GB2295226A

GB2295226A - Apparatus for determination of an applanation surface for the measurement of eye pressure

Info

Publication number: GB2295226A
Application number: GB9523115A
Authority: GB
Inventors: Ehrhard Dammann; Norbert Klose; Peter Voigt; Klaus Mohr
Original assignee: Jenoptik Technologie GmbH
Current assignee: Jenoptik Technologie GmbH
Priority date: 1994-11-18
Filing date: 1995-11-10
Publication date: 1996-05-22
Also published as: GB9523115D0; JPH08299282A; DE4441081A1

Description

1 ' is 2295226 APPARATUS FOR EXACT DETERMINATION OF AN APPLANATION SURFACE

FOR THE MEASUREMENT OF EYE PRESSURE The invention is directed to an apparatus for measuring eye pressure, for example an arrangement for exact determination of an applanation surf ace by means of a tonometer, in particular an automatic applanation tonometer.

In automatic tonorneters, the effective Pressing force and applanated surface are detected during an applanation process in order to determine intraocular pressure. For the most part, the applanation surface is detected optically in dfferent ways.

One principle for determining the surface optically consists in measuring the portion of reflected fight which is absent from the total reflection at the end face of the contact-pressure body or pressing body during contact with the comea, this absent portion being proportional to the respective dimensions of the applanation surface.

A solution of this kind is known, e.g., from DE-PS 39 31 630. In this instance, a measuring body in the form of a prism is used. The prism has at least one beveled side surface at which the bundle of beams entering through the base surface of the prism is so reflected that total reflection occurs at the end face of the prism parallel to the base surface (with air as external medium). When the end face of the prism contacts the eye, the light in the region of the applanation surface is refracted out of the prism and results in a weakened intensity of the integral absorbed light bundle.

For exact determination of the applanation surface it is essential that the end face of the pressing body be free of deposits before every measurement. However, after every contact with the eye there remain deposits which must be removed by special cleaning. In so doing, the original value of the measurement light (intensity value 10) is advisably always adjusted again. However, since the light intensity itself does not constitute a sufficiently constant value, this step by itself still leads to substantial measurement errors.

The most important source of error is the temperature dependence of the light source and receiver. A possible temperature drift within the range of normal fluctuations in i 2 room temperature between 150 C and 30 C by itself leads to 10 fluctuations of 0% when using an LED.

Another source of error occurs every time the pressing body is changed. this change being necessary in order to disinfect the pressing body. Accordingly, in view of the low permissible positional tolerances, considerable intensity losses can easily occur due to a slight misalignment when replacing the pressirig body.

For this reason, the object of the present invention is to find a new possibility for exact determination of the applanation for detg eye pressure by mean of a tonometer in which erroneous measurements due to the sources of error mentioned above are prevented.

According to the invention there is provided an apparatus for the determination of an applanation surface for measuring eye pressure by means of a tonometer comprising a light source, an optical element having a reflecting surface for contact with the eye to produce an applanation surface, the amount of light from the light source totally reflected by the reflecting surface being inversely dependent on the area of the applanation surface, a first light receiver arranged to receive and transform the reflected light, and an evaluating unit arranged to calculate the applanation surface from the change in intensity resulting from the reduced total reflection, in which a second receiver is provided for monitoring the reflective condition of the reflecting surface of the optical element; wherein an element for coupling out light is arranged between the light source and the optical elements, the coupled out light being guided to the second receiver as a control beam; and the evaluating unit includes means for comparing the light intensities received by the first and second light receiver to evaluate whether an error condition of the total reflecting surface of the optical element is present.

According to another form of the invention, in an arrangement for exact determination of an applanation surface for measuring eye pressure by means of a tonometer with a Eght source, a prism used as pressing body, a receiver, and an evaluating unit, wherein the prism has an end face producing an applanation surface on the eye, at which end face a bundle of beams proceeding from the light source is totally reflected in contact vAth air and this total reflection is partially canceled when contacting the eye, and wherein the light reflected at the end face is directed to and 3 transformed in the receiver, and the evaluating urd arranged downstream contains means for calculating the applanation surface ftom the change in intensity resulting ftom the reduced total reflection, the object is met according to the invention in that a second receiver is provided for monitoring the cleanness of the end face of the prism, wherein the second receiver is arranged closely adjacent to the first receiver and receives a control value, in that an element for coupling out light is arranged between the fight source and prism in the beam bundle proceeding from the light source, wherein the coupled out fight is guided to the second receiver as a control beam, and in that the evaluating unit contains means for forming a control value quotient from the base intensity value of the first receiver and the control value of the second receiver as well as storage means for storing a normal value formed in this way and a subtractor for calculating an error value from the normal value and the actual control value quotient, wherein a value failing below a permissible error value serves as a precondition for initiating an applanation measurement process.

The element for coupling out light advantageously contains a semitransparent mirror which is preferably a dielectric beam-dividing coating arranged on a glass plate. The splitting ratio is advisably I: 1. A deflecting mirror in the form of a concave mirror is advantageously arranged downstream of the semitransparent mirror in order to focus light on the second receiver.

When using a divergent fight source such as an LED, an annular diaphragm with a mirror coating which allows the fight to pass in an unobstructed manner through its central opening to the prism in a definpd beam bundle can advantageously be used as the element for coupling out fight. The mirror coating of the diaphragm is advantageously constructed as a concave mirror which focusses the coupled out fight on the second receiver.

A third possibility for realizing the element for coupling out fight consists in the use of a fiber-optic fight guide which reaches into the peripheral area of the beam bundle of the light source at one end and is guided to the second receiver at the other end. The fiber-optic light guide is advisably constructed as a fiber ring at the periphery of the beam bundle of the light source.

The second receiver is advantageously of the same constructional type and from the same production batch as the first receiver.

The evaluating unit advisably contains additional means for suppressing the influence of temporary fluctuations in intensity of the light source, wherein the intensity measurement generated by the applanation surface is advisably put in a ratio to the actual control value.

The invention is based on the idea that the substantial errors in determining the applanation surface at a tonometer are brought about by deposits on the pressing body and alignment errors of the pressing body and that the temperature drift of the light source and receiver are additionally superposed on these errors so that they cannot be neatly separated.

Therefore, the path taken by the invention is to provide a control channel which first compensates for the effect of temperature and accordingly makes it possible to detect and assess the external error effects (deposits on the pressing body and n- iisalignment of the pressing body). In so doing, the aforementioned external factors are subject to criteria which prevent an (erroneous) measurement from being initiated when a tolerance is exceeded.

1 With the arrangement according to the invention it is possible to increase the accuracy of measurement of the applanation surface and to improve the reproducibility of the measurements. This has a decisive positive effect on the accuracy with which eye pressure is measured in a tonometer and makes the automation of eye pressure measurement independent from the experience of the user. Moreover, as a secondary result, the temperature dependence of applanation surface measurement is substantially compensated for.

Embodiments of the invention will now be described, by way of example, with reference to the drawings of which:

Fig. 1 shows a basic variant of the optical part of the arrangement according to the invention; Fig. 2 shows an advantageous development of the optical part of the invention; Fig. 3 shows a second optimized construction of the optical part of the invention.

In a basic variant of the invention shown in Fig. 1, the arrangement according to the invention has a light source 1, an element 2 for coupling out light, a pressing body with total reflection at the end face forrning the contact surface with the eye, two lenses 9 and 10, and two receivers 6 and 8.

The light of the light source 1, preferably an LED, is collimated by means of 0 -P the lens 9 and is radiated into the pressing body which is advantageously a special prism 5 NAq'th a basic trapezoidal shape. When air is the medium bounding the end face of the prism 5, total reflection of the parallel light is achieved within the prism 5, this light being focussed on the receiver 6 by lens 10 upon exiting from the prism 5. In the absence of eye contact - depending on the operating state of the tonometer - the receiver 6 supplies an (electronically transformed) base intensity value 10 and with eye contact, depending on the dimensions of the applanation surface,- supplies an intensity measurement Im. A second receiver 8 supplying a so-called control value IK is arranged directiy adjacent to the receiver 6. The two receivers 6 and 8 are advantageously photoreceivers of one and the same batch which are to be considered as having identical sensitivity with respect to their in-u- nediate proxirruty.

6 The light striking the second receiver 8 is branched off directly from the fight bundle exiting from the fight source I by means of an element 2 for coupling out light and is advisably deflected to the receiver 8 by a mirror 7. As is conventionaL the element 2 coupling out fight can be a semitransparent miffor, a bearn-dividing cube or a glass plate with a dielectric beam-dividing coating which divides the light of the light source I into a measurement beam. 3 and a control beam 4, preferably in a ratio of I: 1.

The element 2 coupling out fight can be arranged in the divergent light or in the collimated fight of the light source 1; that is, it can be an-anged upstream of the lens 9 - as shown in Fig. I - or downstream of lens 9. In order to prevent fight losses in the control beam 4 resulting from the increasing beam cross section when the element 2 coupling out light is an-anged in the divergent fight beam, the mirror 7 is advisably constructed as a concave mirror.

Fig. 2 shows another variant of the coupling out of light for the control beam 4.

This variant makes use of the fact that the light of an LED has a relatively large divergence so that a peripheral area of the beam bundle is not collimated by the lens 9 in any case. This relatively large peripheral area is advantageously used to form the control beam 4 by means of an annular diaphragm I I which is slightly inclined (up to a maximum of 45). Focussing means are required due to the relatively great divergence of ihe light. This requirement can be met, as described in Fig. 1, by a beam-deflecting mirror 7 in the form of a concave miffor or the diaphragm I I itself may be shaped as a concave mirror and focusses the light directly on the receiver 8 which is aligned directly with the latter as the case may be.

Fig. 2 shows the same basic construction as that shown in Fig. 1. However, Fig. 2 shows an eye applanation at the end face of the prism 5. As a result, the receiver 6 records a light fluctuation in the form of an intensity measurement IM depending on the applanated surface, while the base intensity value 10 (output signal indicated in dashed lines) was available before without eye contact. Fig. I illustrates the opposite case without eye applanation with the possible subsequent detection of the intensity measurement IM (dashed line).

The latter state is also the basis for Fig. 3. The optical coupling out of light for generating the control value IK is again modified in this case. The coupling out of light is -4.

effected in this constructional variant by means of a fiber-optic light guide 12 which guides the light from the periphery of the light bundle of the divergent light source I to the second receiver 8. For this purpose, the fiber-optic Fight guide 12 advantageously has a bundle of fibers which is arranged concentrically around the measurement beam 3 in a holder 13, preferably of cast resin, and ensures the approximate equality of the base intensity value 10 and control value IK.

The transformed receiver signals are evaluated in an evaluating unit (not shown) which was expanded, according to the invention, to include means for forming a control value quotient from the base intensity value 10 and control value 1K, storage means for storing a normal value formed in this way, and a subtractor for calculating an error value from the normal value and the actual control value quotient.

The control value quotient k4K is formed to characterize the cleanness of the prism 5. Concretely, the intensity of the light source I IS received and its fluctuations detected by means of the second receiver 8 at least via a portion of the light cone of the light source 1.

This light source intensity (control value 10 is then put in a ratio to the intensity (base intensity value 10) incident on the first receiver 6 in the "idling" operation of the tonometer (Without eye contact). For one, this ensures that fluctuations in intensity due to temperature influences are eliminated since the effect of temperature changes on the base intensity value 1.0 and on the control value IK is of equal proportion. This is also ensured on the receiver side by means of the closely adjoining arrangement of the receivers 6 and S.

As a result of this basic normalization or scaling, the control value quotient I04K represents a measurement for the optical state of the prism 5, indeed not only for the cleanness of the end face, but also for the position of the prism 5 which is subject to exacting tolerances. The latter is highly important since the prism 5 is frequently removed and replaced in order to carry out disinfection.

For the purpose of objective control of these demonstrated error values in determining the applanation surface, a once-only normal value of the control value quotient U is measured and stored in the best clean and aligned state of the prism 5.

OIIK)Norrn Immediately before every measurement of the applanation surface of the eye, the actual value of the control value quotient (10110actwl is detected.The evaluating unit then takes the 8 dillerence of the normal value and actual value of the control value quotient and compares this difference with a given tolerance. An applanation process is initiated only when fdbg below this tolerance (104K)Nxm - (IO'K)al ""T- This ensures a sensitive control of the optical state at the prism 5 w1fich automaticaDy blocks an erroneous measurement of the applanated surfitce. The error source can be eUminated in a targeted manner and evaluated in the new compulsorily instituted control measurement by manual cleaning and/or readjustment of the prism 5, wherein the applanation process is initiated automatically when the tolerance is maintained.

In order also to substantially eliminate the effect of temporary fluctuations of the light source I on the measurement of the applanation surface, the following measured value:

I N I m I r.

p instead of the ratio of the intensity measurement IM (eye contact) to the base intensity value 10 (prior to applanation), is advantageously used as an intensity measurement for calculating the is applanation surface, so that IN represents a scaled intensity measurement value.

The time curve of the value of the applanation surface accordingly results from the time curve of the quotient of the synchronously received values of IM and 1K. In this way, measurement of the applanation surface is freed to a great extent ftom all substantial sources of error in optical surface measurement.

1 List of Reference Numbers 1 2 3 4 5 6 7 8 9 11 12 13 10,K IM light source element coupling out light measurement beam control beam prism receiver mirror second receiver lens lens diaph;?-,.,n-. fiber-optic light guide holder basic intensity value control value intensity measurement 9

Claims

CLAIMS:

1. An apparatus for the determination of an applanation surface for measuring eye pressure by means of a tonometer comprising a light source, an optical element having a reflecting surface for contact with the eye to produce an applanation surface, the amount of light from the light source totally reflected by the reflecting surface being inversely dependent on the area of the applanation surface, a first light receiver arranged to receive and transform the reflected light, and an evaluating unit arranged to calculate the applanation surface from the change in intensity resulting from the reduced total reflection, in which a second receiver is provided for monitoring the reflective condition of the reflecting surface of the optical element; wherein an element for coupling out light is arranged between the light source and the optical element, the coupled out light being guided to the second receiver as a control beam; and the evaluating unit includes means for comparing the light intensities received by the first and second light receiver to evaluate whether an error condition of the total reflecting surface of the optical element is present.

2. An apparatus as claimed in claim 1 in which the second light receiver is situated closely adjacent the first light receiver.

3. An apparatus as claimed in claim 1 or claim 2 in which the evaluating unit is arranged to form a control value quotient from the light intensity received by the first receiver and the light intensity of the control is 11 beam received by the second receiver, and compares that value with a stored normal value to evaluate whether an error condition is present.

4. An apparatus as claimed in any preceding claim in which the light coupling element is a semitransparent mirror.

5. An apparatus as claimed in claim 4 in which the semitransparent mirror is a dielectric beam-dividing coating which is arranged on a glass plate and which divides the intensity in a ratio of 1:1.

6. An apparatus as claimed in any preceding claim in which a mirror for deflecting the coupled out light to the second receiver is arranged downstream of the light coupling element wherein the mirror is preferably a concave mirror.

7. An apparatus as claimed in any preceding claim in which when using a divergent light source, preferably an LED, the light coupling element is an annular diaphragm with a mirror coating which allows the light to pass in an unobstructed manner through its central opening to the optical element in a defined beam bundle.

8. An apparatus as claimed in claim 7 in which the mirror coating of the diaphragm is constructed as a concave mirror which focuses the coupled out light on the second receiver.

9. An apparatus as claimed in claim 1 in which the light coupling element is a fiber-optic light guide which lies on the periphery of the beam bundle generated by the 2_ light source.

10. An apparatus as claimed in claim 9 in which the fiber-optic light guide is constructed as a fiber ring at the periphery of the beam bundle of the light source.

11. An apparatus as claimed in any preceding claim in which the second receiver is of the same constructional type and from the same production batch as the first receiver.

12. An apparatus for measuring eye pressure substantially as herein described with reference to Fig. 1 or Fig. 2 or Fig. 3 of the drawings.

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