Reducing energy consumption for determining a biometric parameter using a laser

Abstract

The invention relates to a system for determining a biometric parameter of a body comprising a blood perfused tissue, and a method for controlling such system. The system comprises a laser and one or more photodetectors. The laser emits coherent light to the tissue. The photodetector(s) receive(s) a portion of the light and generate a corresponding output signal. The system is switchable between a low-power mode and an active mode. In the low-power mode, the system operates at a lower rate of energy consumption than in the active mode. In the active mode, the system determines the biometric parameter. In the low-power mode, the system controls the laser to emit light to the tissue, determines a measure of motion based on the photodetector signal(s), and switches to the active mode if the measure of motion satisfies a condition.

Description

[0001] REDUCING ENERGY CONSUMPTION FOR DETERMINING A BIOMETRIC PARAMETER USING A

[0002] LASER

[0003] Field of the invention

[0004] The present invention relates to a system and method for determining a biometric parameter of a body, the body comprising a blood perfused tissue. In particular, the invention relates to the determination of a biometric parameter using a system that comprises a laser and one or more photodetectors.

[0005] The present invention further relates to a computer program for performing said method, and for a non-transitory computer readable medium storing said computer program.

[0006] Background of the invention

[0007] It is known to perform biometric measurements using a laser and a photodetector. An example of such biometric measurement is laser Doppler velocimetry (LDV). When LDV is used for measuring blood flow in a body, it is commonly referred to as laser Doppler flowmetry (LDF). The term 'velocimetry' may suggest that velocity is measured but the blood flow signal obtained by LDV is in fact a scalar and contains no direction information. The blood flow signal is therefore related to speed of the blood. For this reason, the present disclosure uses the term LDF.

[0008] Description of the invention

[0009] A drawback of known systems for determining a biometric parameter is their relatively high energy consumption.

[0010] An object of the invention is to overcome this drawback, or at least provide an alternative system. In particular, the invention aims to reduce energy consumption of determining a biometric parameter of a body using a system comprising a laser and a photodetector.

[0011] This aim is achieved by the system for determining a biometric parameter of a body, according to claim 1. The system comprises a laser and one or more photodetectors. The laser is configured to emit coherent light to the blood perfused tissue. Each photodetector is configured to receive a portion of the coherent light, e.g. a portion of coherent light scattered by the blood perfused tissue. Each photodetector generates an output signal based on the received portion of the coherent light. The system is switchable between a low-power mode and an active mode. In the active mode, the system is configured to operate at a first rate of energy consumption. In the active mode, the system is further configured to determine the biometric parameter. In the low-power mode, the system is configured to operate at a second rate of energy consumption that is lower than the first rate of energy consumption. In the low-power mode, the system is further configured to: control the laser to emit coherent light to the blood perfused tissue, determine a measure of motion based on the output signal of at least one of the one or more photodetectors; determine whether the measure of motion satisfies a first condition; and switch to the active mode if the measure of motion satisfies the first condition.

[0012] In other words, the system is operated at a low rate of energy consumption for detecting the level of motion, and switches to the more energy consuming mode for determining the biometric parameter only when the detected motion satisfies a predetermined first condition. The first condition applies when relatively little motion is present, e.g. the body is in rest or moves slowly. Therefore, the more energy consuming mode of operating the system is avoided when too much motion is present to perform a measurement of sufficient quality. Thus, the overall energy consumption of the system is reduced.

[0013] For example, operating the system at the lower rate of energy consumption in the low-power mode comprises any one or more of: operating one or more processors of the system at a lower processing speed in the low-power mode than in the active mode; using less processors of the system in the low-power mode than in the active mode; using a lower sample rate in the low-power mode than in the active mode (e.g. by controlling one or more analog-to-digital convertors and / or processors); reducing or switching off signal amplification in the low-power mode (e.g. by controlling one or more pre-amplifiers); reducing a rate of data transfer in the low-power mode (e.g. by reducing frequency of data transmission by switching off data transmission); in a system with multiple photodetectors: reducing the number of photodetectors that are read-out in the low-power mode as compared to the active mode (e.g. by not powering some of the photodetectors and associated front-end electronics); switching off biometric sensors of the system, such as non-coherent light sources for PPG measurement; and / or operating the laser at a lower rate of energy consumption than in the active mode (described in more detail below), e.g. operating the laser at a duty cycle of < 50%, preferably < 25 %, more preferably < 10%.

[0014] In one example, the system comprises a first processor and at least one second processor, wherein the first processor has a lower rate of energy consumption than the second processor, wherein the first processor is used in the low-power mode, while the at least one second processors is not used in the low-power mode. In the active mode, the at least one second processor is used, and optionally also the first processor. For example, the first processor is a dedicated processor, such as an ASIC, configured for determining the measure of motion.

[0015] For example, the first condition comprises a threshold condition. For example, if the measure of motion increases for increasing motion, the threshold condition is satisfied if the measure of motion is below a predetermined threshold. Similarly, if the measure of motion decreases for increasing motion, the threshold condition is satisfied if the measure of motion exceeds a predetermined threshold.

[0016] Each photodetector generates an output signal that is indicative of intensity of the light it receives. Specifically, the photodetectors are at least sensitive to light of wavelengths corresponding to the wavelength of the light emitted by the laser.

[0017] The system includes a set of one or more photodetectors for detecting light emitted by the laser. Optionally, the system includes a second set of one or more photodetectors, e.g. one or more photodetectors for detecting light from a different light source. For example, the system further comprises a non-coherent light source and one or more photodetectors configured to receive light emitted by the non-coherent light source.

[0018] The system comprises hardware for processing the output of the photodetector(s). The hardware for example comprises electronic circuits (digital and / or analog) and / or integrated circuits. The system, for example, comprises one or more microprocessors, DSPs, FPGAs, ASICs or other logic circuitry. For example, the system comprises one or more processors and a memory for storing software for execution by the one or more processors.

[0019] The biometric parameter for example comprises one or more of: a laser doppler flowmetry (LDF) parameter, a photoplethysmogram (PPG) parameter, heart rate, heart rate variability, artery stiffness, blood pressure, breathing rate.

[0020] Preferably, the laser comprises a semiconductor laser, such as a laser diode or quantum dot laser.

[0021] Preferably, the laser emits infrared light (e.g. light with a wavelength of 750 - 1000 nm, preferably 750 - 850 nm).

[0022] Preferred embodiments are defined in the dependent claims and in the following paragraphs.

[0023] In an embodiment, the system is configured to not determine the biometric parameter in the low-power mode. In other words, the system is configured to only determine the biometric parameter in the active mode.

[0024] For example, the determination of the biometric parameter comprises performing an algorithm (e.g. implemented by hardware and / or software of the system). The algorithm for example comprises frequency transforming the output signal, which is an energy consuming operation. In this example, the system is configured to perform the algorithm in the active mode, while not performing the algorithm in the low-power mode.

[0025] Preferably, in the active mode, the laser is used to determine the biometric parameter. In a preferred embodiment, the system is configured to, in the active mode, control the laser to emit coherent light to the blood perfused tissue and to determine the biometric parameter using at least one output signal as generated by the photodetector(s) that receive the laser light as scattered by the blood perfused tissue.

[0026] In an embodiment, the system is configured to, in the active mode, operate the laser at a third rate of energy consumption. The system is further configured to, in the lower-power mode, operate the laser at a fourth rate of energy consumption that is lower than the third rate of energy consumption.

[0027] For example, operating the laser at a third rate of energy consumption comprises operating the laser at a first light intensity, whereas operating the laser at the fourth rate of energy consumption comprises operating the laser at a second light intensity that is smaller than the first light intensity.

[0028] In an alternative embodiment, the system comprises an additional laser for use in the determination of the biometric: the additional laser having a higher output power than the laser used for determining the measure of motion. The additional may be referred to as "the high-power laser", while the other laser is referred to as "the low-power laser". The system is further configured to, in the low-power mode, control the lower-power laser to emit coherent Light to the blood perfused tissue, while controlling the high-power laser to be switched off. The system is further configured to, in the active mode, control the high-power laser to emit coherent light to the blood perfused tissue, while preferably controlling the low-power laser to be switched off.

[0029] In an exemplary embodiment, the additional laser ("high-power laser") emits light at a smaller wavelength and / or at a higher intensity than the laser ("low-power laser")

[0030] In a preferred embodiment, the system is configured to operate the laser with a first duty cycle in the active mode, and to operate the laser with a second duty cycle in the low-power mode, wherein the second duty cycle is lower than the first duty cycle.

[0031] Preferably, in the low-power mode the system operates the laser at a duty cycle of < 50%, preferably < 25%, more preferably < 10%. For example, the laser is switched on and off according to a sequence of 100 ms "on" and 900 ms "off". In another example, the laser is operated according to a sequence of 50 ms "on" and 950 ms "off".

[0032] Preferably, when the laser is operated in the active mode, the laser is controlled to have a duty cycle that exceeds 50%, more preferably exceeds 75%, most preferably the duty cycle is 100% (i.e. laser is continuously switched on for the duration of the active mode). In an embodiment, the system is configured to, in the active mode, switch to the low-power mode after a predetermined time, wherein preferably the predetermined time is in the range of 30 seconds - 5 minutes.

[0033] In an embodiment, the system is configured to, in the active mode, determine the number of cardiac cycles from at least one output signal of the one or more photodetectors, and to switch to the low-power mode after determining the biometric parameter for a predetermined number of cardiac cycles, the predetermined number being an integer > 1. For example, the predetermined number is 20-100 cardiac cycles.

[0034] In a preferred embodiment, the system is configured to determine the measure of motion also in the active mode. In this embodiment, the system is further configured to determine, in the active mode, whether the measure of motion satisfies a second condition, and to switch to the low-power mode if the measure of motion satisfies the second condition. Particularly, the second condition is indicative for a high degree of motion.

[0035] In an embodiment, the biometric parameter comprises a laser doppler flowmetry, LDF, parameter and / or a photoplethysmogram PPG, parameter.

[0036] Preferably, the laser comprises a semiconductor laser, such as vertical cavity surface-emitting lasers (VCSELs), edge-emitting lasers (EELs), horizontal cavity surface-emitting lasers (HCSELs), vertical external-cavity surface-emitting lasers (VECSELs), quantum-dot lasers (QDLs), quantum cascade lasers (QCLs).

[0037] In an embodiment, the laser comprises a semiconductor laser with integrated photodetector (e.g. an integrated photodiode).

[0038] The semiconductor laser and photodetector may be integrated into a monolithic structure. For example, the photodetector is integrated in an intra-cavity, stacked, or adjacent configuration.

[0039] Preferably, the system comprises a VCSEL with integrated photodetector (also referred to as "VIP").

[0040] In a further embodiment, the one or more photodetectors comprise the integrated photodetector and at least one additional photodetector. The system is configured to determine the measure of motion based on the output signal of the integrated photodetector and to determine the biometric parameter based on the output signal of the additional photodetector.

[0041] The inventors found that the output signal of the integrated photodetector is particularly advantageous for determining the measure of motion. On the other hand, the inventors found that the biometric parameter is more accurately determined using an additional photodetector. In this embodiment, a single light source (e.g. the VCSEL of the VIP) is preferably used, with at least two photodetectors (the integrated photodetector and the external photodetector). Optionally, more than one additional photodetector is provided, e.g. 2-4 additional photodetectors are provided. For example, the distance between the laser and the one or more additional photodetectors is 0.5 - 8 mm of the laser, preferably 1 - 4 mm, more preferably 1.2 - 1.8 mm.

[0042] In an embodiment, the system comprises a wearable device comprising the laser and the one or more photodetectors. In a first example, the wearable device further comprises one or more processors configured for determining the biometric based on at least one output signal of the one or more photodetectors. In a second example, the wearable device comprises a communication module, preferably a wireless communication module, to communicate at least one output signal of the one or more photodetectors, digitized by front-end electronics of the wearable device, to a remote computing system for determining the biometric. For example, the remote computing system is a server, a computer, a tablet, or a smartphone. In the second example, the wireless communication is preferably only used in the active mode, not in the low-power mode, which further reduces energy consumption. In both examples, determining the measure of motion and determining whether the first condition is satisfied is preferably done by the hardware and / or software included in the wearable device.

[0043] The wearable device for example comprises a patch, a smart watch, a wrist band, an arm band, a chest band, glasses, or a ring. In an embodiment, the wearable device is an in-ear device, also referred to as a hearable. For example, the wearable device is an ear bud or a hearing aid. Due to their limited size, in-ear devices typically have limited battery capacity. Thus, the reduction of energy consumption that embodiments of the invention provide, are particularly advantageous in the context of in-ear devices.

[0044] The invention further relates to a method for controlling a system for determining a biometric parameter of a body, the body comprising a blood perfused tissue. The method comprises controlling a laser of the system to emit coherent light to the blood perfused tissue. The method further comprises obtaining at least one output signal from one or more photodetectors of the system, each of the one or more photodetectors receiving a portion of the coherent light. The method further comprises switching between a low-power mode and an active mode. In the active mode, the method comprises: operating the system at a first rate of energy consumption, and determining the biometric parameter. In the low powermode, the method comprises: operating the system at a second rate of energy consumption that is lower than the first rate of energy consumption, controlling the laser to emit coherent light to the blood perfused tissue; determining a measure of motion based on the at least one output signal, determining whether the measure of motion satisfies a first condition, and switching to the active mode if the measure of motion satisfies the first condition.

[0045] The same technical effects as described above in relation to the system apply to the method. Moreover, any features of the system described above can similarly be applied in the method. Preferably, the method is performed using the system of any of the embodiments of this disclosure. For example, the system comprises one or more processors configured to perform the steps of the method.

[0046] The invention further relates to a computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method any of the embodiments of this disclosure. The invention further relates to a non- transitory computer readable medium storing said computer program.

[0047] Brief description of the drawings

[0048] In the following, example embodiments will be described with reference to the drawings, wherein:

[0049] Figure 1 illustrates a first embodiment of a system according to the invention;

[0050] Figure 2 illustrates primary elements of a VCSEL with integrated photodetector ("VIP");

[0051] Figure 3 illustrates the use of a VIP and the Doppler effect for determining blood flow;

[0052] Figure 4 illustrates components of the output signal of the VIP;

[0053] Figure 5 illustrates a second embodiment of the system according to the invention;

[0054] Figure 6 shows a flow diagram of a method according to the invention;

[0055] Figure 7-10 show examples of wearables according to different embodiments of the invention; and

[0056] Figure 11 shows a variant of the embodiment of Figure 5, but with a curved shape of the device, for easier fit to the body, e.g. on the wrist or finger or arm.

[0057] Detailed description of the drawings

[0058] In figure 1, a system 10, in the form of a hearable, 10 is illustrated having a housing 12 configured to be provided in an ear canal of a person. A VCSEL with integrated photodetector 14 ("VIP") is provided on or in the housing so as to be able to emit laser light in a direction away from the housing and to receive laser light travelling in the opposite direction. An additional photodetector 15 is provided on or in the housing. The additional photodetector 15 is arranged to detect laser light, emitted by VIP 14 and scattered by tissue. In the drawing, VIP 14 and photodetector 15 are arranged on different sides of the housing 12. Alternatively, the VIP 14 and photodetector 15 are positioned on the same side of the housing 12.

[0059] In the housing 12, a sound generator 16 (often called a receiver) may be provided for outputting sound from a spout 18, usually in the direction of the ear drum of the ear canal. Further, a processor 20 is provided, often in the housing 12, for receiving output signals from the VIP 14 and the photodetector 15 and processing these output signal for determining a biometric parameter. Optionally, processor 20 is configured to control the laser 14 and / or sound generator 16 and / or other components of the system 10.

[0060] The processor 20 may be configured to communicate with other electronic components via a connector, communication port or cable 22, which may also carry power and / or other signals for the housing 12, sound generator 16 or the like.

[0061] In the present context, cf. figure 2, a VIP 14 could be a Vertical Cavity Surface Emitting Laser with Integrated Photodetector 144. A vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer. We note that the term VCSEL includes Vertical external cavity surface emitting lasers (VECSEL).

[0062] In figure 2, a VIP 14 is seen having an active region or optical cavity 141 provided between two reflecting portions 142 and 143, often formed by Bragg reflectors. Light 150 is emitted from the optical cavity 141 through the reflecting portion 143, which is not completely reflecting. Light 152 received into the cavity 141 will interfere with the operation of the VCSEL in a manner which will be detectable by the detector 144.

[0063] The use of a VIP for determining a biometric parameter is illustrated in figure 3. The laser light 150 is launched onto the skin 161 of a tissue portion 160, such as an ear canal, arm, chest, nose or wrist. Inside the tissue, blood is flowing. In blood, different constituents exist, such as red blood cells 162. The blood flows in arteries, veins, capillaries and at an average velocity V (averaged over the exposed volume), which may vary over time in amplitude and direction, and blood cells have individual velocity vectors which are indicated by arrows. Clearly, the velocity of the flowing blood will depend on the heartbeat of the person. Also, the stiffness of the blood vessels will affect the velocity, as the velocity will be higher in stiffer blood vessels not able to expand to compensate for an increasing blood pressure due to the heart pumping. In the present illustration, the blood flows at velocity V in an upward direction.

[0064] Part of the emitted laser light 150 will be reflected by the skin surface and revert toward the element 143 to re-enter the optical cavity 141. Part of the laser light 150 will penetrate the skin and be scattered by the blood and in particular the particles or cells 162 thereof. This scattering will both redirect the light but will also impart, due to the Doppler effect, a wavelength shift thereof. Some of this light, portion 152, is scattered back towards the optical cavity 141. As the optical cavity 141 forms part of the laser of the VIP 14, the scattered light 152 entering the optical cavity 141 will affect the light generation of the laser and will affect both the amplitude and the frequency of the light emitted and thus in the optical cavity.

[0065] The detector 144 is positioned so as to determine the power of the light in the optical cavity 141. Thus, the frequency and amplitude of this power is affected by the light reflected from the blood cells 162 and thus the velocity thereof. Thus, from the signal output of the detector 144, this velocity may be determined.

[0066] The spectral power density S(v) of the photo current fluctuations of the output signal of the detector 144 may be determined from the output signal during a number of time intervals, such as every 10ms. Then, the first moment Mi may be determined for each point in time: v2

[0067] Mi (t) = v ■ S (v, t) dv

[0068] Vl

[0069] Where v is the frequency, vi=lkHz, for example, and v2=20kHz, for example. The upper and lower frequency limits can be varied in order to achieve optimal separation between the movement portion of the signal and the blood flow velocity portion.

[0070] This Mi will be a measure of the blood flow velocity. Then, the moments Mi may be plotted against time to illustrate the variation over time of this velocity. From this, the heart rate, for example, may be determined. Optionally, the 0thmoment is used to normalize Mi.

[0071] Also, the use of an analog circuit may be chosen to bandpass filter and integrate over time, in order to arrive at the blood flow. Yet another manner would be to use a number of Mi bands and pick the best one for the determination.

[0072] It could be preferred to calculate the spectral power density at pre-defined points in time from the signal acquired over a defined time interval preceding the present point in time, which could be selected to be the interval between two sequential points in time.

[0073] The k-th moment of the spectral power density is: v2 Mk(t) = vk- S(v, t) dv Vl

[0074] It is noted that the system measures the velocity of the blood cells moving in the tissue, and not necessarily the blood flow. The velocity is the distance which an object (solid, liquid or gas) moves with respect to time (i.e., the distance travelled per unit of time). In the case of blood flowing in a vessel, the velocity is often expressed in the units of cm / sec. In contrast, flow is the volume of a liquid or gas that is moving per unit of time. For blood flowing in a large vessel, flow is often expressed in the units of ml / min (cm3 / min; 1 ml = 1cm3). In some situations, the tissue portion 160 moves relative to the VIP 14, e.g. when the person wearing the system 10 is running. Naturally, this movement will in itself give rise to a Doppler shift and amplitude modulation of the light 152. This Doppler shift is seen both in the light reflected from the surface of the skin 161, from scattering from within the tissue 160 as well as from the light scattered by the blood 162.

[0075] Thus, the output signal of the VIP 14 will be a superposition of the artifacts caused by movement of the tissue 160 relative to the VIP 14 and that caused by the movement of the blood in the tissue 160. Figure 4 illustrates the components of the VIP output signal 180 and the later splitting up thereof into the signal 182, being a high frequency portion, relating to the Doppler shift and amplitude modulation caused by the velocity of the blood 162 and the signal 184, being a low frequency portion, relating to the amplitude modulation caused by the relative movement between the VIP 14 and the tissue 160. This relative movement may be caused by the user moving, such as running. For hearable 10, the movement of the user may cause the housing 12 to move inside the ear canal. Generally, movement of the user can induce movement of reflective layers / structures in the tissue relative to the sensor 14. Alternatively or additionally, the tissue 160 may swell periodically due to the blood pressure variations caused by the heart beating inducing movement of reflective layers / structures in the tissue relative to the sensor 14. The latter two types of movement are also relevant for wearable systems that have the photodetector and / or laser in close or direct contact with the blood perfused tissue. Particularly, movement of reflective layers / structures and swelling of tissue also affect the output signals in wearable systems designed for close or direct contact of the photodetector (and optionally laser) with the skin of the user, such as bands or patches for wrist, chest or arm.

[0076] However, it has been realized that the changing distance between VIP 14 and reflective tissue layers / structures, will be present as deterministic discrete frequencies in the VIP output, whereas the biometric information relating to the blood flow is present in the VIP's output in the form of a broadband signal that resembles stochastic noise. The bandwidth of the biometric information can be for example between 1kHz and 50kHz.

[0077] Thus, in this embodiment, the output signal of the VIP 14 is processed into biometric signals independent of movement artifacts. This processing can be performed in several ways. In one situation, the biometric signals are in a higher frequency band than the movement artifacts. Then, the average of the power spectral density of the noise between e.g. 10-50kHz may be representative of the blood flow velocity without movement artifacts.

[0078] According to some embodiments, frequency filtering is applied to the output signal 180. For example, the output signal is frequency filtered to obtain a high frequency signal 182 and a low frequency signal 184. The low frequency signal 184 is used for determining a measure of motion. The high frequency signal 182 is optionally used for determining a biometric parameter. For example, the low frequency signal 184 is obtained by applying a low pass filter to the output signal 180, wherein the low pass filter has a corner frequency of 10 kHz or less, e.g. 100 Hz - 1 kHz. For example, the high frequency signal 182 is obtained by applying a high pass filter to the output signal 180, wherein the high pass filter has a corner frequency of equal to or greater than 1 kHz, e.g. 10-50 kHz.

[0079] In fact, even if movement artifacts will appear at higher frequencies, their deterministic nature makes them distinguishable from the broadband signal. These artifacts can be removed by the signal processing by using distinguishable features such as amplitude, spectral density, and time dependency.

[0080] Thus, the VIP signal 180 may optionally be used for the determination of the biometric parameter, such as a velocity of the blood flow or blood cells. Other parameters may be determined based on the VIP output signal, such as a perfusion of the tissue, heart rate, heart rate variability, artery stiffness, blood pressure, breathing rate or the like. In figure 4, a peak detection is performed on the high frequency signal 182 (alternatively, it could be used on the signal 180) to identify heart beats and thus e.g. pulse or heart rate.

[0081] These parameters can be used to determine conditions of the system's user, such as health related conditions as atrial fibrillation or the like, his / her stress level, fitness level or the like. Clearly, the blood velocity for the same person with the same pulse will increase with increased arterial stiffness.

[0082] Heartrate induced tissue movement is also present in the tissue. Under sedentary conditions this movement is visible in the low frequency content of the photodetector signal. The heartrate induced tissue movement can be used as a redundancy check or even as a fundamental signal for metrics like e.g. heartrate.

[0083] It is noted, referring to figure 4, that the heartrate movement signal is, as the biometric signal in the noise, caused by the pulsating of the heart. Therefore, the signals will have the same period. This fact can be used to distinguish body movement artifacts, as seen in the signal 184, from heart beat induced motion artifacts with the frequency seen at 183. Actually, a signal (not illustrated) could be derived from the signal 180 with the frequency of the peaks.

[0084] Due to the fact that the biometric derived from the noise signal 182 is a measure for the blood flow velocity, and that the signal, derivable with the frequency 183, is a measure for the amount of blood, the two signals will have different shapes. This difference will depend other tissue parameters, such as artery stiffness, blood pressure, and the like, which then also could be determined and potentially monitored over time.

[0085] In this context, it is then noted that the output of the photodetector 144 may be used for generating information on amplitude variation caused by relative movement between the VIP 14 and the tissue or element in question. It is noted that the tissue may move with the blood pressure pulses. Then, this movement will have a, relatively small, part of pulsatile information, which may be used for determining e.g. heartbeat.

[0086] In general, it is noted that a VIP 14 may, especially when used rather close to the tissue in question, be used without further optics, which allows the optical sensor to remain compact, such as for use in small housings and volumes. Alternatively, other optical components, such as lenses, slits, reflectors or the like, may be provided outside of the VCSEL for protection thereof and / or beam forming and the like.

[0087] The VIP is particularly suitable for determining the measure of motion (e.g. based on the low frequency component 184 of the VIP's output signal). Preferably, an additional photodetector is provided for determining the biometric parameter. This is illustrated in figure 5, which shows a second embodiment of a system according to the invention.

[0088] Figure 5 shows a cross section view of a body-worn device or system 510 and a cross- sectional view of a region of blood perfused tissue 560. The device 510 comprises a VIP 514 and an additional photodetector 515. The VIP emits coherent light 550 that penetrates the skin 561 and other parts of the tissue 560 at exposed tissue region 564. In the example, the VIP emits infrared light with a wavelength of 850 nm.

[0089] Discontinuities of optical properties in the tissue 560 can scatter the light in other directions than that of the incident direction, wherein moving discontinuities 562, e.g. blood cells, moving in blood vessels can Doppler-shift the light. The VIP 514 receives scattered light 552 from the tissue 560. The additional photodetector receives scattered light 562 from the tissue 560.

[0090] The general amount of light absorbed in the tissue 560 is related to the amount of blood therein, as the absorption of the blood reduces the amount of scattered light, and can thus be used to derive a measure of blood volume (e.g. PPG). The Doppler shifting is related to a blood speed and can thus be used to derive a measure of blood speed (e.g. LDF). Both the measure of blood speed and the measure of blood volume can be derived from the output signal 566 of the VIP 514 and / or the output signal 568 of the photodetector 515.

[0091] Preferably, the output signal 568 of the photodetector 515 is used for determining one or more biometric parameters, while the output signal 566 of the VIP 514 is used for determining a measure of motion. As described above, determining a measure of motion based on the VIP's output signal 556, may for example comprise low pass filtering.

[0092] Figure 6 illustrate a method according to an embodiment of the invention. The method is for example implemented using hardware and / or software. For example, the method is implemented using one or more processors and associated memory that stores software for execution by the one or more processors. Preferably, the method is performed using the system according to figure 5. The method includes two modes: a low-power mode 602 and an active mode 604. In step S606 of the low-power mode 602, a laser (such as VCSEL 514) is controlled to emit light at a relatively low rate of energy consumption. For example, the laser is controlled according to a duty cycle of less than 50%, preferably a duty cycle of 1-10%. For example, the laser is controlled to switch on for 100 ms, then switch off for 900 ms, and so on. The laser light is emitted to a blood perfused tissue. In a next step S608, a measure of motion is determined from at least one output signal from the one or more photodetectors that receive light scattered by the blood perfused tissue. For example, the output signal 566 of VIP 514 is used to determine a measure of motion (e.g. body motion that can affect the quality of the determination of the biometric parameter, such as LDF or PPG). In a subsequent step S610, it is determined whether the measure of motion satisfies a predetermined condition that is indicative for a relatively low level of motion. For example, the measure of motion is compared to a threshold level (e.g. step S610 checks whether the measure of motion is less than a predetermined threshold). If the condition of step S610 is not satisfied ("N"), the method returns to step S606 and thus remains in the low-power mode 602. If the condition of step S610 is satisfied ("Y"), the method proceeds to step S612 of the active mode 604.

[0093] In step S612 of the active mode, the laser (e.g. VCSEL 14) is controlled to emit light at a relatively high rate of energy consumption, i.e. higher than the rate of energy consumption in the low-power mode 602. For example, the laser is controlled according to a duty cycle of 100%. In a next step S614 a biometric parameter, such as a LDF or PPG parameter, is determined from at least one output signal of the one or more photodetectors. For example, the output signal 568 of the photodetector 515 is used to determine the biometric parameter. Optionally, more than one output signal is used to determine the biometric parameter, e.g. both output signals 566, 568 are used.

[0094] Subsequently, in step S616 it is determined whether to return to the low-power mode 602 or to remain in the active mode 604. For example, step S616 determines whether a predetermined time (e.g. 30 - 300 seconds) time has passed since entering the active mode 604. If the predetermined time has passed ("Y"), the method returns to step S606 of the low- power mode 602. If the predetermined time has not yet passed ("N"), the method returns to step S612. Thus, steps S612 and S614 are repeated until the predetermined time has passed.

[0095] In another example, step S616 determines whether a predetermined number of cardiac cycles (e.g. 5-100 cardiac cycles) have passed. If the predetermined number of cardiac cycles have passed ("Y"), the method returns to step S606 of the low-power mode 602. If less than the predetermined number of cardiac cycles have passed ("N"), the method returns to step S612. In other words, steps S612 and S614 are repeated until the predetermined number of cardiac cycles have been passed, after which the method returns to step S606 of the low-power mode 602. In another example, step S616 determines whether a level of motion satisfies a predetermined condition, indicative of a relatively high level of motion. For example, the system is configured to determine the measure of motion also in the active mode, e.g. based on the output signal 566 of VIP 514. The predetermined threshold condition for example includes whether the measure of motion exceeds a threshold level. If step S616 determines that a relatively high level of motion is present ("Y"), the method returns to step S606 of the low-power mode 602. If step S616 determines that a relatively high level of motion is not present C'N"), the method remains in the active mode 604 and returns to step S612.

[0096] If desired, multiple conditions can be checked in step S616. For example, the method returns to step S606 if a predetermined time has lapsed or if a relatively high level of motion is detected or both, and remains in the active mode 604 otherwise. In another example, the method returns to step S606 if a predetermined number of cardiac cycles have passed or if a relatively high level of motion is detected or both, and remains in the active mode 604 otherwise.

[0097] Figure 7 illustrates an embodiment of the invention in the form of a hearable. Figure 8 illustrates a jogger wearing four different embodiments of the invention. Figure 9 illustrates an embodiment of the invention in the form of a patch. Figure 10 illustrates an embodiment of the invention in the form of a ring.

[0098] The system according to the invention may be provided as wearable. For example, the system is provided as a hearable 714, such as an ear bud or a hearing aid, as seen in figure 7. The hearable comprises an element, such as a so-called dome 715, for positioning the hearable near the blood perfused tissue, such as in the ear canal. The hearable 714 comprises a laser 714, such as a VCSEL with integrated photodetector.

[0099] A wearable according to the invention may be fixed relative to the blood perfused tissue, e.g. any portion of the user's skin. For example, the wearable forms part of a watch, ring, head band, arm band, clothing, when fixed using a resilient fixture, such as a waist band, arm band or the like.

[0100] Figure 8 illustrates a jogger wearing different embodiments of the system of the invention, such as in the form of a watch 810, a patch 910, glasses 1110 or a chest band 1210, each comprising a system according to the invention.

[0101] Figure 9 illustrates the patch 910 having two adhesive portions 971 and 972 and a central portion comprising a window behind which a semiconductor laser with integrated photodetector 914 is provided transmitting radiation through the window.

[0102] Figure 10 illustrates a ring 1010 having a window behind which a semiconductor laser 1014 is provided so as to launch radiation out through the window. The ring 1010 further comprises a photodetector (not shown) for receiving the coherent light scattered by the tissue of a finger of the user.

[0103] Figure 11 schematically illustrates a variant of figure 5, wherein the device 510' has a more rounded shape for fitting around at least a portion of a rounded body part, such as a wrist, a finger or an ear lobe. For example, device 510' is part of a watch 810 (figure 8). Although figure 11 may suggest the device 510' is located at some distance to the surface of the tissue 560, preferably the device 510' is in close contact with the tissue 560, e.g. the laser 514 and photodetector 515 of the device 510' are pressed against the tissue (e.g. by a band).

[0104] Embodiments

[0105] 1. A system for determining a biometric parameter of a body, the body comprising a blood perfused tissue, the system comprising: a laser for emitting coherent light to the blood perfused tissue; and one or more photodetectors, each configured to receive a portion of the coherent light and generate an output signal based on the received portion of the coherent light, wherein the system is switchable between a low-power mode and an active mode, wherein: in the active mode the system is configured to operate at a first rate of energy consumption, and the system is configured to determine the biometric parameter; and in the low-power mode the system is configured to operate at a second rate of energy consumption that is lower than the first rate of energy consumption, and the system is configured to: o control the laser to emit coherent light to the blood perfused tissue, o determine a measure of motion based on the output signal of at least one of the one or more photodetectors, o determine whether the measure of motion satisfies a first condition, and o switch to the active mode if the measure of motion satisfies the first condition.

[0106] 2. The system according to embodiment 1, wherein the system is configured to not determine the biometric parameter in the low-power mode.

[0107] 3. The system according to embodiment 1 or embodiment 2, wherein the system is configured to, in the active mode, control the laser to emit coherent light to the blood perfused tissue and to determine the biometric parameter based on the output signal of at least one of the one or more photodetectors.

[0108] 4. The system according to embodiment 3, wherein the system is configured to, in the active mode, operate the laser at a third rate of energy consumption, and the system is configured to, in the lower-power mode, operate the laser at a fourth rate of energy consumption that is lower than the third rate of energy consumption.

[0109] 5. The system according to embodiment 4, wherein the system is configured to, in the active mode, operate the laser with a first duty cycle, and the system is configured to, in the low- power mode, operate the laser with a second duty cycle that is lower than the first duty cycle.

[0110] 6. The system according to embodiment 1 or 2, further system comprising an additional laser having a higher output power than the laser, wherein the system is configured to: in the low-power mode: control the laser to emit coherent light to the blood perfused tissue and control the additional laser to be switched off; and in the active mode: control the additional laser to emit coherent light to the blood perfused tissue and preferably control the laser to be switched off.

[0111] 7. The system according to any of the preceding embodiments, wherein the system is configured to, in the low-power mode, operate the laser at a duty cycle of < 50%, preferably < 25%, more preferably < 10%.

[0112] 8. The system according to any of the preceding embodiments, wherein the system is configured to: operate one or more processors of the system at a lower processing speed in the low- power mode than in the active mode; and / or using less processors of the system in the low-power mode than in the active mode; and / or using a lower sample rate in the low-power mode than in the active mode (e.g. by controlling one or more analog-to-digital convertors and / or processors); and / or reducing or switching off signal amplification in the low-power mode (e.g. by controlling one or more pre-amplifiers); and / or reducing a rate of data transfer in the low-power mode (e.g. by reducing frequency of data transmission by switching off data transmission); and / or switching off biometric sensors of the system, such as non-coherent light sources for PPG measurement.

[0113] 9. The system according to any of the preceding embodiments, wherein the system comprises multiple photodetectors, and the system is configured to read-out a lower number of photodetectors in the low-power mode than in the active mode (e.g. by switching off some of the photodetectors and associated front-end electronics in the low-power mode).

[0114] 10. The system according to any of the preceding embodiments, wherein in the active mode, the system is configured to switch to the low-power mode after a predetermined time, wherein preferably the predetermined time is in the range of 30 seconds - 5 minutes. 11. The system according to any of the preceding embodiments, wherein in the active mode, the system is configured to determine a number of cardiac cycles from at least one output signal of the one or more photodetectors, and to switch to the low-power mode after determining the biometric parameter for a predetermined number of cardiac cycles, the predetermined number being an integer > 1.

[0115] 12. The system according to any of the preceding embodiments, wherein the system is configured to determine the measure of motion also in the active mode, and the system is further configured to determine, in the active mode, whether the measure of motion satisfies a second condition, and to switch to the low-power mode if the measure of motion satisfies the second condition.

[0116] 13. The system according to any of the preceding embodiments, wherein the biometric parameter comprises a laser doppler flowmetry, LDF, parameter and / or a photoplethysmogram PPG, parameter.

[0117] 14. The system according to any of the preceding embodiments, wherein the laser comprises a semiconductor laser.

[0118] 15. The system according to embodiment 14, wherein the laser comprises an integrated photodetector, such as a photodiode.

[0119] 16. The system according to embodiment 15, wherein the one or more photodetectors comprise the integrated photodetector and at least one additional photodetector, wherein the system is configured to determine the measure of motion based on the output signal of the integrated photodetector and to determine the biometric parameter based on the output signal of the additional photodetector.

[0120] 17. The system according to any of the embodiments 14-16, wherein the laser comprises a vertical cavity surface-emitting laser (VCSEL), such as a VCSEL with integrated photodetector (VIP).

[0121] 18. The system according to any of the preceding embodiments, comprising a wearable device that comprises the laser and the one or more photodetectors, wherein preferably the wearable device is an in-ear device, such as a hearing aid.

[0122] 19. A method for controlling a system for determining a biometric parameter of a body, the body comprising a blood perfused tissue, the method comprising : controlling a laser of the system to emit coherent light to the blood perfused tissue; obtaining at least one output signal from one or more photodetectors of the system, wherein each of the one or more photodetectors receives a portion of the coherent light; and switching between a low-power mode and an active mode, wherein the method comprises: in the active mode: o operating the system to operate at a first rate of energy consumption, and o determining the biometric parameter; and in the low-power mode: o operating the system to operate at a second rate of energy consumption that is lower than the first rate of energy consumption, o controlling the laser to emit coherent light to the blood perfused tissue, o determining a measure of motion based on the at least one output signal, o determining whether the measure of motion satisfies a first condition, and o switching to the active mode if the measure of motion satisfies the first condition.

[0123] 20. The method of embodiment 19, wherein the biometric parameter is not determined in the low-power mode.

[0124] 21. The method of embodiment 19 or 20, comprising: controlling, in the active mode, the laser to emit coherent light to the blood perfused tissue and determining the biometric parameter based on the output signal of at least one of the one or more photodetectors.

[0125] 22. The method of embodiment 21, comprising : operating, in the active mode, the laser at a third rate of energy consumption, and operating, in the lower-power mode, the laser at a fourth rate of energy consumption that is lower than the third rate of energy consumption.

[0126] 23. The method of embodiment 22, comprising : operating, in the active mode, the laser with a first duty cycle, and operating, in the low-power mode, the laser with a second duty cycle that is lower than the first duty cycle.

[0127] 24. The method of embodiments 19 or 20, wherein the system further comprises an additional laser having a higher output power than the laser, and the method comprises: in the low-power mode: controlling the laser to emit coherent light to the blood perfused tissue and controlling the additional laser to be switched off; and in the active mode: controlling the additional laser to emit coherent light to the blood perfused tissue and optionally controlling the laser to be switched off.

[0128] 25. The method of any of the embodiments 19-24, comprising : operating, in the low-power mode, the laser at a duty cycle of < 50%, preferably < 25%, more preferably < 10%.

[0129] 26. The method of any of the embodiments 19-25, comprising : in the active mode, switching to the low-power mode after a predetermined time, wherein preferably the predetermined time is in the range of 30 seconds - 5 minutes. 27. The method of any of the embodiments 19-26, comprising : in the active mode, determining a number of cardiac cycles from at least one output signal of the one or more photodetectors, and switching to the low-power mode after determining the biometric parameter for a predetermined number of cardiac cycles, the predetermined number being an integer > 1.

[0130] 28. The method of any of the embodiments 19-27, comprising : determining the measure of motion also in the active mode, and determining, in the active mode, whether the measure of motion satisfies a second condition, and switching to the low-power mode if the measure of motion satisfies the second condition.

[0131] 29. The method of any of the embodiments 19-28, wherein determining the biometric parameter comprises: performing laser doppler flowmetry, LDF to determine a LDF parameter; and / or performing a photoplethysmogram, PPG, measurement to determine a PPG parameter.

[0132] 30. The method of any of the embodiments 19-29, wherein the laser comprises a laser with integrated photodetector and the one or more photodetectors comprise the integrated photodetector and at least one additional photodetectors, the method comprises: determining the measure of motion based on the output signal of the integrated photodetector and determining the biometric parameter based on the output signal of the additional photodetector.

[0133] 31. A computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method according to any of the embodiments 19-30.

Claims

CLAIMS1. A system for determining a biometric parameter of a body, the body comprising a blood perfused tissue, the system comprising: a laser for emitting coherent light to the blood perfused tissue; and one or more photodetectors, each configured to receive a portion of the coherent light and generate an output signal based on the received portion of the coherent light, wherein the system is switchable between a low-power mode for measuring motion and an active mode for determining the biometric parameter, wherein: in the active mode the system is configured to operate at a first rate of energy consumption, and the system is configured to: o control the laser to emit coherent light to the blood perfused tissue; and o determine the biometric parameter based on the output signal of at least one of the one or more photodetectors; and in the low-power mode the system is configured to operate at a second rate of energy consumption that is lower than the first rate of energy consumption, and the system is configured to: o control the laser to emit coherent light to the blood perfused tissue, o determine a measure of motion based on the output signal of at least one of the one or more photodetectors, o determine whether the measure of motion satisfies a first condition indicative for a relatively low level of motion, and o switch to the active mode if the measure of motion satisfies the first condition.

2. The system according to claim 1, wherein the system is configured to not determine the biometric parameter in the low-power mode.

3. The system according to claim 1 or 2, wherein the system is configured to, in the active mode, operate the laser at a third rate of energy consumption, and the system is configured to, in the lower-power mode, operate the laser at a fourth rate of energy consumption that is lower than the third rate of energy consumption.

4. The system according to claim 3, wherein the system is configured to, in the active mode, operate the laser with a first duty cycle, and the system is configured to, in the low-power mode, operate the laser with a second duty cycle that is lower than the first duty cycle.

5. The system according to any of the preceding claims, wherein the system is configured to, in the low-power mode, operate the laser at a duty cycle of < 50%, preferably < 25%, more preferably < 10%.

6. The system according to any of the preceding claims, wherein the system is configured to: operate one or more processors of the system at a lower processing speed in the low- power mode than in the active mode; and / or using less processors of the system in the low-power mode than in the active mode; and / or using a lower sample rate in the low-power mode than in the active mode; and / or reducing or switching off signal amplification in the low-power mode; and / or reducing a rate of data transfer in the low-power mode; and / or switching off biometric sensors of the system, such as non-coherent light sources for PPG measurement.

7. The system according to any of the preceding claims, wherein the system comprises multiple photodetectors, and the system is configured to read-out a lower number of photodetectors in the low-power mode than in the active mode.

8. The system according to any preceding claim, wherein in the active mode, the system is configured to switch to the low-power mode after a predetermined time, wherein preferably the predetermined time is in the range of 30 seconds - 5 minutes.

9. The system according to any preceding claim, wherein in the active mode, the system is configured to determine a number of cardiac cycles from at least one output signal of the one or more photodetectors, and to switch to the low-power mode after determining the biometric parameter for a predetermined number of cardiac cycles, the predetermined number being an integer > 1.

10. The system according to any preceding claim, wherein the system is configured to determine the measure of motion also in the active mode, and the system is further configured to determine, in the active mode, whether the measure of motion satisfies a second condition indicative for a relatively high level of motion, and to switch to the low- power mode if the measure of motion satisfies the second condition.

11. The system according to any preceding claim, wherein the biometric parameter comprises a laser doppler flowmetry, LDF, parameter and / or a photoplethysmogram, PPG, parameter.

12. The system according to any preceding claim, wherein the laser comprises a semiconductor laser with integrated photodetector, preferably a VCSEL with integrated photodetector.

13. The system according to claim 12, wherein the one or more photodetectors comprise the integrated photodetector and at least one additional photodetector, wherein the system is configured to determine the measure of motion based on the output signal of the integrated photodetector and to determine the biometric parameter based on the output signal of the additional photodetector.

14. The system according to any of the preceding claims, comprising a wearable device that comprises the laser and the one or more photodetectors, wherein preferably the wearable device is an in-ear device, such as a hearing aid.

15. A method for controlling a system for determining a biometric parameter of a body, the body comprising a blood perfused tissue, the method comprising : controlling a laser of the system to emit coherent light to the blood perfused tissue; obtaining at least one output signal from one or more photodetectors of the system, wherein each of the one or more photodetectors receives a portion of the coherent light; and switching between a low-power mode for measuring motion and an active mode for determining the biometric parameter, wherein the method comprises: in the active mode: o operating the system to operate at a first rate of energy consumption,o controlling the laser to emit coherent light to the blood perfused tissue, o determining the biometric parameter based on the output signal of at least one of the one or more photodetectors; and in the low-power mode: o operating the system to operate at a second rate of energy consumption that is lower than the first rate of energy consumption, o controlling the laser to emit coherent light to the blood perfused tissue, o determining a measure of motion based on the at least one output signal, o determining whether the measure of motion satisfies a first condition indicative for a relatively low level of motion, and o switching to the active mode if the measure of motion satisfies the first condition.

16. A computer program comprising instructions which, when executed by one or more processors, cause the one or more processors to perform the method according to the preceding claim.

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