Abstract:
A contact lens shaped measuring device ( 1 ) includes sensor ( 3 ) having a protrusion ( 14 ) towards the cornea ( 22 ). The measuring device ( 1 ) is flexible to a degree that it is flattened by a closing eye lid and the protrusion creates an indentation of the cornea. The occurring force on the protrusion is measured by the sensor. Applying a constant lid acceleration/deceleration model to the movement of the lid and a mechanical model to the cornea, the tension of the cornea is determined and deduced from the force measured with the lid closed, yielding the true intraocular pressure. In an alternative, the protrusion is characterized by a discontinuity in its shape, or the sensor is subdivided, each subsensor being characterized by a protrusion of different shape. With the values obtained as extrema and at the discontinuity or with different protrusions, the cornea tension can be obtained by a (linear) extrapolation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for monitoring biomechanical properties of the eye, more particularly intraocular pressure or characteristics of the cornea, and a device for measuring eye properties of that kind. 
         [0002]    Conventionally, IOP (cf. Glossary) is measured by applanation of the cornea, e.g. by the Goldman applanometer. Devices of this type exert about centrally pressure on the cornea up to a predefined applanation and measure the force required. However, due to the central impact on the cornea, a significant dependence on the curvature of the cornea and its stiffness exists. 
         [0003]    Recently, pressure sensors have been developed measuring the pressure or force exerted in a peripheral region of the cornea. One such sensor is described in British patent application 1017637.8 in the name of the University of Dundee et al. which is not yet published. The pressure sensor is a contact lens with a circumferentially embedded pressure sensor. The pressure sensor bears toward the cornea an elevation. If the lid closes over the contact lens, it is flattened and the elevations are pressed on the cornea. On the basis of a suitable calibration, which accounts for individual mechanical properties of the eye and the lid, the IOP can be monitored over extended periods, e.g. 24 hours. 
         [0004]    The front side of the sensor portion, i.e. opposite the cornea, may be constituted by a material significantly stiffer than the cornea so that the pressure of the lid is transmitted to the sensor without significant deterioration. 
         [0005]    Generally, the sensor is of the type of a variable capacitor coupled to an inductance to constitute a resonance circuit. The inductance serves as the antenna so that the resonance frequency can be wirelessly determined. 
         [0006]    For example, the frame of glass may be provided with a suitable antenna and the emitter electronics may be implemented in the frame. Thereby, monitoring is possible with only a minimum impairment to the monitored person. 
         [0007]    However, the requirement of an adaptation and periodical recalibration of the system remains, which is burdensome and expensive. 
       SUMMARY OF THE INVENTION 
       [0008]    Therefore it is an object of the present invention to provide a method and device which require less calibration efforts. 
         [0009]    Another object is to provide method and device allowing the monitoring of other properties of the eye than the IOP. 
         [0010]    Method and devices attaining at least the first object are defined in the independent claims. The further claims define preferred embodiments thereof. 
         [0011]    Such a method is a method for monitoring biomechanical properties of the eye, using a measuring arrangement which comprises a measuring device designed to be born in the manner of a contact lens, the measuring device comprising a force detector in operative engagement with a protrusion directed towards the cornea of an eye once the measuring device is applied to the eye, the measuring device being flexible in order to be pressed against the cornea by a lid of the eye moving over the measuring device, wherein the method comprises the following steps:
       measuring and storing force values and indication of time when the force values occurred;   determining the time t start  when the force values start to be significantly greater than zero, time t stop  at which the force values after t start  are no more substantially greater than zero, equivalent to the times when the protrusion first gets in contact with the cornea respectively disconnects therefrom;   determining a maximum force value between t start  and t stop  in order to determine the phases of eye closing, eye being closed, and eye opening;   deriving from the force values and the corresponding time values between t start  and t End  a value indicative of the tension of the cornea;   providing the shape of the protrusion with at least one discontinuity so that at least one significant step in the force values is observed, with the step being attributable to an indentation depth predefined by the discontinuity;   additionally to providing a protrusion as set forth above, performing an extrapolation, preferably a linear extrapolation towards indentation depth 0, or a non-linear fit using at least two of the measured value pairs of a) maximum force and corresponding indentation depth; and b) the pairs of force values and corresponding indentation depths defined by the at least one discontinuity.       
 
         [0018]    Preferred developments thereof are a method wherein at least one of the following is performed: 
         [0019]    the shape of the protrusion is provided with at least one discontinuity so that at least one significant step in the force values is observed, with the step being attributable to an indentation depth predefined by the discontinuity
       the movement of the lid is determined based on a movement of constant deceleration and acceleration;   the tension of the cornea is determined by determining the parameters of a model of the cornea by inserting measured force values and time values, or values derived thereform, in equation of the model.       
 
         [0022]    The object is attained by a device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having at least one protrusion directed towards the eye when applied to an eye, the device having such an overall flexibility the a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion, wherein the protrusion has at least two portions of different height. 
         [0023]    An alternate device is a device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having a protrusion directed towards an eye when applied to the eye, the device having such an overall flexibility the a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion, wherein the protrusion has flanks which comprises at least one step creating a transition zone from a larger protrusion to a smaller protrusion. 
         [0024]    A further alternate device is a device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having a protrusion directed towards an eye when applied to the eye, the device having such an overall flexibility the a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion, wherein the sensor is subdivided in at least to subsensors, the subsensor having protrusions of different size, so that the subsensors timely shifted touch the cornea when a lid closes over the device. 
         [0025]    Further variants and developments of the method and the device are given in the description and the claims. 
         [0026]    The invention will be further described by preferred embodiments with reference to the Figures. 
     
    
     
       BRIEF DESCRITION OF THE DRAWINGS 
         [0027]      FIG. 1  Top view on a monitoring lens; 
           [0028]      FIG. 2  Enlarged cross-section through a sensor, with indented situation indicated by broken lines; 
           [0029]      FIG. 3  Cross-section through an eye with monitoring lens; 
           [0030]      FIG. 4  Mechanical model; 
           [0031]      FIG. 5  Hysteresis-diagram for uniform knob; 
           [0032]      FIG. 6  Flow diagram: Indentation depth interpolation according to constant acceleration model for the eye lid; 
           [0033]      FIG. 7  Flow diagram: Computation of hysteresis, interpolation only; 
           [0034]      FIG. 8  Top view on lens with two sensors; 
           [0035]      FIG. 9  Partial cross-section of the sensor of  FIG. 8 ; enlarged; 
           [0036]      FIG. 10  Force-time diagram of the lens of  FIG. 8 ; 
           [0037]      FIG. 11  Extrapolation diagram; 
           [0038]      FIG. 12  Cross-section of knob with transition; 
           [0039]      FIG. 13  Force-time diagram of knob of  FIG. 12 ; 
           [0040]      FIG. 14  Cross-section through monitoring lens with sensor having varying knob shape; 
           [0041]      FIG. 15  p-H-diagram of sensors with transition. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0042]      FIG. 1  shows a “monitoring lens”  1 , i.e. a contact lens with a sensor  3 . The sensor  3  is an annular arrangement of a pressure sensing device. Preferably, it is a capacitive sensor. A coil  4  is arranged as an annular winding on the outside. Sensor  3  and coil  4  build a resonance circuit with a resonance frequency dependent on the force exerted on the sensor. A schematic cross-section through sensor  3  is shown in  FIG. 2 . An essentially U-shaped rigid frame  5  is closed by a membrane  7 . On the bottom of the frame  5  (upper side in  FIG. 2 ), one electrode  9  is provided. The other electrode  10  is attached to the interior face of membrane  7 . Preferably, and as shown in dashed lines as the indented state, the second electrode  11  is fixed (glued, soldered or the like) to the center of membrane  7  so that its flexibility is less reduced. If the electrode  10  is attached to the membrane by its entire surface, the membrane-electrode stack has a significantly increased stiffness. Furthermore, by the only quite small attachment zone  8 , it is avoided that the second electrode  11  is bent in the indented state. 
         [0043]    On the exterior face of membrane  7 , a so-called knob  14  is provided. As it is to be pressed on the cornea  16 , it is of a soft, resilient and biocompatible material like silicone or a hydrogel. 
         [0044]    Further indicated by dash-dotted lines is the indented cornea  16  of an eye. The indentation occurs under the influence of the lid when closed, either by blinking or during sleep. 
         [0045]    Still to be remarked that the height of the knob  17  and the displacement of the membrane are emphasized for illustration purposes. Movement of the membrane reduces the indentation of the cornea and the forces exerted, hence reduces the signal amplitude. 
         [0046]    The knob is to create indentation of the cornea in the magnitude of micrometers, hence its shape may have a generally flatter aspect, cf. GB 1017637.8. 
         [0047]    The only minimal indentation of the cornea still constitutes an important advantage of the monitoring lens over the prior art applanometers. Due to the only minimal impact on the eye considered in its entirety, it is reasonable to assume that the true IOP is not influenced by the measurement, i.e. is considered as constant with regard to the time needed for a spontaneous blink, i.e. the time scale of the dynamic measurement where during a blink of the lid, a measurement cycle is performed. 
         [0048]      FIG. 3  shows a partial section through an eye  18  (lens  19 ; iris  20 ; cornea  22 ) bearing a monitoring lens  1  with sensor  3 . Noteworthy in the passive state, the knob  14  does not touch the cornea. 
         [0049]    A basic finding in the context of the present invention consists in that during closing, the lid is first accelerated by a constant value, then slowed down by substantially the same value. 
         [0050]    The same applies during the opening of the lid, but with lower acceleration values. 
         [0051]    Furthermore, the mechanical behavior of the cornea during indentation can be described by a viscoelastic model  23  ( FIG. 4 ) (D. H. Glass, C. J. Roberts, A. S. Litsky, P. A. Weber, A Viscoelastic Biomechanical Model of the Cornea Describing the Effect of Viscosity and Elasticity on Hysteresis, IOVS 49 (2008) 3919-3926.). 
         [0052]    In series to the first elastic element  24  lay in parallel the viscous element  26  and the second elastic element  28 . These elements represent the viscoelastic behavior of the cornea. The model explains the hysteresis in a curve  29  showing the measured pressure p vs. indentation H as shown in  FIG. 5 . 
         [0053]    With measuring the pressure p trigger  at a given indentation H trigger  while closing (upper branch  30 ) and opening (lower branch  31 ) the eye, p c  resp. p o , the hysteresis τ═=═Δp hy =P c −P o  can be determined. Thereby, viscoelastic and other properties of the cornea and the eye are accessible additionally to the IOP. 
         [0054]    Furthermore, the model  23  allows to calculate the contribution p cornea  to the uncompensated pressure IOP raw . The model  23  is characterized by the following equations (cf.  FIG. 4 ): 
         [0000]    
       
         
           
             
               
                 
                   σ 
                   = 
                   
                     
                       σ 
                       V 
                     
                     + 
                     
                       
                         σ 
                         
                           E 
                           2 
                         
                       
                        
                       
                           
                       
                        
                       
                         ( 
                         
                           = 
                           
                             σ 
                             
                               E 
                               1 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   ɛ 
                   = 
                   
                     
                       ɛ 
                       
                         E 
                         1 
                       
                     
                     + 
                     
                       
                         ɛ 
                         VE 
                       
                        
                       
                           
                       
                        
                       
                         ( 
                         
                           
                             ɛ 
                             VE 
                           
                           = 
                           
                             
                               ɛ 
                               V 
                             
                             = 
                             
                               ɛ 
                               E 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       ( 
                       
                         
                           σ 
                           
                             E 
                             1 
                           
                         
                         = 
                       
                       ) 
                     
                      
                     
                         
                     
                      
                     σ 
                   
                   = 
                   
                     
                       E 
                       1 
                     
                      
                     
                       ɛ 
                       
                         E 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     σ 
                     V 
                   
                   = 
                   
                     η 
                      
                     
                       
                          
                         
                           ɛ 
                           VE 
                         
                       
                       
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     σ 
                     
                       E 
                       2 
                     
                   
                   = 
                   
                     
                       E 
                       2 
                     
                      
                     
                       ɛ 
                       VE 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0055]    With σ corresponding to p(t) and ε to H(t), the contribution Pcornea during the period t IOP0  to t IOP1 , i.e. on the measured pressure during the quasi-steady state while the lid is closed, can be determined and in consequence the true or compensated IOP be obtained. In practice, a non-linear fit algorithm may be used to resolve the equations of the model. 
         [0056]    A further property of the monitoring lens  1  is that its flattening is quantitatively correlated with the movement of the lid. In other terms, the position s lid  of the lid in the direction of the opening/closing movement is a measure for the distance between cornea  16 ,  22  and frame  5  and therefore also a measure for the indentation H of the cornea  16  by the knob  14  once the knob  14  touches it. Preferably, when the lid is entirely closed, the frame  5  touches the cornea  16 ,  22 , and a maximum or steady-state indentation H knob =H knob,max  occurs. 
       EXAMPLE 1 
       [0057]    In order to determine the characteristic values of the hysteresis curve  29  of  FIG. 5 , the finding is used that the acceleration values during opening (a open ) and closing (a close ) of the lid can be derived from the delay between occurrence of maximal force, corresponding to the closed lid, and the time when the force is zero again, i.e. the lid is opened to a degree that the knob  14  does no more exert a sensible force on the cornea. As explained above, the indentation depth H is then accessible by interpolating the position of the lid using the general equation 
         [0000]        s   lid =½ a t   2   +v   0    t+s   lid0    (6)
 
         [0000]    with
   a: the acceleration of the lid   s lid : position of the lid at time t   s lid0 :position of the lid at t=0: on closing: s lid0 =0; on opening: s lid0 =final position of the when closed, i.e. at t IOP0 .   t: time   v 0 : the velocity of the lid at time t =0 defined as the moment when the knob  14  hits the cornea  22  on lid closing; on lid opening: v 0 =0   
 
         [0063]    For the application of this equation, it shall be recalled that the measured time interval covers only the phases when the lid is decelerating during the closing movement and accelerating during the opening movement. 
         [0064]    The relation between H and s 
         [0000]        H=f ( s   lid )   (7)
 
         [0000]    is determined by calibration of the lens  1  and stored. In practical terms, s lid  may also correspond to the angle of the lid. 
         [0065]    The force F(t) sensed by the knob  14  when indenting the cornea is generally unambiguously related to the pressure p although the contact surface varies with the indentation depth H. The relationship between F(t) and p(t) for a given knob shape can be empirically determined and used as a lookup table or a numerically determined function, e.g. a polynomial, or a set of functions, e.g. a (cubic) spline. Empirically, rounded shapes of the knob have been found suited, i.e. knobs with rounded apex (obviously also avoiding irritations of the cornea). 
         [0066]    Furthermore, it is possible to determine a shape of the knob where the dependence of pressure is linearly dependent on the measured force, hence the following equation applies: 
         [0000]        p ( t )= c   p   F ( t )   (8)
 
         [0000]    with
       c p  a constant       
 
         [0068]    which significantly simplifies the calculations. 
         [0069]    In another approach, the pressure may additionally be dependent on the interpolated indentation depth, e.g. 
         [0000]        p ( t )= g ( H ( t )) F ( t )   (9)
 
         [0000]    with
       g(H(T)): function of H(t), characteristic for the knob; defined analytically or by discrete values and interpolated as necessary; may be determined by (FEA) simulation or measurement;       
 
         [0071]    The shape is, with sufficient precision, independent of an individual eye. The exact shape can be found e.g. by numeric methods on the basis of mechanical properties of the cornea, e.g. by Finite Element Analysis FEA. 
         [0072]    The evaluation of the sensor signal is shown by flow diagrams in  FIGS. 6 and 7 . The sensor furnishes its values with a sufficient high rate to an evaluation device, e.g. constituted by an embedded controller, possibly integrated in glasses, or a separate station to which data received by the monitoring circuitry in the glasses are continuously (wireless) or periodically transferred. Hence, the data may be locally stored and transferred later on to a evaluation station, or immediately evaluated. 
         [0073]    Eyelid closing occurs within typically 75 ms, with the period used for the measurement (t start  to t IOP0 ) in the range of 1 to 3 ms, and lid opening within about 3 times those periods (i.e. totally about 225 ms, t IOP1  to t end  about 3 to 9 ms), while the lid remains typically closed during a spontaneous blink for about 16 ms. In view of the movement of the lid, the force data have to be sampled sufficiently fast, at least 5 values are required per branch  30 ,  31  of the hysteresis curve. As transfer rates from the sensor to the emitter/receiver is limited, it is preferred to base the interpolation for determining the acceleration values from the opening phase which occurs over a significantly longer period. Empirically and from theoretical considerations, a data acquisition rate of about 5 kHz is sufficient. Higher rates tend to improve the performance. 
         [0074]    The time t max  when the lid is closed is determined as the point in time where the force signal is at its maximum F max . Practically, as the period the lid is closed is taken as the interval from t IOP0  to t IOP1  where the force F(t) remains above a predetermined threshold. As such, it is taken e.g. a percentage of the maximum signal. The percentage is at most 10%, preferably at most 5%, and most preferably at most 2%, i.e. the threshold F th  is defined to be at least 0.9 F max , or 0.95 F max  or 0.98 F max . 
         [0075]    The time period t End −t IOP1  (t End : point in time where the sensor signal gets 0 again, i.e. the knob does no more exert a measurable force of the cornea) is used  32  as an index in a generic lookup table  33  ( FIG. 6 ). The table  33  furnishes the values of a open  and a close  for the acceleration during opening of the lid and the deceleration during closing of the lid. According to the preferred embodiment, the knobs are designed such that they measurably contact the cornea only in the periods where the lid is slowing down in lid closing and accelerating in lid opening. 
         [0076]    Using a close  and the times t start  (time when a measurable force signal first occurred, i.e. the knob starts to exert pressure on the cornea) and t IOP0  (time where the force signal passes F th , indicative of that the lid is closed and its movement has stopped), the lid velocity v close0  at t start  is determined  34 : 
         [0000]        v   start   =a   close ( t   IOP0   −t   start )   (10)
 
         [0077]    Now, the indentation depth H(t) is determined  36  resp.  37  for closing and opening based on the positions of the lid which is given by the equations: 
         [0000]        s   close   =v   start ( t−t   start )−½ a   close ( t−t   start ) 2 ;   (11)
 
         [0000]        s   open =½ a   open ( t−t   IOP1 ) 2   +s   0 ;   (12)
 
         [0000]    with
       s 0  the closed position of the lid, e.g. given as the final value of s close  
 
for the closing resp. opening movement of the eye.
       
 
         [0079]    The pressure p(t) is determined on the basis of the sensor signal, i.e. the force exerted on the knob by the eye. 
         [0080]    With the measured indentation H(t), the known time t when these indentations have been measured, and p(t), the hysteresis curve p(H(t)) can be constructed and the true IOP can determined by compensating the influence of the cornea on the measured uncompensated IOP raw  which is the pressure measured as an average in the quasi-steady state period from t IOP0  to t IOP1 . This separation is done by considering the IOP as a constant on the measurement time scale, i.e. for the duration of a blink as set forth above. 
         [0081]    For the non-linear fit, the period t start  to t End  is subdivided in three segments: 
         [0082]    A) Lid closing, increasing force: t start  to t IOP0    
         [0083]    B) Lid about closed, force about constant: t IOP0  to t IOP1    
         [0084]    C) Lid opening, force decreasing: t IOP1  to t End    
         [0085]    A) and C) are characterized by that the sensor frame  5  is not yet or no more in touch with the eye  18 , although the cornea is indented. During these phases, the viscoelastic model  23  is applied. 
         [0086]    In phase B, however, the frame  5  touches the cornea  22 . Therefore, independently of the position of the lid (which is now closed), indentation depth is assumed to be constant because the sensor is steady with respect to the eye, and the viscous component has a sufficiently short relaxation, that its contribution can be neglected. In this phase, the pressure component of the cornea is determined by the elastic elements E 1  and E 2  only. 
         [0087]    During phases A and C, based on the viscoelastic model, the non-linear fit can be determined on the basis of the [H(t), p(t)] value pairs yielding the parameters E 1 , E 2 , and η of the model. Using E 1  and E 2  for calculating the reduction of the IOP by the tension of the cornea: 
         [0000]    
       
         
           
             
               
                 
                   
                     p 
                     cornea 
                   
                   = 
                   
                     
                       
                         
                           E 
                           1 
                         
                          
                         
                           E 
                           2 
                         
                       
                       
                         
                           E 
                           1 
                         
                          
                         
                           E 
                           2 
                         
                       
                     
                      
                     
                       [ 
                       
                         
                           H 
                           knob 
                         
                         
                           α 
                           anat 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0000]    with
       α anat  a constant;
 
and
       
 
         [0000]        IOP=IOP   raw   −p   cornea    (14)
 
         [0000]    furnishes the true IOP. 
         [0089]    For the non-linear fit, the equations given above can be used in a numerical equation solver or they can be combined to a reduced set of equations, down to only one equation, and then subjected to a usual non-linear fitting algorithm. As a criterion for approximating the correct solution, the principle of least squares error is suitable. 
         [0090]    Starting with the mentioned equations, the following particular solution of this system of differential equation can be derived: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       p 
                       cornea 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         H 
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                        
                       
                         E 
                         1 
                       
                     
                     
                       
                         α 
                         anat 
                       
                        
                       
                         [ 
                         
                           1 
                           + 
                           
                             
                               
                                 E 
                                 1 
                               
                               
                                 E 
                                 2 
                               
                             
                              
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                    
                                   
                                     
                                       - 
                                       t 
                                     
                                     / 
                                     τ 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where 
         [0091]    H(t) depth of indentation of cornea at time t 
         [0092]    τ=η 1 /E 1  hysteresis constant 
         [0093]    In deriving the formula, it is supposed that the model of constant acceleration of the lid is applicable and that the reaction force of the monitoring lens is negligible with respect to the force the lid exerts on the lens. 
         [0094]    Applying a non-linear fit to this formula in connection with equation 14 and using the results of the measurement (measured pressure on the cornea indentation depth at different times t and the hysteresis constant τ) yields E 1  and E 2 . 
         [0095]    Furthermore, a viable approach is to set E 1 =E 2  (D. H. Glass, Characterization of the Biomechanical Properties of the in vivo Human Cornea (Thesis), Ohio State University, 2008, cf. p. 59). 
         [0096]    Still to be mentioned that the hysteresis constant τ can be determined by determining the pressure at equal indentations H trigger  during opening and closing the eye. For determining the values for these indentations, interpolation techniques known per se may be used. The hysteresis τ is supposed to furnish information of the healthy state of the cornea and the eye and is used in the non-linear fit. 
         [0097]      FIG. 7  shows the flow diagram of calculating τ. The initial steps are identical with the determination of IOP explained above, although not shown in  FIG. 6 : 
         [0098]    The force signal of the sensor is read  39  until it significantly deviates  40  from zero. This point in time is defined  41  as t start . Force signals F sensor  are recorded and stored  42  at 5 kHz until it is determined  43  that it no more deviates significantly from zero. This point in time is stored  44  as t end . 
         [0099]    In the recorded data duples (F sensor,i , t), the maximum F sensor,max  value is searched  45 , and the corresponding time t max  is recorded. The data are postprocessed for determining properties of the eye or values indicative of its health state, in particular the IOP as set forth above. 
         [0100]    Next step is again determining  32  the parameters of the movement of the lid using the look-up table  34  and therefrom  36  the indentation depths H 1  for the F 1  values. 
         [0101]    Furthermore, the time t trigger0  is determined in a predefined position between t start  and t max  (or as an alternative t IOP0 ) and, and the corresponding point in time t trigger1  for the lid opening phase between t max  (alternative: t IOP1 ) and t end  as the point in time where the same indentation as at t trigger0 , occurs, wherein intermediate values are interpolated. Alternatively, instead of defining t trigger , H trigger  may be determined. Suitable values for t trigger0  are the middle of the mentioned intervals, or H trigger =½ H knob,max  (max. indentation depth). 
         [0102]    For t trigger  or H trigger , the force values F trigger0  and F trigger1  during closing resp. opening the eye are calculated  46  using interpolation as required. The difference F trigger0 −F trigger1  yields  47  a measure of τ. 
         [0103]    In a preferred variant, the threshold force value F th  is determined in an autocalibration cycle. The device determines whether the eye is closed longer than a predefined time t th . The start and stop criterion is whether the force is greater than zero resp. zero again. Then the initial and final values representing closing and opening the eye are discarded. The resulting period is significantly longer than the usual time for these movements, e.g. at least ½ s or at least 1 s. The force values of this period of closed eye are postprocessed. Preferably, an averaging is included. F th  is then set to be a small percentage lower than the obtained steady state force value, cf. above. 
         [0104]    For an autocalibration, the patient may be asked to close the eyes for a few seconds, or periods of extended lid closure may be used and automatically detected, e.g. sleep. Manual triggering and controlling is possible as well, where even discarding of initial and final values may be avoided. 
         [0105]    Accordingly, t th  may be a few seconds, e.g. at least 2 s, preferably at least 5 s or even at least 10 s. 
       EXAMPLE 2 
       [0106]    In order to avoid the computational effort of Example 1, the sensor can be modified the way that at least two segments of distinctly different height H knob (i), i≧2, are present, Preferably, the segmentation is at least mirror-symmetrical in view of the about mirror-symmetrical movement of the lids. 
         [0107]      FIG. 8  shows an arrangement with two large segments S 0   51  which constitutes the first sensor. Between them two smaller segments S 1   53  are provided, constituting sensor  2 . They are characterized by a knob significantly lower than the knob of S 0 . 
         [0108]    For the data transfer, two antennas  55 ,  56  are provided for sensor  51  resp.  53 , each extending over the whole periphery. 
         [0109]    However, the antenna/sensor capacitor arrangements are responsive to different frequencies so that the sensors are capable to furnish data independently and without additional measures for avoiding conflicts. 
         [0110]    Besides the antennas  55 ,  56 ,  FIG. 9  also shows the differing heights H knob,s0    58  and H knob,s1    59 . 
         [0111]    The effect of the different knob elevations is depicted in  FIG. 10 . The points in time when sensor S 1   53 , too, gets in contact with the cornea or looses contact immediately defines the times t trigger10    61  and t trigger11  corresponding to a well-defined indentation H S0  by the sensor S 0   51 . 
         [0112]    Furthermore, sensor S 1   53  furnishes by itself a maximum quasi steady state force value  63  in addition to the maximum steady state value  62  of sensor s 0   51 . 
         [0113]    In a first alternative, the force values F max,S0    62  and F trigger,S0    64  of sensor S 0  at t max  (eyelid closed, maximum force signal) resp. at t trigger10  which is now a well-defined indentation depth by S 0 , are used. Additionally, or alternatively to the force signal of S 0  at t trigger11 , the maximum force signal of the second sensor can be used. Another usable value is the force F trigger1,S0  at t trigger11 , which is, however, more difficult to determine due to its more complicate history, yet because of the slower lid movement, the time resolution is better. 
         [0114]    These values obtained at steady-state may be used for a linear extrapolation to H=0 yielding directly the true IOP as shown in  FIG. 11 . The straight line through pressure p max,S0    62  at H knob,S0    59  and pressure p trigger    64  at indentation H knob,S0 H knob,S1    67  (indentation by sensor S 0  when sensor S 1  just touches the cornea) is extended to the ordinate to yield the true IOP  68 . 
         [0115]    In case of the values obtained at points in time (t trigger10 ) where the indentation is changing, dynamic effects are to be taken in account, and by experience, a non-linear fit has to be performed on the basis of the biomechanical model as explained above. 
         [0116]    The force or pressure values at t trigger10  and t trigger11  may be used to calculate the hysteresis parameter τ, too. 
         [0117]    The concept is not limited to two sensors. Further sensors each of different knob may be provided, and the sizes of the sensors are not restricted to S 1  being significantly smaller than S 0 . 
       EXAMPLE 3 
       [0118]    The knob is provided with a transition  70 . In  FIG. 12 , the transition is a transition to a broader knob shape. In other terms, from the apex  72  to the transition  70 , the shape of the knob corresponds to a small knob, and from transition  70  down to the membrane  7 , the shape of the basis  71  corresponds to a bigger knob. 
         [0119]    When the knob  14  is pressed on the cornea  16 , the signals obtained are those as if the knob has the shape of the small knob (portion  73  in  FIG. 13 ). If the indentation increases, the cornea gets in touch with the transition  70  at time t trigger0    74 . As the contact zone is stepwise increased, also the force signal of the sensor shows a step increase  74 . Afterwards, the signal  75  merely corresponds to that of a virtual bigger knob. 
         [0120]    During opening of the eye, the same occurs in the inverse sense at t trigger1    75 : The force signal shows a sharp decay  77  when the cornea no more touches the broader basis  71 . 
         [0121]    As shown in  FIG. 14 , the same effect may be obtained by a small knob shape  79  over one part of the sensor extension, and the remainder  80  being shaped as a significantly bigger knob. In this case, the transition zone is preferably wave-shaped so that a smooth transition occurs to avoid an irritation of the eye. 
         [0122]    The resulting p/H diagram is shown in  FIG. 15 . Essentially the part  81  toward the closed eye is shifted to higher pressure values. Furthermore, the shift zone defines a predetermined point in time, and the pressures p trigger0    83  and p trigger1    85  allow to determine the hysteresis τ. 
         [0123]    The transition depth H trigger    87  once again defines a point in time where the indentation depth either corresponds to the apex  72  or the difference between greater knob height and smaller knob height in the variant of  FIG. 14 . 
         [0124]    The pressure p trigger0    83  defines a pressure at indentation H trigger    87 . The upwards shifted part  81  of the hysteresis curve is normalized or corrected by using the effective contact surface (the continuous line indicate a calculation assuming a uniform shape) yielding the curve  88  (dashed line). 
         [0125]    The thereby obtained vertex point  90  is p max  at H knob , i.e. frame  5  in contact with the cornea  22 . The two points obtained (p trigger0    83 , p max 90 ) allow a linear extrapolation as explained above for  FIG. 11  yielding the true IOP  63 . 
         [0126]    From the foregoing examples, the one skilled in the art is capable to conceive numerous variants and alterations without leaving the scope of the invention which is solely defined by the claims. 
         [0127]    In particular, the following may apply:
       With respect to Examples 2 and 3, there may be more knobs of different height or transitions, possibly even in combination, e.g. two knobs each with a transition. The advantage would be to have more points for determining the IOP by extrapolation.   Other mechanical models of the cornea may be used, e.g. the Standard Linear Solid model or models developed for soft tissue, e.g. M. N. Tanahq, M. Higashimori, M. Kaneko, I. Kao, IEEE Transactions on Biomedical Eng., 58/3 (2011), 509.       
 
         [0130]    Glossary
   a acceleration (of the lid)   FEA Finite Element Analysis   F max  maximum force   F(t), Fi force at time t, resp. ith force value   F th  threshold value of sensor signal   H indentation depths of knob in the cornea   H knob,max  height of the knob with respect to the surface of the sensor frame =maximal indentation   H max  maximum indentation   H trigger  a predefined indentation value   IOP intraocular pressure   P c , p o  pressure measured at a given indentation H trigger  during eye closing resp. opening   p cornea  pressure of cornea opposed to IOP   p(t), pi pressure dependent on time t, resp. ith pressure   p trigger0/1  pressure at time t trigger0/1      S 0  segment with larger knob   S 1  segment with smaller knob   s lid  position of the lid   t end  time when knob looses contact with cornea in lid opening   t IOP1 , t IOP0 start time of lid opening, end time of lid closing   t IOP0  time of passage of force signal through F th  during lid closing; end time of lid closing   t start  time when knob starts to exert pressure on the cornea, i.e. a force signal first occurs   t trigger0,1  triggering times, where H trigger0/1  occurs   v velocity (of the lid)   v start  lid velocity at t start