate thereof by a metal ring 59 press fit into the 
carrier ends. 
An A.C. signal of constant amplitude is applied to shaft 30 and a high 
input impedance amplifier receives the signal to ground through the 
capacitive impedance presented by dielectric 36. The signal from the 
impedance divider is proportional to the impedance presented by the 
capacitive coupling to the cornea. 
In operation the area threshold signal is adjusted to trigger recording of 
force signal when a preselected area of the cornea is applanated. For 
direct comparison with the Goldmann tonometer, this preselected area can 
be chosen to be 3.06 mm diameter, which value is conventional for the 
Goldmann instrument. Clearly, the footplate area and the trigger point of 
the area proportional signal are parameters of choice for the instrument 
designer. The footplate diameter itself is selected on design 
considerations which are independent of the Goldmann standard area except 
that this diameter is large compared with the area to be applanated. 
The response of the present apparatus is noted to produce signals which are 
monotonic functions of applanation in contrast to the complex signal 
obtained from the prior art instrument of Mackay and Marg. 
It is recognized in the field of tonometry that the cornea is not perfectly 
flexible and the finite rigidity of the cornea provides an apparent 
increment of the measured pressure. It is also known that surface tension 
forces operate between the lacrimal fluid and the exterior corneal surface 
to reduce the applied force required for a given deformation. These two 
effects are oppositely directed and it has been determined that for the 
standard applanated area of prior art (3.06 mm diameter) the two effects 
are of approximately equal magnitude: thus, for this standard area, the 
pressure derived from independent area and force sensors need not be 
corrected for these two effects (within the accuracy of their 
cancellation). In copending U.S. Ser. No. 599,967, tonometric measurements 
are carried out relative to a standard reference force for applanation. 
As above described, one embodiment of the subject tonometer monitors both 
parameters in a substantially continuous manner and sampling and 
digitizing both parameters to obtain concurrent corresponding values while 
the applied force is varied. It is recognized that the response of the 
cornea or other deformable membrance to the applanation may be 
continuously monitored in both the area signal and the force signal to 
establish the continuous, two-dimensional response function rather than a 
particular discrete point on that function. One thereby obtains access to 
a wealth of information latent in the shape of the response function. 
Corneal rigidity, hysteresis, corneal bending and like quantifiable 
parameters are thereby accessible to measurement and study. It is noted 
that the choice of foot plate area was selected in the prior art to 
substantially minimize a corneal rigidity effect. One may well wish to 
measure the effect of corneal rigidity and other attributes which may 
contribute to effective diagnoses. The localization of regions of this 
generalized two-dimensional function space for study is accomplished in 
straightforward fashion by constraint of signals or by constraints imposed 
upon the recorded two-dimensional data. 
A more detailed exposition of a preferred clinical embodiment with 
reference to corneal tonometry is shown in FIG. 2a. A probe case 50 of 
cylindrical symmetry contains LVDT windings 56 which effectuate sensing 
the displacement of the LVDT core 54. The LVDT comprises three windings: a 
driven primary and two symmetrically situated secondaries, connected in 
series opposed form. The flux arising from the driven primary links the 
core and the two secondaries. With the core 54 at zero displacement, 
symmetrically disposed with respect to the secondaries, equal opposed 
voltages are induced across the secondaries for a net null signal. Upon 
displacement of the core 54, the voltages across the secondaries become 
unbalanced and a difference signal obtained from the LVDT exhibits phase 
and amplitude dependent upon the direction and magnitude of the 
displacement. This signal is then processed to yield a waveform faithfully 
reproducing the motion of the core. 
The LVDT core 54 is supported near one end of shaft 30 with foot plate 32 
at the other end. Hypodermic syringe stock is recommended as an excellent 
available stock for shaft construction. A thin insulating film constitutes 
the dielectric of a capacitive coupling between foot plate 32 and the 
cornea. The insulating film 36 may be a polymeric coating, a glass or 
fused silicon dioxide. Polyurethane, mylar, polyethylene, polyester, 
epoxy, acrylic and the like are all very good examples for this purpose 
because these substances exhibit relativeley low toxicity and because they 
exhibit high dielectric constants. Glass or silicon dioxide films have the 
advantage of superior chemical and dimensional stability as well as damage 
resistance owing to their hardness. Certain other materials, such as 
anodized aluminum, are also suitable. 
A spring carrier 58 is mechanicaly secured to the probe case 50 to support 
the shaft 30 via support springs 40 and to provide electrical coupling 
thereto. An appropriate spring suspension which has been employed for this 
application is shown in FIG. 2A wherein a metal foil is etched to remove 
annular segments as shown in FIG. 1b. The resulting flat springs are 
secured to the spring carrier by a press fit ring. The shaft 30 is secured 
to the central hole by bonds 60 formed from known conducting epoxy resin. 
Capactive coupling to the cornea through foot plate 32 and dielectric film 
36 results in an area proportional signal if spurious currents through 
stray capacitances can be eliminated or compensated. For this purpose, the 
probe structure incorporates a guard conductor shell 62 surrounding the 
foot plate 32, shaft 30 and spring suspension. Insulating shell 64 
isolates the guard conductor 62 from the probe case 50 and insulator 66 
likewise isolates the spring carrier 58 from guard conductor 62. 
A preamplifier 68 preferably housed inside of guard conductor 62 comprises 
a differential amplifier 70 for comparison of the foot plate signal 
separately from the parasitic currents arising from the stray 
capacitances. 
A preferred structural variation of the above described probe is shown in 
FIG. 2b. The footplate 32 is here joined mechanically to hollow shaft 30 
through insulated collar 31. An electrical coupling from footplate 32 to 
preamplifier 70 is realized from an insulated conductor 69 which is 
carried coaxially in hollow shaft 30. The springs 40 are electrically 
isolated from the shaft 30 and the latter is, in this variation, driven to 
guard potential. 
It is useful at this point to consider the amplitude of the desired 
capacitive current resulting from contact between the cornea and foot 
plate 36. Consider a representative thickness of 0.001 inch (25.4 microns) 
and a relative dielectric constant of 3.6 for the dielectric film 36. 
Glasses, and in particular fused silicon dioxide exhibit dielectric 
constants in this range and many common polymeric coatings have similar 
dielectric constants. 
Oscillator 72 provides an AC excitation at a frequency which for present 
purposes can be assumed as 10 KHz. Under the assumption of a 10 volt peak 
signal the AC current through the foot plate 32 is very nearly 6 
microamperes. While this is not difficult to measure directly, the effects 
of parasitic capacitance (which can induce currents that reach or exceed 
this value) are effectively removed by floating the guard shell conductor 
and the entire preamplifier to foot plate potential. The signal is 
amplified to the point where interwinding capacitance introduces neligible 
effects, at which point the signal is returned to ground through 
transformer coupling 74. This area proportional signal is again amplified 
by linear amplifier 76, phase detected against the oscillator reference 
signal in sync detector 77 from which there is obtained a DC signal 
representative of the applanation area. In the same fashion, the LVDT 
excitation is amplified by amplifier 78 and phase detected against the 
oscillator reference at sync detector 79 to yield a force proportional DC 
signal. These signals may then be presented to further processing means 
and display. 
The signal processing accomplished at processor 80 includes the simple 
area-triggered output discussed above as well as processing of somewhat 
greater generality to extract selected parameters. Processing of the area 
and force proportional signals may be carried out with full generality 
following the standard approach suggested in FIG. 3b wherein each signal 
is sampled at a sampling rate which is derived from a convenient clock 
such as might be obtained from oscillator 72 by a simple divider, the 
output of which initiates conversion at each of the analogue to digital 
converters (ADC). When conversion is complete in both ADCs a DATA RDY 
signal so informs the processor thereby requesting a read operation 
directed to the data latches now holding the digitized area and force 
data. When the data have been read successfully a RESET signal is issued 
to the ADCs. 
A more specialized apparatus may be obtained following FIG. 3a wherein the 
area proportional signal is compared with a reference level to gate a 
pressure proportional datum to a processor and display. It is noted that 
further operations on the pressure proportional signal, if desired, are 
symbolically contained within the processor unit. An example of such 
optional processing would be an averaged sampling of the sensor transient 
on both the rising and falling sides, corresponding to rise and fall of 
the applanation condition. (There are technical reasons which tend to 
reduce the value of sampling on the falling portion of the transient for 
simple intraocular pressure measurement.) The logic unit processes the 
signal in accordance with the relationship of the derived parameter 
(intraocular pressure, for example) to the force response transient, which 
in the present apparatus contrasts with the transient waveform of he 
McKay-Marg instrument. The work of Mackay and Marg suggest that the 
intraocular pressure is proportional to the amplitude of the relative 
minimum of the transient force response waveform. The operational 
principle underlying the transient waveform of the present apparatus 
yields a monotonic function, which when properly scaled at a selected area 
magnitude accurately measures the intra-ocular pressure. Further optional 
processing, already alluded to herein, includes multiple sampling at known 
succesive values of the area proportional signal to yield a two parameter 
analysis of the corneal behaviour. The details of this aspect of the 
processing are outside the scope of the present invention. 
One will readily appreciate that alternative pressure sensing means can be 
employed in the form of a piezo-electric transducer for direct sensing of 
the applied force. A piezoelectric transducer is conceptually illustrated 
in FIG. 4 wherein a footplate 32 is supported as in other embodiments by 
shaft 30. The distal end of shaft 30 is bonded to a bimorph piezoelectric 
element 300. The latter typically comprises a conductor such as an 
aluminum disk 301 bonded to a peizoelectric crystal 302. The force applied 
to the footplate in contact with the cornea is transmitted through shaft 
30 to the bimorph disk 300 causing the latter to assume a slightly concave 
shape, thereby inducing radial tensile stresses in the peizo crystal. A 
potential developed between the plane surfaces of the bimorph sensor 300 
is proportional to the transient applied force and the resulting voltage 
pulse is directed to a high impedance amplifier 303. A pulse with 
amplitude proportional to the applied force is thereby obtained for use in 
processing as in the above described embodiments. 
Another transducer for obtaining a force proportional signal is illustrated 
in FIGS. 5a and 5b. This variable reluctance sensor is structuraly similar 
to the LVDT with the distinction that no AC excitation, AC amplifier nor 
synchronous detection are employed. As distinct from the LVDT apparatus, 
which develops a signal proportional to the absolute displacement of the 
LVDT core, the variable reluctance sensor yields a signal proportional to 
dz/dt, the rate of displacement along the z axis of a permanent magnet 87 
with respect to windings 88a and 88b. In principle this signal can be 
integrated in integrator 89 to yield the z displacement. Sufficient 
integration is inherent in an AC amplifier exhibiting approximately -20 
db/decade rolloff over the appropriate frequency range (about 1 to 100 
Hz). The resulting pulse amplitude is therefore force proportional through 
the displacement proportionality and may be treated as in the above 
embodiments. 
In the above described embodiments, electrical measurements of applanated 
area proportional signal and applantion force proportional signal are 
combined to yield the desired pressure proportional signal. Another 
embodiment utilizes an area proportional signal as before, but the probe 
now comprises a structure supported directly on the cornea of a supine 
patient, as schematically illustrated in FIG. 6. The applanation of the 
cornea under gravity against the footplate 32 defines a standard force and 
it is only necessary to measure the resulting area proportional signal as 
discussed herein to obtain a signal which is transformed to a pressure 
indicia. It is recognized that this embodiment differs from standard 
practice in Goldmann tonometry in the precise principle of a fixed 
applanating force and variable degree of applanation whereas standard 
practice as well as above described embodiments emphasize a fixed 
reference applanation area and variable applanation force. 
The application of the invention has been described in the context of 
measuring corneal applanation, eg. the hydrostatic pressure of the eye. 
Rather wider uses of the method and apparatus will occur to equipment 
designers. For example, the examination of bloat in cattle, distension of 
internal organs and subcutaneous conditions generally, pressure 
measurements in flexible tubing, automobile tires and similar flexible 
bodies (provided tsuch bodies can be rendered sufficiently electrically 
conductive) are all appropriate applications of the apparatus and method 
here described. 
It will be apparent that many changes could be made in the above method and 
apparatus and many apparently different embodiments of this invention 
could be made without departing from the scope thereof; it is therefore 
intended that all matter contained in the above description and shown in 
the accompanying drawings shall be interpreted as illustrative and not in 
a limiting sense.