A tonometer for measuring an intraocular pressure of an eye using an air pulse

Abstract

Disclosed is a method of measuring intraocular pressure value of eye (308), the method comprising steps of: sending first excitation air pulse having first parameter values towards eye; measuring first signal related to corneal oscillations caused by sent first excitation air pulse; determining signal quality indicator of first signal; adjusting first parameter values to second parameter values if, determined signal quality indicator is outside of predetermined range; sending second excitation air pulse having second parameter values towards eye; measuring second signal related to corneal oscillations caused by sent second excitation air pulse; and determining intraocular pressure value from measured second signal. Disclosed also is a tonometer (200, 300) for measuring intraocular pressure of an eye, wherein the tonometer comprises an air pulse generator (202, 400), a corneal oscillation measurement sensor (204), and a controller (206).

Description

[0001] A TONOMETER. FOR MEASURING AN INTRAOCULAR PRESSURE OF AN

[0002] EYE USING AN AIR PULSE

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to methods for measuring intraocular pressures of eyes. Moreover, the present disclosure relates to tonometers for measuring intraocular pressures of eyes.

[0005] BACKGROUND

[0006] The accurate measurement of intraocular pressure (IOP) is pivotal in ophthalmic diagnostics. It serves as a fundamental parameter for assessing ocular health and detecting abnormalities, particularly in conditions such as glaucoma which causes damage to optic nerve leading to progressive vision impairment. Tonometer is an instrument used for measuring IOP.

[0007] The tonometer may be a rebound tonometer, a dynamic contour tonometer, a non-contact (Air-Puff) tonometer, and a Goldmann applanation tonometer (GAT).

[0008] Currently used non-contact (Air-Puff) tonometers might cause infection to person operating the tonometer as the air buff might spread decrease from the eye of the patient. Furthermore, if the force of the air buff is large it might cause discomfort to the patient. In addition, contact tonometer's have problem of making a physical contact with a surface of the eye, thus potentially causing infections and discomfort. One of the problems with conventional air puff tonometer or Goldmann applanation tonometer is that those require eye to be in applanation state to measure IOP. As an example, in the air puff tonometer the intensity of the air flow is slowly increased so that the cornea bends over the applanation state and then decreased so that the cornea returns to its normal state. IOP is measured at the applanation state so that optical methods define the applanation state and system registers how much force / intensity is needed to create the applanation. This might take relatively long time and thus being uncomfortable for a user. Furthermore, said (Imbert-Fick) principle of measuring with air buff tonometer requires applanating the cornea from a reasonably large area (several millimeters in diameter). As the area is large, total force required by the air buff is also relatively high. Large area is also more difficult to align as part of the air buff might be directed to eye lid also. Therefore, in light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks.

[0009] SUMMARY

[0010] The aim of the present disclosure is to provide a method and a tonometer to induce corneal oscillation in a cornea of the eye and analyzing various characteristics of the oscillation to determine the IOP of the eye. The aim of the present disclosure is achieved by a tonometer and a method for measuring an intraocular pressure of an eye as defined in the appended independent claims to which reference is made to. Advantageous features are set out in the appended dependent claims.

[0011] Throughout the description and claims of this specification, the words "comprise" , "include", "have", and "contain" and variations of these words, for example "comprising" and "comprises" , mean "including but not limited to", and do not exclude other components, items, integers, or steps not explicitly disclosed also to be present. Moreover, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Indeed, present disclosure differs further on conventional method as it is not based on the Imbert-Fick principle and thus applanation state is not needed. Instead, in present disclosure, a gentle air impulse displaces the cornea from its normal position, the air impulse is stopped, and thereafter the cornea oscillates as a damped harmonic oscillator returning to the normal state. Oscillations and movements of the cornea are monitored with displacement sensor or a like and those can be used to derive for example intra ocular pressure (via experimental observations, via physiological model etc.).

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is an illustration of a flowchart depicting steps of a method for measuring an intraocular pressure of an eye using air pulse, in accordance with an embodiment of the present disclosure;

[0014] FIG. 2 is an illustration of schematic diagram of a tonometer for measuring an intraocular pressure of an eye using air pulse, in accordance with an embodiment of the present disclosure;

[0015] FIG. 3 is an illustration of an environment where with reference to FIG. 2 is in use, in accordance with an embodiment of the present disclosure;

[0016] FIG. 4 is a schematic illustration of operational flow of an air pulse generator in accordance with an embodiment of the present disclosure; and

[0017] FIGs. 5A-5E are graphical illustrations of an amplitude of corneal oscillation plotted against time, in accordance with an embodiment of the present disclosure.

[0018] DETAILED DESCRIPTION OF EMBODIMENTS

[0019] The following detailed description illustrates embodiments of the present disclosure and ways in which they can be implemented. Although some modes of carrying out the present disclosure have been disclosed, those skilled in the art would recognize that other embodiments for carrying out or practicing the present disclosure are also possible.

[0020] In a first aspect, the present disclosure provides a method for measuring an intraocular pressure value of an eye, the method comprising: sending a first excitation air pulse having first parameter values towards the eye; measuring a first signal related to corneal oscillations caused by the sent first excitation air pulse; determining a signal quality indicator of the first signal; adjusting the first parameter values to second parameter values if, the determined signal quality indicator is outside of a predetermined range; sending a second excitation air pulse having the second parameter values towards the eye; measuring a second signal related to corneal oscillations caused by the sent second excitation air pulse; and determining the intraocular pressure value from the measured second signal.

[0021] The aforementioned method allows to perform measurement of the intraocular pressure (IOP) value of the eye without making any direct contact with the eye, thereby reducing risk of contamination and discomfort in the eye. The aforementioned method involves the utilization of an air pulse to induce oscillations in a corneal surface of the eye which is subsequently analyzed to determine the IOP value of the eye. Moreover, the method involves assessing the signal quality related to corneal oscillations and afterwards regulating a force exerted on the eye by the air pulse. Further, the method allows adjusting the force of the air pulse by changing a release time, a velocity and a volume of the air pulse based on the assessment thereby minimizing an overall time consumption in multiple adjustment to achieve optimum force of the air pulse. Furthermore, the aforementioned method provides an accurate and reliable measurement of IOP.

[0022] In a second aspect, the present disclosure provides a tonometer for measuring intraocular pressure of an eye, the tonometer comprising: an air pulse generator; a corneal oscillation measurement sensor; a controller communicably coupled to the air pulse generator and to the corneal oscillation measurement sensor, the controller configured to: set first parameters for the air pulse generator; initiate the air pulse generator to generate a first excitation air pulse using the first parameters to oscillate a cornea of the eye; use the corneal oscillation measurement sensor to measure a first signal related to the corneal oscillations; determine a signal quality indicator of the measured first signal; adjust the first parameters to second parameters, if the determined signal quality indicator is outside of a predetermined range; set the second parameters for the air pulse generator; initiate the air pulse generator to generate a second excitation air pulse using the second parameters to oscillate the cornea of the eye; use the corneal oscillation measurement sensor to measure a second signal related to the corneal oscillations; and determine the intraocular pressure value from the measured second signal.

[0023] The aforementioned tonometer measures the IOP of the eye by dynamically adjusting the force exerted by the air pulse on the corneal surface of the eye. Moreover, the tonometer is configured to assess the quality of the signal related to the corneal oscillation and based on the assessment, the force exerted on the eye by the air pulse is adjusted thereby reducing number of iterations required for achieving an optimum force of the air pulse. Furthermore, the tonometer incorporates unique constructional feature of prompt closing of the air pulse generator after the air pulse is directed towards the eye in order to achieve a rapid pressure equalization within the air pulse generator to achieve accurate IOP measurement value.

[0024] Throughout the present disclosure, the term "intraocular pressure" refers to a pressure exerted by the vitreous fluid inside an eye chamber that retains the shape of the eye, provides support to a retina of the eye, and acts as a shock absorber, protecting the delicate structures of the eye from external forces. Measuring the intraocular pressure (IOP) is essential in determining and assessing a health condition of the eye and is carried out by the tonometer.

[0025] Throughout the present disclosure, the term "first excitation air pulse" refers to a controlled burst of air directed towards the eye at a first time instant, which is capable of exerting certain force on the corneal surface of the eye. The force exerted by the first excitation air pulse on the corneal surface of the eye depends on a mass of air and a velocity at which the first excitation air pulse is sent towards the eye. Moreover, the force exerted by the first excitation air pulse might be adjusted based on factors such as corneal thickness, and ocular conditions (including previous IOP value) of the eye if known prior to the measurement. Alternatively, a default values for the air pulse can be used. According to one embodiment the default values depend on patient age, gender and other pre-known factors. The term "corneal surface" refers to the transparent outer surface membrane of the eye. The term "first parameter values" as used herein refers to specific parameter values at which the first excitation air pulse is sent towards the eye. Optionally, the first parameter values comprise values for parameters such as at least one: a diameter of an exit aperture of a nozzle of a tonometer, a pressure inside of the nozzle and a time duration of releasing air from the exit aperture. The first parameter values are relevant in configuring a volume of the first excitation air pulse and a velocity at which the first excitation air pulse is directed towards the eye, as the velocity and volume of the first excitation air pulse affects the force exerted at the corneal surface of the eye. Moreover, the first parameter values comprise a shape or waveform of the first excitation air pulse. The shape or waveform of the first excitation air pulse affects the force exerted at the corneal surface of the eye.

[0026] Notably, the first excitation air pulse is configured to cause deformation on the cornea surface by exerting the force thereon. The application of the first excitation air pulse is terminated rapidly thus removing the pressure from the corneal surface. Moreover, the first excitation air pulse sent towards the eye, is set at the first parameter values. The first parameter values can be based on default values, previous known IOP, the requirement and ocular condition experienced by the person who is undergoing a procedure for IOP measurement. The first parameter values are configured before initiating the air pulse generator to produce and release the first excitation air pulse.

[0027] Optionally, the first parameter values of the first excitation air pulse comprise at least one of: a first force at a surface of the eye; a first size of the first excitation air pulse at the surface of the eye; and a first duration of the first excitation air pulse.

[0028] In this regard, the term "first force" as used herein refers to the force exerted by the first excitation air pulse on the surface of the eye (i.e., the corneal surface). The first force depends on the pressure of the first excitation air pulse inside the nozzle of the tonometer. The higher the pressure of the first excitation air pulse inside the nozzle, the higher will be the force exerted by the first excitation air pulse on the surface of the eye. Throughout the present disclosure, the term "first size" refers to an area of the first excitation air pulse making a contact with the corneal surface. The first size is dependent on a value of the diameter of the exit aperture of the nozzle. In other words, the wider the exit aperture (i.e., the higher value of the diameter of exit aperture), the larger the first size of the first excitation air pulse. Throughout the present disclosure, the term "first duration" refers to the time duration of releasing the first excitation air pulse. A technical effect is that the first parameter values comprises a variety of parameters that affect the measurement of the IOP. As an example, if duration is too small cornea might not experience sufficient force from the air and thus measurement is not possible. Alternatively, if duration is too long the patient might have time to react to the air impulse and blinks the eye thus making error to the measurement.

[0029] Throughout the present disclosure, the term "corneal oscillations" refers to imposition and rapid removal of the impact of the first excitation pulse that causes an oscillation on corneal surface. Notably, the corneal oscillations are used in determining the IOP. The term "first signal" as used herein refers to a signal containing information related to the various characteristics of the corneal oscillations caused by the first excitation air pulse. As the impact of the first excitation air pulse causes the corneal oscillations, values of the various characteristics of the corneal oscillations depends on the pressure exerted by the first excitation air pulse. The first signal corresponding to the corneal oscillations is dependent upon the first excitation air pulse sent and provides insight into adjustments required to obtain the corneal oscillations that corresponds to accurate measurement of the IOP of the eye. The first signal is essential to determine quality of the corneal oscillation and how much adjustment is required in the first parameters so that a next excitation pulse is able to exert a required force on the corneal surface and thereby providing accurate measurement of the IOP value.

[0030] Throughout the present disclosure, the term "signal quality indicator" refers to a value used to assess whether the corneal oscillations caused by the first excitation air pulse exhibit the necessary characteristics for accurate measurement of the IOP. The signal quality indicator is used to evaluate whether the corneal oscillations meet a predefined criteria for each of these characteristics. If the corneal oscillations exhibit the required characteristics within an acceptable ranges, the signal quality indicator may be classified as good and no further adjustments in the first parameter values are required and the IOP measurement can be carried out with accuracy. Conversely, if the corneal oscillations do not meet the required criteria, the signal quality indicator may be classified as poor and further assessment or adjustments is necessary. Notably, by determining the signal quality indicator, the accuracy and reliability of the measurement of the IOP is determined. In this regard, the signal quality indicator may be determined by one or more of the following: (i) amplitude of first oscillation, (ii) deviation of oscillation frequency, (iii) decay factor (rate of amplitude reduction), (iv) ratio of decreasing to increasing pea k-to- peak amplitudes, or (v) comparison of corneal oscillation profile with a reference profile.

[0031] Optionally, the step of determining the signal quality indicator comprises: calculating a deviation of an oscillation frequency of corneal oscillations caused by the sent first excitation air pulse from the measured first signal; and using the deviation as the indicator of the signal quality.

[0032] Deviation of the oscillation frequency can be calculated from the measured first signal. As an example, how to calculate the deviation of the oscillation frequency of a periodic signal from its average, the following steps could be taken: First, the time between each cycle of the signal should be measured across several cycles. Next, from these measurements, the average cycle time can be calculated. The average frequency can then be determined as the inverse of the average cycle time. For each measured cycle time, the corresponding frequency is thus computed. Finally, the variation of each frequency from the average is evaluated, either by measuring the absolute difference or by calculating the standard deviation to understand the distribution of frequencies around the average.

[0033] Standard deviation = sqrt(sum(fi-faVe)2 / N), wherein is frequency of cycle i, fave is average frequency and N number of cycles considered.

[0034] In this regard, the calculated deviation is compared with a normal deviation range to determine if the calculated deviation is within the normal deviation range or outside the normal deviation range for ascertaining the levelling as the signal quality indicator. When the calculated deviation is within the normal deviation range, it indicates that the signal quality indicator can be classified as good. A technical effect is that the signal quality indicator is effectively determined based on the deviation of the oscillation frequency

[0035] Optionally, the step of determining the signal quality indicator comprises: fitting parameters of a physiological model of the eye to the first signal; calculating a confidence level of the parameters; and using the confidence level as the indicator of the signal quality.

[0036] In this regard, the parameters of the physiological model of the eye refers to at least one of: corneal properties (such as its curvature, thickness, refractive index, and biomechanical properties) that influence response of the eye to external stimuli, aqueous humor dynamics; vitreous humor properties; ocular surface properties (such as tear film, conjunctiva, and other components of the ocular surface), any other properties exhibited by the eye (both in a healthy state and in a state of experiencing any ocular condition). The first signal of the corneal oscillation are mapped to the parameters of the physiological model of the eye, based on which the confidence level of the each of the parameters is calculated. The confidence level is calculated utilizing at least one of: a confidence interval estimation, a hypothesis testing technique, any other suitable technique to quantify a reliability of the first signal referring to various characteristics of the corneal oscillations. The calculated confidence level is considered as the signal quality indicator of the first signal. The higher value of the confidence level suggests that the signal quality indicator may be classified as good one and no further adjustment is required and vice versa. A technical effect is that the parameters of the physiological model of the eye are effectively used to determine the signal quality indicator.

[0037] Optionally, the step of determining the signal quality indicator comprises: determining pea k-to- peak amplitudes of a plurality of corneal oscillations; comparing consecutive peak-to-peak amplitudes of the plurality of corneal oscillation to each other's; calculating ratio between a number of occurrences in which the amplitudes decreases between two consecutive peak-to-peak amplitudes in respect to a number of occurrences in which the amplitudes increases between two consecutive peak-to-peak amplitudes; and using the ratio as the indicator of the signal quality.

[0038] In this regard, the corneal measurement sensor is configured to measure the peak-to-peak amplitudes of the plurality of corneal oscillations caused by the first excitation air pulse. The calculated ratio provides insight into stability and / or consistency of the first signal over time where a higher ratio of decreasing amplitudes may indicate more fluctuations or instability in the first signal, while a lower ratio may suggest greater stability. The calculated ratio also identifies trends or patterns in the signal's behavior. For example, a consistently high ratio of decreasing amplitudes may indicate a gradual attenuation or damping of the signal over time, while a consistently low ratio may suggest a more periodic or cyclic pattern. The ratio is considered as another signal quality indicator wherein higher stability indicates that the signal quality indicator may be classified as good and thereby eliminating need of further adjustment. A technical effect is that it is straightforward and computationally light to assess the quality of the signal by comparing peak amplitudes.

[0039] Optionally, the step of determining the signal quality indicator comprises determining amplitude of the first corneal oscillation and using the amplitude as indicator of the signal quality. In this regard, the term "amplitude of the first corneal oscillation" refers to a maximum displacement or deformation of the corneal surface caused by the first excitation air pulse. The amplitude is dependent on the force exerted by the first excitation air pulse at the corneal surface. A lower value of the amplitude, than a predefined standard amplitude value, indicates that the applied pressure at the corneal surface by the first excitation air pulse is not sufficient to induce the corneal oscillation which can be utilized for measuring the IOP. Therefore, a lower amplitude value of the corneal oscillation caused by the first excitation air pulse when compared to the predefined standard amplitude value, indicates that adjustments to the first parameters needs to be made, before sending out a next excitation pulse. A technical effect is that the amplitude of the first oscillation is effectively taken into account for determining the signal quality indicator. It may be appreciated that the amplitude of the first oscillation is directly related to the energy delivered to the cornea and the biomechanical properties of the eye, therefore it is a simple, robust, and easily quantifiable metric that can be automatically extracted by the tonometer and reducing the need for operator intervention. Moreover, the amplitude of the first oscillation is less susceptible to noise and artifacts than more complex signal features, making it a reliable first check for signal quality. Beneficially, by using the amplitude of the first oscillation, rapid, real- time feedback is provided on whether the initial measurement conditions are suitable, allowing the system to adapt instantly. Moreover, by ensuring that only signals with sufficient amplitude are used for IOP determination, the method reduces the risk of underestimation or overestimation of intraocular pressure due to weak or ambiguous corneal responses. Furthermore, specific amplitudes of the first oscillation enable automatic tailoring of the air pulse parameters to each patient's unique corneal biomechanics, improving accuracy across a wide range of eye types and conditions. Furthermore, using the amplitude of the first oscillation minimizes the likelihood of false readings caused by insufficient corneal excitation, poor alignment, or patient movement. Additionally, by quickly identifying and correcting suboptimal measurement conditions, the number of repeated measurements may be reduced, saving time and improving comfort for both patient and clinician.

[0040] Optionally, the step of determining the signal quality indicator comprises determining decay factor of the corneal oscillations and using the decay factor as the indicator of the signal quality. In this regard, the term "decay factor" used herein refers to a rate at which the amplitude of the corneal oscillations decreases over time. Herein, the term "decay" may refer to, for example, reduction in amplitude, damping of the corneal oscillations, attenuation of the corneal oscillations, reduction in amplitude of the corneal oscillations, etc., that indicate decrease in amplitude of the corneal oscillations over time. In other words, the term "decay" may be interchangeably used to denote "damping", "attenuation" , "reduction" , and so on. Notably, when the first signal corresponding to the corneal oscillation caused by the first excitation air pulse dies out faster indicating the decay factor to be higher than a standard value, it indicates that the contact time of the first excitation air pulse or the pressure at the corneal surface is of lower value and the first parameter value needs adjustment in order to obtain accurate IOP measurement. Furthermore, we can analyze decay factor (or damping factor as another name) of the oscillations. If the decay factor is within a predetermined range, we can conclude that the signal quality is good. A technical effect is that the decay factor is effectively taken into account for determining the signal quality indicator.

[0041] Notably, the signal quality indicator is compared with the predetermined range, after determining the signal quality indicator. The predetermined range pertains to nominal values for each of the signal quality indicators defined as per various guidelines of ophthalmic practices. The first parameter values are adjusted to the second parameter values utilizing the adjustment mechanism, based on the comparison between the signal quality indicator and the predetermined range. The second parameter values are set by adjusting the diameter of the exit aperture and the timing the opening as well as closing of the controllable valve. In this regard, the accuracy in adjustment made to the first parameter values (i.e., adjusting the parameter values from first parameter values to the second parameter value) are ensured by utilizing the closed loop feedback mechanism. Beneficially, the closed loop feedback mechanism, based on the signal quality indicator derived from the first corneal oscillation signal, enables a more accurate and reliable intraocular pressure measurement. In particular, the closed loop feedback mechanism is used to dynamically adjust air pulse parameters for improved measurement accuracy. Herein, the closed loop feedback mechanism may be performed in real-time. By adjusting the second parameter values based on real-time feedback, the corneal oscillation signal is curated within a range suitable for accurate IOP measurement. Moreover, the adjustment accounts for individual differences in corneal properties, eye anatomy, and patient condition, leading to more reliable results across diverse populations.

[0042] Throughout the present disclosure, the term "second excitation air pulse" refers to the excitation air pulse sent after assessing the signal quality indicator corresponding to the first excitation air pulse. In this the second excitation air pulse is sent towards the eye, after the second parameter values are set. The second parameter values are adjusted so as to ensure that the second excitation air pulse exerts an optimum pressure or force at the corneal surface of the eye resulting in corneal oscillations which can be assessed to provide accurate IOP measurement.

[0043] Optionally, the second parameter values of the second excitation air pulse comprise at least one of: a second force at surface of the eye; a second size of the second excitation air pulse at the surface of the eye; and a second duration of the second excitation air pulse.

[0044] In this regard, the second parameter values are set to the second force value, the second size value and the second duration corresponding to the second excitation air pulse. In order to set the second parameter values, the controllable valve and the air pressure inside the nozzle are adjusted, thereby regulating at least one of the following: the force exerted by the second excitation air pulse on the surface of the eye, the size or volume of the second excitation air pulse, and the duration that the second excitation air pulse remains in contact with the corneal surface of the eye. Notably, the second parameter values are different from the first parameter values. The second excitation air pulse is configured exert the second force on the corneal surface of the eye which is of a different value from the first force exerted by the first excitation air pulse. Moreover, the second excitation air pulse is of the second size which is different from the first size of the first excitation air pulse. Further, the second excitation air pulse is released for the second duration different from the first duration for the first excitation air pulse. Furthermore, prior to release of the second excitation air pulse, the exit aperture is adjusted to a second value by customizing the diameter of the exit aperture so as to generate air pulse of the second size at the eye. The pressure inside the nozzle is adjusted and the second excitation pulse is released for the second duration so that the second excitation air pulse exerts the second force at the eye for the second duration. A technical effect is that the second parameter values comprises a variety of parameters that are adjusted for measuring the accurate IOP values.

[0045] Throughout the present disclosure, the term "second signal" used herein refers to a signal that contains information related to the various characteristics of the corneal oscillations corresponding to the second excitation air pulse. Notably, the second signal is different from the first signal. The second signal and values of the various characteristics of the corneal oscillations corresponding to the second excitation air pulse are measured, by detecting and analyzing movements and deformations of the corneal surface. The detection and analysis technique such as confocal microscopy, optical coherence tomography (OCT), or ultrasound or any other suitable techniques are utilized to assess the corneal oscillations and measure the second signal.

[0046] The second excitation air pulse exerts the second force on the corneal surface of the eye. As a response, a reactive force is exerted back by the corneal surface which is relative to the IOP of the eye. The second signal pertaining to the various characteristics of the corneal oscillations corresponding to the second excitation air pulse also includes information on the reactive force that is exerted back by the corneal surface. The IOP is determined by analyzing the values of the various characteristics of the corneal oscillations corresponding to the second excitation air pulse. As changes in the IOP directly influence the corneal oscillations caused by the second excitation air pulse, by analyzing the relationship between the corneal oscillations and the IOP, the controller calculates the pressure within the eye.

[0047] The present disclosure also relates to the tonometer as described above. Various embodiments and variants disclosed above, with respect to the aforementioned method, apply mutatis mutandis to the tonometer. Throughout the present disclosure, the term "tonometer" refers to a medical device used to measure the intralocular pressure (IOP). Throughout the present disclosure, the term "air pulse generator" refers to a component of the tonometer for producing the first excitation air pulse and the second excitation air pulse which is directed onto the corneal surface of the eye during IOP measurement.

[0048] Optionally, the air pulse generator comprises: a nozzle having an exit aperture; a pressure generation means configured to generate a pressure inside of the nozzle; and a controllable valve configured to release air from the exit aperture; wherein the first and the second parameters comprise a diameter of the exit aperture, a pressure inside of the nozzle and a time duration of releasing air from the exit aperture.

[0049] In this regard, the term "nozzle" refers to a device that controls a flow of the first and the second excitation air pulse. Notably, the air pulse generator utilizes the pressure generation means to generate the first and the second excitation air pulse of certain pressure inside of the nozzle. The term "pressure generation means" used herein refers to an air compressor (for example a reciprocating air compressor, a rotary screw air compressor, a centrifugal air compressor, and the like) that is configured to generate excitation air pulse. The controllable valve may refer to one of: a solenoid valve, a diaphragm valve, a check valve, a pressure relief valve, a control valve and any such suitable valve capable of swift opening and closing. The controllable valve is configured to release air from the pressure generation means via the exit aperture of the nozzle. The exit aperture has a diameter which can be adjusted to make an opening of the exit aperture wide or narrow. The controller utilizes the adjustment mechanism to adjust the diameter of the exit aperture to control the amount of air released while directing each air pulse, the duration of release of the air pulse, and the pressure of air pulse. Moreover, by controlling the movement of the controllable valve, the air pressure inside the nozzle can be controlled swiftly. The duration for which the air pulse is released is adjusted considering at least one of: the set pressure inside the nozzle, and the intended force which should be exerted on the corneal surface by the air pulse. By adjusting the diameter of the exit aperture and controlling the opening and the closing of the controllable valve, the amount of air released while directing each of the first and second excitation air pulse, the duration of release of the excitation air pulse, and the pressure of excitation air pulse can be controlled, and thus, inadequate, or excessive air pressure leading to inaccurate measurement is avoided. For example, a larger nozzle diameter produces a broader, potentially less intense pulse, while a smaller nozzle diameter focuses the pulse, possibly increasing its intensity at a given pressure. A technical effect is that the air pulse generator is able to effectively generate the first and the second excitation air pulse which are sent towards the eye.

[0050] Throughout the present disclosure, the term "corneal oscillation measurement sensor" refers to a sensing device configured to sense and measure the corneal oscillations induced at the corneal surface by the first excitation air pulse and the second excitation air pulse. The corneal oscillation measurement sensor is also configured to transmit information corresponding to the corneal oscillation to the controller present in the tonometer. The corneal oscillation measurement sensor is configured to measure various characteristics of corneal oscillations observed on the corneal surface after the excitation air pulse impacts on the corneal surface. The phrase "various characteristics of the corneal oscillation" used herein refers to: an amplitude, a frequency, a damping factor, a duration, a phase, a value corresponding to pea k-to- peak amplitude, and other characteristics. The various characteristics of the corneal oscillation are dependent on the force of the excitation air pulse exerted on the corneal surface. As an example, the corneal oscillation measurement sensor is a sensor which measures displacement of corneal surface from its "normal, non-disturbed, level" as a function of time. This measurement of displacement can be analyzed to find, among others, the frequency of the oscillations. The measured signal can be analyzed for example by using fast Fourier transformation (FFT) as an example to find frequency component. In general displacement measurement as a function of time can be done to obtain information on corneal oscillations (and movements) during impact of air pulse and after the impact (post oscillation). This information can be used to calculate various parameters. As an example, intra ocular pressure is function of oscillation frequency after the impact.

[0051] Throughout the description, the term "controller" refers to a computational device that is operable for controlling the overall operation of the tonometer. The controller is configured to control the air pulse generator for adjusting the force exerted on the eye by the excitation air pulse directed towards the eye of the person, for accurate measurement of the IOP of the eye. In an example, the controller may be any one of an embedded microcontroller, a microprocessor, a field-programmable gate array (FPGA), digital signal processor (DSP), an application-specific integrated circuit (ASIC), a programmable logic circuits, an on-the chip control system, and any other suitable control module configured to receive input from the corneal oscillation measurement sensor pertaining to the corneal oscillation; to process information available in the received input; and to control the components of the air pulse generator such as the nozzle, the diameter of the exit aperture of the nozzle and the valve, thereby adjusting the force exerted on the eye by the excitation air pulse. The controller may be implemented as an internal component of the tonometer, an external component of the tonometer, or a combination thereof.

[0052] The controller is configured to open the controllable valve of the air pulse generator to release the excitation air pulse and close the controllable valve afterwards to allow a pressure equalization between the air pulse generator and an atmospheric air. In this regard, the controller can adjust the nozzle diameter, either automatically (based on feedback from the signal quality indicator) or manually (via a user interface). Herein, the nozzle diameter may be automatically decreased if the initial signal amplitude is too low or increased if the signal is too strong or causes discomfort. It may be appreciated that the adjustment can be fine-tuned in small increments. Moreover, the controller is configured to adjust the diameter of the exit aperture within an optimal diameter value, wherein the optimal diameter value ranges from 0.05 mm to 3.5 mm. The diameter can be for example the diameter can be, for example, from 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7,

[0053] 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4,

[0054] 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, 2.0, 2.05, 2.1,

[0055] 2.15, 2.2, 2.25, 2.3, 2.35, 2.4, 2.45, 2.5, 2.55, 2.6, 2.65, 2.7, 2.75, 2.8,

[0056] 2.85, 2.9, 2.95, 3.0, 3.05, 3.1, 3.15, 3.2, 3.25, 3.3, 3.35, 3.4, 3.45 mm up to 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7,

[0057] 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4,

[0058] 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, 2.0, 2.05, 2.1,

[0059] 2.15, 2.2, 2.25, 2.3, 2.35, 2.4, 2.45, 2.5, 2.55, 2.6, 2.65, 2.7, 2.75, 2.8,

[0060] 2.85, 2.9, 2.95, 3.0, 3.05, 3.1, 3.15, 3.2, 3.25, 3.3, 3.35, 3.4, 3.45 or 3.5 mm. Moreover, the controller is configured to utilize an adjustment mechanism for operating the controllable valve and adjusting the diameter of the exit aperture. Furthermore, the controller is configured to utilize a closed loop feedback mechanism to ensure that the operation of the controllable valve (i.e., opening and closing) and the adjustment made in the diameter of the exit aperture are performed accurately. Beneficially, by adjusting the nozzle size, the air pulse may be optimized for different eyes, conditions, or measurement requirements, thereby, improving the accuracy and comfort of the IOP measurement. For example, a smaller or larger nozzle may be chosen depending on the patient's corneal properties or the initial signal quality.

[0061] The first parameter values are configured by controlling the controllable valve and the diameter of the exit aperture of the air pulse generator. The adjustment of the diameter of exit aperture and the controllable valve may be performed by utilizing the adjustment mechanism integrated with the tonometer. The controller is configured to trigger the adjustment mechanism that controls the diameter of the exit aperture and functioning of the controllable valve, to set the first parameter values. The controller may utilize the closed loop feedback mechanism to ensure that the diameter of the exit aperture is adjusted correctly, and the opening as well as the closing of the controllable valve is swift and smooth, in order to control the amount of air released while directing each air pulse, the duration of release of the air pulse, and the pressure of air pulse.

[0062] The corneal oscillation measurement sensor is configured to measure the values of the various characteristics of the corneal oscillations and provide an input to the controller correspondingly. The corneal oscillation measurement sensor is positioned in proximity to the corneal surface, detects the movements and deformations of the corneal surface caused by the first excitation air pulse. The corneal oscillation measurement sensor may utilize various detection techniques, such as confocal chromatic sensor (CCS) , optical coherence tomography (OCT), or ultrasound, to capture and analyze the corneal oscillation. In general, a distance sensor with a resolution in micrometer range can be used.

[0063] Optionally, the diameter of the exit aperture is adjusted to a first value, and the pressure inside of the nozzle is set to generate an air pulse having a set force at a surface of an eye when in use. In this regard, prior to release of the first excitation air pulse, the controller is configured to adjust the exit aperture to the first value by modifying the diameter value of the exit aperture utilizing the adjustment mechanism. Moreover, by timing the opening and closing of the controllable valve alongside the adjustment of the diameter of the exit aperture, the controller is configured to set the pressure inside of the nozzle in order to generate an air pulse having a first force at the surface (corneal surface) of an eye when the tonometer is in use. A technical effect is that the tonometer is able to effectively generate the air impulse having the desired set force at the surface of the eye.

[0064] Optionally, the nozzle aperture is dimensioned to form an air pulse of a set size at the surface of the eye when in use. In this regard, the size of the air pulse (i.e., the first or the second excitation air pulse) is dependent on the diameter of the exit aperture; the larger the exit aperture the larger the size of the air pulse. The exit aperture is dimensioned by the controller utilizing the adjustment mechanism for adjusting the diameter of the exit aperture. In an implementation, the exit aperture can be dimensioned manually utilizing a user interface. In this regard, the controller is configured to provide a plurality of adjustment instructions at the user interface and track the adjustment performed while updating the plurality of adjustment instructions in real time. A technical effect is that the nozzle aperture is dimensioned effectively to form the air pulse of the desired set size at the surface of the eye.

[0065] Optionally, the corneal oscillation measurement sensor is one of: confocal chromatic sensor, distance sensor. Notably, the confocal chromatic sensor, and the distance sensor the suitable sensing devices capable of sensing the corneal oscillations which are of very small magnitude. It will be appreciated that the confocal chromatic sensor, and the distance sensor are well-known in the art. A technical effect is that the corneal oscillation measurement sensor is selected from sensors that are cost effective and easy to implement. Optionally, the first signal is a corneal oscillations profile as a function of time and the signal quality indicator is determined by comparing the profile with a reference profile. The term "reference profile" used herein refers to: a dataset of corneal oscillations obtained from a healthy population with normal ocular characteristics, a plurality of standards for corneal oscillations obtained from published literature and / or a plurality of clinical guidelines. In addition to medical history of the patient might be used as the reference profile. The reference profile may also be devised based on at least one of: ophthalmic calibration standards; known physical properties of the eye; a personalized baseline profile of the person undergoing the IOP measurement procedure, based on historical data or initial measurements; visual inspection or qualitative assessment by the healthcare professional, an ophthalmic technician or any other trained individual who is operating the tonometer to carry out measurement of IOP of the eye. The measured corneal oscillations profile indicates various characteristics of the corneal oscillation at the eye surface caused by the first excitation air pulse and measured by the corneal measurement sensor. The measured corneal oscillations profile is expressed as the function of time. The measured corneal oscillations profile is also expressed as a function of the signal quality indicator. The measured corneal oscillations profile is compared with the reference profile, thereby providing insight into the properties of the first excitation air pulse and if there is any requirement of adjusting the first parameter values of the air pulse generator before sending out the next excitation air pulse to measure the IOP of the eye. In this regard, the comparison of the corneal oscillations profile with the reference profile may be performed using statistical or mathematical similarity measures (such as amplitude, frequency, damping, shape, etc.) or by visual inspection by a healthcare provider or a clinician, to generate the signal quality indicator. Notably, a close match indicates a high-quality signal suitable for IOP determination. If not, the controller may prompt adjustments (e.g., change air pulse parameters) or flag the IOP measurement as unreliable. A technical effect is that the comparison of the corneal oscillations profile with the reference profile enables to enhance the effectiveness and quality of the determined signal quality indicator.

[0066] EXPERIMENTAL PART

[0067] Several corneal oscillations were measured using a tonometer having an air pulse generator to generate and direct an air pulse towards eye using different parameter values (pressure inside nozzle, diameter of exit aperture and time duration of release of the air pulse). The measurements were carried out on eye having known intraocular pressure value.

[0068] In one experiment, the parameters values are set at: air pulse pressure was kept at 1 bar (pressure inside nozzle was 2 bar including atmospheric pressure of lbar); diameter of exit aperture was kept at 0.3 millimeter; and time duration of release of the air pulse was kept at 5 millisecond (msec), known IOP of an eye used for the experiments was 20 millimeter of Hg (mmHg). A graph (as shown in FIG. 5A) was plotted illustrating movement of the corneal surface as measured with a displacement sensor. In the experiment a laser based displacement sensor was used (first signal related to corneal oscillations) where x-axis shows time in milliseconds (msec) and y-axis shows corneal displacement in micrometers.

[0069] As observed from the plotted graph, the corneal oscillation frequency had a small deviation indicating to good signal quality indicator. The oscillation amplitude and the signal decay factor are within a predetermined range. In addition, a ratio between a number of occurrences in which the amplitudes decreases between two consecutive peak to peak amplitudes in respect to a number of occurrences in which the amplitudes increases between two consecutive peak to peak amplitudes was inside the predetermined range. These good signal quality indicators provided evidence that the set parameter values (0.3mm diameter, 1 bar pressure inside the nozzle and duration of 5 msec) were found suitable for IOP measurement from the oscillation frequencies of the eye with known IOP of 20 mmHg.

[0070] In another experiment the parameter values were kept the same as in FIG. 5A, however, the known IOP of eye was 40 mmHg. As observed from the plotted graph in FIG. 5B, the corneal oscillation frequency had a large deviation, the oscillation amplitude signal is lower than in FIG. 5A and the signal decay factor was not within defined ranges. In addition, a ratio between a number of occurrences in which the amplitudes decreased between two consecutive peak to peak amplitudes in respect to a number of occurrences in which the amplitudes increased between two consecutive peak to peak amplitudes was not inside the predetermined range. Based on these signal quality indicators, the set parameter values (0.3mm diameter, 1 bar pressure inside the nozzle and duration of 5 msec) were deemed not to be suitable for IOP measurement from the oscillation frequencies of the eye with known IOP of 40 mmHg. This demonstrates that there is a need to modify parameters depending on target eye (patient or condition of the patient). Parameters which work for one IOP value were surprisingly found out not to work for another IOP values.

[0071] In other experiments illustrated in FIG. 5C and FIG. 5D, the pressure inside the nozzle was adjusted from 1 bar to 1.5 bar while keeping other parameter values same as the previous experiment i.e., diameter of exit aperture was 0.3 mm, and duration of air pulse was 5 msec. In FIG. 5C known IOP of eye was 20 mmHg and in FIG. 5D the known IOP of eye was 40 mmHg. Graphs (as shown in FIG. 5C and 5D) were plotted where the x-axis indicated the time in millisecond (msec) and the y-axis indicated the corneal displacement in micrometers. Based on the signal quality indicators, the set parameter values were deemed not to be suitable for IOP measurement. Yet in another experiment, the air nozzle diameter was adjusted from 0.3 mm to 0.6 mm, the air pulse duration was 5 milliseconds, and pressure of air pulse was 1.5 bar. The known IOP of eye was 20 mmHg. A graph (as shown in FIG. 5E) was plotted where the x-axis indicated the time in milliseconds and the y-axis indicated the corneal displacement. The oscillation amplitude was unnecessary high compared to FIG. 5A that can cause sensation in the eye and discomfort for the patient. Furthermore, the oscillation signal contained irregularities that do not represent good signal quality based on the signal quality indicators. Therefore, the exit aperture value of 0.6 mm was considered as not optimal for IOP measurement.

[0072] For a short duration and low-pressure air pulse, for example, with a duration less than 1 msec and pressure inside the nozzle less than 1 bar, no detectable oscillation was recorded.

[0073] Based on the experiments it was found out that sending a first excitation air pulse having first parameter values towards the eye (cornea) and measuring a first signal related to the corneal oscillations provides surprising insight i the selected parameter range for the measurement device (air impulse tonometer) is feasible or not. It was also found that adjusting the first parameters to second parameter values, based on the signal quality indicator, can improve the measurement signal to a level in which the intraocular pressure value of the eye can be measured accurately.

[0074] DETAILED DESCRIPTION OF THE DRAWINGS

[0075] Referring to FIG. 1, illustrated is a flowchart depicting steps of a method for measuring an intraocular pressure of an eye, in accordance with an embodiment of the present disclosure. At step 102, a first excitation air pulse having a first parameter values is sent towards the eye. At step 104, a first signal related to corneal oscillations caused by the sent first excitation air pulse is measured. At step 106, a signal quality indicator of the first signal is determined. At step 108, if the determined signal quality indicator is outside of a predetermined range, then the first parameter values are adjusted to second parameter values. At step 110, a second excitation air pulse having the second parameter values is sent towards the eye. At step 112, a second signal related to corneal oscillations caused by the sent second excitation air pulse is measured. At step 114, the intraocular pressure value is determined from the measured second signal.

[0076] Referring to FIG. 2, illustrated is a schematic illustration of a tonometer 200 for measuring an intraocular pressure of an eye using air pulse, in accordance with an embodiment of the present disclosure. As shown, the tonometer 200 comprise an air pulse generator 202, a corneal oscillation measurement sensor 204 and a controller 206 communicably coupled to the air pulse generator 202 and to the corneal oscillation measurement sensor 204, the controller 206 configured to set first parameters for the air pulse generator 202; initiate the air pulse generator 202 to generate a first excitation air pulse using the first parameters to oscillate a cornea of the eye; use the corneal oscillation measurement sensor 204 to measure a first signal related to the corneal oscillations; determine a signal quality indicator of the measured first signal; adjust the first parameters to second parameters, if the determined signal quality indicator is outside of a predetermined range; set the second parameters for the air pulse generator 202; initiate the air pulse generator 202 to generate a second excitation air pulse using the second parameters to oscillate the cornea of the eye; use the corneal oscillation measurement sensor 204 to measure a second signal related to the corneal oscillations; and determine the intraocular pressure value from the measured second signal. Optionally, the air pulse generator 202 comprises a nozzle 208 having an exit aperture 210, a pressure generation means 212 and a controllable valve 214. An excitation air pulse generated by the air pulse generator exits through the exit aperture 210 of the nozzle 208. Referring to FIG. 3, illustrated is schematic illustration of an environment in which a tonometer 300 is implemented, in accordance with an embodiment of the present disclosure. As shown, the tonometer 300 is positioned with respect to a head 302 of a person. The tonometer 300 is configured to send an excitation air pulse 304 towards a surface 306 of the eye 308.

[0077] Referring to FIG. 4, illustrated is a schematic illustration of operational flow of an air pulse generator 400, in accordance with an embodiment of the present disclosure. As shown, the air pulse generator 400 comprises a pressure generation means 402, a controllable valve 404, a nozzle 406 and an exit aperture 408. At step SI, the air pulse generator 400 is initiated while a controllable valve 404 is in closed position, thus a pressure inside the pressure generation means 402 is set at Pl and the pressure inside the nozzle 406 is at Patm '-e- atmospheric pressure. In such situation, no excitation air pulse passes through the nozzle 406 and the exit aperture 408 of a diameter D. At step S2, the controllable valve 404 is opened thus the pressure in the pressure generation means 402 and the nozzle 406 are set at Pl while an excitation air pulse 410 exits from the exit aperture 408. At step S3, the controllable valve 404 is closed. The pressure inside the pressure generation means 402 is set at Pl. The pressure in the nozzle 406 is now set a value P(t) which is a function of a duration t of release of the excitation air pulse 410. The pressure inside the nozzle 406 is to be neutralized to the atmospheric pressure Patm / ar|d this causes an additional volume of excitation air pulse 412 (which is a residual air left in nozzle 406 after release of the excitation air pulse 410) added to the excitation air pulse 410. At step S4, the neutralization of pressure inside the nozzle 406 is complete and the pressure value inside nozzle is set at Patmar|d the pressure in pressure generation means 402 is Pl. The excitation air pulse 410 which exits the exit aperture 408 is now a combination of additional volume of excitation air pulse 412 and the excitation air pulse 410, which are measured as function of duration t. The duration t comprises a release time tl of the excitation air pulse 410 and a trailing time t2. At step S5, a graphical illustration of a pressure intensity I of the excitation air pulse is plotted against the duration. It is observed from the graphical illustration that longer trailing time t2 results in longer application of pressure from the excitation air pulse.

[0078] Referring to FIGs. 5A-5E, illustrated are graphical illustrations of an amplitude of corneal oscillation plotted against time, in accordance with an embodiment of the present disclosure. As shown in FIGs. 5A-5E, a plurality of graphs are plotted for the amplitude of corneal oscillation (depicted on x-axis) against time (depicted on y-axis) while changing parameter values.

Claims

CLAIMS1. A method for measuring an intraocular pressure value of an eye (308), the method comprising: sending a first excitation air pulse having first parameter values towards the eye; measuring a first signal related to corneal oscillations caused by the sent first excitation air pulse; determining a signal quality indicator of the first signal; adjusting the first parameter values to second parameter values if, the determined signal quality indicator is outside of a predetermined range; sending a second excitation air pulse having the second parameter values towards the eye; measuring a second signal related to corneal oscillations caused by the sent second excitation air pulse; and determining the intraocular pressure value from the measured second signal.

2. A method according to claim 1, wherein the first parameter values of the first excitation air pulse comprise at least one of: a first force at a surface (306) of the eye (308); a first size of the first excitation air pulse at the surface of the eye; a first duration of the first excitation air pulse.

3. A method according to claim 1 or 2, wherein the second parameter values of the second excitation air pulse comprise at least one of: a second force at a surface (306) of the eye (308); a second size of the second excitation air pulse at the surface of the eye; a second duration of the second excitation air pulse.

4. A method according to any of the preceding claims, wherein the step of determining the signal quality indicator comprises: calculating a deviation of an oscillation frequency of the corneal oscillations caused by the sent first excitation air pulse from the measured first signal; and using the deviation as the indicator of the signal quality.

5. A method according to any of the preceding claims, wherein the step of determining the signal quality indicator comprises: fitting parameters of a physiological model of the eye (308) to the first signal; calculating a confidence level of the parameters; and using the confidence level as the indicator of the signal quality.

6. A method according to any of the preceding claims, wherein the step of determining the signal quality indicator comprises: determining peak to peak amplitudes of plurality of oscillations; comparing consecutive peak to peak amplitude to each others; calculating a ratio between a number of occurrences in which the amplitudes decrease between two consecutive peak to peak amplitudes in respect to a number of occurrences in which the amplitudes increase between two consecutive peak to peak amplitudes; and using the ratio as the indicator of the signal quality.

7. A method according to any of the preceding claims, wherein the step of determining the signal quality indicator comprises determining amplitude of the first oscillation and using the amplitude as indicator of the signal quality.

8. A method according to any of the preceding claims, wherein the step of determining the signal quality indicator comprises determining a decay factor of the corneal oscillations and using the decay factor as the indicator of the signal quality.

9. A tonometer (200, 300) for measuring intraocular pressure of an eye (308), the tonometer comprising: an air pulse generator (202, 400); a corneal oscillation measurement sensor (204); a controller (206) communicably coupled to the air pulse generator and to the corneal oscillation measurement sensor, the controller configured to: set first parameters for the air pulse generator; initiate the air pulse generator to generate a first excitation air pulse using the first parameters to oscillate a cornea of the eye; use the corneal oscillation measurement sensor to measure a first signal related to the corneal oscillations; determine a signal quality indicator of the measured first signal; adjust the first parameters to second parameters, if the determined signal quality indicator is outside of a predetermined range; set the second parameters for the air pulse generator; initiate the air pulse generator to generate a second excitation air pulse using the second parameters to oscillate the cornea of the eye; use the corneal oscillation measurement sensor to measure a second signal related to the corneal oscillations; and determine the intraocular pressure value from the measured second signal.

10. A tonometer (200, 300) according to claim 9, wherein the air pulse generator (202, 400) comprises: a nozzle (208, 406) having an exit aperture (210, 408); a pressure generation means (212, 402) configured to generate a pressure inside of the nozzle; anda controllable valve (214, 404) configured to release air from the exit aperture; wherein the first and the second parameters comprise a diameter of the exit aperture, a pressure inside of the nozzle and a time duration of releasing air from the exit aperture.

11. A tonometer (200, 300) according to claim 10, wherein the diameter of the exit aperture (210, 408) is adjusted to a first value, and the pressure inside of the nozzle (208, 406) is set to generate an air impulse having a set force at a surface (306) of an eye (308) when in use.

12. A tonometer (200, 300) according to claim 10 or 11, wherein the nozzle (208, 406) aperture is dimensioned to form an air impulse of a set size at the surface (306) of the eye (308) when in use.

13. A tonometer (200, 300) according to any of the claims 9-12, wherein the corneal oscillation measurement sensor (204) is one of: a confocal chromatic sensor, a distance sensor.

14. A tonometer (200, 300) according to any of the claims 9-13, wherein the first signal is a corneal oscillations profile as a function of time and the signal quality indicator is determined by comparing the corneal oscillations profile with a reference profile.

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