Patent Description:
The present invention relates to apparatus and methods of analyzing a material comprising at least one analyte. The apparatus comprises a measurement body having a contact surface suitable to be brought in thermal contact or pressure-transmitting contact with said material, said thermal or pressure-transmitting contact permitting heat or pressure waves generated by absorption of excitation radiation in the material to be transferred to said measurement body.

The apparatus further comprises an excitation radiation source configured for irradiating excitation radiation into the material to be absorbed therein, and a detection device for detecting a physical response of the measurement body, or of a component included therein, to heat or a pressure wave received from said material upon absorption of said excitation radiation, and for generating a response signal based on said detected physical response. Herein, the response signal is indicative of the degree of absorption of excitation radiation. The present invention is not limited to any specific physical response to heat or pressure waves received from said material upon absorption of the excitation radiation, nor to any specific way of detecting this physical response in a manner that allows for generating a response signal that is indicative of the degree of absorption of excitation radiation. Various physical responses and corresponding detection methods have been previously proposed for these types of analyte measurement procedures by the present applicant and are briefly summarized below, and each of them may be applied in the present invention.

For example, the detection device may comprise a light source for generating a detection light beam travelling through at least a portion of said measurement body or a component included in said measurement body, and said physical response of the measurement body to heat or pressure waves received from said material upon absorption of said excitation radiation may be a local change in the refractive index of said measurement body or said component. In this case, the detection device may be configured for detecting one of a change in the light path or a change in the phase of detection light beam due to said change in refractive index of the material of the measurement body or the component included in the same.

For example, in various methods and devices described in detail in two earlier applications of the present applicant, published as <CIT> and <CIT>, both of which are included herein by reference, the measurement body is transparent for said detection light beam, and the detection light beam is directed to be totally or partially reflected at a surface of said measurement body that is in thermal contact with said material. In this case, the detection device may comprise a photodetector, in particular a position sensitive photodetector which is capable of detecting a degree of deflection, in particular a deflection angle of said detection light beam due to said local change in refractive index. Accordingly, in this case, the physical response to the heat or pressure waves received by the measurement body is a local change in refractive index, and the response signal is the detected degree of deflection which is indeed found to be indicative of the degree of absorption of the excitation radiation.

In alternative variants suggested by the present applicant, as for example disclosed in international application<CIT>, included herein by reference, said detection device may comprise an interferometric device allowing for assessing said change in phase of the detection beam and generating a response signal indicative of said change in phase. In this case, the physical response of the measurement body (or a component included therein) to heat or pressure waves received from said material upon absorption of said excitation radiation is again a local change in index of refraction, while the response signal is in this case an interferometric signal reflecting a change in the phase of the detection beam due to the local change in refractive index.

In yet alternative embodiments, the measurement body or a component in said measurement body may have electrical properties that change in response to a local change in temperature or a change in pressure associated therewith, and said detection device comprises electrodes for capturing electrical signals representing said electrical properties. Various possible setups are disclosed in <CIT>, included herein by reference. For example, the measurement body may comprise sections having piezoelectric properties, and pressure changes associated with received heat lead to electrical signals that can be recorded with the electrodes. In this case, the change in pressure resembles the physical response of the measurement body or of a component included therein, to heat received from the material upon absorption of the excitation radiation, which is detected using the piezoelectric properties of the measurement body and the electrode, and which leads to electrical signals representing the aforementioned response signal that is indicative of the degree of absorption of excitation radiation. In yet further variants, a temperature change due to received heat can be directly measured using very sensitive temperature sensors.

Note that in the following description, the physical response of the measurement body to heat received from the material is described in detail. However, it is to be understood that in various embodiments of the method and apparatus of the invention, the material is in pressure transmitting contact with the measurement body, and the physical response of the measurement body is a response to pressure waves received from the material. Herein, the expression "pressure transmitting contact" shall include all relations that allow the transfer of pressure waves from the material to the measurement body, and in particular, an acoustically coupled relation, where the coupling could be established by a gas, a liquid or a solid body. All detailed explanations given in connection with the thermal contact and physical response to heat received by the measurement body from the material shall be understood in combination with scenarios including pressure transmitting contact and physical response to pressure waves, where applicable, without explicit mention.

The apparatus may further be configured for conducting an analyzing step, in which said analyzing is carried out based, at least in part, on said response signal. For this purpose, the apparatus may comprise a control system comprising one or more processors which are programmed to carry out the analysis. If one is for example interested in determining the concentration of the analyte in the material, the excitation radiation may be chosen to have wavelengths that are characteristic for the absorption spectrum of the analyte, for example associated with absorption peaks thereof. Since the response signal is indicative of the degree of absorption of excitation radiation, in this case the response signal is directly related to the concentration of the analyte in the material. Accordingly, the analyzing step may be based at least in part on a measure of the concentration of the analyte in the material, and in some non-limiting applications, it may actually amount to determining this concentration.

For example, apparatus of the type summarized above have been employed by the applicant for non-invasive measuring of a glucose level of a user. In this specific application, the "analyte" is formed by glucose, and the "material" is the skin of the user. It has been previously demonstrated that this approach allows for very precise measurement of glucose concentration in the interstitial fluid within the skin of a person, which is found to be directly related with and hence representative of the glucose content of the patient's blood. Shown in <FIG> of the present application is the result of a Clark's error grid analysis taken from <CIT>, demonstrating that the above apparatus and analysis method allow for predicting the actual glucose concentration of a person very precisely.

Nevertheless, it would be desirable to improve the accuracy and reliability of the analysis results even further.

<CIT> discloses an apparatus for analyzing a material comprising an excitation emission device for generating at least one electromagnetic excitation beam, in particular an excitation light beam, having at least one excitation wavelength, further comprising a detection device for detecting a reaction signal, and a device for analyzing the material on the basis of the detected reaction signal.

<CIT> discusses the problem of prior art biological attribute measuring devices, namely that measurement data is apt to vary because of unstable contact due to difficulty in controlling a thickness of a saliva layer and pain caused during the measurement period. Disclosed is a biological attribute measuring device including a contact device having a portion, at least part of which is a curved surface, for abutting against a measuring region to apply light to the measuring region, a light detecting means for detecting at least one selected from the group consisting of transmitted light, reflected light, scattered light and transmitted/reflected light from the measuring region and a measuring means for measuring a specific component in the measuring region based on the detection result.

<CIT> discloses noninvasive methods, devices, and systems for measuring various blood constituents or analytes, such as glucose. In an embodiment, a light source comprises LEDs and super-luminescent LEDs. The light source emits light at at least wavelengths of about <NUM>, about <NUM>, and about <NUM>. In an embodiment, the detector comprises a plurality of photodetectors arranged in a special geometry comprising one of a substantially linear substantially equal spaced geometry, a substantially linear substantially non-equal spaced geometry, and a substantially grid geometry.

<CIT> discloses a device for analyzing a substance, comprising: a measurement body, which has a measurement surface and is to be brought at least in part into contact with the substance in the region of the measurement surface for the purpose of measuring; a laser device, particularly having a quantum cascade laser (QCL), a tunable QCL and/or a laser array, preferably an array of QCLs, in order to generate one or more excitation beams at different wavelengths, preferably in the infrared or medium infrared spectral range, which is directed to the substance; and a detection apparatus which is integrated at least in part in the measurement body or connected thereto and comprises the following: a source for coherent detection light and a first optical waveguide structure which can be or is connected to the source for the detection light, which guides the detection light, and has a refractive index which is dependent at least in portions on the temperature and/or pressure, wherein the first optical waveguide structure has at least one portion in which the light intensity depends on a phase shift of detection light in at least one part of the first optical waveguide structure due to a change in temperature or pressure.

An object underlying the present invention is to provide an apparatus and method for analyzing a material as set forth above that allow for improving the accuracy or reliability of the analysis results.

This problem is solved by an apparatus and method according to claims <NUM> and <NUM>, respectively. Preferable embodiments are defined in the dependent claims.

According to one aspect of the invention, an apparatus for analyzing a material comprising at least one analyte, is provided, said apparatus comprising.

According to this aspect of the invention, said detection light beam is directed to be totally or partially reflected at said contact surface that is in thermal or pressure-transmitting contact with said material.

Note that the concept of the detection light beam being "deflected" relates to the total change in angle or change in impingement position at the detector, or in other words, how the detected position of the detection light beam differs from its position without excitation of and absorption by the material. This "deflection" is hence the accumulated effect that the local change of refractive index has on the detection light beam along its light path. A closer inspection reveals that in many cases, part of the deflection of the light beam due to the local change in refractive index occurs before the detection light beam is reflected at a surface of the measurement body that is in thermal or pressure-transmitting contact with the material, which in this case is formed by the front surface of the protrusion. Accordingly, the local change in refractive index typically also leads to a shift of the location precisely where on the surface the detection light beam will be reflected.

In view of this understanding, according to this aspect of the invention, the contact surface is curved in at least one principal direction. This curvature implies that said change in location where the detection light beam is reflected is also accompanied by a change of angle of incidence, and hence leads to a corresponding change in the angle of reflection as well. Accordingly, using a curved reflection surface, the total deflection assessed by the detection device, such as the shift in position detected with a position sensitive detector, can be increased.

In a preferred embodiment, said curvature in said at least one principal direction corresponds to a radius of curvature in a range of <NUM> to <NUM> , preferably <NUM> to <NUM>.

In a preferred embodiment, said curvature in said at least one principal direction is one of concave or convex.

According to the claimed invention, the detection light beam prior to and after reflection at said front surface define a detection light plane, and said at least one principal direction lies within said detection light plane or forms an angle with the detection light plane that is less than <NUM>°, preferably less than <NUM>°. This way, it is ensured that the predominant effect of the curvature on the deflection is in the detection light plane.

In a preferred embodiment, the detection light source is arranged such that said detection light beam is irradiated into said measurement body at an entrance surface, propagates through a portion of said measurement body and exits from the measurement body at an exit surface, wherein the detection beam - in absence of any deflection due to said local change in refractive index - impinges on the exit surface at an angle of <NUM>° or more, preferably <NUM>° or more and most preferably <NUM>° or more with respect to the normal to the exit surface, such that the detection beam is refracted upon exiting from the exit surface of the measurement body, wherein the orientation of the exit surface with respect to the detection light beam is such that said deflection of the detection light beam in response to said heat or pressure waves being transferred to said measurement body increases said angle of said detection light beam with respect to the normal to the exit surface.

In previous designs of the applicant, the shape of the measurement body was generally chosen such that the detection beam would be perpendicular to the entrance and exit surfaces, to avoid losses due to reflection and to avoid refraction, which at first sight would only make the optical setup more complicated. However, according to this embodiment, the detection light source is arranged such that the detection light beam is deliberately refracted at least at the exit surface in a manner defined above. Since usually, the refractive index of the measurement body will be higher than that of the surrounding, an increase in the angle of the detection light beam to the normal to the exit surface will lead to an even larger increase in the angle of the refracted light beam, such that the deflection of the light beam as detected at the detection device is further increased, thereby leading to a larger response signal. This way, the signal-to-noise ratio can be increased. The effect will generally be larger, the more the angle of incidence of the detection beam deviates from the normal to the exit surface. However, it has to be of course avoided that the "critical" angle of total reflection is ever reached. Moreover, for angles close to this critical angle, the proportion of light of the detection light beam reflected at said exit surface will increase, thereby attenuating the intensity of the refracted detection light beam that actually reaches the detection device, such as a photodetector. Accordingly, the best choice for the angle of incidence may be a compromise between a larger degree of refraction and sufficient intensity of the refracted detection light beam. At any rate, the deviation of the angle of incidence from the normal to the axis surface should be at least <NUM>°, and it is preferably at least <NUM>° and most preferably at least <NUM> °. While this embodiment is advantageously used together with the curved contact surface, it is also possible to employ it in embodiments without such curved contact surface.

In a preferred embodiment, the detection light source is arranged such that said detection light beam is irradiated into said measurement body at an entrance surface, propagates through a portion of said measurement body and exits from the measurement body at an exit surface, wherein a focusing lens is attached to or formed integrally with the entrance surface for focusing said detection light beam entering into said measurement body in at least one dimension and/or a collimating lens is attached to or formed integrally with the exit surface for collimating said detection light beam in at least one dimension.

The inventors have noticed that the quality of the measurement is improved if the detection light beam is focused when it is reflected on the contact surface, which is also the area where it will interact with a thermal lens formed in the measurement body. For a clear characteristic deflection, it is advantageous if the diameter of the detection beam in this area is comparatively small, which can be achieved with said focusing lens. In other words, the purpose of the focusing lens is not necessarily to truly focus the detection light beam at some focal point, but rather to reduce its diameter at least in the area where it interacts with the thermal lens. However, this focusing implies that the detection light beam spreads out on its way towards the detection device. This is usually not of much concern if the detection device, such as a position sensitive detector, is arranged directly adjacent or at least close to the exit surface of the detection beam. However, the inventors found out that the signal-to-noise ratio of the measurement can be further increased if the distance between the exit surface and the detector is increased, as this will lead to a larger degree of deflection, for example manifested by a larger shift of the position where the detection light beam impinges on a position sensitive detector. Note that herein, the "larger degree of deflection" does not relate to a larger deflection angle, which would be one possible meaning of "degree of deflection", but a larger effect of the deflection as detected by the detection device. For example, the distance between the reflection at the contact surface of the measurement body and the detection at the detection device may be at least <NUM>, in some embodiments even <NUM> or more, thereby introducing a sort of leverage to the deflection as detected by the detection device. However, when the detection device is located at an appreciable distance from the exit surface, it is advantageous if the detection light beam is collimated after exiting the measurement body, to keep the diameter of the detection beam constant. It is nevertheless emphasized that it is not always necessary that the focusing and the collimation is done in both dimensions, in many practical applications, a certain spread in one of the directions can even be desired, as will be explained below. Accordingly, the focusing lens and/or other collimating lens have to be effective only at least in one dimension. Indeed, in preferred embodiments, at least one of said focusing lens and said collimating lens is a cylinder lens focusing and collimating the detection light beam at least predominantly in one dimension, respectively.

Moreover, by attaching the focusing lens and/or the collimating lens to the measurement body, or by even more preferably forming them integrally with it, no separate adjustment of these lenses during assembly of the apparatus or even during use of the apparatus is necessary. While this embodiment is advantageously used together with the curved contact surface, it is also possible to employ it in embodiments without such curved contact section.

In a preferred embodiment, said detector comprises a position sensitive detector on which said detection light beam impinges, wherein said position sensitive detector is sensitive for detecting shifts in position of the detection light beam impinging thereon in at least one sensing direction. Moreover, said position sensitive detector is arranged such that said deflection of said detection light beam leads to a shift of the position of the detection light beam impinging thereon in said at least one sensing direction. Finally, a cylinder lens is provided in the light path of the detection light beam for shaping the profile of the detection light beam such that the diameter of the detection light beam impinging on said position sensitive detector in said sensing direction is at least <NUM> times as large, preferably at least <NUM> times as large as the diameter of the detection light beam in a direction orthogonal to said sensing direction. The inventors noticed that when using a position sensitive detector that is sensitive for detecting shifts in the position of the detection light beam impinging thereon in at least one sensing direction, the signal-to-noise ratio, and in some embodiments, the linearity of the sensor output, can be increased if the beam profile is such that the light spot formed on the position sensitive detector is elongate in the sensing direction in the manner defined above. This is particularly true for position sensitive detectors measuring current differences at their respective ends. The elongate shape of the light spot according to this aspect of the invention is established using such cylinder lens. While this embodiment is advantageously used together with the curved contact surface, it is also possible to employ it in embodiments without such curved contact surface.

In a preferred embodiment, said cylinder lens is a collimating lens arranged in said light path of the detection light beam between its reflection at said contact surface and said position sensitive detector, wherein said cylinder lens is arranged to collimate said detection light beam at least predominantly (but possibly exclusively) in a dimension orthogonal to said sensing direction of said position sensitive detector, wherein said cylindrical collimating lens is preferably formed integrally with an exit surface of said measurement body at which the detection light beam exits from the measurement body.

In addition or alternatively, the position sensitive detector may be arranged at an angle deviating from <NUM>° from the detection light beam, such that due to this angle, an elongate light spot is formed on the position sensitive detector having its larger extension in the sensing direction.

In a preferred embodiment, the apparatus further comprises a beam splitter for splitting a source light beam into said detection light beam and a reference light beam, wherein said reference light beam is likewise directed to be totally or partially reflected at a surface of said measurement body that is in thermal or pressure-transmitting contact with said material, but in a region where any effect of heat or pressure waves received from the material upon absorption of excitation radiation is negligible. Moreover, said apparatus comprises a further detection device for detecting a degree of deflection, in particular a deflection angle, of the reference light beam after its reflection at said contact surface, wherein said detection device preferably comprises a photodetector, in particular a position sensitive photodetector.

This reference light beam is exposed to the same types of external influences as the detection light beam, except for the heat or pressure waves due to absorption of excitation radiation. Accordingly, by measuring possible deflections of the reference light beam, these external influences can be accounted for and removed from the measurement results obtained with the detection light beam. While in the preferred embodiment, the additional reference beam is combined with the curved contact surface, it is also possible to employ it in embodiments without such curved contact surface.

According to a further aspect of the invention, on said contact surface, a protrusion is formed, said protrusion having a front surface facing said material and being in contact with the material when the material is brought in contact with the contact surface, and in that said excitation radiation is irradiated into the material through said front surface of said protrusion.

The inventors have noticed that one critical aspect of the measurement procedure carried out by the apparatus is a reliable and consistent transmission of the excitation radiation into the material. In some of the apparatus described by the present applicant in the prior applications cited above, excitation radiation was guided through the measurement body such as to enter the material at the interface between the contact surface of the measurement body and the material, and it was seen that at this interface, indeed the excitation radiation could generally be coupled very well into the material. This has been found to be particularly true in applications where the material was formed by a fingertip of a user, and where the apparatus was used for measuring the glucose content in the skin. In this case, the fingertip was placed firmly on the contact surface of the measurement body, thereby establishing sufficient optical coupling to allow the excitation radiation to enter the material through the contact surface of the measurement body.

However, extensive research has shown that imperfect and especially unstable optical coupling can be a source of inaccuracy of the measurement. In particular, the inventors noticed that the optical coupling may change during the course of a single measurement, i.e. without intentionally moving the fingertip on or even off the contact surface. If the optical coupling changes during the course of the measurement, this leads to a variation of the intensity of the excitation radiation actually absorbed by the analyte, and hence to a change in the response signal which change is unrelated to the absorptivity of the analyte at the excitation radiation wavelength or the analyte concentration. In other words, loss of optical coupling during part of the measurement could be misinterpreted as reduced absorptivity at the given excitation wavelength. Assessing the analyte spectrum typically involves measuring the absorption at a plurality of characteristic wavelengths, for example wavelengths corresponding to peaks or local absorption minima of the analyte absorption spectrum, and further involves mathematical combinations of response signals associated with different wavelengths, for example subtracting a response signal obtained at a local minimum of the absorption spectrum from that of an absorption peak. It is therefore understandable that in case the optical coupling and hence the effective intensity of the excitation radiation in the material changes between measurements are different wavelengths, or even during a measurement at a certain wavelength, artefacts and inaccuracies in the measurement results may occur.

It was not obvious for the inventors that an unstable optical coupling would be a significant source of error, and it is even less apparent precisely why the optical coupling between the contact surface and the material should significantly change during the measurement, since the fingertip is not intentionally moved for the duration of the measurement. One possible cause could be that the user inadvertently fails to keep the contact pressure between the finger and the contact surface constant. Another possible cause could be that the user inadvertently moves the fingertip slightly on the contact surface, and that a very small movement could have unexpectedly large effects. This could for example be the case where the fingertip moves between a position where the excitation radiation enters the skin at an epidermal ridge of the fingertip and a position where it enters the skin at a position between two epidermal ridges, where reduced optical coupling may occur.

Irrespectively of the precise underlying reasons, the inventors noticed that the optical contact and its consistency can be improved if a protrusion is formed on the contact surface, said protrusion having a front surface facing the material and being in contact with the material when the material is brought in contact with the contact surface, and if the excitation radiation is irradiated into the material through said front surface of said protrusion. Namely, at the front surface of the protrusion, the local contact pressure is found to be significantly higher than on a flat contact surface if the same total force is applied by the finger against the contact surface. This local increase in contact pressure allows for a better optical coupling, and in particular, a more consistent optical coupling during the course of the measurement.

Note that the protrusion does not only allow for an improved optical coupling, but also for an improved thermal or pressure transmitting coupling. Accordingly, the protrusion will also in many cases promote an improved transfer of heat or pressure waves generated by absorption of excitation radiation in the material to be transferred to the measurement body. While not an embodiment of the presently claimed invention, is also considered herein to use such a protrusion even if the excitation radiation is not irradiated into the material through the front surface thereof. The additional advantages of the protrusion can be exploited both in embodiments including a curved part of the contact surface where the detected light beam is reflected, but also independently therefrom.

In a preferred embodiment, the front surface is flat. However, the invention is not limited to this, and particularly in cases where the detection relies on a reflected detection beam, as will be described below, a curved front surface may also be of advantage. That is to say, in some embodiments, the "curved part of the contact surface", where the detection light beam is reflected, may be formed by the curved front surface of the protrusion. In particular, in cases where the curved part or portion of the contact surface is convex, this curved portion may form a protrusion in itself.

In a preferred embodiment, said protrusion has a footprint area of less than <NUM><NUM>, preferably less than <NUM><NUM>, more preferably less than <NUM><NUM>, even more preferably less than <NUM><NUM> and most preferably less than <NUM><NUM>.

In a preferred embodiment, said protrusion has a tapering shape with one or more sidewalls tapering towards said front surface. This tapering shape implies that the front surface may be smaller than the footprint area, and hence leads to an even higher local contact pressure. The tapering sidewalls also add to the stability of the protrusion. Moreover, in some embodiments, where the detection relies on a reflected detection beam, the tapering sidewalls make it easier for the detection light beam to enter the protrusion while keeping the contact surface small, as will be apparent from the description of detailed embodiments below.

In some embodiments, said protrusion has a footprint which is of circular, oval, or square shape.

In particularly preferred embodiments, the protrusion is ridge shaped, having a longer extension in a first direction and a shorter extension in a second direction orthogonal to the first direction, wherein the longer extension exceeds the shorter extension by a factor of at least <NUM>, preferably of at least <NUM>, more preferably of at least <NUM>, and most preferably of at least <NUM>. Herein, the expression that "the longer extension exceeds the shorter extension by a factor of at least <NUM>" would mean that if the shorter extension was <NUM>, the longer extension would be at least <NUM>.

In a preferred embodiment, a pressure sensor is provided for measuring the contact pressure between the material and the measurement body. Herein, the apparatus preferably further comprises a control system configured for receiving signals from said pressure sensor indicating the contact pressure between the material and the measurement body, wherein said control system is configured to check whether said contact pressure is below a predetermined threshold value. In case it is found that the contact pressure is below said threshold value, the control system is configured to one or more of.

In other words, while the protrusion helps for establishing high contact pressure precisely where it is needed, i. at the front surface where the excitation radiation is coupled into the material, the reliability can be even further improved if the contact pressure is monitored, and if insufficient contact pressure is indicated to the user, such that it can be corrected. Moreover, by preventing the analyte measurement process from starting, or interrupting an analyte measurement process that is already underway, it can be avoided that incorrect measurement results are obtained in case of insufficient contact pressure.

In a preferred embodiment, said apparatus further comprises a clamping device, said clamping device comprising a clamping member movable between an open position in which the clamping member is moved away from the contact surface of the measurement body, and a closed position, in which it is close to said contact surface, said clamping member being biased towards the closed position. The material can be placed on the contact surface when the clamping member is in the open position, and said clamping member is suitable for pressing said material against the contact surface due to the biasing force towards the closed position. This way, a predefined contact pressure can be ensured.

In a preferred embodiment, the aforementioned said pressure sensor is arranged on said clamping device. While in the preferred embodiment, the clamping device is combined with the curved contact surface, it is also possible to employ it in embodiments without such curved contact surface.

In a preferred embodiment, said measurement body is transparent for said excitation radiation, wherein said excitation radiation source is configured for providing said excitation radiation as an excitation beam. Moreover, the excitation radiation source is arranged such that said excitation beam is irradiated into said measurement body at an entrance surface thereof, propagates through a portion of said measurement body and exits from the measurement body at said contact surface. In previous apparatus, the applicant ensured that the excitation radiation beam impinges onto the entrance surface at an angle of <NUM>°, such as to avoid refraction and excess reflection of the excitation radiation beam at the entrance surface. However, extensive research has revealed that a further cause of inadvertent variation of the excitation radiation actually reaching the material is a possible interference of the excitation radiation emitted from the excitation radiation source with excitation radiation that is reflected back from the entrance surface of the measurement body. This interference indeed was found to lead to fluctuations in the intensity of excitation radiation in the material, and to hence immediately lead to variations in the response signals that is unrelated to the analyte concentration. Moreover, the inventors found that by slightly tilting the angle of incidence of the excitation beam, this effect can be suppressed, and the accuracy and reliability of the measurement can be improved. Accordingly, in this embodiment, the excitation beam is directed to impinge on the entrance surface at an angle of <NUM>° or less, preferably <NUM>° or less, and most preferably <NUM>° or less. This way, the inadvertent interference can be reliably prevented. A further advantageous effect of this is that it can be prevented that excitation radiation is reflected back into the excitation radiation source which could possibly be damaged by it. On the other hand, the angle of incidence should not deviate too much from <NUM>°, such as to avoid losses due to excessive reflection. Accordingly, in this embodiment, the angle of incidence should be <NUM>° or more, preferably <NUM>° or more and most preferably <NUM>° or more. While this embodiment is advantageously used together with the protrusion on the contact surface, it is also possible to employ it in embodiments without such protrusion.

In a preferred embodiment, said excitation beam impinges on the contact surface of the measurement body at an angle of <NUM>° ± <NUM>°, to thereby keep losses due to reflection at the contact surface to a minimum.

In a preferred embodiment, the entrance surface and the contact surface at the respective portions thereof where the excitation beam enters and leaves the measurement body, respectively, are inclined with respect to each other with an angle of <NUM>° or more, preferably <NUM>° or more, and most preferably <NUM>° or more, and <NUM>° or less, preferably <NUM>° or less and most preferably <NUM>° or less. Graphically speaking, the measurement body according to this embodiment may have a slightly "wedge-like" shape, which allows for establishing both, the slight inclination of the excitation beam at the entrance surface and the orthogonal orientation thereof at the contact surface.

In preferred embodiments, said material is human tissue, in particular human skin, and said analyte is glucose present in the skin, in particular in the interstitial fluid thereof.

In preferred embodiments, said excitation radiation is generated using an array of lasers, in particular quantum cascade lasers, each having a dedicated wavelength.

In alternative preferred embodiments, said excitation radiation is generated using at least one tunable laser, in particular at least one tunable quantum cascade laser.

In a preferred embodiment, some or all of said excitation wavelengths are in a range of <NUM> to <NUM>, preferably <NUM> to <NUM>. In alternative embodiments, some or all of said excitation wavelengths are in a range of <NUM> to <NUM>. This wavelength range is for example useful for detecting absorption of CH<NUM> and CH<NUM> vibrations in fatty acids.

Also enclosed herein is a method for analyzing a material comprising at least one analyte, said method comprising.

In a preferred embodiment of the method, said front surface is flat.

In a preferred embodiment of the method, said protrusion has a footprint area of less than <NUM><NUM>, preferably less than <NUM><NUM>, more preferably less than <NUM><NUM>, even more preferably less than <NUM><NUM> and most preferably less than <NUM><NUM>.

In a preferred embodiment of the method, said protrusion has a tapering shape with one or more sidewalls tapering towards said front surface.

In a preferred embodiment of the method, said protrusion has a footprint which is of circular, oval, or square shape.

In a preferred embodiment of the method, the protrusion is ridge shaped, having a longer extension in a first direction and a shorter extension in a second direction orthogonal to the first direction, wherein the longer extension exceeds the shorter extension by a factor of at least <NUM>, preferably of at least <NUM>, more preferably of at least <NUM>, and most preferably of at least <NUM>.

In a preferred embodiment of the method, the contact pressure between the material and the measurement body is measured.

Preferably, the method further comprises a step of checking whether said contact pressure is below a predetermined threshold value, and in case it is found that the contact pressure is below said threshold value, carrying on one or more of the following steps:.

Preferably, the method further comprises a step of fixing said material to the contact surface using a clamping device, said clamping device comprising a clamping member movable between an open position in which the clamping member is moved away from the contact surface of the measurement body, and a closed position, in which it is close to said contact surface, said clamping member being biased towards the closed position, wherein said material is placed on the contact surface when the clamping member is in the open position, and wherein said clamping member presses said material against the contact surface due to the biasing force towards the closed position.

In a preferred embodiment of the method, said pressure sensor is arranged on said clamping device.

In a preferred embodiment of the method, said measurement body is transparent for said excitation radiation,.

In a preferred embodiment of the method, said excitation beam impinges on the contact surface of the measurement body at an angle of <NUM>° ± <NUM>°.

In a preferred embodiment of the method, the entrance surface and the contact surface at the respective portions thereof where the excitation beam enters and leaves the measurement body, respectively, are inclined with respect to each other with an angle of <NUM>° or more, preferably <NUM>° or more, and most preferably <NUM>° or more, and <NUM>° or less, preferably <NUM>° or less and most preferably <NUM>° or less.

In a preferred embodiment of the method, said detection comprises generating a detection light beam travelling through at least a portion of said measurement body or a component included in said measurement body,.

In a preferred embodiment of the method, said measurement body is transparent for said detection light beam, said detection light beam is directed to be totally or partially reflected at a surface of said measurement body that is in thermal or pressure-transmitting contact with said material, and wherein said detection comprises detecting a degree of deflection, in particular a deflection angle, of the detection light beam after its reflection at said contact surface, due to said local change in refractive index, wherein said detection is preferably carried out using a photodetector, in particular a position sensitive photodetector.

In a preferred embodiment of the method, said detection light beam is directed to be totally or partially reflected at said front surface of said protrusion that is in thermal or pressure-transmitting contact with said material.

In a preferred embodiment of the method, the front surface of the protrusion is curved in at least one principal direction. Herein, said curvature in said at least one principal direction corresponds to a radius of curvature in a range of <NUM> to <NUM>, preferably <NUM> to <NUM>. Said curvature in said at least one principal direction is one of concave or convex.

In a preferred embodiment of the method, the detection light beam prior to and after reflection at said front surface defines a detection light plane, and said at least one principal direction lies within said detection light plane or forms an angle with the detection light plane that is less than <NUM>°, preferably less than <NUM>°.

In a preferred embodiment of the method, the detection light beam prior and after reflection at said front surface defines a detection light plane, and said first direction is parallel with said detection light plane or forms an angle with the detection light plane that is less than <NUM>°, preferably less than <NUM>°.

In a preferred embodiment of the method, the detection light source is arranged such that said detection light beam is irradiated into said measurement body at an entrance surface, propagates through a portion of said measurement body and exits from the measurement body at an exit surface, wherein the detection beam impinges - in absence of any deflection due to said local change in refractive index - on the exit surface at an angle of <NUM>° or more, preferably <NUM>° or more and most preferably <NUM>° or more with respect to the normal to the exit surface, such that the detection beam is refracted upon exiting from the exit surface of the measurement body, wherein the orientation of the exit surface with respect to the detection light beam is such that said deflection of the detection light beam in response to said heat or pressure waves being transferred to said measurement body increases said angle of said detection light beam to the normal to the exit surface.

In a preferred embodiment of the method, said detection light beam is irradiated into said measurement body at an entrance surface, propagates through a portion of said measurement body and exits from the measurement body at an exit surface, wherein a focusing lens is formed integrally with the entrance surface for focusing said detection light beam entering into said measurement body in at least one dimension and/or a collimating lens is formed integrally with the exit surface for collimating said detection light beam in at least one dimension. Herein, at least one of said focusing lens and said collimating lens is preferably a cylinder lens focusing and collimating the detection light beam at least predominantly in one dimension, respectively.

In a preferred embodiment of the method, said detector comprises a position sensitive detector on which said detection light beam impinges, wherein said position sensitive detector detects shifts in position of the detection light beam impinging thereon in at least one sensing direction,.

In a preferred embodiment of the method, said cylinder lens is a collimating lens arranged in said light path of the detection light beam between its reflection at said contact surface and said position sensitive detector, wherein said cylinder lens collimates said detection light beam at least predominantly in a dimension orthogonal to said sensing direction of said position sensitive detector, wherein said cylinder collimating lens is preferably formed integrally with an exit surface of said measurement body at which the detection light beam exits from the measurement body.

In a preferred embodiment of the method, a source light beam is splitted into said detection light beam and a reference light beam, wherein said reference light beam is likewise directed to be totally or partially reflected at a surface of said measurement body that is in thermal or pressure-transmitting contact with said material, but in a region where any effect of heat or pressure waves received from the material upon absorption of excitation radiation is negligible, and wherein a degree of deflection, in particular a deflection angle, of the reference light beam after its reflection at said contact surface is detected, preferably using a photodetector, in particular a position sensitive photodetector.

In a preferred embodiment of the method, said detection comprises using an interferometric device allowing for assessing said change in phase of the detection beam and generating a response signal indicative of said change in phase.

In a preferred embodiment of the method, said measurement body or a component in said measurement body has electrical properties that change in response to a local change in temperature or a change in pressure associated therewith, and wherein said detection device comprises electrodes for capturing electrical signals representing said electrical properties.

In a preferred embodiment of the method, an optical fiber is embedded in said measurement body, a detection light source is provided at one end of said fiber for coupling detection light into said optical fiber and a mode detector is provided at the other end of said fiber, wherein using said mode detector, changes in optical modes of said detection light in response to the heat or pressure waves received by the measurement body from said material are detected, wherein said changes in optical modes preferably comprise a shift or a rotation of an interference pattern of optical modes at the mode detector.

In a preferred embodiment of the method, said material is human tissue, in particular human skin, and said analyte is glucose present in the skin, in particular in the interstitial fluid thereof.

Preferably, the method further comprises a step of generating said excitation radiation using an array of lasers, in particular quantum cascade lasers, each having a dedicated wavelength.

Preferably, the method further comprises a step of generating said excitation radiation using at least one tunable laser, in particular at least one tunable quantum cascade laser.

In a preferred embodiment of the method, some or all of said excitation wavelengths are in a range of <NUM> to <NUM>, preferably <NUM> to <NUM>.

It is to be understood that both the foregoing general description and the following description are exemplary and explanatory only and are not restrictive of the methods and devices described herein. In this application, the use of the singular may include the plural unless specifically stated otherwise. Also, the use of "or" means "and/or" where applicable or unless stated otherwise. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to various implementations of the example embodiments as illustrated in the accompanying drawings. The same reference signs will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.

<FIG> is a schematic illustration of the measurement principle underlying the analyte measurement procedure summarized above and described in more detail in the following. While the method and apparatus of the invention are suitable for analyzing various materials comprising at least one analyte, the following description will focus on specific embodiments where the material is the skin of a patient and the analyte is glucose within the interstitial fluid of the skin. It is to be understood that all details and explanations given in the following with specific reference to glucose measurement are considered in relation to other materials and analytes as well, where applicable, without explicit mention in the following.

In the illustration of <FIG>, a fingertip <NUM> of a user is brought in thermal contact with a contact surface <NUM> of a measurement body <NUM>. In an alternative implementation, which is not shown, the fingertip may be coupled acoustically to the measurement body through an acoustic cell which may include a hollow space filled with a liquid or gas allowing for a transfer of pressure waves to the measurement body. An excitation beam <NUM> is irradiated to the measurement body <NUM> through air or through a waveguide (not shown) and then through the measurement body <NUM> and into the skin at the fingertip <NUM>. In order to determine the concentration of the glucose in the skin, in particular in the interstitial fluid of the skin, various wavelengths of the excitation radiation <NUM> are chosen one after the other or at least partially at the same time for absorption measurement, such that from the measured absorption values the concentration of the glucose can be determined. In <FIG>, absorption spectra are shown for different concentrations of glucose in water, wherein the contribution of the absorption by water has been subtracted. As is seen therein, the glucose molecule has several characteristic absorption peaks in the mid infrared region at wave numbers ranging between <NUM>-<NUM> and <NUM>-<NUM>, corresponding to wavelengths ranging from <NUM> to <NUM>, respectively. In between adjacent absorption peaks, local absorption minima are seen, which are indicated by vertical arrows without wave numbers in <FIG>. As is apparent from <FIG>, particularly the difference in absorption at the absorption peaks and the local absorption minima are characteristic for the glucose concentration. Accordingly, in order to be able to determine the glucose concentration, it is preferable to measure the absorption at some or all of the absorption peaks and at some or all of the local absorption minima and potentially also at some point between the maxima and minima. These wavelengths are referred to as a "analyte(glucose)-characteristic-wavelengths" herein. While wavelengths exactly at the absorption peaks or local absorption minima are the preferred choices for the glucose-characteristic wavelengths, wavelengths close to the peaks/local minima but in an individually defined distance from them may also be used. Accordingly, as understood herein, "analyte-characteristic-wavelengths" are also wavelengths where the difference in absorption to that at the closest absorption peak or the closest local absorption minimum is less than <NUM>%, preferably less than <NUM>% of the difference in absorption between the closest absorption peak and closest local absorption minimum.

The intensity of the excitation beam <NUM> is time modulated with a certain frequency f, such that the excitation radiation, in this case excitation light has alternating intervals of high intensity and low or even vanishing intensity. Without wishing to limit the modulation to any particular waveform, high intensity intervals are referred to as "excitation light pulses" in the following. During the excitation light pulses, excitation light having the glucose-characteristic-wavelength will be absorbed, such that the radiation energy will be converted to heat. Since the glucose molecules relax from the excited state within approximately <NUM>-<NUM> s, the generation of a corresponding heat pulse and/or pressure wave can be regarded as occurring instantaneously for all practical purposes.

Accordingly, along with the excitation light pulses, local heat pulses are generated at the absorption site, leading to a temperature field that varies as a function of space and time and that could be referred to as a thermal wave. As was explained above, the term thermal "wave" is somewhat misleading, since the travel of heat through the material is not governed by a wave equation, but by a diffusion equation instead. However, the notion of a "heat wave" is correct at least to the extent that heat pulses propagate from within the skin to the surface <NUM> of the measurement body <NUM> and into the measurement body <NUM> similarly to what one is used to from wave propagation. A thermal gradient <NUM> that is caused by such a heat pulse is schematically shown in <FIG>.

The heat received by the measurement body <NUM> from the skin of the finger <NUM> causes a physical response that can be detected with one of various possible detection devices which are devised for generating a response signal based on the physical response, wherein this response signal is indicative of the degree of absorption of the excitation light. Various ways of detecting the physical response and generating suitable response signals will be described below.

However, irrespective of the precise way of detecting the physical response, it is worth noting that the maximum depth underneath the surface of the skin in which the absorption can be detected by means of heat pulses travelling to the measurement body <NUM> is found to be limited to a good approximation by the thermal diffusion length µt of the skin, which is defined as <MAT> and which depends on the density ρ, the specific heat capacity Cp, and the thermal conductivity kt of the material as well as on the modulation frequency f of the excitation light. In other words, by selecting the modulation frequency f, a depth can be defined up to which any absorption of the excitation light is reflected in the heat pulses received at the measurement body <NUM>.

With reference again to <FIG>, in the embodiment shown, the physical response to the absorption heat received from the skin is a change in refractive index in an area close to the surface <NUM> of the measurement body <NUM> where the heat gradient <NUM> is transiently formed. This local change in refractive index forms what could be regarded as a thermal lens that can be detected by means of a detection light beam <NUM>. The detection beam <NUM> is passing through the thermal lens or heat gradient region <NUM> and then reflected at the interface of the measurement body <NUM> and the skin of the finger <NUM>. Whenever a heat pulse is received from the skin, a local change in refractive index occurs, and this leads to a deflection of the detection beam <NUM> by the interaction with the material of the measurement body in the region of the thermal lens. In <FIG>, reference sign 22b corresponds to the non-deflected detection beam <NUM>, whereas reference sign 22a corresponds to the detection beam when it is deflected due to the thermal lens formed in the heat gradient region <NUM>. This deflection can be measured and forms an example of the aforementioned response signal. The degree of the deflection is indicative of the amount of heat received, and hence the degree of absorption of the excitation light <NUM> in the skin of the finger <NUM>. Herein, the "degree of deflection" may refer to a deflection angle, but more generally corresponds to any deviation between the detection light beams detectable by the corresponding detection device.

<FIG> shows a more detailed sectional view of an apparatus <NUM> that relies on the measurement principle as illustrated with reference to <FIG>. The apparatus <NUM> comprises a housing <NUM> which includes the measurement body <NUM>, having a top surface (contact surface) <NUM> on which a finger <NUM> rests. Within the housing <NUM>, an excitation light source <NUM> is provided, which generates the excitation light beam <NUM>. In the embodiment shown, the excitation light source <NUM> comprises an array of quantum cascade lasers each having a dedicated wavelength. For example, the array of quantum cascade lasers could include individual quantum cascade laser elements with wavelengths corresponding to the absorption peaks and local minima shown in <FIG> (i.e. the glucose-characteristic-wavelengths), as well as other wavelengths that can be used for reference measurements, or for detecting other substances that could be disturbing to the measurement of the glucose, for example lactate or albumin. The laser array may irradiate an excitation beam directly into and through the measurement body <NUM>, but it may as well irradiate into a light waveguide (not shown) which couples the laser array with the measurement body and leads the excitation beam in a curved or non-curved manner to the measurement body <NUM>. A light waveguide may as well be used in the case of generation of the excitation beam by a single tunable laser.

The apparatus <NUM> further comprises a light source <NUM>, for example a laser, for emitting the detection beam <NUM>, as well as a position-sensitive detector <NUM> which allows for detecting the deflection of the detection light beam <NUM>. Note that as understood herein, the term "light beam" is not restricted to light in the visible range, although in preferred embodiments, the detection light beam <NUM> will indeed be in the visible range of the light spectrum. The measurement body <NUM> in this case is transparent for both, the excitation light beam <NUM> as well as the detection light beam <NUM>. In addition, a camera <NUM> or another imaging device is provided that allows for taking images of the contact surface <NUM> of the optical medium <NUM> in the direction from the inside of the measurement <NUM> body to the finger <NUM> , to thereby record a fingerprint of the finger <NUM> resting on the contact surface <NUM>. This fingerprint can be processed by a control unit <NUM> such as to identify a user via his or her fingerprint. The control unit <NUM> also serves for controlling the light sources <NUM> and <NUM> for the excitation light and the detection light, respectively, as well as the sensor <NUM>. The control unit <NUM> is also in wireless connection with an external data processing device <NUM> to exchange data. For example, via the wireless connection, user-specific calibration data can be retrieved by the control unit <NUM> for the user that is identified via the fingerprint. The control unit <NUM> and the external data processing device <NUM> together form an example of a "control system" as referred to herein.

The control system can be comprised by one or more processors, microcontrollers, computers, ASICs, FPGAs, or the like. The control system may be distributed, as indicated in <FIG>, with various components in data communication with each other, or could be formed by a single control unit, such as the control unit <NUM>, which would be devised for all of the control functionalities described herein. The control system may be generally embodied in hardware, in software, or a combination of both.

As is further seen in <FIG>, the excitation and detection light sources <NUM> and <NUM>, as well as the position sensitive detector <NUM> are all attached to a common carrier structure <NUM>. This means that these components can be preassembled with precision on this structure <NUM>, such that they do not need to be individually adjusted or calibrated when assembling the apparatus <NUM>. One or more of the excitation and/or detection light sources <NUM> and <NUM>, as well as the position sensitive detector <NUM> may also be directly mounted on the measurement body <NUM> in order to avoid additional adjustment or calibration.

In addition, the apparatus <NUM> comprises a corneometric device <NUM> that allows for measuring the water content of the skin. Corneometric devices for measuring the water content in the upper layer of the skin are per se known in the art and need not be described in detail here. For example, known corneometric devices measure the impedance, in particular capacitive impedance of the skin using two interdigital electrodes to which an AC voltage is applied. The corneometric device <NUM> of <FIG> is in contact with the fingertip <NUM> when the latter rests on the contact surface <NUM> of the measurement body <NUM>.

The apparatus also comprises a pH-sensor <NUM> for measuring the pH value of the skin. pH sensors for measuring the pH value on surfaces, including those of the skin are per se known from prior art and need not be described in detail herein. pH sensors for measuring the pH value of the skin are commercially available for medical but also for cosmetic purposes.

<FIG> shows results of a Clarke's error grid analysis obtained with an apparatus of the type shown in <FIG>, illustrating that with the measurement procedure described with reference to <FIG>, indeed very reliable blood sugar concentrations can be measured in a purely non-invasive manner. The data shown in <FIG> are taken from <CIT> and do not yet reflect improvements of the present invention. The present invention allows for improving the reliability of the method even further, as will be described below.

<FIG> schematically shows an apparatus <NUM> which relies on the same general principle involving absorption of heat pulses received by the measurement body <NUM> from the material <NUM> as that of <FIG> and <FIG>, but differs in the physical response exploited and the way the corresponding response signals are generated. Such an apparatus <NUM>, as well as a large number of variations thereof are described in detail in <CIT> incorporated herein by reference, such that a detailed description may be omitted herein. As before, the apparatus comprises a measurement body <NUM> having a contact surface <NUM> which is brought in contact or coupling with the skin of a finger <NUM>. Also, a source <NUM> for an excitation light beam <NUM> with modulated intensity is provided, which is irradiated into a region <NUM> underneath the surface of the skin <NUM> and absorbed therein. In this embodiment, the excitation light beam <NUM> runs through a bore <NUM> in the measurement body <NUM> indicated by hashed lines through the measurement body <NUM>, such that the material of the measurement body <NUM> itself need not be transparent for it.

A control unit <NUM> is provided for modulating the intensity of the excitation light beam <NUM>. This can generally be done in various ways, including a mechanical chopper or an element having a transmissivity or reflectivity that can be electronically controlled. However, in preferred embodiments, the intensity is modulated by modulating the on/off times of the excitation light source <NUM> as well as the operating current during the on-times thereof.

A thermal wave caused by the time varying absorption of the intensity modulated excitation beam <NUM> in the region <NUM> of the skin <NUM>, which is symbolically represented by arrows <NUM>, enters the measuring body <NUM> where it can be detected in a detection region <NUM> which has piezoelectric properties. Pressure changes associated with received heat <NUM> or pressure waves lead to electrical signals in the form of voltage changes that can be recorded with electrodes 6a to 6d, which are connected via conducting leads <NUM> with an evaluation device <NUM> for analyzing the material (the skin of figer <NUM>), which may be a digital processing device, for example a microcontroller or processor or a computer. In this case, the change in pressure resembles the physical response of the measurement body <NUM>, or other component included therein, to heat received from the material <NUM> upon absorption of the excitation radiation, which is detected using the piezoelectric properties of the measurement body <NUM> or parts of it or piezoelectric elements embedded into the measurement body and the electrodes 6a to 6d, and which leads to electrical signals representing the response signal that is indicative of the degree of absorption of excitation radiation <NUM>.

In alternative variants suggested by the present applicant, as for example disclosed in international application <CIT> included herein by reference, the detection device may comprise an interferometric device which may be embedded into the measurement body and which is allowing for assessing said change in phase of a first part of the detection beam with respect to a second part of the detected beam, wherein only one of the parts of the detection beam passing through a measurement arm is affected by the effects of the heat or pressure wave in the measurement body, and generates on the output side of the interferometric device a response signal indicative of said change in phase in the measurement arm. In this case, the physical response of the measurement body <NUM> (or a component included therein) to heat received from said material <NUM> upon absorption of said excitation radiation <NUM> is again a local change in index of refraction, while the response signal is in this case an interferometric signal reflecting a change in the phase of one part of the detection beam due to the local change in refractive index. This is schematically illustrated in <FIG>, where a measurement body <NUM> is shown which is to be brought in contact with the material (such as a finger, not shown in <FIG>). In this case, the measurement body <NUM> may be a silicon substrate in which a light guiding structure <NUM> is provided, which forms an interferometric device <NUM>. The interferometric device <NUM> forms a Mach-Zehnder interferometer, having a measurement arm 60a and a reference arm 60b. Detection light <NUM> generated by a detection light source <NUM> is fed into the light guiding structure <NUM> and is splitted by a splitter 60c into a portion or a part of the detection beam travelling along the measurement arm 60a and a portion or part travelling along the reference arm 60b, which portions are then united by a combiner 60d. The measurement body <NUM> is used or arranged such that the reference arm 60a is exposed to heat received from the skin upon absorption of excitation light, but not, or at least to a much lesser extent, the reference arm 60b. Due to received heat, the refractive index in the measurement arm 60a will change, which in turn leads to a phase shift of the detection light <NUM> travelling along the measurement arm 60a. Since the light travelling along the reference arm 60b is unaffected by the heat received, there will be a change in relative phase of the two portions of light combined by the combiner 60d, which leads to an interference pattern that can be detected using a detector <NUM> It should be noted that a camera as shown in <FIG> for detecting and analyzing a fingerprint may as well be combined with measurement bodies <NUM> and apparatus as shown in <FIG>.

<FIG> shows a schematic illustration of an apparatus <NUM> according to an embodiment in a side sectional view. <FIG> shows the same apparatus <NUM> in a front sectional view. In the embodiment shown in <FIG> and <FIG>, the material is again the skin of a finger <NUM> of a user, and the analyte to be assessed is glucose content in the skin, and in particular, in the interstitial fluid therein. The embodiment of <FIG> and <FIG> allows for ensuring a reliable and consistent transmission of excitation radiation <NUM> into the skin of the finger <NUM>. The measurement body <NUM> shown in <FIG> and <FIG> is transparent for excitation radiation <NUM>, and has, in addition to the contact surface <NUM>, an entrance surface <NUM> for the excitation radiation <NUM>, which is the bottom surface in the illustration of <FIG> and <FIG>.

The measurement body <NUM> further has an entrance surface <NUM> for the detection light beam <NUM>, which in the illustration of <FIG> corresponds to the left side wall, and an exit surface <NUM> corresponding to the right side wall. Integrally with the entrance surface <NUM> and exit surface <NUM>, a corresponding focusing lens <NUM> and collimating lens <NUM>, respectively, are formed. In the shown embodiment, the focusing and collimating lenses <NUM> and <NUM> are formed monolithically with the remainder of the measurement body <NUM>. In other embodiments, the focusing and collimation lenses <NUM> and <NUM> could be formed separately from the measurement body <NUM>, but could be attached to the entrance and exit surfaces <NUM>, <NUM>, respectively, such that no individual adjustment thereof is necessary.

Moreover, on the contact surface <NUM> of the measurement body <NUM>, a protrusion <NUM> is formed. The protrusion <NUM> has a front surface <NUM> that is in contact with the skin of the finger <NUM>, and through which the excitation radiation <NUM>, which in the present embodiment is formed by an excitation light beam <NUM> in the mid-IR range, is radiated into the skin. The protrusion <NUM> has four side walls <NUM>, which are each tapering towards the front surface <NUM>. This way, the area of the front surface <NUM> is smaller than the footprint area of the protrusion <NUM> on said contact surface <NUM>. As is seen from the comparison of <FIG> and <FIG>, the protrusion <NUM> is ridge shaped, having a longer extension in a first direction which is the x-direction lying in the paper plane of <FIG>, and a short extension in a second direction orthogonal to the first direction, which is the y-direction lying in the paper plane of <FIG>.

Finally, on the contact surface <NUM>, pressure sensors <NUM> are disposed, which measure the contact pressure between the finger <NUM> and the contact surface <NUM>. The pressure sensors <NUM> are connected with a control system, such as the control unit <NUM> of <FIG> (not shown).

Next, the function of the various features shown in <FIG> and <FIG> will be explained. As is seen in <FIG>, the entrance surface <NUM> for the excitation radiation <NUM> is not parallel to the contact surface <NUM> or the front surface <NUM> of the protrusion <NUM>. Instead, the measurement body is slightly wedge-shaped. Moreover, the excitation light source <NUM> is arranged such that the excitation beam <NUM> impinges on the entrance surface at an angle that deviates from <NUM>°.

This is different from the arrangement shown for example in <FIG>, where the excitation beam is deliberately caused to impinge onto the entrance surface at an angle of <NUM>°, such as to avoid refraction and excess reflection of the excitation radiation beam at the entrance surface. However, as was explained above, with such an arrangement, part of the excitation radiation <NUM> will be reflected from the entrance surface <NUM> and may interfere with the excitation radiation <NUM> emitted from the excitation radiation source <NUM>. The inventors found that this interference can lead to fluctuations in the intensity of excitation radiation <NUM> in the skin of the finger <NUM>, thereby leading to artificial variations in the response signals that are completely unrelated to the analyte concentration. However, when avoiding perpendicular incidence of the excitation beam <NUM> on the entrance surface <NUM>, interference between the incoming radiation <NUM> and the reflected radiation <NUM>' shown in <FIG> can be suppressed, and the accuracy and reliability of the measurement as a whole can be improved.

In preferred embodiments, the angle of incidence should deviate from <NUM>° only by a few degrees, if at all. Favorable angles of incidence could be <NUM>° or less, preferably <NUM>° or less, and more preferably <NUM>° or less. The optimum choice for the angle will also depend on the distance between the excitation radiation source <NUM> and the entrance surface <NUM>. The deviation from <NUM>° should not be chosen larger than necessary for reliably avoiding the undesired interference effects. In preferred embodiments, the angle of incidence is therefore <NUM>° or more, preferably <NUM>° or more, and most preferably <NUM>° or more.

The protrusion <NUM> has the special technical effect that the local contact pressure between the finger <NUM> and the measurement body <NUM> is increased. More precisely, the increased contact pressure occurs at the front surface <NUM> of the protrusion <NUM>, which is where the excitation light beam <NUM> is coupled from the measurement body <NUM> into the skin of the finger <NUM>. This increased contact pressure allows for ensuring a good and reliable optical coupling between the measurement body <NUM> and the skin.

The significant improvement obtainable with this protrusion <NUM> came as a surprise to the inventors, because in general, sufficient optical coupling was obtained in previous apparatuses of the applicant having an entirely flat contact surface <NUM>, such that it was not apparent that the additional manufacturing costs and increased complexity involved with providing the protrusion <NUM> would be worth the effort.

However, the inventors found out that although the optical coupling with an entirely flat contact surface <NUM> generally appears satisfactory, particularly inconsistent or unstable optical coupling could be a source of inaccuracy of the measurement. As was explained in the summary above, the inventors noticed that the optical coupling may change during the course of a single measurement, i.e. without intentionally moving the fingertip on or even off the contact surface. This was found in some cases to cause a variation of the intensity of the excitation radiation actually absorbed by the analyte, and hence to a change in the response signal which was unrelated to the absorptivity of the analyte at the excitation radiation wavelength or the analyte concentration. In other words, loss of optical coupling during part of the measurement could be misinterpreted as reduced absorptivity at the given excitation wavelength. As was also explained above, assessing the analyte spectrum typically involves measuring the absorption at a plurality of characteristic wavelengths, for example wavelengths corresponding to peaks or local absorption minima of the analyte absorption spectrum, and further involves mathematical combinations of response signals associated with different wavelengths. For example, a response signal obtained at a local minimum of the absorption spectrum may be subtracted from that of an absorption peak to give a value representing the concentration of the glucose in the skin. Clearly, any variation in the optical coupling and hence the effective intensity of the excitation radiation in the material between measurements at different wavelengths, or even during a measurement at a certain wavelength, may lead to artefacts or inaccuracies in the measurement results.

As was explained in the summary of the invention, it is not entirely clear precisely why the optical coupling between the contact surface and the material should change during the measurement, e.g. whether it is because the user fails to keep the contact pressure between the finger and the contact surface constant, or whether the user inadvertently moves the fingertip on the contact surface. Irrespectively of the precise underlying reasons, the inventors noticed that the optical contact can be significantly stabilized using a protrusion such as the protrusion <NUM> as shown in <FIG> and <FIG>, having front surface <NUM> in contact with the skin of the finger <NUM> with a local increased contact pressure. The front surface may have a size of less than <NUM><NUM>, in particular less than <NUM><NUM> and may be flat or curved in a concave or convex manner.

To further ensure constant contact pressure during the measurement, pressure sensors <NUM> are provided. The pressure sensors <NUM> generate signals indicating the contact pressure between the finger <NUM> and the contact surface <NUM> of the measurement body <NUM>. The signals are conveyed to a control system (not shown) which is configured to check whether the sensed contact pressure is below a predetermined threshold value. If this is found to be the case, this is indicated to the user by means of a suitable output device, such as a display, a light signal, an acoustic signal or the like, such that the user can be prompted to increase the contact pressure. Moreover, the control system is configured to prevent an analyte measurement process from starting while the contact pressure is below the threshold, thereby avoiding measurements to be carried out that have doubtful quality and possibly have to be repeated, which can lead to impatience or frustration of the user. Moreover, if during the measurement it is found that the contact pressure falls below the threshold, the analyte measurement process is interrupted, again giving the user the opportunity to resume the original contact pressure, such that the measurement can be completed. A pressure sensor could also be located underneath the protrusion <NUM> in the measurement body <NUM> and be implemented as a piezoelectric element (not shown) which may be transparent for the excitation beam.

Note that each of the features and functionalities explained so far with reference to <FIG> are related with reliably coupling a consistent amount of excitation radiation into the skin of the finger <NUM>, and are as such independent of the specific type of physical response of the measurement body (or a component included therein) to heat or pressure waves received from the skin of finger <NUM> or of the detection device generating corresponding response signals. Accordingly, these features can be used in combination with any of the variants shown in <FIG>, <FIG>, <FIG>.

In the embodiment of <FIG>, the physical response to heat or pressure waves received by the measurement body <NUM> is a local change in refractive index, and this physical response is detected via a deflection of a detection light beam <NUM> that is reflected at the front surface <NUM> of the protrusion <NUM>. As is seen in <FIG>, the detection light beam <NUM> is generated by a detection light source <NUM>, and the deflection of the detection light beam <NUM> is detected using a position sensitive detector (PSD) <NUM>, which may also be referred to as a position sensitive device. As understood herein, the "deflection of the detection light beam <NUM>" denotes the total deviation of the detection light beam at the corresponding detection device, and with specific reference to the embodiment of <FIG>, it denotes the shift of the position of the detection light beam <NUM> on the PSD <NUM>. This deviation is the combined effect of all changes to the propagation of the detection light beam <NUM> along its light path that are caused by the local change in refractive index.

The specific ridge-shaped geometry of the protrusion <NUM> shown in <FIG> and <FIG> has been adapted to this detection setup. The detection light beam <NUM> prior to and after reflection at the front surface <NUM> of the protrusion <NUM> define a detection light plane, which in the embodiment of <FIG> and <FIG> corresponds with the x-z-plane, i.e. the paper plane of <FIG>, and this detection light plane coincides with the first, longer direction of the ridge-shaped protrusion <NUM>. Indeed, as is apparent from <FIG>, this larger extension of the protrusion <NUM> in the detection light plane is necessary such that the incoming and reflected detection light beams <NUM> fit into the protrusion <NUM>. Conversely, as is apparent from <FIG>, in a direction perpendicular to this detection light plane, i.e. the "second direction" of the ridge-shaped protrusion <NUM>, the extension can be made much smaller, to thereby keep the surface area of the front surface <NUM> as a whole small and hence increase the local contact pressure.

Moreover, the focusing lens <NUM> at the entrance surface <NUM> of the detection light beam <NUM> allows for keeping the detection light beam diameter <NUM> narrow in the region where it is reflected on the front surface <NUM> of the protrusion <NUM>, which is also the region where the thermal lens (not shown in <FIG>) will be formed. A narrow beam diameter promotes a clear and characteristic deflection of the detection light beam <NUM> at the thermal lens, which itself is only of comparatively small size.

The collimating lens <NUM> at the exit surface <NUM> of the detection beam <NUM> allows for keeping the diameter of the detection beam <NUM> at least nearly constant on its travel between the exit surface <NUM> and the PSD <NUM>. This allows for increasing the distance between the PSD <NUM> and the exit surface <NUM>, which means that any deflection angle acquired by the detection beam <NUM> will lead to a larger shift of the position where the detection beam <NUM> impinges on the PSD <NUM>, thereby increasing the signal-to-noise ratio of the response signal. For example, the distance between the site of reflection at the front surface <NUM> of the protrusion <NUM> and the PSD <NUM> may be <NUM> or more, in some embodiments even <NUM> or more.

In the embodiment shown in <FIG>, both, the focusing lens <NUM> and the collimating lens <NUM> are shown as spherical lenses which have focusing and collimating effect in both principal directions. However, in other embodiments, especially the collimating lens <NUM> could be a cylinder lens, which has its collimating effect only in the y-direction in <FIG>, while different from what is shown in <FIG>, the detection light beam <NUM> could spread out in the x-z-plane, i.e. the paper plane of <FIG>. This means that the light spot of the detection light beam <NUM> on the PSD <NUM> will be oblong, with the longer diameter being parallel to the sensing direction of the PSD <NUM>, which is also the direction in which the light spot will move upon deflection of the detection light beam <NUM>. This elongate shape of the light spot has found to likewise lead to a better signal-to-noise ratio and a better linearity of the response signal.

As understood herein, the "deflection" of the detection light beam <NUM> relates to the total deviation of the detection had been <NUM> from its "undisturbed" light path, i.e. without the local change in refractive index due to heat or pressure waves received by the measurement body <NUM>, as measured by the detection device, such as the PSD <NUM>. This "deflection" is hence the accumulated effect that the local change of refractive index has on the detection light beam <NUM> along its light path. In practice, the deflections will always tend to be small, and in order to get more accurate and reliable measurement results, it is important to raise the signal-to-noise ratio of the response signal. One possible approach has been explained above with reference to <FIG>, namely by increasing the distance between the exit surface <NUM> and the PSD <NUM>. Further ways to improve the signal-to-noise ratio will be discussed with reference to <FIG> below.

Without wishing to be bound by theory, <FIG> illustrates a mechanism of the deflection as currently understood by the inventors, that is fully consistent with the actual measurements. In <FIG>, the unperturbed detection light beam is shown in solid line, with an incoming portion <NUM> and an outgoing portion 22b. When an excitation radiation pulse <NUM> is absorbed in the skin of <FIG>, as explained above, a heat pulse is generated that travels through the skin and into the measurement body <NUM>, where it causes a local change in refractive index, which is referred to as a "thermal lens" herein and schematically shown at reference sign <NUM> in <FIG>. The thermal lens <NUM> is in this embodiment found to be a region of increased refractive index, which leads to the refraction as indicated by the dashed refracted and reflected detection light beam 20a. The deviation of the reflected detection light beam 22a from the unperturbed reflected detection light beam 22b is referred to as the "deflection" herein.

Note that in <FIG>, the entrance and exit surfaces <NUM> and <NUM> of the measurement body <NUM> are angled such as to form a right angle with the incoming and outgoing detection light beam <NUM>. This orthogonal arrangement of the optical boundary is the natural choice in the field, as it allows for reducing reflection and also for avoiding diffraction, which would make the optical setup more complicated. However, in the embodiment of <FIG>, at least the exit surface <NUM> is arranged such as to not be perpendicular to the reflected detection light beam 22b. Instead, the detection light beam 22b forms an angle α<NUM> with respect to the normal to the exit surface <NUM>, such that the undisturbed detection light beam 22b is refracted when exiting the measurement body <NUM> with an angle β<NUM> that is larger than α<NUM>, since the refractive index of the measurement body <NUM> is higher than that of the surrounding, which in the present embodiment is air.

The deflected light beam 22a is likewise refracted at the exit surface <NUM>. However, due to the interaction with the thermal lens <NUM>, the angle of incidence α<NUM> is larger than in the undisturbed detection beam 22b, and according to Snell's law, the angle of refraction β<NUM> is considerably bigger than β<NUM>. In other words, the difference between the refracted angles, β<NUM> - β<NUM> is larger than the difference between the angles of incidence α<NUM> - α<NUM> , i.e. β<NUM> - β<NUM> > α<NUM> - α<NUM>, such that the deflection of the reflected detection light beam 22a as measured by the PSD <NUM> (not shown in <FIG>) is increased. This allows for further increasing the signal-to-noise ratio.

Finally, with reference to <FIG>, an embodiment is shown where the contact surface <NUM> of the measurement body <NUM> is curved in the region where the detection light beam <NUM> is reflected. In other words, the measurement body in this region has a concavely curved recess. As is schematically shown in <FIG>, part of the deflection of the light beam <NUM> due to the local change in refractive index occurs before the detection light beam <NUM> is reflected at a surface of the measurement body that is in thermal or pressure-transmitting contact with the finger <NUM>. This means that the local change in refractive index also leads to a shift of the location precisely where on the surface the detection light beam <NUM> will be reflected.

In view of this understanding, in the embodiment shown in <FIG>, the contact surface <NUM> has a curved portion <NUM>. Due to this curvature, a change in location precisely where the detection light beam <NUM> is reflected on the curved portion <NUM> is accompanied by a change of angle of incidence, and hence leads to a corresponding change in the angle of reflection as well, as can be seen in <FIG>. Accordingly, using a curved reflection surface, the total deflection assessed by the detection device, such as the shift in position of the impinging detection light beam <NUM> detected with the position sensitive detector <NUM>, can be increased, thereby further allowing for increasing the signal-to-noise ratio. Note that in <FIG>, the curved portion <NUM> is shown to be formed in the otherwise flat contact surface <NUM>, but a curved portion could likewise be formed in the front surface <NUM> of the protrusion <NUM> shown in <FIG> and <FIG>. moreover, while in the embodiment shown in <FIG>, the curved portion <NUM> was concave, similar effects can be obtained with convexcurved portions (not shown) as well, since in this case too, the local change in refractive index will be accompanied by a change in the position where the detection light beam impinges on the curved portion, and hence a change in angle of incidence.

Moreover, in <FIG>, the curved portion <NUM> is shown to have a spherical shape i.e. having a same or similar curvature in two principal directions. However, in other embodiments the curved portion <NUM> could be curved predominantly or even exclusively only in one direction, for example having the shape of a section of a cylinder (with the section plane parallel to the cylinder axis). This is particularly advantageous in case of concave curved portions <NUM>, where in this case the finger <NUM> could be placed parallel to the longitudinal axis of the concave curved portion, allowing for a particularly good contact on the curved surface of the curved portion. As was mentioned above, in preferred embodiments, the radius of curvature of the curved portion <NUM> in at least one principal direction is in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>. In preferred embodiments, the width of the curved region <NUM> in the principal direction is at least <NUM>, and at most two times the radius of curvature.

As was mentioned above, in many embodiments it is advantageous if the light spot of the detection light beam <NUM> on the PSD <NUM> has an elongate shape, for example and elliptic shape with its long axis being parallel to the detection direction. This elongate shape can be obtained for example by collimating the detection light beam <NUM> only in a direction orthogonal to the detection direction, as was explained with reference to <FIG> above. However, in addition or alternatively, the elongate shape of the light spot can be obtained by tilting the PSD <NUM> in the detection light plane with respect to the detection light beam <NUM>, as shown in <FIG>, such that it impinges on the PSD <NUM> with an angle deviating from <NUM>°. For example, the angle of incidence onto the detecting surface of the PSD <NUM> could be less than <NUM>°, preferably less than <NUM>°, and most preferably less than <NUM>°.

<FIG> and <FIG> show a yet further apparatus <NUM> which is similar to that of <FIG>, in a top view and in a perspective view, respectively. Similar to the apparatus of <FIG>, the apparatus of <FIG> and <FIG> comprises a detection light beam <NUM> which is reflected on a contact surface <NUM> of a measurement body <NUM>, and a detection device, such as a PSD <NUM>, that allows for detecting a deflection of the detection light beam <NUM> due to interaction with a thermal lens indicated at reference sign <NUM>. In the embodiment of <FIG> and <FIG>, the detection light beam <NUM> is derived from a source light beam <NUM> by means of a splitter <NUM>. The splitter <NUM> transmits a portion of the source light beam <NUM> which forms the detection light beam <NUM>, and reflects another portion, which forms a reference light beam <NUM>. Using a mirror <NUM>, the reference light beam <NUM> is likewise directed to be totally or partially reflected at the surface <NUM> of the measurement body <NUM> at a location that is close to the reflection location of the detection light beam <NUM>, and in particular also in a region where the finger <NUM> (not shown) in operation would contact the contact surface <NUM>. However, the reflection point of the reference light beam <NUM> on the contact surface (or more precisely, at the interface between the contact surface <NUM> and the material, i.e. finger <NUM>) is sufficiently far away from the area where the excitation beam <NUM> is absorbed, such that any effect of heat or pressure waves received from the finger <NUM> upon absorption of the excitation radiation <NUM> is negligible. This is illustrated in <FIG> and <FIG>, where it can be seen that the thermal lens <NUM> does not extend to the region where the reference light beam <NUM> is reflected on the contact surface <NUM> of the measurement body <NUM>.

In <FIG>, reference signs <NUM> indicate the points where the detection light beam <NUM> and the reference light beam <NUM> enter and leave the measurement body <NUM>, and are mainly shown to assist in imagining the three-dimensional structure. Note that in the schematic illustration of <FIG>, no refraction of the detection and reference light beams <NUM>, <NUM> at the entrance and exit surfaces is shown for simplicity. A further detection device <NUM> is provided for detecting a degree of deflection of the reference light beam <NUM>, and in the embodiment shown, it is formed by an identical type of PSD as the PSD <NUM>.

It is seen that the reference light beam <NUM> will be exposed to all or almost all the same types of noise, vibrations, perturbations or external influences as the detection light beam <NUM>, except for the effect of the thermal lens <NUM>, or in other words, the heat or pressure wave received due to absorption of the excitation light beam <NUM>. Accordingly, all or at least most types of external effects that could lead to a deflection of the detection light beam <NUM>, other than those attributable to the absorption in the material, will also influence the reference light beam <NUM>, and can be measured by the additional detector <NUM>. Then, the measurement result of the additional detector <NUM> with respect to the reference light beam <NUM> can be used to correct for these effects in the measurement result of the PSD <NUM> with respect to the detection light beam <NUM>, to thereby improve the measurement signal quality.

With reference to <FIG>, a further embodiment of an apparatus is shown, which comprises an optical fiber <NUM> embedded in a measurement body <NUM>. A detection light source <NUM> is provided at one end of said fiber <NUM> for coupling detection light into said fiber <NUM>. At the other end of the fiber <NUM>, a mode detector <NUM> is provided. The mode detector <NUM> is suitable for detecting changes in optical modes of said detection light in response to the heat or pressure waves received by the measurement body <NUM> from said material. For example, the mode detector <NUM> could comprise a camera suitable for visualising the modes, and more precisely, an interference pattern of the optical modes. On the right of <FIG>, an image generated by such mode camera is schematically shown, in which optical modes <NUM>, and more precisely, an interference pattern of optical modes, can be seen in a certain rotational orientation.

<FIG> shows the same apparatus as <FIG>, in which however a thermal gradient <NUM> is formed due to heat or pressure waves received from the material, such as a finger <NUM> (not shown in <FIG>). This will lead to a transient deformation of the optical fiber <NUM> which is shown in the enlarged portion of <FIG>, where the deformation is highly exaggerated for illustration purposes. Such a transient deformation of the optical fiber <NUM> will lead to a change in the optical modes as detected by the mode camera <NUM>. In the exemplary embodiment illustrated in <FIG>, the change in the modes amounts to a rotation of the interference pattern of the modes, as seen by comparison of the schematically shown mode images in <FIG>. In other embodiments, the change in the modes could for example correspond to a shift of the interference pattern of the modes.

In the embodiment shown, the mode detector <NUM> comprises a processor (not individually shown) configured for detecting changes in the modes based on an image analysis of the camera images. As mentioned above, detectable changes in optical modes may comprise a shift or a rotation of an interference pattern of optical modes within the fiber and also on the mode camera <NUM>. The distance of the shift or a rotation angle is hence a quantitative parameter that is associated with the amount of heat or intensity of a pressure wave received from the material, and hence ultimately indicative of the amount of excitation light absorbed by the material. The apparatus of <FIG> is advantageous in that it is very simple robust and needs hardly any adjustment of optical components. It is particularly useful for portable apparatus.

<FIG> shows a side view and perspective view of an apparatus <NUM> according to a further embodiment. The apparatus <NUM> is a portable glucose measuring device having a size similar to that of a small smart phone. In the upper view of <FIG>, a measurement body <NUM> having a contact surface <NUM> is shown which in this case has a curved portion similar to that of <FIG>. On the contact surface <NUM>, a finger <NUM> can be placed in a manner shown in <FIG>, where the finger <NUM> is only schematically represented by a cylindrical structure. While the further details of the apparatus are not shown in <FIG> and <FIG>, the measurement principle of the device is similar to that of <FIG>, using a detection light beam (not shown) that is reflected on a curved surface. As was explained with reference to <FIG>, this leads to particularly large deflections of the measurement beam and hence a particularly high signal-to-noise ratio.

Further shown in <FIG> is a clamping device <NUM>, comprising a clamping member <NUM>. The clamping member <NUM> is pivotably mounted at a first end (left end in the figure) and biased by a torsion spring <NUM> into a closed position shown in the upper view of <FIG>, in which it is close to the contact surface <NUM>. At the second end of the clamping member <NUM>, a handle member <NUM> is provided allowing for gripping the clamping member <NUM> and rotating it against the biasing force of the torsion spring <NUM> into an open position in which the clamping member <NUM> is moved away from the contact surface <NUM> of the measurement body <NUM>. The finger <NUM> can be placed on the contact surface <NUM> when the clamping member <NUM> is in the open position, and said clamping member <NUM> is suitable for pressing said finger <NUM> against the contact surface <NUM> due to the biasing force towards the closed position. This way, a predefined contact pressure can be ensured. Close to the second end of the clamping member <NUM>, a cushion <NUM> is formed, which rests on the finger <NUM> when it is held by the clamping member <NUM> in a manner shown in <FIG>. In the embodiment shown, a pressure sensor (not shown) is provided in the cushion <NUM>, monitoring the contact pressure in a similar manner as the pressure sensors <NUM> shown in <FIG>. Note that the clamping mechanism is not limited to use in a handheld device, but could also be provided e.g. on a tabletop device or any other variant. Moreover, the biasing force of the clamping member <NUM> need not be generated by a torsion spring such as the torsion spring <NUM>, but could also be provided by the clamping member <NUM> acting as a leaf spring. Instead of the torsion spring <NUM>, there could be an adjustable mount allowing for adjusting the rest position of the clamping member <NUM> (leaf spring) and hence the biasing force created thereby.

Claim 1:
An apparatus (<NUM>) for analyzing a material (<NUM>) comprising at least one analyte, said apparatus (<NUM>) comprising
a measurement body (<NUM>) having a contact surface (<NUM>) suitable to be brought in thermal contact or pressure-transmitting contact with said material (<NUM>), said thermal or pressure-transmitting contact permitting heat or pressure waves generated by absorption of excitation radiation in the material (<NUM>) to be transferred to said measurement body (<NUM>),
an excitation radiation source (<NUM>) configured for irradiating excitation radiation (<NUM>) into the material (<NUM>) to be absorbed therein, and
a detection light source (<NUM>) for generating a detection light beam (<NUM>) travelling through at least a portion of said measurement body (<NUM>) or a component included in said measurement body, wherein said detection light beam (<NUM>) is directed to be totally or partially reflected at said contact surface (<NUM>), wherein said detection light beam (<NUM>) is deflected upon heat or pressure waves generated by absorption of excitation radiation in the material (<NUM>) being transferred to said measurement body (<NUM>), and
a detector (<NUM>) for detecting a degree of deflection, in particular a deflection angle, of the detection light beam (<NUM>) after its reflection at said contact surface (<NUM>),
characterized in that said contact surface (<NUM>) of the measurement body is curved in at least one principal direction in the area where the detection light beam (<NUM>) is reflected,
wherein the detection light beam (<NUM>) prior to and after reflection at said contact surface (<NUM>) defines a detection light plane, and wherein said principal direction lies within said detection light plane or forms an angle with the detection light plane that is less than <NUM>°, preferably less than <NUM>°.