HK1252243B - Method and arrangement for eye measurements - Google Patents

Description

Field of the invention

Intraocular pressure (IOP) plays a major role in the pathogenesis of open angle glaucoma, one of the leading causes of blindness. There are about 150 million people with glaucoma globally, about half of which are unknowingly affected and without diagnosis. The prevalence of glaucoma increases with aging of the human population and it is expected that this will increase by 30% the number of glaucoma cases during the next decade. The only way to currently treat glaucoma is by lowering the intraocular pressure (IOP).

An IOP measurement is the most practical way of screening open angle glaucoma. However, screening large parts of the population is needed to find undiagnosed cases.

The other type of glaucoma is the narrow angle glaucoma that causes a sudden IOP increase that may cause blindness in a few days. Since one per mille of the population is affected with acute narrow angle closure glaucoma, it is mandatory to diagnose acute glaucoma by measuring IOP in community emergency departments of general medicine. Consequently it would be beneficial if every doctor's office would have means to measure IOP.

State of the art

Contact methods (e.g. Goldmann tonometry, Mackay-Marg tonometry) for measuring IOP mostly require use of an anesthetic to carry out the measurement and are thus impractical e.g. for screening large human populations. Also non contact air impulse tonometer has been on the market for decades. The drawback of air impulse tonometers is patient discomfort experienced due to the air impulse.

The US patent application document US 2010/0249569 A1 presents a non-contact ultrasonic tonometer for IOP measurements, which employs piezo-electric transducers to excite wave signals into the eye. The positions of said transducers have to be exactly measured, which makes the IOP measurement procedure complex and slow. Also temperature variations cause error and uncertainty in the IOP measurement information together with possible errors in said position measurements. The eye shape also introduces bias, i.e. error, into the measurement.

The patent document US 6030343 A presents a method that is based on an airborne ultrasonic beam that is reflected from the cornea - the same beam measures and actuates the eye. The actuation is done by a narrow band ultrasonic tone burst, which deforms the cornea, and the system measures the phase shift from the deformed eye.

The patent documents US2004193033 and US5251627 describe a non-contact measurement method by means of linear excitation (e.g. loudspeakers or ultrasonic transducers).

The prior art solutions suffer from difficulties to achieve a convenient and low-cost device for measuring IOP precisely and comfortably for the patient by non-contact measurements.

The patent document US5148807 A describes a non-contact, non-invasive tonometer utilizing a technique of angle modulation of high frequency sound waves or high frequency light waves to determine intraocular pressure of an eye. The angle modulation technique can be either frequency modulation or phase modulation. US5148807 discloses an intraocular pressure measurement arrangement according to the preamble of independent claim 1 and an intraocular pressure measurement method according to the preamble of independent claim 9.

The US patent application document US2012277569 A1 describes a method for measuring bio-medical attributes of an eye, such as internal or intraocular pressure. The method comprises applying a pressure disturbance to the eye acoustically and using non-invasive optical techniques to perform measurements of vibrations or measurements of time varying relative location of one or more surfaces or structures of the eye in a manner correlated with the pressure disturbance.

Short description of the invention

The object of the present invention, which is defined in the appended claims, is to achieve a contactless, fast and advanced device and method to measure IOP without need for anaesthetics. An object of the invention is to achieve an IOP reading that is both precise, i.e. unbiased and features small uncertainty in the IOP estimate. This is achieved by an intraocular pressure measurement arrangement for measuring pressure of an eye of a patient. The arrangement comprises means for detecting at least one of acoustic reflectivity, optical reflectivity, optical path difference, positioning of intraocular pressure measurement arrangement with respect to the eye, orientation of intraocular pressure measurement arrangement with respect to the eye, shape of cornea and corneal thickness, at least one source for producing an excitation pressure pulse which is in the form of a nonlinear acoustic wave or a nonlinear mechanical wave and transmitted by air to the eye to generate at least one surface wave to the eye, means for triggering data acquisition of the pressure measurement arrangement, means for detecting at least one surface wave from a distance from the eye to extract surface wave information, and means for determining the pressure of the eye based on said surface wave information.

The invention is based on detection at least one of acoustic reflectivity, optical reflectivity or optical path difference, positioning of intraocular pressure measurement arrangement with respect to the eye, orientation of intraocular pressure measurement arrangement with respect to the eye, shape of cornea and corneal thickness, and on production of the nonlinear acoustic wave or the nonlinear mechanical wave from a distance coupling to the eye of the patient to generate at least one surface wave to the eye, and on triggering of data acquisition of the eye pressure measurement arrangement.

The invention enables patient and user friendly use with no need to touch sensitive surfaces of an eye together with advanced methods to process measurement information in order to extract quantitative pressure information of the eye. One benefit is that the invention can be utilized from one patient to another with less risk for contamination as contact to the eye is avoided.

In accordance with the invention, there are provided a new intraocular pressure measurement arrangement and a new intraocular pressure measurement method as defined in the appended independent claims.

Short description of figures

Figure 1

presents a preferred embodiment of the measurement arrangement with solenoid-driven impacting device capable of non-linear wave excitation and optical pickup means.

Figure 2

presents an exemplary embodiment of the measurement arrangement with a spring-loaded impacting device capable of non-linear excitation and optical pickup means.

Figure 3a-b

present a device capable of non-linear wave excitation based on rotating rods or strips impacting against a target / targets

Figure 4

presents means for non-linear excitation based on a whiplash effect generating a non-linear wave in air.

Figure 5.

Presents laser based means for non-linear wave excitation.

Figure 6.

Presents arrangements to control propagation of excited wave.

Figure 7.

Presents a low power wide beam measuring arrangement

Figure 8.

Presents examples of different photodetector arrays.

Figure 9.

Presents wide beam arrangement's capability to measure corneal shape and/or curvature.

Figure 10.

Presents the different path and angle of reflected lightbeam from front and rear surface of cornea, which can be used in determining the thickness of cornea.

Figure 11.

Presents means for detecting the position and orientation of the measurement arrangement.

Figure 12.

Presents an example of a signal obtained by one photodetector element in array, used in determining corneal thickness.

Detailed description of the invention

The invention is based on excitation of acoustic waves into air, which then couple to the eye of the patient and generate linear or non-linear waves that travel on the surface of the eye. At least one of time-of-flight, speed of sound, attenuation, frequency content, dispersion of these waves or surface motion is then detected via a single or multiple detector(s). The IOP is subsequently determined from these parameters.

The generation of acoustic waves in air can be done in a multitude of ways, most of which involve either a chemical explosion, plasma burst (either mechanically or electrically or optically generated) or a mechanical impact of two surfaces. The combining factor in all of these is the need for energy: all the methods require large amounts of energy to be released in a short timeframe within a small volume of material. To be commercially viable, this energy storage needs to be safe to the patient and easy to generate by the operator.

In the embodiments according to the present invention is presented non-contacting photoacoustic and ultrasonic intraocular pressure (IOP) measurement techniques, which may have e.g. following requirements: non-contact excitation and detection methods, which are safe for the patient, determination of essentially accurate intraocular pressure (IOP) values, possibility to follow-up of patient's IOP values, and said techniques can be used by a health care professional and/or by a patient and/or a third person in a convenient and ergonomic way with lowered risk for contamination from patient to patient.

Figure 1 presents the preferred exemplary embodiment according to the present invention the excitation system for exciting non-linear waves into air comprises of either one or several solenoids 100a and a weight 101a (e.g. metallic and is magnetized or non-magnetized) made out of a magnetic material with rod 104 attached to the weight. The solenoids 100a are used to move the weight-rod system (101a, 104) within the cavity of the solenoid. This rod then impacts on a surface (impacting target 105a) that can be either hard or soft, and the impact between the rod and the surface generates a non-linear wave into air from the other side of the impacting target, i.e. sound emitter 105b The tip of the rod is optimally shaped(conical, corrugated, grooved, patterned, mathematical function). 105a is a target made out of a material (man-made or natural, hard or soft) with optional surface shaping (conical, corrugated, grooved, patterned, mathematical function) at sound emitter 105b. The solenoids may be timed with a microcontroller, and the timing of the solenoids may be used to adjust the strength and duration of the impact and subsequent non-linear wave generation. The solenoids may be driven with a negative current to allow pullback of the rod-weight system and an inner wall 106 may be used to electromagnetically hold the weight-rod system in place when the device is not in operation to facilitate free orientation of the measurement device. An alternative way would be to use the gravity field to hold the weight-rod system. 106 may mechanically insulate the two compartments.

A second set of one or several solenoids 100b may be used to drive a counterweight 101b in opposite direction with the primary weight-rod system to reduce the recoil felt by the operator. This also minimizes the mechanical crosstalk between the excitation and pick-up parts of the system. Safety walls 102, 103 may be used to prevent weights 101a, 101b from exiting the device. Wall 102 is preferably a metallic enclosure with a hole for the rod.

The sound emitter 105b emits an acoustic wave that couples to the eye 109. The wave propagating on the interfaces of the eye is picked up by a vibrometer 107 and an optical beam 108.

The receiving electronics may be triggered by a piezoelectric transducer that can either be attached to the casing of the impactor (102) or that may detect the non-linear wave from afar.

Figure 2 presents one exemplary embodiment according to the present invention. In this embodiment the spring-driven non-linear wave excitation system comprises a rigid structure 207 attached to a spring 200, which moves the weight 201 impacting against a target 202. Sound emitter 203 emits an acoustic wave that is coupled to the eye 206. The wave propagating on the interfaces of the eye is picked up by vibrometer or optical path difference measuring device 205 and optical beam 204.

A rod may be inserted through the weight to allow the operator to arm the device. This weight-spring system may be placed inside a metal casing with a groove and several stopping grooves milled into it. These stopping grooves allow different controlled energy levels to be stored into the spring, and subsequently allow modifying the parameters of the non-linear excitation at release of the stored energy.

Figure 3a presents one exemplary embodiment according to the present invention. The acoustic wave excitation system comprises of a wheel 300 that rotates with rods or strips 301a attached to it near the eye 305. In one embodiment, these rods or strips 301a are made out of an elastic material and may be armed by rotating them against an arming target 302 made out of a hard material: the rods or strips bend 301b, thus storing energy into them. Once the rods or strips have bent enough, the energy is released by the rod or strip rapidly straightening and impacting against an impacting target 304. The non-linear wave is generated in this impact and emitted from sound emitter 303. The parameters of the non-linear excitation may be altered by adjusting the length and shape of the rods or strips, spacing of the rods or strips, the material of the rods or strips and the rotation speed of the wheel.

Figure 3b presents another embodiment in which the wheel 300 rotates with a high angular velocity and rigid rods or strips 301a impact directly against an impacting target 304 and sound is emitted from sound emitter 303 towards the eye 305. In this embodiment, the parameters of the shock wave excitation may be altered by adjusting the rotation speed of the wheel and material of the rods or strip and the target.

Figure 4 presents one exemplary embodiment according to the present invention. In this embodiment the non-linear wave excitation system, located near eye 404, comprises of a weight 400 driven by solenoids with an elastic string 402 attached to it (step 1). The weight is rapidly driven against a target 401 (step 2), and the elastic recoil of the weight from the target rapidly changes the movement direction of the weight (step 3). Alternatively a rapid switch in solenoid current reverses the momentum of the weight. This change in the sign of the momentum of the weight causes the elastic string to change its momentum a short amount of time later, resulting in a whiplash-like motion and generation of a non-linear wave in air near the string tip 403. The parameters of the non-linear wave excitation can be altered by adjusting the length or dimensions or material of the string and the movement velocity as a function of time (velocity profile) of the weight.

Figure 5 presents one exemplary embodiment according to the present invention in which the laser system 501 generates a laser beam 502 focused by optics 503 onto a point 504 on an optically absorbing target 505. Thermoelastic expansion or ablation of absorbing material 505 generates sound waves 507 that are emitted from the sound emitter 506. Optionally the optics 503 are not required. The preferred embodiment has walls 508 that prevent light or sound harmful to the patient from escaping the system. Optionally the laser beam can be focused to a point 509 inside material (gas, liquid or solid) to obtain optical breakdown (plasma generation) and generation of sound waves 507. The sound is transmitted through a thin solid membrane or a thin porous membrane or plate 510 that acoustically couples the gas, liquid or solid to the air outside the device.

Figure 6 presents one exemplary embodiment according to the present invention in which the generated non-linear wavefront 600 may be shaped (604a-604d) with an acoustically reflecting, absorbing, time-delaying, or dispersion removing material 603 with one more many pinholes 602a-602d or slits 602e-602g placed in front of the propagating wavefront. These pinholes or slits may be circular or arbitrary shaped. The pinholes may also converge 602d or diverge towards the eye. The pinholes allow for point-like excitation 602a on the eye surface, and also allow for time-delayed excitation 602b, 602c, and thus shaping the mode propagating on the surface of the eye. Time-delayed excitation or a confocal arrangement of slits 602f, 602g can be used for natural focusing or dispersion removal of the propagating waves into the point of optical detection 601 of the wave on the eye. In addition, this can be used to reduce the effects of interfering modes. The pinhole system in addition eliminates the crosstalk between the excitation and the pickup arising from the non-linear wave propagating in air between the sample and the pickup.

In one exemplary embodiment according to the present invention the patient may be soothed with a psychologically designed excitation sound pattern in combination with optional soothing light, images or music. This increases the repeatability of the measurement by reducing the stress of the patient and resulting eye movement from the measurement situation.

Figure 7 presents one of the exemplary embodiment according to the present invention for detecting corneal surface waves and adjusting tonometer positioning for measurement is using a wide (eg 2-8mm) output beam of coherent light directed to the cornea and detecting reflected light at an angle. If the beam is directed from the front of the eye, the reflection is measured from one or more positions at different angle. If the beam is directed obliquely to the eye the reflected light is measured from the opposite side. The beam can also be directed and detected from several directions.

The light source can comprise a laser 700 or lasers 700, 701 or led lights 700, 701 or superluminescent diodes 700, 701 of one or more different wavelengths, wherein beamsplitter 702 or beamsplitters (plate or cubical) direct the beam(s) 704 to the receiving optics. After collimation optics 703 comprised of positive or negative lenses, the light beam propagating towards the cornea can be modified with e.g. beam expander optics 705 comprising of positive or negative lenses in Kepler or Galilean configuration. The light is directed through a diffraction grating (holographic or grooved), etalon or both 706 to form an interference pattern on the surface of the cornea 707. An intraocular pressure measurement arrangement can have as a means for detecting e.g. a receiver comprising at least three photodetectors in the array 706 and optics (lense or lenses or aspherical) 708 focusing onto or close to the surface of the cornea. Each photo-detector can have its own lens or aperture to enhance the signal and to reduce noise. Exemplary photodetector arrangements 800, 801, 802 are presented in figure 8. An excitation pressure pulse generated by at least one source (e.g. one or more sources presented in Figures 1-5) is transmitted by air to the cornea, generating a surface wave which changes the interference pattern on the corneal surface and the surface wave slightly deflects the cornea forming a local higher-intensity reflection, which both can be observed by the receiver.

In the exemplary embodiment according to the present invention can be used in tonometer positioning, i.e. right orientation and measuring distance, by having a light source 700, 701 and receiver 709 with known angles focused on a certain position. The light source is directed towards the cornea 707 and receiver is in position to detect the reflected beam from the cornea. When the light source 700, 701 and receiver 709 are positioned in such way that the center of the detector array 709 records maximum intensity, the tonometer is correctly positioned in distance and orientation. In one embodiment, the photodetector array consists of only three photodetectors 802. Here, the correct position is achieved when all photodetectors have the same or nearly the same signal intensity. Vertical positioning can be achieved by an accelometer or several accelometers aimed along different axis. A gyroscope can be used in detecting circular movement of the tonometer. These sensors can also be used in detecting tonometer movement changes (circular or directional). The measurement can be started when the tonometer is in right position and not moving or when the movement is minimal. A tonometer software can instruct the user to position the tonometer correctly by for example using arrow symbols in a display.

Figure 9 presents the exemplary embodiment according to the present invention where corneal curvature 901, 902 can be evaluated according to how the central and lateral photodiode signal 906 intensity differ from each other, and in case of at least three detectors, from the total signal intensity of reflected light from the cornea. If the corneal radius is small and the corneal curvature is steep, the reflected beam 904, 905 expands more and received signals of central photodetectors differ more from received signals of peripheral photodetectors in the detector array than in the case of larger corneal radius with less curvature.

The measurement system measures the surface disturbances of the corneal surface and surface waves. The receiver has detector array and the detected waves arrive at different time to each of the detector elements. Thus, the surface-wave velocity can be calculated.

Figure 10 presents measurement of corneal thickness. Optics (1000) guide the beam of light to the corneal surface (1001). The reflected light (1001) from the front surface (1005) of the cornea differs in angle from the bundle of the light reflected (1004) from the back of the corneal surface (1003), because of the corneal thickness and the inner portion is steeper in the curvature than the outside surface. Also the difference in refractive index bends the light. Detection optics (1007) comprising of lenses of spherical or aspherical type (1007) guide the light to the detectors. The difference in the reflected angles (1002, 1004) depends of the corneal thickness and is detected by the photodetectors. Thickness may vary with the position along the cornea (1008) and can be measured by means of multiple detectors described before or by moving the detection system along the cornea. This allows detection of possible stress fields in the cornea.

The reflected light from the front surface of the cornea differs in angle from the bundle of the light reflected from the back of the corneal surface, because of the corneal thickness and the inner portion is steeper in the curvature than the outside surface.

When the corneal surface wave moves, photo-detector elements in the array of the receiver receive the rays reflected from the outside and inside surface of the cornea at different times. If the wave is moving from the receiver to the light source, the light ray reflected from corneal outside surface arrives first and after that the light rays reflected from the rear surface of the cornea. Otherwise, on the contrary, a rear portion of the reflected wave is first detected.

When corneal curvature and surface wave velocity are known the corneal thickness can be calculated based on the measurement data. If more than one wavelength is used in measurement, the accuracy can be improved. Different wavelengths have different properties of refraction. Refraction is the change in direction of propagation of a wave due to a change in its transmission medium. The medium changes when entering cornea, is different inside the cornea and exiting the cornea causing the different wavelengths to differ in corneal rear surface reflected light.

Use of more than one wavelength coherent light beams which has different refracting properties it is possible to measure the disparity between the surface and the rear wall arrival of the different wavelength beams. In this way more parameters are obtained in solving the equation central to calculating the corneal thickness. Thus, the measurement accuracy is improved.

In embodiments according to the present invention the generated non-linear wavefront can be shaped by a pinhole or a wedge or a patterned surface or a waveguide to allow more localized and coherent linear wave excitation on the eye surface. The pinholes can be shaped to allow larger or smaller cones or other topological shapes (e.g. exponential horns) of non-linear waves to pass through. In addition, the pinholes may be shaped in an arc-like pattern which allows natural focusing of the waves on the eye surface. To reduce the mechanical cross-talk between the excitation and the pickup, the excitation part may be suspended in air with impact absorbing springs or acoustic damping material, e.g. foam or rubber. For positioning of the measuring head, a holographic diffraction grating is used to project a rectangular grid on the eye surface. A camera mounted on the measuring head images the reflected image of the grid. Based on the distortion of the image of the grid, corneal surface curvature and position relative to the eye (distance, angular tilt) are calculated. Based on the position data, the measurement head may be moved with a linear stage (piezo or a linear motor) for more precise orientation of the measurement head and to compensate for movement of the operator's hand or the subject's eye.

Figure 11 presents means to detect the measurement arrangement position and orientation. Laser light source 1100 of visible or infrared wavelength incorporating a a holographic grid projects 1101a an image of a grid 1101b on the surface of the cornea 1103. The grid is detected with a camera 1102 focused on the surface of the eye. The grid may consist of any pattern of regular geometric shapes, preferrably squares. Depending on the position of the measurement arrangement with respect to the eye, the projected holographic grid is distorted 1101c, and from the distortion, the position and orientation of the measurement arrangement can be calculated.

Figure 12 presents a preliminary set of data measured with an exemplary arrangement presented in figure 10. Two distinct features can be seen: A first arrival reflected from corneal front surface 1200 followed by a smaller wavecrest 1201 which arise from the bottom surface of the cornea.

An intraocular pressure measurement arrangement according to the present invention for measuring pressure of an eye 202 of a patient comprises means for detecting one or more of following: acoustic reflectivity, optical reflectivity, optical path difference, positioning of intraocular pressure measurement arrangement with respect to the eye, orientation of intraocular pressure measurement arrangement with respect to the eye, shape of cornea and corneal thickness. The arrangement comprises at least one source for producing an excitation pressure pulse which is in the form of a nonlinear acoustic wave or a nonlinear mechanical wave and transmitted by air to the eye 202 of the patient to generate at least one surface wave to the eye. The measurement arrangement according to the present invention further comprises means for triggering data acquisition of the pressure measurement arrangement. At least one surface wave is detected from a distance 201 from the eye 202 by means for detecting to extract surface wave information, and pressure information of the eye is determined based on said surface wave information by means for determining pressure information of the eye.

In different kind of embodiments according to the present invention the measurement arrangement can comprise one or more of the following: means for linear wave pick-up from the eye 202, means for shaping non-linear waves, means for low power wide beam measuring, means for obtaining corneal curvature information, means for obtaining corneal thickness information, and means for determining location and orientation of the measurement arrangement and curvature of the cornea.

In one further embodiment according to the present invention the measurement arrangement can comprise means for triggering in order to calm the patient to reduce measurement spread, and to increase compliance and referrals to other people. Also light and images can be utilized in order to calm the patient for one or more of said purposes.

Next is provided a more detailed description of the presented figures 1-12. In figure 1 is presented means for non-linear wave generation to the eye 202 and linear wave pick-up from the eye 202, said means comprising: (100) coils that drive the impacting device, (101a) metallic (magnetized or non-magnetized) weight with a (104) rod or strip with optional surface shaping (conical, corrugated, grooved, patterned, mathematical function) attached with a (101b) counterweight to reduce the recoil felt by the operator from the excitation. (102) a metallic enclosure with a hole for the rod, with a (103) back wall and an mechanically insulating (106) inner wall separating the two compartments. (105a) is a target made out of a material (man-made or natural, hard or soft) with an (105b) optional surface shaping (conical, corrugated, grooved, patterned, mathematical function). (107) is an optical means transmitting and receiving a laser or light beam (108) for surface wave pick-up from the (109) eye.

In figure 2 is presented another means for non-linear wave generation to the eye 202 and linear wave pickup from the eye 202, said means comprising: A (200) spring is attached to a solid frame (207) and an (201) impacting mass with either a flat or shaped (conical, corrugated, grooved, patterned) surface. This impacting mass hits the (202) solid target. The (203) solid target surface can be shaped (conical, corrugated, grooved, patterned, mathematical function). The generated non-linear wave travels then through air to the surface of the (206) eye, from where it is picked up by (205) optical means transmitting and receiving a laser or a light beam (204).

In figure 3 is presented another means for non-linear wave generation to the eye 202, said means comprising: A (300) a wheel with (301a) protruding elastic or stiff rods or strips attached to it rotates. The (301a) rods or strips may be either smooth or their (304) surface can be shaped (corrugated, grooved, patterned, mathematical function). When rotating, the wheel makes tension in the rods which press against (302) an arming target (Figure 3a), which makes the rods or strips gather potential energy in the form of spring tension (301b). The rod or strip is then released due to the wheel rotation, causing an impact against the (304) impacting target, whose surface may be flat or patterned. The non-linear wave generated then travels to the eye (305). In one alternative embodiment (Figure 3b) of the invention, the (300) wheel rotates at a high angular velocity, and the (301a) rods or strips impact directly against the (304) impacting target, whose surface (303) can be shaped.

In figure 4 is presented another means for non-linear wave generation to the eye, said means comprising: A weight 400, displaced by driving means comprising either of a solenoid or a linear motor or by gravitational means or by pneumatic pressure or by an chemical reaction, hits against an elastic impacting target (401). After the impact, an elastic string (402) lags behind due to Newton's 2nd law and generates a whip-like motion (403), which causes localized supersonic displacement and a subsequent non-linear wave which then couples to the eye (404).

In figure 5 is presented another means for non-linear wave generation, said means comprising: A laser light source (501) transmits a laser beam (502) through focusing optics (503). In one embodiment of the invention, the laser beam is focused (504) on the surface of a target (505), made out of natural or synthetic materials, preferrably metal. The other surface of the target (506) emits a non-linear wave (507). In another embodiment, the laser beam (502) is focused in a spot in the medium (air, water, solid) it travels in (509), and generates an optical breakdown (plasma) which transmits the generated non-linear wave (507) through a membrane (510).

Figure 6 presents one exemplary embodiment according to the present invention in which the generated non-linear wavefront 600 can be shaped (604a-604d) by using means for shaping non-linear waves, said means comprising: an acoustically reflecting, absorbing, time-delaying, and/or dispersion removing material 603 with one more many pinholes 602a-602d and/or slits 602e-602g placed in front of the propagating wavefront. These pinholes or slits may be circular or arbitrary shaped. The pinholes may also converge 602d or diverge towards the eye. The pinholes allow for point-like excitation 602a on the eye surface, and also allow for time-delayed excitation 602b, 602c, and thus shaping the mode propagating on the surface of the eye. Time-delayed excitation or a confocal arrangement of slits 602f, 602g can be used for natural focusing or dispersion removal of the propagating waves into the point of optical detection 601 of the wave on the eye. In addition, this can be used to reduce the effects of interfering modes. The pinhole system in addition eliminates the crosstalk between the excitation and the pickup arising from the non-linear wave propagating in air between the sample and the pickup.

In figure 7 is presented means for low power wide beam measuring, said means comprising laser or lasers of one or more different wavelengths and/or led lights (700, 701) of one or more different wavelengths, optics and beamsplitters (702) in order to direct the beams (704) to the optics. The optics can comprise a collimator (703) and beam expanding optics (705). In one embodiment of the invention, an etalon or a diffraction grating (706) is used to form an interference pattern on the surface of the cornea (707). The reflected light is then collected through receiving optics (708) and recorded with a photodiode array (709). Each photodiode may have its own lens or aperture to improve the signal-to-noise ratio.

In figure 8 is presented an example of a photodetector array configuration consisting of 15 elements (800), 11 elements (801) and 3 elements (802). Photodiodes may be positioned in a rectangular, spherical or irregular grid pattern.

In figure 9 is presented means for obtaining corneal curvature information by evaluating the corneal curvature e.g. according to how the central and lateral photodetector signal intensity differ from each other, and in case of three detectors or also more, the total signal intensity of reflected light from cornea. A light source (900) emits a beam of light (903) which reflects from the surface of the cornea. If the corneal radius is small (902) and the corneal curvature is steep, the reflected beam expands more (904) and central photodetectors received signals (905) differ more from peripheral photodetectors in the detectorarray (906) than in the case of larger corneal radius (901) with less curvature.

In figure 10 is presented means for obtaining the corneal thickness information and/or information on waves travelling on the cornea. The reflected light from the front surface of the cornea (1002) travels through the cornea and differs in angle from the bundle of the light reflected from the back of the corneal surface (1003), because the corneal thickness and the inner portion (1006) is smaller in curvature than the outside surface (1005). A photodetector array (1007) detects the location of the reflected beam. The differences in the corneal thickness (1008) as a function of location can be inferred from the angle if the corneal shape is known.

In figure 11 is presented means for determining the location and the orientation of the measurement arrangement and the curvature of the cornea, said means comprising: a light source (1100) incorporating a holographic diffraction grating projects (1101a) a rectangular grid (1101b) on the surface of the cornea (1103). This projected light (1104) is reflected (1105) from the surface and is detected with a camera (1102). From the distorted image of the grid (1101c), the curvature of the cornea is calculated if the projector (1100) location and orientation is known. The distance between the projector (1100) and the cornea surface (1103) may be determined from the apparent size of the grid.

In Figure 12 is presented a sample of the signal obtained. Activation (e.g. impactor / spark) excitation is created, which arrives to the cornea forming a surface wave. The corneal surface waves are detected by photo-detectors, which can measure the surface disturbances of the corneal surface and surface waves. The receiver has detector array and the detected waves have arrived at different time to each of the detector elements. Thus, the surface-wave velocity can be calculated. Light bundle reflected from the outside surface (1201) of the moving surface wave arrives first to the detector element and after that the light bundle reflected from corneal inside surface (1202). The beams reflected from the rear of the cornea are of lower intensity than reflection from front surface.

On the basis of the present invention can be implemented an ideal tonometer capable of measuring intraocular pressure with fast comfortable measurements without anesthetic and disposable waste operated also by an unskilled operator.

Although the invention has been presented in reference to the attached figures and specification, the invention is by no means limited to those, as the invention is subject to variations within the scope allowed for by the claims.

Claims (14)

1. An intraocular pressure measurement arrangement for measuring pressure of an eye (202), wherein the arrangement comprises:

   - means configured to detect at least one of acoustic reflectivity, optical reflectivity, optical path difference, positioning of intraocular pressure measurement arrangement with respect to the eye, orientation of intraocular pressure measurement arrangement with respect to the eye, shape of cornea and corneal thickness,

   - at least one source configured to produce an excitation pressure pulse which is transmitted by air to the eye (202) to generate at least one surface wave to the eye,

   - means configured to trigger data acquisition of the pressure measurement arrangement,

   - means configured to detect the at least one surface wave from a distance (201) from the eye (202) to extract surface wave information, and

   - means configured to determine the pressure of the eye based on said surface wave informatior,

   characterised in that the at least one source for producing an excitation pressure pulse is configured to form an excitation pressure pulse which is in the form of a nonlinear acoustic wave or is a nonlinear mechanical wave.
2. An intraocular pressure measurement arrangement according to claim 1, wherein the means configured to detect the at least one surface wave comprise a detector array and means configured to determine velocity of the surface wave based on times at which a light bundle reflected from an outside surface (1201) of the moving surface wave arrives at detector elements of the detector array.
3. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to perform linear wave pick-up from the eye (202).
4. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to shape the nonlinear waves.
5. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to perform low power wide beam measuring.
6. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to obtain corneal curvature information.
7. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to obtain corneal thickness information.
8. An intraocular pressure measurement arrangement according to claim 1, wherein the arrangement comprises means configured to determine location and orientation of the measurement arrangement and curvature of the cornea.
9. An intraocular pressure measurement method for measuring pressure of an eye (20) using an intraocular pressure measurement arrangement, wherein the method comprises:

   - detecting at least one of: acoustic reflectivity, optical reflectivity, optical path difference, positioning of intraocular pressure measurement arrangement with respect to the eye, orientation of intraocular pressure measurement arrangement with respect to the eye, a shape of cornea, and a corneal thickness,

   - producing an excitation pressure pulse and transmitting said pulse by air to the eye (202) to generate at least one surface wave to the eye,

   - triggering data acquisition of the pressure measurement arrangement,

   - detecting at least one surface wave from a distance (201) from the eye (202) to extract surface wave information, and

   - determining the pressure of the eye based on said surface wave information,

   characterised in that in the step of producing an excitation pressure pulse the excitation pressure pulse which is formed is in the form of a nonlinear acoustic wave or a nonlinear mechanical wave.
10. An intraocular pressure measurement method according to claim 9, wherein the method comprises determining times at which a light bundle reflected from an outside surface (1201) of the moving surface wave arrives at detector elements of a detector array, and determining velocity of the surface wave based on the determined times.
11. An intraocular pressure measurement method according to claim 9, wherein the method comprises performing linear wave pick-up from the eye (202).
12. An intraocular pressure measurement method according to claim 9, wherein the method comprises shaping the non-linear waves.
13. An intraocular pressure measurement method according to claim 9, wherein the method comprises low power wide beam measuring.
14. An intraocular pressure measurement method according to claim 9, wherein the method comprises obtaining corneal curvature information.