Optoelectronic arrangement and method for operating an optoelectronic arrangement

Abstract

The invention concerns an optoelectronic arrangement, comprising a housing with a bottom contact surface having an elevated region. This region comprises a plurality of openings, wherein three openings of the plurality of openings are arranged in a first row; each of the openings comprising an at least partially transparent cover. Two optoelectronic devices are arranged in a first of the three openings, configured to emit light of different wavelengths. A photodetector is arranged in a second of the three openings, wherein a distance of the optoelectronic components to the photodetector are substantially equal. A light absorbing element is arranged in a central opening of the three openings between the first and second opening. Finally, fastening means configured to exert a pressure of the second region onto a sample surface, said pressure being in the range of 4000 N / m² to 20000 N / m² and particularly between 5000 N / m² and 16000 N / m².

Description

[0001] OPTOELECTRONIC ARRANGEMENT AND METHOD FOR OPERATING AN OPTOELECTRONIC

[0002] ARRANGEMENT

[0003] The present application claims priority from German patent application DE 10 2024 119 715 . 3 dated July 11 , 2024 , the disclosure of which is incorporated herein by reference in its entirety .

[0004] The present invention concerns an optoelectronic arrangement and a wearable device having such arrangement . The invention also refers to a method for operating an optoelectronic arrangement .

[0005] BACKGROUND

[0006] Wearables , like smart watches or phones , but also suitable clothing may comprise additional functionality that provide vital sign monitoring, generally referred to as VSM . Typical VSM application include , but are not limited to heart rate measurement , blood pressure measurement , oxygen saturation measurement and glucose measurements .

[0007] Such measurements are often based on optical data acquisition, where a defined amount of light is emitted onto the subj ect' s skin and the reflected or transmitted light is detected and evaluated . Consequently, an important quality factor is the sensitivity of such measurement .

[0008] A typical application in this regard is based on Photoplethysmography ( PPG) , which obtains the signal strength over time and particularly over a plurality of heart beats . The heartbeat can be seen in light reflected and / or transmitted by arterial blood vessels , as the different volume of blood at the measurement site causes an AC signal in the obtained reflected- and / or transmitted portion of light with a frequency corresponding to the heartbeat . The relative amplitude of such signal , for example the difference between the highest signal level and the lowest signal level relative to the average signal level ( DC ) , is referred to as perfusion index ( PI ) or modulation depth and it provides the information for many of the above-mentioned VSM applications . Applications include but are not limited to heartrate monitoring ( HRM) , blood oxygen saturation ( SPO2 ) , blood pressure measurement ( BP ) , and measurement of the blood glucose concentration . However , it has been observed by the inventor that the sensitivity of the Photoplethysmography measurement depends on various parameters , some of them based on the handling of the measurement device by a user and others based on the set-up of the sensor devices and the data acquisition .

[0009] It is therefore an obj ect of the present application to provide for an optical front-end that comprises an increased sensitivity for Photoplethysmography and stabilizes the PPG signals by operating at a point of optimal or preferred sensitivity, for example , near a peak .

[0010] SUMMARY OF THE INVENTION

[0011] This and other obj ects are addressed by the subj ect matter of the independent claims . Features and further aspects of the proposed principles are outlined in the dependent claims .

[0012] The inventors observed various aspects when using an optical front end to perform optical measurements , to be used in vital sign monitoring applications . For once , it has been found that the actual contact surface containing the emitter and detector should be as small as possible , or within a certain size range , to produce a preferred contact pressure between the optical front end and the wrist while maintaining an acceptably low strap force of the wristband for comfort of wear . More particularly it has been found that if the contact pressure is too small , the PI is also small and / or unstable and / or not well established . The PI is a measure of sensitivity, and it is desirable to achieve the highest possible sensitivity . On the other hand, if the contact pressure is too high, the PI or more particularly the sensitivity of the overall measurement deteriorates again . In order to define an optimal point for operation, it has been found that the contact surface on which the pressure is exerted shall be smaller than the overall surface that is in contact with the user' s tissue for a typical size of smartwatches and / or fitness trackers on the market today .

[0013] Based on a given wrist and tightness of the strap , reducing the contact area causes a pressure increase acting on that area . The contact pressure can be adj usted by using different strap hole positions of the wristband, a wristband with springs to adj ust and / or control the tightness , a belt-type wristband with a continuous size adj ustment , or any other method of equivalently adj usting the tightness with which the watch is held against the wrist . It has been found that the sensitivity can be increased by a factor of 2 . 5 to 4 depending on the wavelength . It has also been observed that the proposed structural changes reduce the strap force necessary to obtain peak PI and, hence , a more comfortable strap force range is possible . In this context , it has been observed that the optimal force to be exerted depends on the wavelength used .

[0014] If the pressure dependence of the PI has been established, for example , by a suitable calibration, a pressure sensor can be used to decorrelate the PI for each measurement . Based on the above calibration, where PI is measured versus contact pressure , one would then operate the device at the pressure reading that has produced the largest PI . Note , that a pressure sensor is not strictly needed to identify the preferred operating point , if the PI is established versus the length of the wristband, for example , by adj usting strap-hole positions , and the length adj ustments of the wristband are repeatable .

[0015] In addition, the structural changes also take the distance between the sensor and the emitter into account , as both sensitivity and signal level are also a function of distance . Generally, the sensitivity increases with separation, since light propagates a longer distance in tissue and through deeper lying layers before reaching the detector : hence , there is more interaction with the pulsatile blood flow and the AC component is therefore enhanced . Secondly, the DC level decreases with separation, since tissue is a turbid medium -- exhibiting scattering and absorption of light -- with overall an approximately exponential attenuation . Consequently, with a relatively amplified AC coefficient and a weakening DC level , the sensitivity = AC / DC = PI will increase .

[0016] Crosstalk, which is a portion of light not having sufficiently interacted with the sample , yet reaching the detector is of great concern . The inventor proposes to reduce cross talk by deploying a dedicated light absorbing element between the emitter and the detector . Suppression of the crosstalk by such an element as proposed directly reduces the DC value of the signal level and hence raises the AC / DC ratio and thus the PI .

[0017] It is suitable to arrange an additional photodiode ( PD ) as a light absorbing element in between the source and the detector at the nominal distance : the shorter separation between emitter and central photodiode can be used for wavelengths that are strongly attenuated by tissue , for example , green light , while otherwise it is used as an absorber and thus helps to suppress crosstalk when the diode on the far-side is used, for instance together with red- and IR wavelengths .

[0018] Finally, it has been found that emitted light may be reflected directly into or close to the emitter . Such light is usually lost . However , by light recycling measures , the forward emission towards the probe sample is increased again and the signal level ( DC ) increased .

[0019] In some aspects , the inventor proposes an optoelectronic arrangement , comprising a housing with a bottom contact surface , said bottom contact surface having a first region and a second region, the first region surrounding the second region . The second region is elevated with respect to the first region . Hence , the bottom contact surface comprises a protruding area , said area forming the second region .

[0020] The protruding area , that is the second region, comprises a smaller area compared to the bottom contact surface , thereby causing a higher local contact pressure when the bottom contact is pressed against a sample probe , particular if the sample probe comprises a flexible or yielding surface upon exerting pressure .

[0021] The second region now comprises a plurality of openings , wherein three openings of the plurality of openings are arranged in a first row . Each of the openings also comprise an at least partially transparent cover . A partially transparent cover in this regard shall be understood as a cover that comprises a very high transmission in certain portions of the visible , UV or infrared spectrum, but may outside those portions optionally act as a filter . Typical transparent covers include Si02 , Sapphire or other glasses . They are usually resistant to scratches or dirt and other substances to ensure a high transmission over a long time .

[0022] In this regard, each of the at least three openings can be flush with a surface of the second region . However, it is also possible that they are either recessed with regard to the surface of the second region or protrude the second region . Combinations are also possible . For example , the opening with the light emitting device underneath may protrude , while the openings with the light detection devices may be recessed or vice versa . Each opening or some of those may comprise lenses , while other openings may be flush .

[0023] A first optoelectronic device arranged in a first of the three openings . This optoelectronic device comprises a first optoelectronic component configured to emit light of a first wavelength, in particular red light . The optoelectronic device also comprises a second optoelectronic component configured to emit light of a second wavelength, in particular infrared light .

[0024] The optoelectronic arrangement further comprises a first light detection device , in particular a photodetector , a photodiode and the like . The first detection device is arranged in a second of the three openings . In accordance with the proposed principle , a distance of the first optoelectronic component to the first light detection device is substantially equal to a distance of the second optoelectronic component to the first light detection device . In other words , the first and second optoelectronic components are substantially spaced equally apart from the first light detection device . The first light detection device is configured to detect light of the first and the second wavelength . It has now been observed that for the given second region protruding above the surrounding first region, there is a certain pressure or force range being exerted onto the probe sample to obtain good results for optical measurements , the determination of a perfusion index as well as any measurement derived therefrom like for example glucose or oxygen saturation measurements . The latter is usually calculated from the ratio of PI at red wavelength to the PI at IR wavelength; the ratio of Pls at different wavelengths is also called the ratio of ratios and denoted with the letter R .

[0025] In some aspects , the pressure of the second region onto a sample surface should be in the range of 4500 N / m2to 20000 N / m2and particularly between 5000 N / m2and 16000 N / m2.

[0026] This pressure may be dependent on the size of the elevated region and / or the elevation as such as tissue is a compressible substance . For example in some aspects , the force exerted on the elevated region may be in the range of 0 . 65 N and 2 . 6 N and particular in the range of 0 . 65 N and 2 . 5 N . The size of the elevated region may be in the range of 1 . 2 cm2to 1 . 5 cm2for example .

[0027] In some aspects , the proposed optical arrangement comprises a strap or bracelet that can be wrapped around the wrist of a user to be sampled and is configured to exert a force and / or pressure in the above- mentioned range . Those forces and / or pressures may be of particular use if the emitted light is in the above-mentioned portions of the spectrum.

[0028] In some further aspects , the strap and the optical arrangement is configured to be wrapped around a forearm or wrist of a user , such that the plurality of openings arranged in the first row are substantially parallel to the arm of the user . In other aspects , the optical arrangement with the strap is configured such that , when worn by a user, the optical path lies substantially parallel to the forearm of the user and not perpendicular to it .

[0029] It has been observed that the quality of the measurements is almost equal in both directions , with a slight preference towards a direction parallel to the forearm. Hence it may be generally suitable in some aspects to arrange the first row parallel to the forearm in case of wearables .

[0030] Further in accordance with the proposed principle of the above- mentioned arrangement , a light absorbing element is arranged in a central opening of the three openings between the first and second opening , said light absorbing device is configured to absorb light of the first and the second wavelength . In some aspects , the light absorbing element may be a photodiode , or a device similar to the first light detection device .

[0031] The proposed optical arrangement implements various improvements based on the various observations . As a result , the overall perfusion index and signal level are significantly improved, which in turn enables more advanced applications , such as blood glucose detection or blood pressure measurement , or allow reducing the overall power consumption for standard applications . Particularly in wearables with its limited power resource , the reduced consumption results in a longer cycle between recharging .

[0032] In some aspects , the optical arrangement further comprises a fourth and a fifth opening arranged in a second row intersecting the first row . Such intersection may be perpendicular . Such arrangement of the openings may result in the shape of a cross or a T . In some aspects , one of the three openings of the first row, in particular the central opening , is arranged between the fourth and fifth opening . In some other aspects , the central opening is arranged at an intersection of the cross or T . In both cases , a light absorbing element is arranged in the central opening .

[0033] In this regard a second optoelectronic device can be arranged in the fourth opening in some aspects , said optoelectronic device comprising a first optoelectronic component configured to emit light of the first wavelength, in particular red light and a fourth optoelectronic component configured to emit light of the second wavelength, in particular infrared light . Likewise , a second light detection device , in particular a photodetector or any other device similar to the first light detection device may be arranged in the fifth opening , wherein a distance of the first optoelectronic component to the second light detection device is substantially equal to a distance of the fourth optoelectronic component to the second light detection device , the second light detection device configured to detect light of the first and the second wavelength .

[0034] Depending on the vital sign monitoring (VSM) application, different wavelengths for optical measurement may be required or at least suitable . For PPG measurement used to determine the oxygen saturation, one can use red and infrared light and the resulting ratio of perfusion indices , which is also called the ratio of ratios , for example , (AC / DC ) red / (AC / DC ) IR . Different wavelengths also have different penetration depth into the probe sample . Hence , it is proposed in some aspects that the first and / or second optoelectronic device comprises a third optoelectronic component configured to emit light of a third wavelength, in particular green light .

[0035] It has been observed that green light is attenuated in tissue much more strongly than red / IR light ; however , the PI for green light is also much larger than that for red or infrared light at the same separation distance between emitter and detector . Therefore , to reduce the overall power consumption, the separation between third optoelectronic component and the first light detecting device is selected to be smaller than the distance between the first and / or second optoelectronic component and the first light detecting device . Likewise , a distance of the third optoelectronic component to the second light detection device is smaller than a distance of the first and second component of the second optoelectronic device to the second light detection device .

[0036] In some embodiments , the first and / or second optoelectronic device may comprise a third optoelectronic component configured to emit light of a third wavelength, in particular green light . Centers of the respective first , second and third optoelectronic components form edges of a virtual triangle with the center of the third optoelectronic component being closest to the respective central opening . The light absorbing element arranged in the central opening may be a third light detection device configured to detect light of the first , second and third wavelength . It has been found that a photodetector suitable to detect light of the third wavelength and therefore also of the first and second wavelength acts as a light absorber . By arranging said light absorbing element between an optoelectronic device and the light detection device , it has been observed that the crosstalk is significantly reduced, and the sensitivity enhanced . The reason behind this fact lies most likely in light being reflected close to the sample surface , which then reaches the location between an optoelectronic device and the light detection device . The light absorbing element absorbs such light , thereby helping to suppress crosstalk .

[0037] The light absorbing element can be a photodetector, which can also be used to detect green light . Hence , the photodetector at the central position can -depending on the desired optical measurement- act as detector or as light absorber , or both at the same time . When operated as an absorber, the photodetector may simply be switched off or it may be operated with reverse bias in the photo-cell mode to best absorb all incoming light .

[0038] Some aspects concern the arrangement of the optoelectronic components of the optoelectronic device . In some aspects , the first and / or second optoelectronic device comprise a package with a first recess , in which the first and second optoelectronic components are arranged . It is also possible in some aspects that each of the first and second optoelectronic components are arranged in separate recesses of the optoelectronic device . To further increase the light emission therefrom, the recess or recesses may comprise beveled sidewalls . To further improve so-called light recycling for example light that is reflected or scattered from the tissue back into the recess , the sidewalls may comprise a material with a high reflectivity for light of the first and / or second wavelength . Examples of such material contain TI02 particles .

[0039] In some aspects , in which a third optoelectronic component is present in the optoelectronic device , the package may comprise a second recess , in which the third optoelectronic component is arranged . As for the other recesses , said second recess comprising -in particular beveled- sidewalls having a high reflectivity for light of the third wavelength . It may also have a low reflectivity for light of the first and second wavelength, hence also acting as a light absorbing element for light of such wavelengths .

[0040] In some further aspect , the optoelectronic arrangement comprises a pressure sensor for determining at least one of pressure and force acting upon the second region and / or the first region . In some aspects , the pressure sensor is arranged close to one of the optoelectronic component and / or the light detecting devices . In some aspects , the pressure sensor is arranged close to the central opening . The pressure sensor is configured to determine the pressure and / or the force acting upon the second region and / or the first region . In this regard, the determination may be periodically conducted or in response to an optical measurement . Correlating a pressure measurement to one or more optical measurements allows compensating variations of the pressure during different measurements . Furthermore , pressure measurement enables decorrelation of the actual optical measurement , thereby compensating for any variation of fastening means .

[0041] In some aspects , the optical arrangement comprises a control circuit coupled to the optoelectronic components , the light detection devices and the pressure sensor for controlling the individual elements and obtaining and evaluating the measured signals .

[0042] Some aspects concern measures for improving the signal to noise ratio . For example , the first and / or second light detection device can comprise a color filter configured to block light in a portion of the spectrum different from the first and second wavelength . Such color filter can also be arranged in or on the transparent cover . In some further aspects , the at least partially transparent cover over the first and / or second light detecting device comprises a color filter or color filtering properties configured to block light in a portion of the spectrum different from the at least one of the above-mentioned wavelengths . In some other aspects , the first region of the bottom contact surface comprises a light absorbing coating, in particular a black coating . Alternatively or additionally, the first region of the bottom contact surface can comprise a light absorbing material , in particular a blackened material . A suitable material which is highly absorbent for light of in the red and infrared portion of the spectrum is Polyether Ether Ketone ( PEEK) . Such material is a high-performance engineering plastic with outstanding resistance to harsh chemicals , and excellent mechanical strength and dimensional stability, which makes it suitable for the second region not to be damaged upon fall , scratches and the like . In a further embodiment , the second region may also comprise a light absorbing coating , in particular a black coating . One may use the same material as described above , for example Polyether Ether Ketone ( PEEK) .

[0043] In another aspect , the material of the second and optionally also the first region comprises a lower thermal conductivity, in particular a heat conduction significantly lower than those of metals . It has been observed that a high heat flow properties result in a degradation of the PI signal quality . This may be due to the fact the heat of the skin is transported by the material away from the region . As a result , the diameter of blood vessels is shrinking and thus reducing the PI . Consequently, the second region should not contain metal , but rather a plastic .

[0044] Some aspects concern the shape and form of the second region . As stated previously, the second region elevated above the first region should be small to exert a certain pressure on the contact surface . Hence , it can be suitable in some instances , if the shape of the second region follows the arrangement of the plurality of openings in the second region . For example , if the arrangement of the plurality of openings in the second region resembles a cross or a T , then the second region may also have the form of a stylized cross or a T in top view . The edges can be rounded . Some other possible forms are a circle or an oval . Some further aspect concern fastening means, which can be attached to the optical arrangement. The fastening means are configured to exert an adjustable pressure of the bottom contact surface onto a sample surface, the sample surface comprising in particular skin tissue. This provides a possibility to adjust the pressure, thereby shifting the PI to its respective optimum. In some further aspects, the fastening means may comprise a stretchable band or other material, such that the restoring force exerted by the stretchable material remains constant over a long range of stretching of the material. This provides the possibility to have a constant force and thus pressure exerted on the second region almost independent of the circumference of the wrist.

[0045] In another aspect, the elevation of the second region with respect to the first region is in the range of 0.5 mm to 3.5 mm and in particular between 1.0 mm to 2.0 mm and in particular less than 1.75 mm. It has been observed that an elevation within such ranges provides good results. Using a different elevation may also change the necessary force to be applied by the fastening means .

[0046] In some other aspects, the separation between a center of the first optoelectronic device, in particular a common center of the first and second optoelectronic components of the first optoelectronic device to the first light detecting device is in the range of 5 mm to 15 mm and in particular between 7.5 mm and 12.5 mm and in particular between 7.5 mm and 10 mm and in particular between 8.0 mm and 9.0 mm. Likewise, a distance between a center of the second optoelectronic device, in particular a common center of the first and second optoelectronic components of the second optoelectronic device to the second light detecting device is in the range of 5 mm to 15 mm and in particular between 7.5 mm and 12.5 mm and in particular between 7.5 mm and 10 mm and in particular between 8.0 mm and 9.0 mm.

[0047] The above range reflects the observation that a distance smaller than 5 mm results in a lower perfusion index. The above ranges however offer a good trade-off between the overall size of the device and the signal quality for optical measurements. In some further aspects , a distance between a center of the third optoelectronic components to the third light detecting device is in the range between 2 . 5 mm and 5 mm and in particular between 3 mm and 4 mm. Hence , the distance between the emitter for green light and the detector is significantly smaller and slightly less than half the previous distance .

[0048] In some aspects , the first and second optoelectronic devices are substantially identical in construction . They may comprise the similar emitters and / or optoelectronic components of the same type . In some aspects , the first and second light detection devices are identical in construction . Hence , first and second light detection devices may comprise a photodetector of the same or similar type . In some aspect , wherein the first , second and third light detection devices are substantially identical in construction . Consequently, three photodetectors can be used in some aspects , which are of the same or similar type and particularly configured to detect light in ranges of the light spectrum covered by the first , second and third wavelength .

[0049] The proposed principle offers a good compromise between competing requirements . For a high sensitivity it would be best to separate optoelectronic components and the photodetector "infinitely" far away from each other, resulting in low DC signal and poor SNR . At the same time to achieve high contact pressure for low strap forces , the contact area has to be kept small , which means the optoelectronic components and the photodiode should be closer together . For low supply currents of the optoelectronic components but high DC signal counts , which are required for best SNR, it is necessary to bring the optoelectronic components as close as possible to the photodetector .

[0050] The proposed principles implement a small contact area with large enough distances between the optoelectronic components and the light detection devices for red and IR wavelengths . The components and light detection devices are placed on diametrically opposed positions around a circle or another suitable shape . The diameter and area of shape is chosen such, that a relatively high sensitivity is obtained together with sufficient DC signal for red and IR wavelengths . The signal levels are further increased by the light recycling properties of the proposed arrangement .

[0051] Some further aspects concern a wearable device , in particular a watch . The wearable device comprises an optoelectronic arrangement in accordance with the proposed principle and a strap forming an adj ustable fastening means to exert a pressure of the bottom contact surface onto the skin tissue . The housing of the optoelectronic arrangement can also correspond to a housing of the wearable as such; hence , the individual elements of the optoelectronic arrangement are arranged within the housing of the wearable .

[0052] The wearable also comprises an evaluation and control unit coupled to the first and / or second optoelectronic device , the first and / or second light detecting device and the light absorbing device . It is configured to operate those devices to obtain data and determine from said data at least one of heart rate , oxygen saturation and glucose concentration .

[0053] In some further aspects , the wearable may also comprise a pressure sensor and / or a temperature sensor . This enables to obtain the contact pressure during the measurement and subsequently calibrate the measurement or indicate the user to adj ust the fastening of the fastening means / strap . Similarly, a measurement by the temperature sensor may be used to calibrate or decorrelate the subsequent optical measurement .

[0054] In some aspects , the housing ( or optoelectronic device in more general terms ) is implemented as a ring , earbud, watch or any other wearable , that may fit in some aspects into a user' s usual environment and can be carried continuously . Said ring, earbud, watch or any other wearable is in communicative connection with the evaluation unit or a mobile or any other device implementing the evaluation and control unit . The ring, earbud, watch or any other wearable may cover a larger portion of the user' s skin, for example wrap around the finger , clipped to the ear from both sides and the like . They may contain several emitters and detectors at various locations , thus allowing not only to measure at one spot but at several spots at the same time- or sequentially . As a result thereof , s kin irregularity or other issue can be overcome and the overall measurement quality improved .

[0055] Apart from wearable- and handheld devices , other applications are possible . For example , the optoelectronic arrangement can be implemented in medical devices or laboratory equipment , for example for test and measurement purposes . Those devices again can be stationary or mobile . Some more aspects concern mobile displays in which the detectors are directly implemented . In such applications , the display LEDs for example for the red and green and even blue color can be used as emitter in accordance with the proposed principle .

[0056] A finger is placed directly on the display surface and then illuminated by the display for obtaining the first and / or second signal . Likewise , the proposed principle can be implemented in VR or AR glasses and devices .

[0057] Another application concerns safety issues , for example during certain labor-intensive work but also while driving a vehicle and the like . It is possible to implement such optoelectronic arrangements for example to obtain the perfusion index and from there the noise spectrum during driving . This enables for example to warn drivers of potential health threats while driving .

[0058] In some further aspect , a method for operating an optoelectronic arrangement in accordance with the proposed principle is proposed . The method comprises the step of providing an optoelectronic arrangement with a housing having a bottom contact surface , said bottom contact surface having a first region surrounding an elevated second region . The second region comprises a plurality of openings , wherein three openings of the plurality of openings are arranged in a first row; each of the openings comprising an at least partially transparent cover that is flush with a surface of the second region .

[0059] The optoelectronic arrangement is fixed onto a user' s skin to probe a human tissue , such that a defined contact pressure is exerted onto the user' s s kin by the second region of the bottom contact surface , with a smaller contact pressure being exerted onto the user' s s kin by the first region of the bottom contact surface . The contact pressure of the second region onto a sample surface is in the range of 4000 N / m2to 20000 N / m2and particularly between 5000 N / m2and 16000 N / m2.

[0060] Light of a first wavelength and / or a second wavelength is then emitted from a first opening towards the user' s skin to interact with the tissue thereof . Light being reflected into the second opening , located between the first and a third opening , is absorbed by a light absorbing element , thereby reducing any potential crosstalk . Light being reflected into the third opening is detected . The detected components are evaluated to obtain a perfusion index , or any other value related to the optical measurement .

[0061] In some aspects , a force is exerted onto the second region of the bottom contact surface in the range of 1 . 25 N to 4 . 5 N and in particular between 1 . 75 N and 4 N and in particular between 2 N and 3 . 5 N . In some aspects this force is exerted by the second region onto the skin or tissue . In this regard and throughout this application Newton' s law of mechanics apply, meaning that the second region is pushing with a force onto the tissue and the tissue is pushing back with an equal and opposite force .

[0062] In some aspects , the light of the first and second wavelength is emitted using a time division scheme . Furthermore , in some aspects , light of a third wavelength, for example green light , can be emitted from a first opening towards the user' s skin to interact with the tissue thereof . Light being reflected into the second opening , located between the first and a third opening, is detected and evaluated to obtain a perfusion index or any other value related to the optical measurement . Hence , light reaching the second opening can either be absorbed, reducing the cross talk and / or detected and processed further .

[0063] In some aspects , the pressure onto the tissue surface and / or the second region is measured either shortly before , shortly after or during the various optical measurements . The pressure is then used in some aspects , to normalize the measurement and compensate the optical signal with a correction depending on the determined pressure . In some aspects , the user may be advised to exert different pressures , for example by adj usting the fastening means . During the adj ustments , the optical measurements are performed to obtain a correlation between the optical signal and the pressure or force exerted by the second region onto the tissue . The calibration can be repeated and / or updated with further pressure measurements during subsequent optical measurements .

[0064] SHORT DESCRIPTION OF THE DRAWINGS

[0065] Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments and examples described in detail in connection with the accompanying drawings in which

[0066] Figure 1 shows a perspective view of an embodiment of an optoelectronic arrangement in a smartwatch in accordance with some aspects of the proposed principles ;

[0067] Figure 2 illustrates a top view of the embodiment of an optoelectronic arrangement in a smartwatch in accordance with some aspects of the proposed principles ;

[0068] Figure 3 shows a more detailed view of the second region of an optoelectronic arrangement in accordance with some aspects of the proposed principle ;

[0069] Figures 4A and 4B show exemplary measurements illustrating the relationship of the DC and PERFUSION INDEX in dependence to the distance at a given s kin position in accordance with some aspects of the proposed principle ;

[0070] Figure 5 illustrates a schematic top view of an optoelectronic component used in an optoelectronic arrangement in accordance with some aspects of the proposed principle ; Figure 6 shows side views of an optoelectronic component used in an optoelectronic arrangement in accordance with some aspects of the proposed principle ;

[0071] Figure 7 illustrates two diagrams showing the pressure and force vs the perfusion index at different distances for the red, infrared and green wavelength illustrating some aspects of the proposed principle ;

[0072] Figure 8 shows a diagram illustrating the perfusion index vs . the pressure and force at different wavelengths in accordance with some aspects of the proposed principle ;

[0073] Figure 9 is a diagram illustrating the effect on the PI at different pressures levels to outline some aspects of the proposed principle ;

[0074] Figure 10 shows the DC level vs various pressure levels for different distances between the emitter and the detector in accordance with some aspects of the proposed principle ;

[0075] Figure 11 illustrates the SNR and MNR vs the contact pressure for different wavelengths and different distances between the emitter and the detector in accordance with some aspects of the proposed principle ;

[0076] Figures 12A and 12B show several diagrams illustrating different curvatures of signals levels vs distance for different contact pressures ;

[0077] Figure 13 is a diagram showing the frequency spectrum and relative phase between different harmonics to illustrate some aspects of the proposed principle ;

[0078] Figures 14A and 14B show two diagrams illustrating the perfusion index PI at different strap hole positions for two differently implemented optoelectronic components used in an optoelectronic arrangement in accordance with some aspects of the proposed principle .

[0079] DETAILED DESCRIPTION The following embodiments and examples disclose various aspects and their combinations according to the proposed principle . The embodiments and examples are not always to scale . Likewise , different elements can be displayed enlarged or reduced in size to emphasize individual aspects . It goes without saying that the individual aspects of the embodiments and examples shown in the figures can be combined with each other without further ado , without this contradicting the principle according to the invention . Some aspects show a regular structure or form. It should be noted that in practice slight differences and deviations from the ideal form may occur without , however, contradicting the inventive idea .

[0080] In addition, the individual figures and aspects are not necessarily shown in the correct size , nor do the proportions between individual elements have to be essentially correct . Some aspects are highlighted by showing them enlarged . However , terms such as "above" , "over" , "below" , "under" "larger" , "smaller" and the like are correctly represented with regard to the elements in the figures . So it is possible to deduce such relations between the elements based on the figures .

[0081] Figure 1 illustrates a perspective view ( up-side down) of an optoelectronic arrangement in accordance with some aspects of the proposed principle . The arrangement is integrated in a smartwatch 1 as a wearable device , with the strap being removed from its fastening location 41 for better illustration . Still , the watch 1 is fastened around a user' s wrist , with the strap being adj ustable such that the watch is fastened with the strap for tight , snug, or loose fit .

[0082] The watch comprises a housing that corresponds to the housing 10 of the optoelectronic arrangement . The watch may comprise electronic circuitry providing a variety of different functions . The watch also comprises a plurality of different sensors , some of them relevant for the purpose of the proposed principle . The bottom contact surface 11 of the housing 10 is pressed by the strap onto a user' s wrist or more generally onto his / her skin surface . By adj usting the strap , the overall contact pressure can be adj usted and varied . The bottom contact surface 11 comprises a first region 12 surrounding a centrally located circular second region 13 . As illustrated, the second central region 13 is elevated and protrudes the first region 2 of the bottom contact surface . The height between the two regions lies in the range between 1 . 0 mm and 1 . 5 mm and is in the present embodiment around 1 . 2 mm.

[0083] The surface of the first and second region may usually be blackened or made of black or dark material . The surface material and, more generally, the material of both regions is made of a material with a low heat conduction coefficient and in particular a lower heat conduction coefficient than those of metals . Typical material suitable with low heat conduction coefficient include glass , sapphire and plastics . The latter can be doped with a black material , which further provides a high absorption of light of various wavelengths , thereby reducing the crosstalk and improving the measurements , as explained further below in greater detail . One material suitable to form the first and / or second region is PEEK or Polyether Ether Ketone . The material comprises a low heat conduction coefficient but is also highly stable with excellent mechanical strength and dimensional stability .

[0084] The remaining portion of the housing 10 may comprise metal or any other suitable material . The housing 10 also contains power supply, evaluation and control circuitry . In some instance , the top side ( here on the bottom and not visible ) may also comprise a display to visualize the measurement results . Buttons 42 are arranged on the side to select certain functionality of the wearable or adj ust parameters thereof .

[0085] The second region 13 comprises five openings 20 to 24 , which are located onto the elevated region 13 . The openings are arranged forming a crosslike structure with a central opening 22 intersecting two rows of openings . The first row comprising openings 20 , 21 and 22 are substantially perpendicular to the fastening means ( not shown herein) and thus substantially parallel to a forearm of a user when the watch is worn by the user . The second row comprises openings 23 , 24 and 22 is arranged such that it runs substantially perpendicular to the forearm . Each opening comprises a substantial quadratic shape with sightly rounded edges . As illustrated, the overall second region 13 is only slightly larger in its diameter than the length of each row . The openings 20 to 24 fit into the second region 13 , but the overall space from the outer edge of an opening to the edge of the second region 13 is small and preferably only as large as the mechanical stability requires . As a result , the second region is as small as possible to increase the pressure exerted by the fastening means onto the user' s skin .

[0086] Each opening 20 to 24 comprises a cover that is flush with the surface of the second region and transparent to light of certain wavelength, as explained further below . These wavelengths can be for example red, infrared and green . The cover is reflective or absorbent , that is not transparent for wavelengths that are not suited for the measurements proposed herein . Typical materials for the cover include all kinds of glass , but also transparent plastics , as long as they are resistant to scratches and do not age under the various light spectra .

[0087] Each opening is arranged above an optoelectronic device or a detecting device . Figure 2 shows the top view of the bottom contact surface 11 with the devices being present . Two openings , namely opening 20 of the first row of openings and opening 23 of the second row of openings , each comprise a light emitting device 50 and 50 ' , respectively . The light emitting devices are of the same type and configured in the present example to emit light within the red, infrared and green spectrum. More particular, the first row, perpendicular to the fastening means or strap 43 attached to element 41 comprises opening

[0088] 20 with light emitting device 50 and a second opening 21 opposite the first opening at the other end of second region 13 . The second opening

[0089] 21 comprises a light detecting device implemented as a photodiode 60 .

[0090] The second row is perpendicular to the first row and thus parallel to the strap 43 . The second row comprises a third opening 23 at the left edge of the second region 13 with the light emitting device 50 ' placed therein and a fourth opening 24 on the other end of the second region 13 with a light detecting device in form of a photodiode 60 ' . Photodiodes 60 and 60 ' are both from the same type and configured in the present example to detect light within the red, infrared and green spectrum. A rotation by 45 ° , or some other angle , is also possible . However , it has been observed that specifically the measurement path that is substantially parallel to the forearm is producing somewhat preferable PPG signals .

[0091] Furthermore , central opening 22 , the central opening for both rows also contains a photodiode 61 as a light absorbing element , designed to absorb light of the red, infrared and green spectrum . Depending on its operation, the photodiode 61 acts as light absorbing element and / or as light detecting element , the latter in case the devices 50 or 50 ' emit green light . Photodiode 61 is in this embodiment of the same type as photodiode 60 and 60 ' . However, it is instructive to point out that a centrally located light emitting device , which is a popular design choice in current products for vital sign monitoring, does not satisfy the same function . Due to the larger bandgap of devices emitting green light , which are often placed in this position, red and IR light at the correspondingly longer wavelengths than green light , or -- more generally -- any light with lower photon energy than the bandgap of the green light emitting device , cannot be ( substantially) absorbed and is thus reflected . Therefore , a common green light emitting device , referred to as green LED is not a preferred absorber for red and IR light .

[0092] Similarly, and as a differentiating aspect , the centrally located light detection device 61 cannot feature a filter that reflects longer wavelengths , as is customarily done in some products to suppress ambient light at red and IR wavelengths . With a reverse bias , the centrally located light detection device can be operated in a mode that maximally absorbs the incoming light by effectively reducing the probability for generated electron-hole pairs to recombine .

[0093] In the present embodiment , the first and second rows are positioned to form a cross . However , it is also possible to select opening 21 with element 60 ' as central opening . In such embodiment , the shape of the two rows would resemble a "T" , with the opening 24 being the central opening for the row having openings 20 , 24 and 21 . While this embodiment may be useful in some aspects , it does not offer the symmetrical structure as the embodiment presented in Figures 1 and 2 .

[0094] The embodiment also contains an optional pressure sensor and an optional temperature sensor , both are not shown in these embodiments . The pressure sensor is arranged closely to region 13 , for example beneath it . This would allow to measure the pressure exerted onto the elevated second region . In operation of the arrangement , one can emit light from one of devices 50 , 50 ' in openings 20 and 23 , respectively . When the emitters 50 emit red or infrared light , the photodiode 61 in the central opening is switched off or operated in reverse mode , thereby acting as a light absorbing element . When the central photodiode is used for measurement concurrent to the photodiode on the far side , it still acts as an absorber , thus suppressing crosstalk between the source and the distant PD . Hence , light that is reflected from the user' s skin back to the second opening is absorbed therein and will not reach any of the other two openings 21 or 24 . If green light is emitted, the photodiode 61 in central opening 22 is activated to detect light interacting the with user' s tissue and reflected back to the central opening . Consequently, depending on the operation, the photodiode 61 in central opening 22 acts as light absorbing device to reduce the overall cross talk or as light detecting device . It is also possible , to use the other photodiodes to record PPG at three different distances with the same device . This is helpful if one wishes to calculate the curvature of the DC-signal with distance ( separation) from the source .

[0095] Another improvement is achieved by the configuration of the arrangement of light emitters and light detecting device as shown in Figure 1 and 2 as well as in more detail in Figure 3 . Figure 3 illustrates a PCB with the various devices attached to it , which is arranged directly below a second region such that the various devices are located beneath the covered openings in the second region . The PCB 130 comprises a shape of a circle or circumference LI in top view with a given radius . In some aspects , as shown for example in Figures 1 or 2 , the circle LI can correspond to the outer edge of the second region . To this extent , the embodiment of Figure 3 would resemble the second region in an arrangement according to the present invention . The second region comprises a pressure sensor 28 close to the central opening that is configured to determine the pressure or force exerted on the elevated second region close to the central opening . While only one pressure sensor is shown, one will note that different sensors located at different positions in the first and second region are suitable , as the accuracy of measurement of the perfusion index is highly dependent on the pressure . More particular , one may use three sensors to define a plane parallel to the interface of the optical front end with the tissue : the three pressure sensors bear the construction of the optical front end, i . e . , region two , region one , or both regions .

[0096] In an alternative embodiment , the region 130 may not be elevated with regard to the bottom surface of the arrangement . However , as seen in the Figure 3 , the dashed lines L2 resembling a cross-like shape correspond to a region that is elevated . The elevated region contains the openings with the respective device arranged therein . As seen from the Figure , the elevated region by the dashed line L2 comprises an even smaller surface than the circular region LI .

[0097] In both embodiments , photodiodes 60 , 60 ' and 61 each of the same type are located on the PCB and carrier 130 below openings 24 , 21 and 22 , respectively . Each photodiode comprises an active detecting area occupying the large amount of the overall opening , for example 80% to 95% of the opening , to collect as much light as possible . Bonding wires are connecting the photodiode to contact areas at an edge of each package , although other configurations for mechanical and electrical connection can be implemented .

[0098] The light emitting devices 50 and 50 ' below openings 23 and 20 are implemented as a so called 3-in-l configuration . Such configuration is explained in greater detail in Figure 5 further below . The distance DI from one light emitting device for example 50 ' to the opposite photo diode 60 for detecting the red and infrared light portion is about 8 . 5 mm measured from the respective centers as shown in the Figure 3 . The distance D6 is between the red and infrared light emitting devices of elements 50 and 50 ' to the adj acent photodiode . The distance D2 between the component configured to emit green light of the light emitting devices 50 and 50 ' to the central photodetector is less , in the range of about 3 mm to 4 mm and in particular about 3 . 5 mm. The smaller separation required is due to substantially stronger attenuation of green light over distance compared to red and IR . Distances D3 correspond to the distance between the green emitting lighting diode and the photodetector 60 , respectively . They are in the range 5 to 6 mm and thus more than 50% larger than distance D2 .

[0099] In this regard, Figures 4A and 4B show the relationship between the DC portion and distance between the emitter and the respective photodiodes , as well as the perfusion index vs the distance , respectively . Figure 4A shows the change of the DC portion of the detected signal versus the distance . The points at appr 8 . 5 mm distance correspond to the configuration wherein the optoelectronic components for red and infrared light in the embodiment of Figure 3 are distanced from the center of the photodiode by 8 . 5 mm ( distance DI ) , for example the distance between the opposite elements 50 ' and 60 or between elements 50 and 60 ' . Likewise , the second points at appr 4 . 5 mm correspond to the distance between the component of red and infrared light of devices 50 and 50 ' to the central photodiode 61 , that is distance D4 . The right point for the curve marked as GREEN corresponds to a distance of 7 . 5 mm between the components 502 for green light in devices 50 and 50 ' to the outermost photodiode 60 and 60 ' , distance D7 . The left point at distance 3 . 5 mm corresponds to distance D2 shown in Figure 3 , that is the distance between the device for green light and the central photodiode 61 .

[0100] The measurements indicate that the central photodiode 61 generates a 5 . 5 times higher signal in connection with the red and infrared optoelectronic components than the photodiodes 60 and 60 ' , which are located further away from the respective light components . However, the perfusion index PI measured by those diodes 60 and 60 ' as given in Figure 4B is about 3 times higher, which provides a better result for light in the red and infrared spectrum. These Figures stipulate to use a larger distance for optical measurements in the red and infrared portion of the spectrum . In contrast , the central photodiode 61 generates a 36 times higher signal for the green portion at the smaller distance , while the PI is increased only by 2x with the diode 61 on the outer side .

[0101] As a result , the red and infrared light generates 5 . 5 times more photo current at the central photo diode 61 in comparison to the outer photodiode 60 , 60 ' , but the perfusion index is larger by a factor of 3 , which is more desirable . If a reflector is placed in position of the central photodiode 61 , for example in the form of a light emitting diode with a band gap larger than the energy for red or infrared light , an artificial secondary light source will be created at the central position, which would contribute to the PPG signal due to its reflection of signal portions reflected from the tissue into the central opening . This is undesirable because the actual or effective distance is then no longer clearly defined and may also be more dependent on the s kin type than before . Further, -and also quite relevant- , the perfusion index would be reduced due to additional crosstalk and due to the general distance dependence of the sensitivity .

[0102] At green light , the central photodiode 61 generates a 36 times higher signal ( depending on skin type and other possible influencing factors ) than the one of opposite photodiodes 60 and 60 ' ; in contrast , the perfusion index PI would improve by a factor of 2 times with the greater distance .

[0103] It is the general preference that the modulation depth / perfusion index is larger than the noise level . Ideally, this noise is mainly shot noise . Since shot noise scales with the square root of the signal , a 36 times greater signal means that the SNR has improved by a factor of six compared to the outer photodiodes 60 and 60 ' ; conversely, it is possible to improve the perfusion index by only a factor of two ( for green light ) when using the outer photodiodes 60 and 60 ' . Therefore , for green light , it is more desirable to use the central diode 61 .

[0104] For red and IR light , with a gain of only 5 . 5x in signal level , the signal to noise ratio may be improved by ~2 . 3x with the central photodiode 61 , but the loss in sensitivity is 3x, which makes it more desirable to use the outer photodiodes 60 and 60 ' for red and IR . This principle seems legitimate for skin type 2 but should be reasonably applicable also to some other s kin types .

[0105] Another aspect is based on light recycling techniques employed in some aspects with the proposed principle . Light that is reflected from the human skin and tissue directly back into the recess of the emitter device is usually lost . Hence , a larger supply current is to be used to obtain the same signal strength . Figures 5 and 6 illustrate several aspects thereof aiming to reduce the necessary supply current and hence power . It has been found in this regard that light that is otherwise lost to the application is recycled in the package and thus may recontribute to the signal . The gain is very substantial : about 2x compared to no light recycling at all , and still substantially 10% to 2x depending on wavelength over conventional solutions . In addition, it has been observed that light recycling causes an increase in the PI over no light recycling .

[0106] Figure 5 shows a more detailed view with the respective dimension for a lighting device used in the proposed optical arrangement in a 3-in- 1 configuration . The dimensions given in the embodiment of Figure 5 are based on design and may vary depending on the implementation . The light emitting device 50 comprises two recesses 55 and 55 ' which are of slightly different size and shape . They are centered on an axis and are pointing in the same direction as the PDs . Two lighting components 500 and 501 are placed in the first recess 55 , whereas the first component 500 is configured to emit red light , while the second component 501 is configured to emit infrared light . Both components are arranged in a symmetrical manner such that their distance to the light detecting device as explained previously are substantially equal . The second recess 55 ' contains another light emitting component 502 configured to emit green light . In the present embodiment , the emission surface for component 502 is larger and appr . 4 times the emission surface of one of components 500 and 501 , respectively . The recess 55 and 55 ' are separated by a barrier to avoid crosstalk from green light into the red light or infrared light . The crosstalk would lead to fluorescence at the respective red and IR wavelengths , when excited by photons at the green wavelength . In addition, the barrier makes for recesses with relatively compact emissive areas , which results in a well-defined distance for PPG and a correspondingly well-defined PI .

[0107] The sidewalls can be made of different material and surfaces to improve reflection . Figure 6 shows two different embodiments . In the left configuration, the sidewalls 55 are straight and not beveled . Further, the bottom surface and the sidewalls are made of a non-reflecting material . Consequently, only a portion of light , for example appr 45% is inj ected deeper into the probe sample and the human skin, while another portion being reflected into the recess is absorbed and therefore lost for any interaction . The maj ority of light is reflected at the various interfaces , for example between the covers of the light emitting device and air or the human tissue .

[0108] In contrast thereto , the right embodiment comprises beveled sidewalls 55 with the surface 550 covered by a highly reflective material . A reflective material is also applied on the bottom to contribute to the light recycling properties . Such material may comprise TiO2 , aluminum, silver or another suitable highly reflective material . In an example , one can use a silicone matrix with approximately a 30% filling of TiO2 for undercast und sidewall coating . As a result , light being reflected from the interface to air or to the human s kin surface without interacting with human tissue is reflected into the recess and subsequently recycled . It is reemitted by the reflective material and therefore receives a further opportunity to contribute to the measurement . The cycle repeats until the remaining light has been consumed due to residual absorption in the package .

[0109] It has been now observed that the contact pressure of an optical front end and the optical arrangement in accordance with the proposed principle with the human s kin and the wrist in case of a smartwatch is central to the performance of optical vital sign monitoring applications .

[0110] If the contact pressure is too small , the PI is either small- or at least not sufficiently stable . In some cases , it can even not be established in a predictable and repeatable way . If the contact pressure exceeds a certain level , the pressure exerted presses the blood vessels and change its optical behavior . The high pressure causes a deterioration of the PI . Consequently, it has been found that the pressure to be exerted onto the user' s wrist has an optimum, which slightly depends on the wavelength, but generally lies in the range of 8000 N / m2to 12000 N / m2with a force of appr IN for the exemplary front end presented herein .

[0111] Figure 7 shows the dependence of the perfusion index on contact pressure ( associated with or converted from strap hole positions ) for green, red, and IR light and at two different distances . The X-axis represent the discrete strap hole positions . Those strap holes correspond to repeatable and determined pressure values . The Y-axis shows the strength of the perfusion index . The left diagram illustrates the perfusion index for green light at 3 . 5 mm and 7 . 5 mm, which corresponds to distances D2 and D7 , respectively . The right diagram the perfusion index for red and infrared at distances D4 and DI . A clear maximum perfusion index is observed for each wavelength . The maximum values for the red light and infrared light are located at the same pressure or force , while for green light the maximum value is slightly shifted to a lower pressure and force . The perfusion index also increases with increasing distance for red and infrared light as shown in the right drawing . The same behavior is true for green light . In any case , the pressure dependence of the PI is somewhat different for different wavelengths and, depending on the application, one might wish to optimize the perfusion index for the wavelength ( s ) of interest .

[0112] Moreover, the perfusion index peaks at substantially the same pressure level for all distances between the emitter and the detector . Since perfusion index is a measure of sensitivity, the best sensitivity is achieved when the perfusion index is at a maximum .

[0113] Figure 8 illustrates a diagram showing the perfusion index vs the pressure and force at different wavelengths in accordance with some aspects of the proposed principle . The X-axis represent the pressure exerted onto the second region and the force acting thereupon with a logarithmic scale . The Y-axis shows the perfusion index . There is a clear range , in which the perfusion index is optimal or close to a maximum plateau with a pronounced decrease at lower or higher pressures . Consequently, to obtain good and repeatable results over a longer time period and many consecutive measurements , one should aim to maintain the pressure in a range where the PI is at a maximum. In case the pressure is known, but not necessarily in the optimal range , one can utilize the known pressure to correct the measured perfusion index ( or any other optical measurement ) . The pressure dependence can be measured and quantified with a curve like the one illustrated in Figure 8 . Subsequently decorrelation of the DC , AC, and PI is possible utilizing the pressure sensor .

[0114] Figure 9 shows a measurement of the heartbeat over time at different pressure levels . The measurement is taken with infrared light . As the perfusion index is correlated with the amplitude and DC level of the heartbeat , the observation becomes more tangible by showing raw-data normalized to the respective DC-level for different pressure levels around the optimal level at strap hole position 6 ( corresponding to a pressure of about 12000Pa ) . The strap hole position 5 is slightly below the maximum, for example , the strap is slightly too loose , position 6 is right at the maximum, and strap hole position 7 is beyond the optimal pressure , for example , the strap is slightly too tight / the contact pressure is already too high .

[0115] The left diagram of Figure 9 clearly shows that the modulation depth rises sharply when applying the pressure at the optimal value . The increase in modulation corresponds to a factor of roughly 4x . When the pressure is further increased beyond the optimal value , amplitude of the heartbeat decreases again in a similar , nearly symmetrical fashion .

[0116] The right diagram of Figure 9 is a close-up that allows to examine in more detail the differences in modulation and also the differences in the morphology of the PPG-wave : although the curves looks similar and the traces for the pressure corresponding to strap hole positions 5 and 7 could be scaled up to approximately match the curve of pressure corresponding to strap hole position6 , there are small differences in the shapes of the curves as well . Those will be explained in greater detail below .

[0117] Similarly, Figure 10 shows the DC level vs strap hole position for different distances between the emitter and the detector in accordance with some aspects of the proposed principle . The left diagram illustrates the curves for green light at two differences distances D2 and D7 . The right drawing is the DC value for red and infrared light at distances D4 and DI , respectively . The Figure shows the dependence of the DC level on contact pressure . Overall , the greater the pressure , the higher the DC level . As one can also use the DC-level to extract information related to the glucose concentrations or the change thereof over time a stable contact pressure is suitable and desired . As it can be seen from the right drawings , a pressure too high does not strongly deteriorates such measurements purely based on DC level . At lower pressure level one can observe that the slope of the respective DC levels is different for green and red light . While the slope for the green curve comprises a more gradual onset , the DC level at red and infrared light changes more substantially within one strap hole position .

[0118] Figure 11 illustrates two further dependencies of the PPG signal on contact pressure . It is possible to extract the white-card loop-back equivalent SNR from PPG data , which is provided as a function of contact pressure in the left diagram of Figure 11 . The diagram illustrates pronounced dips in the signal to noise-ratio SNR around the pressure corresponding to strap hole position 6 for all colors and at all distances . The curves indicate a coupling of the optical signal to physiological processes in tissue other than the heartbeat . However , apart from that , the SNR seems to be relatively independent of pressure , following by-and-large the DC curves , as one would expect . The perfusion index is largest at the pressure corresponding to strap hole position 6 with the SNR being lowest there as well . It is reasonable to assume that perfusion index and SNR are correlated . Moreover , it seems plausible to assume that coupling of the second region to the living tissue is most effective at this pressure and hence additional physiological variability, which may not be directly associated with the heartbeat but with the living specimen as such, for example , muscle movement , also best couples to the PPG measurement system and hence increases the variability of the signal .

[0119] The right diagram illustrates the modulation to noise ratio MNR which is also of interest to PPG measurement systems . It is a quantitative measure of the system performance considering both sensitivity and noise : the larger the perfusion index and the larger the SNR, the greater the MNR . The value is calculated from the modulation amplitude due to the heartbeat divided by the standard deviation o of the measurement and it is equivalent to multiplying the PI with the SNR : it follows : where MNR is the modulation to noise ratio and SNR is the signal to noise ratio given by SNR = DC / o .

[0120] Figure 12A shows two diagrams illustrating different curvatures of signal levels vs distance for different contact pressures and more particularly the curvature of the signal level with the separation between source and detector . The distances between the three LEDs for red, infrared and green vary and are available for each light source in the above shown design . Hence , simultaneous measurements at the different distances become possible . Likewise , it is possible to quantify the curvature of the DC-signal . The results indicate that the curvature , which is due to the approximate Lambert-Beer law in a turbid medium, is a function of the contact pressure .

[0121] Furthermore , the change in curvature can be used to extract the blood glucose concentration . It is advisable to reduce and remove other influences that can alter curvature with distance . The left diagram of Figure 12A shows the signal level ( DC ) for red light as a function of the strap hole position (X-axis ) for three different distances between the source and the detector . As already discussed, the signal levels tend to increase steadily with increasing contact pressure , although this increase becomes smaller . The right diagram of Figure 12A illustrates vertical cross-sections at the various pressures corresponding to strap hole positions 3 to 9 and plot the DC level over distance for the three distances available , here 4 . 6 , 6 . 0 , and 8 . 5 mm, shown for red light . The curvature can be extracted by normalizing the signal levels to the respective maxima at 4 . 6 mm, taking the logarithm, normalizing again to the minimum, and then subtracting a straight line going from 0 to 1 . The result is the plot shown in Figure 12B on the left side . The end points are pinned to zero and the resulting mid-point deviation is an indication of the signal curvature with distance . The right diagram shows the mid-point deviation plotted over the various pressure points .

[0122] Especially for small contact pressures , there is a strong slope . At the optimal pressure point around position 6 , the perfusion index is at a maximum while the slope results in a minimum and then stays approximately flat or slightly increases again on average , when the strap force increases further . In summary, the results indicate that it is essential to keep contact pressure steady and ideally at a level where the perfusion index is at a maximum to be able to utilize the signal curvature as a function of distance .

[0123] Figure 13 is a diagram showing the frequency spectrum and relative phase between different harmonics to illustrate some aspects of the proposed principle . The spectrum is derived from a measurement with the infrared light . Three curves are obtained, one at the maximum perfusion index corresponding to the optimal exerted force and pressure , one at a slightly smaller pressure and one at a larger pressure . As expected, the curve for strap hole position 6 provides the largest amplitudes . On the right-hand side , the relative phase between the second harmonic and the fundamental is then illustrated as D21 . The two other curves correspond to the relative phase between the third harmonic and the fundamental marked as D31 and between the third and second harmonics marked as D32 . Curve D21 exhibits a constant slope throughout and especially crossing the optimal pressure at strap hole position 6 of the maximum perfusion index . However , the other relative phases , such as D31 and D32 show a more constant behavior with a moderate peak at the maximum perfusion index at pressure corresponding to position 6 .

[0124] Although the behavior is weak, the relative phase between the harmonics appears to depend on the contact pressure and thus the PPG wavemorphology is also affected by the strap force . This aspect of PPG measurement is critical for all of the more modern / recent applications , such as blood pressure monitoring and the determination of Glucose concentrations , because they are relying critically on the PPG wave morphology .

[0125] Figures 14A and 14B show two diagrams illustrating the perfusion index PI at different strap hole positions for the two differently implemented optoelectronic components shown in Figure 6 . Figure 14A shows the measurement using a component with reflective surfaces similar to the right implementation shown in Figure 6 . The signal level particularly for the green portion shows a maximum at around a strap hole position 7 with a PI value of approximately 0 . 016 and 0 . 014 , respectively . The next curve , namely the IR level at 90 ° shows its maximum for strap hole position 7 and 8 with a smaller PI level of 0 . 005 . in contrast thereto Figure 14B illustrates the perfusion index PI measured with an optoelectronic component that does not feature the light recycling functionality . Similar to the measurement of Figure 14A, the maximum PI for green light is at strap hole position 7 , but with a value of 0 . 005 that is 2 . 5-3 times lower than with light recycling functionality . A similar decrease is given for the other measurements of the red and infrared light .

[0126] The two measurements clearly indicate that light recycling functionality by providing a reflective material on the sidewalls and the bottom of the light components increases of the PI throughout the measured wavelengths . The implemented functionality supports a reduction of the overall power consumption of the optoelectronic components without reducing the PI level below a suitable threshold . The combination of an optimized pressure of the elevated region on the human tissue together with the light recycling functionality seems to significantly boost the overall PI measurement . LIST OF REFERENCES

[0127] I optoelectronic arrangement

[0128] 10 housing

[0129] II bottom contact surface

[0130] 12 first region

[0131] 13 second region

[0132] 20, 21 openings

[0133] 22 central opening

[0134] 23, 24 openings

[0135] 41 fastening

[0136] 42 button

[0137] 43 fastening means, strap

[0138] 50, 50' light emitting device

[0139] 51, 52 recess

[0140] 55 sidewall

[0141] 60, 60' light detecting device

[0142] 61 light absorbing device

[0143] 500,501 optoelectronic component

[0144] 502 optoelectronic component

[0145] 550 bevelled sidewall

[0146] LI, L2 circumference

Claims

CLAIMS1 . Optoelectronic arrangement , comprising : a housing with a bottom contact surface , said bottom contact surface having a first region and a second region, the first region surrounding the second region, wherein the second region is elevated with respect to the first region; the second region comprising a plurality of openings , wherein three openings of the plurality of openings are arranged in a first row; each of the openings comprising an at least partially transparent cover ; a first optoelectronic device arranged in a first of the three openings , said optoelectronic device comprising a first optoelectronic component configured to emit light of a first wavelength, in particular red light and a second optoelectronic component configured to emit light of a second wavelength, in particular infrared light ; a first light detection device , in particular a photodetector, arranged in a second of the three openings , wherein a distance of the first optoelectronic component to the first light detection device is substantially equal to a distance of the second optoelectronic component to the first light detection device , the first light detection device configured to detect light of the first and the second wavelength; a light absorbing element arranged in a central opening of the three openings between the first and second opening , said light absorbing device configured to absorb light of the first and the second wavelength, fastening means configured to exert a pressure of the second region onto a sample surface , said pressure being in the range of 4000 N / m2to 20000 N / m2and particularly between 5000 N / m2and 16000 N / m2.2 . Optoelectronic arrangement , comprising : a housing with a bottom contact surface , said bottom contact surface having a first region and a second region, the first region surrounding the second region, whereinthe second region is elevated with respect to the first region; the second region comprising a plurality of openings , wherein three openings of the plurality of openings are arranged in a first row; each of the openings comprising an at least partially transparent cover and the two outer openings arranged close to opposing sides of the second region; a first optoelectronic device arranged in a first of the three openings , said optoelectronic device comprising a first optoelectronic component configured to emit light of a first wavelength, in particular red light and a second optoelectronic component configured to emit light of a second wavelength, in particular infrared light ; a first light detection device , in particular a photodetector, arranged in a second of the three openings , wherein a distance of the first optoelectronic component to the first light detection device is substantially equal to a distance of the second optoelectronic component to the first light detection device , the first light detection device configured to detect light of the first and the second wavelength; the optoelectronic arrangement configured to exert a force by the second region onto a sample surface , said force being in the range of 0 . 55 N to 2 . 2 N and in particular between 0 . 6 N to 2 N and in particular between 0 . 7 N to 1 . 7 N .3 . Arrangement according to one of claims 1 or 2 , further comprising a fourth and a fifth opening arranged in a second row intersecting the first row in particular perpendicular, wherein one of the three openings of the first row, in particular the central opening , is arranged between the fourth and fifth opening and a light absorbing element is arranged in said opening configured to absorb light of the first and the second wavelength; and / or the five openings form the shape of a cross or a T , wherein a light absorbing element is arranged in the central opening at an intersection of the cross or T configured to absorb light of the first and the second wavelength .4 . Arrangement according to claim 3 , wherein a second optoelectronic device is arranged in the fourth opening , said second optoelectronic device comprising a first optoelectronic component configured to emit light of the first wavelength, in particular red light and a fourth optoelectronic component configured to emit light of the second wavelength, in particular infrared light ; a second light detection device , in particular a photodetector , arranged in the fifth opening , wherein a distance of the first optoelectronic component to the second light detection device is substantially equal to a distance of the fourth optoelectronic component to the second light detection device , the second light detection device configured to detect light of the first and the second wavelength .5 . Arrangement according to any of the preceding claims , further comprising at least one of :- a pressure sensor configured to measure a pressure exerted onto at least one of the second region and the first region;- a force sensor configured to measure a force acting upon at least one of the second region and the first region;- a force sensor configured to measure a force exerted by at least one of the second region and the first region .6 . Arrangement according to any of the preceding claims , wherein a size of the second region is between 1 cm2and 2cm2and particularly between 1 . 2 cm2and 1 . 8 cm2and particularly between 1 . 3 cm2and 1 . 8 cm2and particularly between 1 . 4 cm2and 1 . 7 cm2; and / or a distance between two opposing edges of the second region along a virtual line between centers of the first and / or second optoelectronic component and the first light detecting device is between 10 % and 40 % larger than a distance of the first and / or second optoelectronic component to the first light detection device ; and / or a distance between an edge of the second region to one of the first and second opening is less than 15 % and particularly lessthan 10% of a distance between two opposing edges of the second region along a virtual line between centers of the first and second opening .7 . Arrangement according to any of the preceding claims , wherein each of the at least three openings are at least one of : recessed with regard to the surface of the second region; elevated with regard to the surface of the second region; and / or covered by a cover that is flush with a surface of the second region covered by a lens with a curved surface shape .8 . Arrangement according to any of the preceding claims , wherein the first and / or second optoelectronic device comprises a third optoelectronic component configured to emit light of a third wavelength, in particular green light ; and wherein a distance of the third optoelectronic component to the first light detection device is smaller than a distance of the first and second component to the first light detection device ; and / or wherein a distance of the third optoelectronic component to the second light detection device is smaller than a distance of the first and second component of the second optoelectronic device to the second light detection device .9 . Arrangement according to any of the preceding claims , wherein the first and / or second optoelectronic device comprises a third optoelectronic component configured to emit light of a third wavelength, in particular green light ; and wherein centers of the respective first , second and third optoelectronic components form edges of a virtual triangle with the center of the third optoelectronic component closest to the respective central opening .10 . Arrangement according to any of the preceding claims , wherein the light absorbing element arranged in the central opening is a third light detection device configured to detect light of the first ,second and third wavelength, wherein optionally, a distance of the third optoelectronic component configured to emit light of a third wavelength, in particular green light to the light absorbing element is smaller than a distance of the first and second optoelectronic component to the first and / or second light detecting device .11 . rrangement according to any of the preceding claims , wherein the first and / or second optoelectronic device comprise a package with a first recess , in which the first and second optoelectronic components are arranged, said recess comprising -in particular beveled- sidewalls and optionally having a high reflectivity for light of the first and / or second wavelength .12 . Arrangement according to claim 11 , wherein the package comprise a second recess , in which the third optoelectronic component is arranged, said second recess comprising -in particular beveled- sidewalls and optionally having a high reflectivity for light of the third wavelength .13 . Arrangement according to any of the preceding claims , wherein the first and / or second light detection device comprise a color filter configured to block light in a portion of the spectrum different from the first and second wavelength; and / or wherein the at least partially transparent cover over the first and / or second light detecting device comprises a color filter configured to block light in a portion of the spectrum different from the first and second wavelength .14 . Arrangement according to any of the preceding claims , wherein the first region comprises a light absorbing coating , in particular a black coating; or comprises a light absorbing material , in particular a blackened material .15 . Arrangement according to any of the preceding claims , wherein the second region comprises one of : a circular shape ;cross like shape ; a T-shape ; a shape following the arrangement of the plurality of openings in the second region; and a linear shape .16 . Arrangement according to any of the preceding claims further comprising : fastening means to exert an adj ustable pressure of the bottom contact surface onto a sample surface , the sample surface comprising in particular skin tissue ; and / or one of a pressure sensor and / or a temperature sensor, said sensors particularly being arranged in at least one of the first and the second region .17 . Arrangement according to any of the preceding claims , wherein the optical arrangement is oriented in such way that the first row is aligned- substantially parallel to a main direction of liquid containing vessels of the sample probe ,- substantially parallel to a main direction of blood vessels beneath the skin when the optical arrangement is placed onto the s kin; or-substantially parallel to a forearm of a user when attached to the user' s wrist .18 . Arrangement according to any of the preceding claims , wherein the elevation of the second region with respect to the first region is in the range between 0 . 5 mm to 3 . 5 mm and in particular between 1 . 0 mm to 2 . 0 mm and in particular less than 1 . 75 mm; and / or a distance between a center of the first optoelectronic device , in particular a common center of the first and second optoelectronic components of the first optoelectronic device to the first light detecting device and / or a distance between a center of the second optoelectronic device , in particular a common center of the first and second optoelectronic componentsof the second optoelectronic device to the second light detecting device is in the range of 5 mm to 15 mm and in particular between 7 . 5 mm and 12 . 5 mm and in particular between 7 . 5 mm and 10 mm and in particular between 8 . 0 mm and 9 . 0 mm; and / or a distance between a center of the third optoelectronic components to the third light detecting device is in the range between 2 . 5 mm and 5 mm and in particular between 3 mm and 4 mm .19 . Arrangement according to any of the preceding claims , wherein the first and second optoelectronic devices are substantially identical in construction; and / or wherein the first and second light detection devices are substantially identical in construction .20 . earable device , in particular a watch, comprising : an optoelectronic arrangement according to one of the preceding claims ; a strap forming an adj ustable fastening means to exert a pressure of the bottom contact surface onto the s kin tissue ; a pressure sensor to measure the exerted pressure and / or force between the second region and the s kin tissue . an evaluation and control unit coupled to the first and / or second optoelectronic device , the first and / or second light detecting device and the light absorbing device configured to operate those devices to obtain data and determine from said data at least one of : heart rate ; oxygen saturation; glucose concentration; and blood pressure .21 . Method of operating an optoelectronic arrangement , wherein the optoelectronic arrangement comprises :- a housing having a bottom contact surface , said bottom contact surface having a first region surrounding an elevated second region;the second region comprising a plurality of openings , wherein three openings of the plurality of openings are arranged in a row; each of the openings comprising an at least partially transparent cover that is flush with a surface of the second region;- the method comprising :- fixing the optoelectronic arrangement onto a user' s skin with an adj ustable fastening means such that the first and second region of the bottom contact surface each exert a defined contact pressure onto the user' s s kin, with the contact pressure exerted by the first region being smaller than the contact pressure exerted by the second region, wherein the contact pressure of the second region onto a sample surface is in the range of 4000 N / m2to 20000 N / m2and particularly between 5000 N / m2and 16000 N / m2; emitting light of a first wavelength and / or a second wavelength from a first opening towards the user' s s kin to interact with the tissue thereof ;- absorbing light of the first wavelength and / or the second wavelength being reflected into a second opening , located between the first and a third opening, by a light absorbing element ;- detecting light being reflected into the third opening and evaluating the detected light to obtain a value related to the optical measurement , in particular a perfusion index ; optionally determining the perfusion index using the detected light .22 . Method according to claim 21 , wherein the step of absorbing light being reflected into a second opening comprises the step of : detecting light being reflected into a second opening comprises , in particularly by a light detecting device configured to absorb and detect light ; obtaining a value related to the optical measurement from the detected light , in particular a perfusion index .

23. Method according to claim 21 or 22, wherein the second region of the bottom contact surface pressed against the skin or tissue with a force in the range of 0.55 N to 2.2 N and in particular between 0.6 N to 2 N and in particular between 0.7 N to 1.7 N.

24. Method according to any of claims 21 to 23, further comprising: emitting light of a third wavelength, in particular green light, from the first opening towards the user' s skin to interact with the tissue thereof; detecting light being reflected into the second opening and evaluating the detected light to obtain a value related to the optical measurement, in particular a perfusion index.

25. Method according to any of claims 21 to 24, wherein the step of detecting light is conducted by a photodetector arranged in the respective openings .

26. Method according to any of claims 21 to 25, further comprising: measuring the pressure exerted by the second region onto the user' s skin; calibrating at least one of the steps of o determining the perfusion index; o emitting light; and o detecting light using a value derived from measuring the pressure .

27. Method according to any of claims 21 to 26, wherein the step of emitting light and detecting light is conducted such that an optical path lies substantially parallel to blood vessels beneath the user' s skin.

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