Vital function measuring device and method for optical vital sign monitoring

Abstract

This invention relates generally to a vital function measuring device which comprises a first LED and a second LED, wherein the at least two LEDs are of different band gap and are driven by an electrical circuit. To prevent parasitic luminescence due to the excitation of the lower energy LED by the higher energy LED, a negative bias voltage is applied to the lower energy LED while the higher energy LED is operating. Therefore, the electrical circuit enables a first driver circuit to apply a first operating voltage to the higher energy LED, i.e., the first LED, while at the same time a second driver circuit applies a negative bias voltage to the lower energy LED, i.e., the second LED. The core of the invention is hence to modify the circuitry of the vital function measuring device for voltage reversal of one LED by adding a switch that allows for voltage reversal of one LED driver while the other LED is emitting. It also relates to a method for optical vital sign monitoring.

Description

[0001] Vital function measuring device and method for optical vital sign monitoring

[0002] DESCRIPTION

[0003] TECHNICAL FIELD

[0004] The invention relates to a vital function measuring device which comprises a first LED and a second LED . It also relates to a method for optical vital sign monitoring .

[0005] BACKGROUND

[0006] Vital function measuring devices are devices that comprise a measurement device to conduct optical vital sign monitoring, e . g . , bioluminescence measurements , HRM, SpO2 , Blood Pressure measurement , glucose tracking . In general , mobile devices such as smartphones or smartwatches are used for biomonitoring . Smartwatches are worn constantly and thus enable comprehensive monitoring of the relevant parameters .

[0007] The trend in optical vital sign monitoring (VSM) is to arrange several light emitting diodes ( LEDs ) of various wavelengths on one device . Preferably, the LEDs are added to the so-called optical front end ( OFE ) which comprises the optical components such as light sources , optical sensors , dividing walls or optical covers . The OFE is controlled by components of an application-speci fic integrated circuit (AS IC ) . Therefore , certain applications are enabled in addition to today' s heartrate monitoring (HRM) . One example is the measurement of blood oxygenation ( SPO2 ) , requiring red- and IR wavelengths in addition to green, which is the preferred wavelength for HRM . Other LEDs applied for some applications use UV light at or below 400 nm to excite photoluminescence ( PL ) in human tissue and to extract biomarkers of interest from the PL spectrum .

[0008] Since LEDs emitting light at deviating wavelengths are added while the same detectors are being used and, especially, since the LED chips tend to be placed either into the same miniaturi zed package or in proximity on the same printed circuit board ( PCB ) , it is necessary to consider the ef fects of cross-excitation of LEDs emitting at longer wavelengths by LEDs emitting at shorter wavelengths .

[0009] For instance , UV LEDs emit light with a photon energy that is larger than the bandgap of the green LED and therefore the UV photons can be absorbed by the green LED material .

[0010] Accordingly, the green LED material in turn starts to fluoresce at its green wavelength with a certain fluorescence spectrum .

[0011] This phenomenon is generally present for combinations of LEDs with shorter and longer wavelengths that are arranged in proximity . For example , an LED emitting green light may cause red or IR LEDs to fluoresce . An LED emitting UV light may interfere with green, red, and IR LEDs , etc .

[0012] As the primary fluorescence signal remitted by the skin that one wishes to analyze tends to be quite small , the amount of remitted light by the longer wavelength LED due to secondary fluorescence may have a signi ficant influence on the measured signal . Accordingly, the cross-excitation of a green LED in proximity to a UV LED is suf ficient to distort the detectable signal to an extent where the application can no longer work, or at least the accuracy of the measurement is substantially disturbed .

[0013] To prevent the influence of the LED emitting at a shorter wavelength on the LEDs emitting at a longer wavelength, , one approach is to spatially separate the LEDs , such that shortwavelength light does not reach the long-wavelength LED . To some extent , this may be accomplished by dividing walls between the LEDs , which are opaque to the shorter wavelength light . However, the placement of separating walls disadvantageously increases the footprint required for the arrangement and adds to the complexity of mechanical integration of the optical front-end .

[0014] In addition, even though the light from the LED emitting at shorter wavelength has no direct way to excite the longer wavelength LED, it can still be reflected by the tissue which is positioned in proximity . Therefore , it may indirectly excite the longer wavelength LEDs in the neighboring cavity .

[0015] A di f ferent approach to suppress cross-excitation is to place a filter on top of the longer wavelength LED and / or to encapsulate the longer wavelength LED with a casting that absorbs ( suppresses ) the light at the shorter wavelength .

[0016] However, by definition, both techniques require separation like the two- or multi-cavity approach described above and with a similar, disadvantageous impact on the complexity of opto-mechanical integration and increasing footprint . Moreover, the approach is not compatible to LED chips placed directly on a PCB in proximity .

[0017] While the placement of filters on top of an LED or package is cumbersome and expensive , the development of casting materials with di f ferent optical properties for j etting or compression molding is a formidable challenge , since it does require the development of both suitable casting materials as well as the adj ustment of the corresponding production process , e . g . , double j etting of epoxy or silicone compounds or complex molding processes working with two di f ferent mold compounds .

[0018] Further, the addition of filters or attenuating fillers in mold compounds or castings , e . g . , TiCt , will inadvertently and negatively impact the package performance , since some light of the primary wavelength from the longer wavelength LED or chip will be lost .

[0019] An obj ective of the present invention is thus to overcome the disadvantages of the state of the art and to provide a low cost and easy to manufacture vital function measuring device , particularly a vital function measuring device that enables standard- as well as advanced applications , and which may utili ze a plurality of LEDs arranged in such a way as to occupy a smallest possible footprint , yet without excitation of the longer wavelength emitters by light of shorter wavelengths .

[0020] SUMMARY

[0021] With respect to the device the obj ective is met by a vital function measuring device according to claim 1 . The corresponding method is speci fied in claim 15 .

[0022] According to claim 1 the device-related obj ective is met by : a vital function measuring device for measuring a vital function of a living creature comprising :

[0023] • a first LED configured to emit a first light signal at a first wavelength,

[0024] • a second LED, wherein the second LED is configured to emit a second light signal at a second wavelength, wherein the second wavelength is longer than the first wavelength, preferably the second wavelength is at least 100 nm longer than the first wavelength, more preferably at least 250 nm longer than the first wavelength,

[0025] • a sensor configured to receive a reflected first light signal ,

[0026] • a sensor configured to receive a reflected second light signal , • an electrical circuit comprising a first driver circuit to operate the first LED and a second driver circuit to operate the second LED, wherein the electrical circuit is configured to be capable to switch between the application of a positive operating voltage to the second LED and the application of a negative bias voltage to the second LED by the second driver circuit independent of the voltage applied to the first LED by the first driver circuit.

[0027] Advantageously, the parasitic luminescence, especially fluorescence, caused by the emission of the first LED which may excite the second LED can be suppressed. Therefore, no optical separation, e.g. walls between the LEDs, is required. The first LED and the second LED might be optically coupled. The same applies if there are three or even more LEDs present .

[0028] For bioluminescence applications, the invention enables the smallest possible footprint of an optical front end emitting UV-, green-, red-, and IR wavelengths, or a subset thereof.

[0029] The proximity of components-, and thus the minimization of the cross-section (footprint) of the Analogue front end and the ASIC, is a requirement for achieving high enough contact pressure at comfortable strap forces in smartwatches and fitness trackers; the pressure, which is force divided by area, is needed to maximize the sensitivity of e.g. photoplethysmography (PPG) measurements. In particular, for oxygen saturation measurements (SpO2) on the wrist, a maximization of the PI is essential for successful measurements. A high PI is also advantageous for Glucose tracking. The avoidance of a mixing of different wavelengths during the same acquisition phase, enhances the signal quality .

[0030] According to the invention, the first LED is not essentially a light emitting diode, but can generally be any first light source providing a shorter wavelength than the second LED. Preferably, the first light source is built as a light emitting diode or a laser diode , as those are easy to manufacture and low cost while they can be provided very small . Accordingly, the overall area of all LEDs can be constructed advantageously small .

[0031] The sensor configured to receive a reflected first light signal and the sensor configured to receive a reflected second light signal can be built either as one sensor which is able to detect both signals or as two sensors . For the case of two sensors a first sensor receives the reflected first light signal , and the second sensor receives the reflected second light signal .

[0032] According to the invention, the term "a negative bias voltage" means a voltage that is reversed to the operating voltage . According to the invention, the terms "operating voltage" and "positive operating voltage" are used synonymously and do not di f fer in their meaning .

[0033] Advantageously, the electrical circuit is configured to enable the operation of the second LED independently from the operation application of the first LED . Therefore , a di f ferent voltage can be applied to the first and the second LED and even the polarity of the voltages applied to the first and the second LED can be di f ferent .

[0034] The invention is based on the possibility of the vital function measuring device to enable an emission of the first LED and at the same time to apply a negative bias voltage to the second LED .

[0035] In a preferred embodiment , the vital function measuring device comprises an analyzing means , wherein the analyzing means is configured to analyze the received first light signal and the received second light signal . Therefore , the analyzing means is coupled to the sensor or to the sensors . Thereby, the analyzing means is configured to measure the vital function based on the analyzed received first light signal and the received second light signal . The analyzing means is further coupled to an output device , such as a display .

[0036] Preferably, the electrical circuit is configured to apply an operating voltage to the first LED leading to an emission of light of the first LED while at the same time a negative bias voltage is applied at the second LED .

[0037] In a preferred embodiment , each LED channel is reversable independently from all other channels , since the negative bias voltage is applied when one of the other LEDs is emitting light , i . e . , current is flowing in the forward direction .

[0038] Preferably, the amount of reverse bias applied to one LED can be independently controlled .

[0039] This can preferably be reali zed by the electrical circuit , particularly the second driver circuit , comprising a switching component . This switching component is configured to switch the voltage applied to the second LED between the positive operating voltage and the negative bias voltage .

[0040] In a preferred embodiment each driver circuit comprises such a switching component . Accordingly, each LED can be driven by each driver circuit .

[0041] As an option, the second driver circuit comprises a bypass wire and the switching component is configured as a double pole switching means . Thereby the double pole switching means is arranged and configured to switch between the positive operating voltage applied to the second LED and the negative bias voltage applied to the second LED .

[0042] Preferably, the electrical circuit is designed to allow a reversed polarity due to a set of coupled switches that are connected to the LED supply wire (VLED) and to the ground (GND) . During forward operation, the electrical current flows in the operating direction. However, when the switches are used, GND is connected to the anode of the LED and VLEDis connected via the by-pass to the cathode. In this manner the polarity across the P / N-junction can be reversed.

[0043] The configuration enables the second driver circuit to switch between forward mode and reverse mode. According to the invention, the term "forward mode" means the operating mode of the LED, i.e. the LED emits light due to an applied voltage. In reverse mode, a negative bias voltage is applied to the LED, i.e. the applied voltage is reversed to the polarity of the positive operating voltage in forward mode.

[0044] In forward mode, the anode potential, i.e. the potential applied to the anode of the second LED, is connected to an LED supply voltage VLED. In reverse mode, the anode potential is connected via the double pole switching means to ground VGND • The cathode potential, i.e. the potential applied to the cathode of the second LED, is connected to ground VGND in forward mode. In reverse mode, the cathode potential is connected to the LED supply voltage VLEDvia the bypass wire and the double pole switching means. Accordingly, the combination of the bypass wire and the double pole switching means allows to change the polarity of the voltage applied to the second LED.

[0045] In this case, the LED supply voltage is a DC (direct current) voltage and not an AC (alternate current) voltage.

[0046] The second LED is optionally constructed as a laser diode. Therefore, a high energy can be emitted leading to better measurement results.

[0047] In a preferred embodiment, there is a wall between the first and the second LED. This wall is opaque to the shorter wavelength light and may suppress the direct excitation of the second LED by light of the first LED. Accordingly, a smaller negative bias voltage is sufficient to conveniently suppress parasitic luminescence. Preferably, there is a filter on top of the longer wavelength LED and / or the longer wavelength LED can be encapsulated with a casting that absorbs the light of the shorter wavelength . Accordingly, a smaller negative bias voltage is suf ficient to conveniently suppress parasitic luminescence .

[0048] In a preferred embodiment , the vital function measuring device comprises a third LED configured to emit a third light signal at a third wavelength . The third wavelength is longer than the second wavelength .

[0049] In this embodiment , the electrical circuit further comprises a third driver circuit to operate the third LED . Thereby, the electrical circuit is configured to switch between the application of a positive operating voltage to the third LED or the application of a negative bias voltage to the third LED by the third driver circuit , independently of the voltage applied to the first LED by the first driver circuit and / or the second LED by the second driver circuit . Furthermore , all features of the second driver circuit apply as well to the third driver circuit .

[0050] Advantageously, the parasitic luminescence caused by the emission of the first LED or by the emission of the second LED, which either may excite the third LED, can be suppressed . Therefore , no optical separation, e . g . walls between the LEDs , is required . The first LED, the second LED and the third LED might be optically coupled . The same applies i f there are even more LEDs present .

[0051] In a preferred embodiment , there is a wall between the first and the second LED and between the first and the third LED and between the second and the third LED . These walls are opaque to the light of the first LED and / or to the light of the second LED and may suppress the direct excitation of the second LED and / or the third LED by light of the first LED and the second LED, respectively . Accordingly, a smaller negative bias voltage is suf ficient to conveniently suppress parasitic luminescence . Preferably, there is a filter on top of the longer wavelength LED, i . e . , the second LED and / or the third LED, and / or the longer wavelength LED can be encapsulated with a casting that absorbs the light of the shorter wavelength . Accordingly, a smaller negative bias voltage is suf ficient to conveniently suppress parasitic luminescence .

[0052] Preferably, the electrical circuit is configured to apply an operating voltage to the first LED leading to an emission of light of the first LED and to apply at the same time a negative bias voltage to the second LED and to the third LED . Therefore , the parasitic absorption can advantageously be suppressed ef fectively .

[0053] In an optional embodiment , the electrical circuit is configured to apply a positive operating voltage to the first LED leading to an emission of light of the first LED and to apply at the same time a negative bias voltage to the second LED . Furthermore , the electrical circuit is configured to apply a positive operating voltage to the second LED leading to an emission of light of the second LED and to apply at the same time a negative bias voltage to the third LED . Therefore , the parasitic luminescence caused by the emission of the second LED, which may excite the third LED, can be suppressed .

[0054] The third LED is optionally a laser diode .

[0055] In a preferred embodiment , the vital function measuring device comprises an analyzing means , wherein the analyzing means is configured to analyze the received first light signal and the received second light signal and the received third light signal . Therefore , the analyzing means is coupled to the sensor or to the sensors . Thereby, the analyzing means is configured to measure the vital function based on the analyzed received first light signal and the received second light signal and the received third light signal . The analyzing means is further coupled to an output device, such as a display.

[0056] Preferably, photodiodes are used as sensors, e.g. SFH2705, or PIN photodiodes- The photodiodes are usually broadband or equipped with a filter that can suppress the ambient light of longer wavelength (red, IR) . As an option, bandpass filters can be deposited on the photodiodes, which only allow one wavelength or one range of wavelengths to pass through.

[0057] Preferably, the electrical circuit is integrated in an application-specific integrated circuit. As an advantage, the signal-to-noise ratio is high, preferably above 90 dB, more preferred above 95 dB and even more preferred above 100 dB . In a preferred embodiment, the LED driver and the analogue to digital converter for the photodiode are integrated in one ASIC. The ASIC may advantageously provide additional functions such as ECG or bio-impedance. Additional advantages are that the ASIC may minimize the power consumption or it leads to a very high dynamic range of 21 bit or more and it can advantageously integrate certain additional functions such as the possibility of applying offset currents. All these functions can advantageously be realized in a very compact component in order to keep the installation space for smart watches and fitness trackers as small as possible.

[0058] In a preferred embodiment the electronic circuit is integrated in an integrated circuit. As an advantage, the low-noise performance is improved. The term "applicationspecific integrated circuit" is understood to mean an integrated circuit that is configured for a particular use, particularly, an integrated circuit that is configured for a particular application by providing all the functionality, performance, (low-) power properties, driving currents, analogue to digital conversion properties, e.g., sigma-delta converter with high dynamic range, and signal-to-noise ratio performance that is relevant for a successful measurement. In a preferred embodiment of the vital function measuring device , the first wavelength is in the range of 350 nm to 420 nm and the second wavelength is in the range of 450 nm to 600 nm . Here , the excitation of a green LED by the first wavelength in the UV light range might be suppressed .

[0059] In another embodiment , the first wavelength is in the range of 450 nm to 570 nm, preferably 500 nm to 550 nm, and the second wavelength is in the range of 600 nm to 5000 nm, preferably 610 nm to 1600 nm, more preferred 615 nm to 900 nm . Here , the excitation of a red LED or an infrared LED by the first wavelength in the green range might be suppressed .

[0060] In a preferred embodiment , the distance between the first LED and the second LED, i . e . , the gap or slit between the first LED and the second LED, is smaller than 1 mm . Preferably, the gap between the first and the second LED is smaller than 500 pm, more preferred smaller than 300 pm, even more preferred smaller than 100 pm, even more preferred smaller than 50 pm, even more preferred smaller than 10 pm .

[0061] The first LED and the second LED are in one embodiment built in a package within one module or they are placed in one module or the LEDs are mounted in a package on a circuit board or the LEDs are attached to a circuit board .

[0062] Preferably, the overall area, which is covered by the two or more LEDs and the space between the LEDs is smaller than 2 mm2, more preferred smaller than 1 mm2. The small area has a positive impact on the contact pressure at comfortable strap forces in smartwatches and fitness trackers .

[0063] In a preferred embodiment , vital function measuring device is a mobile device .

[0064] In the present invention, the term "mobile device" is understood to mean a vital function measuring device which is designed to be small enough to be held in the hand, especially it is smaller than 50 cm in its largest extension, preferably it is smaller than 20 cm in its largest extension, more preferred it is smaller than 50 mm in its largest extension, even more preferred it is smaller than 20 mm in its largest extension . Such a device comprises a screen as an interface and a computer to carry out arithmetic and / or logic operations .

[0065] As an option, the mobile device comprises a screen and the first LED and the second LED are embedded in this screen .

[0066] Advantageously, the vital sign monitoring can be embedded in display screens . As the area of the LEDs according to the invention is small , there are advantages for mobile devices due to reduced area and reduced material costs .

[0067] Preferably, the mobile device is a smartwatch and comprises a strap to attach the device to a part of the body, in particular to the wrist of a human .

[0068] Preferably, the smartwatch comprises an analogue or digital display which is capable to display the time .

[0069] According to the invention, the term " smartwatch" also includes devices in form of a watch which are capable to display certain information, the time not being an essential information .

[0070] In term of method, the invention relates to a method to for optical vital sign monitoring, preferably on a human body, i . e . an optical vital sign monitoring method . The method uses a vital function measuring device according to the invention, which comprises an analyzing means , and the method comprises the following steps : a) applying a negative bias voltage to the second LED, b) starting an emission of the first LED by an application of a first positive operating voltage for a first duration to the first LED to emit a first light signal for the first duration, c) receiving the first light signal reflected by the human tissue and forward the signal to the analyzing means , wherein the analyzing means analyzes the received first light signal , d) stopping the emission of the first LED by a stopping of the application of the positive operating voltage on the first LED, e ) switching the voltage applied to the second LED from the negative bias voltage to a positive operating voltage to emit a second light signal , and f ) receiving the second light signal reflected by the human tissue and forward the second light signal to the analyzing means , wherein the analyzing means analyzes the received second light signal .

[0071] The process of switching the voltage from the negative bias voltage to a positive operating voltage may include processes where the negative bias voltage is reduced to zero and afterwards the positive operating voltage is applied .

[0072] Advantageously, the process enables a suppression of parasitic luminescence , particularly fluorescence , and a small distance between the LEDs as there is no need for walls between the LEDs . Accordingly, the overall area of the LEDs is small , and the pressure can be increased at comfortable strap forces , especially in smartwatches and fitness trackers . Hence , the measurements of the vital functions is more precise .

[0073] Preferably, the process steps a ) to f ) are conducted orderly . This may ensure an ef ficient suppression of the luminescence .

[0074] In a preferred embodiment , the vital function measuring device comprises a third LED and before step e ) a negative bias voltage is applied to the third LED by a third driving circuit . More preferred, the negative bias voltage is applied to the third LED even before step b ) .

[0075] Preferably, after step f ) steps g) , h) and i ) follow . These steps comprise : g) stopping the emission of the second LED by a stopping of the application of the positive operating voltage on the second LED, h) switching the voltage applied to the third LED from the negative bias voltage to a positive operating voltage to emit a third light signal , and i ) receiving the third light signal reflected by the human tissue and forward the third light signal to the analyzing means , wherein the analyzing means analyzes the received third light signal .

[0076] A further aspect of the invention relates to a vital function measuring device comprising

[0077] • a first LED configured to emit a first light signal at a first wavelength,

[0078] • a sensor configured to receive a reflected first light signal , an electrical circuit comprising a first driver circuit to operate the first LED, wherein the electrical circuit is configured to switch between the application of a positive operating voltage and a negative bias voltage to the first LED . As an advantage , the luminescence caused by light of another LED which is located in proximity or even caused by ambient light can be suppressed .

[0079] A further aspect of the invention relates to a method of operating an LED to suppress a parasitic luminescence of the LED by the application of a reverse bias voltage to the LED while the LED is not operating . The voltage applied to the LED is switched between a forward voltage to operate the LED and a negative bias voltage to prevent the luminescence excited by light at an excitation wavelength . As an advantage , the luminescence caused by light of another LED which is located in proximity or even by ambient light can be suppressed .

[0080] Preferably, the method is applied to a vital function measuring device comprising a first LED configured to emit a first light signal at a first wavelength, a sensor configured to receive a reflected first light signal and an electrical circuit comprising a first driver circuit to operate the first LED, wherein the electrical circuit is configured to switch between the application of a positive operating voltage and a negative bias voltage to the first LED . The method is applied to the first LED . As an advantage , the luminescence caused by light of another LED which is located in proximity or even caused by ambient light can be suppressed when the negative bias voltage is applied .

[0081] In a preferred embodiment , the vital function measuring device comprises an application-speci fic integrated circuit (AS IC ) configured to control the reverse bias logic, wherein the power switches for toggling the supply voltage are provided externally by the user . This approach of fers high flexibility, as users can select power switches best suited to their system ' s requirements . Additionally, by limiting the AS IC' s role to logic control , the design minimi zes silicon real estate , reducing power consumption and costs while maintaining adaptability for di f ferent applications . This embodiment is particularly useful for scenarios where customi zation of power switching components is required .

[0082] In another preferred embodiment , the vital function measuring device comprises an analog integrated circuit ( IC ) embedded within an LED-Emitter Package , wherein the analog IC is configured to switch between a positive operating voltage and a negative bias voltage applied to at least one LED . This design eliminates the need for external power switches , reducing design complexity for the user . By integrating the switching functionality directly into the emitter package , the analog IC can be optimi zed for the speci fic characteristics of the LEDs , ensuring ef ficient performance and improved reliability . This embodiment is especially advantageous for applications requiring a compact , multiemitter system with built-in reverse bias control .

[0083] In a another preferred embodiment , the vital function measuring device comprises a fully integrated AS IC that incorporates both the LED supply and the reverse bias supply, wherein the AS IC is configured to directly switch at least one LED between forward operation and reverse bias without requiring external power switches . This approach provides the highest level of integration, simpli fying implementation and ensuring a streamlined design . By eliminating external switching components , this embodiment reduces system complexity and potential compatibility issues . While it requires additional silicon real estate , it of fers a user- friendly and space-ef ficient solution, making it ideal for applications where minimal external components and simpli fied circuit design are priorities .

[0084] In yet another preferred embodiment , the vital function measuring device comprises an electrical circuit configured to derive the reverse supply voltage either from a separate voltage source or from the LED supply voltage , depending on the speci fic application requirements . This flexibility allows the system to be optimi zed based on power management strategies . A separate reverse supply voltage can enable more precise control over biasing functions , while deriving it from the LED supply simpli fies system design and reduces cost . This embodiment ensures adaptability across di f ferent use cases , allowing for ef ficient integration into various VSM (Vital Sign Monitoring) devices .

[0085] In a preferred embodiment , the method for optical vital sign monitoring comprises an AS IC or an analog IC configured to dynamically control the application of a negative bias voltage to at least one LED to suppress parasitic luminescence , based on predefined operational parameters or real-time sensor feedback . This dynamic control mechanism enhances measurement accuracy by adj usting the reverse bias in response to varying physiological and environmental conditions . By actively suppressing parasitic luminescence , this embodiment ensures high signal quality, making it particularly beneficial for wearable health monitors and medical devices where reliable and consistent measurements are essential . In summary, key elements of the invention are that the vital function measuring device comprises at least two LEDs of di f ferent band gap that are driven by an electrical circuit . To prevent parasitic luminescence due to the excitation of the lower energy LED by the higher energy LED, a negative bias voltage is applied to the lower energy LED while the higher energy LED is operating . Therefore , the electrical circuit enables a first driver circuit to apply a first operating voltage to the higher energy LED, i . e . , the first LED, while at the same time a second driver circuit applies a negative bias voltage to the lower energy LED, i . e . , the second LED .

[0086] The core of the invention is hence to modi fy the circuitry of the vital function measuring device for voltage reversal of one LED by adding a switch that allows for voltage reversal of one LED driver while the other LED is emitting .

[0087] BRIEF DESCRIPTION OF THE DRAWINGS

[0088] Exemplary embodiments of the invention are discussed below with reference to the accompanying drawings .

[0089] FIG . 1 shows a scheme of an arrangement of several LEDs .

[0090] FIG . 2A shows a fluorescence spectrum of a green LED at di f ferent negative bias voltages for an excitation at 400 nm .

[0091] FIG . 2B shows a fluorescence spectrum of a green LED at di f ferent negative bias voltages for an excitation at 380 nm .

[0092] FIG . 3A shows a scheme of a second driver circuit in forward mode ,

[0093] FIG . 3B shows a scheme of a second driver circuit in reverse mode and FIG . 4 shows a combination of a first LED in forward mode and a second LED in reverse mode .

[0094] FIG . 5 illustrates a first implementation of an applicationspeci fic integrated circuit (AS IC ) with integrated reverse bias logic, where external power switches are supplied by the customer .

[0095] FIG . 6 shows a second implementation in which the switching function is embedded within a coordinated LED-Emitter package that includes an analog IC for controlling the reverse bias .

[0096] FIG . 7 depicts a third implementation, featuring a fully integrated AS IC that directly manages both the LED supply and reverse bias supply, eliminating the need for external switching components

[0097] Like features have been designated by like references in the various figures . In particular, the structural and / or functional features that are common among the various embodiments may have the same references and may dispose identical structural , dimensional , and material properties .

[0098] For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail . In particular, the electrical circuits are not shown completely, but the described embodiments being compatible with usual applications .

[0099] DETAILED DESCRIPTION

[0100] A scheme of an arrangement of LEDs 1 is shown in FIG . 1 . A first LED 2 . 1 is positioned in close proximity to a second LED 2 . 2 and a third LED 2 . 3 . The wavelength of the light emitted by the first LED 2 . 1 is the shortest , while the wavelength of the third LED 2 . 3 is the longest . In addition, sensors 3 are arranged at the LEDs 2 . The sensors 3 are configured to detect the reflected signal of the corresponding LED 2 . Accordingly, the light that is emitted by one of the LEDs 2 is reflected back to the arrangement of LEDs 1 under a small angle of under 30 ° . To suppress a fluorescence signal of the second LED 2 . 2 or of the third LED 2 . 3 caused by an excitation by the light of the first LED 2 . 1 , a negative bias voltage is applied to the second LED 2 . 2 and to the third LED 2 . 3 while the first LED 2 . 1 emits light at the shorter wavelength .

[0101] The ef fect of the negative bias voltage applied to a green LED while being excited by light of an excitation wavelength is shown in FIG . 2A and 2B . The excitation wavelength is 400 nm for FIG . 2A and it is 380 nm for the spectra in FIG . 2B .

[0102] In order to determine the influence of the reversed bias voltage , i . e . a negative bias voltage , on the fluorescence of the green LED, the LED was placed on a measurement setup on which electrical contacts are positioned to apply an electrical voltage on the green LED .

[0103] The light of an ultra-violet wavelength was coupled into a fiber and the other end of the fiber is imaged with the help of an achromatic lens into the sample plane . The si ze of the image was approximately 600 pm in diameter and roughly corresponds to the si ze of the green LED, which is 500 x 500 pm2.

[0104] The light emitted by the green LED is collected by a second lens , which is also connected to a fiber, which is coupled into an Oceaninsight QE-PRO FL spectrometer . The spectral range of the spectrometer is from 350 to 1150 nm .

[0105] The receiving lens is positioned in relation to the green LED to avoid any specular reflection of the excitation beam . The fluorescent light is assumed to exhibit with a Lambertian distribution, j ust like it would for normal operation of the green LED with a forward voltage .

[0106] The graph shows a spectrum which comprises an overlay of the excitation signal at 400 nm or 380 nm, respectively, and the fluorescence signal of the excited green LED which emits at about 525 nm . The counts are displayed logarithmically over the wavelength in nm .

[0107] For this measurement , the bias voltage applied to the green LED was varied between a first bias voltage VI of 0 V, where the LED was disconnected from any voltage supply, a second bias voltage V2 of - 1 V, a third bias voltage V3 of -3 V, a fourth bias voltage V4 of -7 V to a fi fth bias voltage V5 of -9 V . A strong decrease of the fluorescence peak is shown for a larger reverse bias voltage . As it can be seen, the ef fect is strong for both excitation wavelengths as shown for 400 nm in FIG . 2A and for 380 nm in FIG . 2B .

[0108] For a bias voltage VI , the fluorescence peak at 525 nm reaches about 10000 counts . The counts do not display a physical quantity, but are proportional to the analogue signal si ze . For an increase of the reversed bias , the fluorescence peak is reduced . For an applied voltage V3 of - 3 V, the fluorescence peak reaches only about ten to twelve percent of the original fluorescence peak at VI , i . e . in comparison to the value without a reversed bias voltage . The fluorescence peak at a bias voltage V5 of - 9 V even leads to a decrease of the peak height to even smaller values .

[0109] For an excitation wavelength of 400 nm as shown in FIG . 2A, it decreases to about 100 counts , i . e . the negative bias voltage leads to a decrease of a factor of about 100 .

[0110] For an excitation wavelength of 380 nm, i . e . for an even higher photon energy, as shown in FIG . 2B the decrease of the fluorescence compared to the fluorescence without a reversed bias voltage is also remarkable . A possible way to switch an LED 2, especially the second LED 2.2, between an operation in forward mode and an operation in reverse mode is shown in FIG. 3A and FIG. 3B. The principle is based on a change of the polarity of the voltage applied to the LED 2. Thereby, for an operation in forward mode an operating voltage is applied to the LED 2 and in reverse mode a reverse bias voltage is applied to the LED 2.

[0111] The LED 2 comprises an anode 4 and a cathode 5. For the circuit arrangement shown in FIG. 3A, i.e. in forward mode, an LED supply voltage VLEDis applied to the anode 4 and ground VGND is applied to the cathode 5. Therefore, the anode is connected to an LED supply wire 7 and the cathode is connected to a ground wire 8. In addition, to regulate the power of the LED 2, a driver 6 is connected to the LED 2.

[0112] For the case shown in FIG. 3A, the LED is operated in forward or operating mode, i.e. the applied voltage is an operating voltage. To decrease the parasitic luminescence, especially fluorescence, of the LED 2, the polarity of the LED 2 shall be changed. Therefore, a double pole switching means 10 is used. The double pole switching means 10 comprises an LED supply switch 11 and a ground switch 12. In operating or forward mode, the LED supply switch 11 is connected to the anode 4 and the ground switch 12 is connected to the cathode 5. In addition, the cathode 5 of the LED 2 is connected to a bypass wire 9. In the case of FIG. 3A, i.e. in forward mode, the bypass wire 9 is open.

[0113] For the case shown in FIG. 3B, the LED is operated in reverse mode, i.e. the voltage applied to the LED 2 is a reverse voltage. Hence, for the case in FIG. 3B, the polarity of the LED 2 is changed in comparison to the forward mode shown in FIG. 3A. In this case, the LED supply switch 11 connects the LED supply wire 9 to the bypass wire 9 and accordingly to the cathode 5 of the LED 2. The ground switch 12 connects the ground VGND, which is a ground potential, to the anode 4 of the LED 2. The scheme in FIG. 4 shows the combination of a first

[0114] LED 2.1, for example an LED emitting UV light, and a second LED 2.2, for example a green LED. Here, the first LED is in forward mode, because the bypass wire 9 is open and the double pole switching means 10 is switched to connect the LED supply voltage VLEDand ground VGND to the first LED 2.1 in forward mode to apply an operation voltage to the first LED 2.1 to emit a first light signal at a first wavelength.

[0115] In contrast, the second LED 2.2 is in reverse mode. The bypass wire 9 is connected to one end of the double pole switching means 10 and the double pole switching means 10 is switched to connect the LED supply voltage VLEDand ground VGND to the second LED 2.1 in reverse mode to suppress any parasitic luminescence such as fluorescence.

[0116] Building on the concept described above, the invention proposes three product implementations designed for VSM (Vital Sign Monitoring) applications, each offering a different level of integration for controlling reverse bias in LED emitters. These solutions allow for efficient suppression of parasitic luminescence while maintaining flexibility in system design.

[0117] The first approach, illustrated in FIG. 5, comprises an ASIC with integrated reverse bias logic, where the required power switches for toggling the supply voltage are supplied externally by the customer or third-party provider. In this configuration, the ASIC manages the control logic, while the actual switching of the reverse bias voltage is handled by external components. This design offers maximum flexibility since customers can select power switches suited to their system's requirements. Additionally, it enables a compact IC design with low power consumption, as the logic switch itself requires minimal real estate. The reverse supply voltage can either be separate from the LED supply or derived from the same source, depending on the specific application. A more integrated approach is shown in FIG . 6 , where the required switching functionality is embedded within a coordinated LED-Emitter Package . This package contains an analog IC that replaces the external power switches needed in FIG . 5 , simpli fying the implementation . By integrating the switching function directly into the emitter package , this solution of fers a compact multi-emitter system with built-in reverse bias capabilities . The customer no longer needs to provide external switches , reducing complexity and improving ease of use . Since the analog IC is designed speci fically for the package , the switching ef ficiency and LED performance can be optimi zed to ensure reliable operation .

[0118] The most advanced solution is depicted in FIG . 7 , where both the LED supply and the reverse bias supply are managed directly within a fully integrated AS IC . In this case , the power switches themselves are incorporated into the AS IC, eliminating the need for any external modi fications . This approach provides maximum convenience , as the AS IC can directly switch between forward operation and reverse biasing without requiring additional components . However, integrating the power switches into the IC layout requires additional silicon real estate , making this approach more complex in terms of IC design . While it of fers the most streamlined and user- friendly implementation, the increased space requirements on the chip must be considered during development .

[0119] These three approaches represent a scalable solution space , allowing for di f ferent levels of integration depending on the application' s speci fic requirements . The first approach provides high flexibility by allowing customers to supply their own external switches . The second approach of fers a balanced trade-of f between integration and flexibility by incorporating the switching function into the LED-Emitter Package . The third approach delivers the most integrated and convenient solution but requires larger silicon areas due to the on-chip power switches . By offering these three solutions, we provide a range of options that balance design complexity, ease of implementation, and integration effort, ensuring adaptability to various VSM applications and optical sensing technologies.

[0120] LIST OF REFERENCE SIGNS

[0121] Arrangement of LEDs 1

[0122] LED 2

[0123] First LED 2.1

[0124] Second LED 2.2

[0125] Third LED 2.3

[0126] Sensor 3

[0127] Anode 4

[0128] Cathode 5

[0129] Driver 6

[0130] LED supply wire 7

[0131] Ground wire 8

[0132] Bypass means 9

[0133] Double pole switching means 10

[0134] LED supply switch 11

[0135] Ground switch 12

[0136] LED supply voltage VLED

[0137] Ground VGND

Claims

CLAIMS1. A vital function measuring device comprising:• a first LED (2.1) configured to emit a first light signal at a first wavelength,• a second LED (2.2) , wherein the second LED (2.2) is configured to emit a second light signal at a second wavelength, wherein the second wavelength is longer than the first wavelength,• a sensor (3) configured to receive a reflected first light signal,• a sensor (3) configured to receive a reflected second light signal,• an electrical circuit comprising a first driver circuit to operate the first LED (2.1) and a second driver circuit to operate the second LED (2.2) , wherein the electrical circuit is configured to switch between the application of a positive operating voltage and a negative bias voltage to the second LED (2.2) independent of the voltage applied to the first LED (2.1) .

2. Vital function measuring device according to claim 1, wherein the second driver circuit comprises a switching component configured to switch the voltage applied to the second LED (2.2) between the positive operating voltage and the negative bias voltage.

3. Vital function measuring device according to claim 2, wherein the second driver circuit comprises a bypass wire (9) and the switching component is configured as a double pole switching means (10) , wherein the double pole switching means (10) is arranged and configured to switch between the positive operating voltage and the negative bias voltage applied to the second LED (2.2) .

4. Vital function measuring device according to one of the claims 1 to 3, wherein the second LED (2.2) is a laser diode.

5. Vital function measuring device according to one of the claims 1 to 4, wherein the vital function measuring device comprises a third LED (2.3) configured to emit a third light signal at a third wavelength, which is longer than the second wavelength, and the electrical circuit comprises a third driver circuit to operate the third LED (2.3) , wherein the electrical circuit is configured to switch between the application of a positive operating voltage or a negative bias voltage to the third LED (2.3) independently of the voltage applied to the first LED (2.1) and / or second LED (2.2) .

6. Vital function measuring device according to claim 5, wherein the electrical circuit is configured to apply a positive operating voltage to the first LED (2.1) and to apply at the same time a negative bias voltage to the secondLED (2.2) and to the third LED (2.3) .

7. Vital function measuring device according to claim 6, wherein the electrical circuit is configured to apply a positive operating voltage to the second LED (2.2) and to apply at the same time a negative bias voltage to the third LED (2.3) .

8. Vital function measuring device according to one of the claims 1 to 7, wherein the electrical circuit is integrated in an application-specific integrated circuit.

9. Vital function measuring device according to one of the claims 1 to 8, wherein the first wavelength is in the range from 350 nm to 420 nm and the second wavelength is in the range from 450 nm to 600 nm.

10. Vital function measuring device according to one of the claims 1 to 8, wherein the first wavelength is in the range from 450 nm to 570 nm, and the second wavelength is in the range from 600 nm to 5000 nm.

11. Vital function measuring device according to one of the claims 1 to 10, wherein the gap between the first LED (2.1) and the second LED (2.2) is smaller than 100 pm.

12. Vital function measuring device according to one of the claims 1 to 11, wherein the first LED (2.1) and the second LED (2.2) are embedded in a display.

13. Vital function measuring device according to one of the claims 1 to 12, wherein the vital function measuring device is a smartwatch, comprising a strap to attach the device to a part of the body, in particular to the wrist of a human.

14. Vital function measuring device according to one of the claims 1 to 13, wherein the vital function measuring device comprises an analyzing means, wherein the analyzing means is configured to analyze the received first light signal and the received second light signal, wherein the analyzing means is configured to measure the vital function based on the analyzed received first light signal and the received second light signal.

15. A Vital function measuring device comprising• a first LED (2.1) configured to emit a first light signal at a first wavelength,• a sensor (3) configured to receive a reflected first light signal, an electrical circuit comprising a first driver circuit to operate the first LED (2.1) , wherein the electrical circuit is configured to switch between the application of a positive operating voltage and a negative bias voltage to the first LED.

16. A method for optical vital sign monitoring, preferably on a human body, applying a vital function measuring device according to claim 14, the method comprising:a) applying a negative bias voltage to the second LED (2.2) , b) starting an emission of the first LED (2.1) c) receiving the first light signal reflected or transmitted by the human tissue and forward the signal to the analyzing means, d) stopping the emission of the first LED (2.1) , e) switching the voltage applied to the secondLED (2.2) from the negative bias voltage to a positive operating voltage to emit a second light signal , f) receiving the second light signal reflected or transmitted by the human tissue and forward the signal to the analyzing means.

17. A method of operating an LED (2) to suppress a parasitic luminescence of the LED (2) by the application of a reverse bias voltage to the LED (2) .

18. Vital function measuring device according to one of the claims 1 to 15, wherein the electrical circuit comprises an application-specific integrated circuit (ASIC) configured to control the reverse bias logic, wherein the power switches for toggling the supply voltage are provided externally by the user.

19. Vital function measuring device according to one of the claims 1 to 15, wherein the electrical circuit comprises an analog integrated circuit (IC) embedded within an LED-Emitter Package, wherein the analog IC is configured to switch between a positive operating voltage and a negative bias voltage applied to at least one LED (2.2, 2.3) .

20. Vital function measuring device according to one of the claims 1 to 15, wherein the electrical circuit comprises a fully integrated ASIC that incorporates both the LED supply and the reverse bias supply, wherein the ASIC is configured to directly switch at least one LED (2.2, 2.3) betweenforward operation and reverse bias without requiring external power switches.

21. Vital function measuring device according to one of the claims 1 to 15 or 18 to 20, wherein the electrical circuit is configured to derive the reverse supply voltage either from a separate voltage source or from the LED supply voltage, depending on the specific application requirements.

22. Method for optical vital sign monitoring according to claim 16, wherein the electrical circuit comprises an ASIC or an analog IC, wherein the ASIC or analog IC is configured to dynamically control the application of a negative bias voltage to at least one LED (2.2, 2.3) to suppress parasitic luminescence, based on predefined operational parameters or real-time sensor feedback.

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