OPTICAL SENSOR MODULE

Abstract

An optical sensor module for measuring both speckleplethysmography (SPG) and photoplethysmography (PPG) signals at human or animal tissue, the optical sensor module comprising: a first light source, for illuminating the tissue for use with SPG measurements, the first light source comprising a laser; a second light source, for illuminating the tissue for use with PPG measurements; and one or more optical sensor(s) for receiving light from the illuminated tissue.

Claims

1. A wearable device comprising an optical sensor module for measuring both speckleplethysmography (SPG) and photoplethysmography (PPG) signals at human or animal tissue, the optical sensor module comprising: a coherent optical output, for illuminating the tissue for use with SPG measurements, the coherent optical output generated by a first light source comprising a laser; an incoherent optical output, for illuminating the tissue for use with PPG measurements; and one or more optical sensor(s) for receiving light from the illuminated tissue wherein: either: the incoherent output is generated by speckle mitigation techniques applied to an output of the laser; or the incoherent output is generated by a second light source separate from the first light source.

2. The optical sensor of claim 1, wherein the incoherent output is generated by speckle mitigation techniques; and wherein the speckle mitigation techniques comprise one or more of the following: a deformable mirror, an optical phase array, raster scanning, angle scanning, wavelength scanning, and wavelength broadening.

3. The wearable device of claim 1, wherein the laser has a wavelength of operation within the range of 620 nm to 1000 nm.

4. The wearable device of claim 3, wherein the wavelength of operation of the laser is 660 nm or 760 nm.

5. The wearable device of claim 1, wherein: the incoherent output is generated by a second light source separate from the first light source; and the second light source is an LED.

6. The wearable device of claim 5, wherein the second light source is an LED operating at infra-red (IR) wavelengths.

7. The wearable device of claim 5, wherein the second light source is an LED operating at a wavelength within the range of 620 nm to 1000 nm.

8. The wearable device of claim 5, further comprising a third light source, the third light source comprising an LED operating at a wavelength within the range of 620 to 800 nm; wherein the first light source is a laser having a wavelength within the range of 620 to 800 nm and the second light source is an LED operating at a wavelength within the range of 800 to 1000 nm.

9. The wearable device of claim 1, wherein: the incoherent output is generated by a second light source separate from the first light source; and the first and second light sources are both lasers and are located on the same photonic integrated circuit.

10. The wearable device of claim 1, wherein: the incoherent output is generated by a second light source separate from the first light source; and the first and second light sources and the one or more optical sensor(s) are configured to carry out SPG and PPG measurements simultaneously.

11. The wearable device of claim 1, wherein the one or more optical sensor(s) comprises an image sensor.

12. The wearable device of claim 11, wherein the same image sensor is used to extract measurements from both the first light source and the second light source.

13. The wearable device of claim 1, wherein the one or more optical sensor(s) is configured to carry out one or more of the following: in-pixel ambient subtraction, in-pixel DC subtraction; near pixel ambient DC subtraction; pixel block statistics calculation; and pixel array statistics calculation.

14. The wearable device of claim 1, wherein the one or more optical sensor(s) includes a processor configured to process captured data in-device and generate PPG and/or SPG output data.

15. The wearable device of claim 1, wherein the one or more optical sensor(s) comprises an event-based image sensor.

16. The wearable device of claim 11, wherein the one or more optical sensor(s) comprises a photodiode for collection of data during PPG measurements and a separate image sensor for collection of data using SPG measurements.

17. The wearable device of claim 1, further comprising one or more processors configured to convert optical measurement(s) at the one or more optical sensor(s) to measurements of one or more of the following: blood pressure, oxygen saturation (SpO2), arterial stiffness, heart rate, heart rate variability, atrial fibrillation, bradycardia, tachycardia, and movement.

18. The wearable device of claim 1, comprising a strap, wherein the optical sensor module is located on the strap.

19. The wearable device of claim 18, wherein, when the wearable device is located on the wrist of a user, one or more of the optical sensor(s) are located over the radial artery of the user.

20. The wearable device claim 18, comprising a module, the module comprising a timepiece or non-optical measurement device, wherein the strap is a wrist strap is connectable to the dorsal module, and wherein the entire optical sensor module is located on the wrist strap.

21. The wearable device of claim 1, further comprising one or more additional optical sensor(s).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0044] FIG. 1 shows a schematic diagram of an optical sensor module according to an embodiment of the present invention;

[0045] FIG. 2 shows a schematic diagram of an optical sensor module according to a further embodiment of the present invention;

[0046] FIG. 3 shows a schematic diagram of an optical sensor module according to a further embodiment of the present invention;

[0047] FIGS. 4A and 4B each show a schematic diagram of three further optical sensor modules according to further embodiments of the present invention;

[0048] FIG. 5 shows a representation of module operation according to a first example;

[0049] FIG. 6 shows a representation of module operation according to a second example;

[0050] FIG. 7 shows a representation of module operation according to a third example;

[0051] FIG. 8 depicts (a) an illustration of the operation of in-pixel ambient/DC removal, and (b) an illustration of the operation of an event image sensor;

[0052] FIG. 9 depicts SPG and ECG data from a first subject;

[0053] FIG. 10 depicts SPG and ECG data from a second subject;

[0054] FIG. 11 depicts SPG data from a third subject;

[0055] FIG. 12 depicts SPG data from a fourth subject;

[0056] FIG. 13 depicts an example process followed where the optical sensor module of some embodiments of the present invention is configured to carry out pixel array statistics;

[0057] FIG. 14 depicts an example process followed where the optical sensor module of some embodiments of the present invention is configured to carry out event-based detection;

[0058] FIG. 15 depicts a smart strap comprising an optical sensor according to some embodiments of the present invention, the smart strap being made of a non-conductive material; and

[0059] FIG. 16 depicts a smart strap comprising an optical sensor according to some embodiments of the present invention, portions of the smart strap being made of a conductive material.

DETAILED DESCRIPTION

[0060] SPG signals are obtained from blood flow speed and are generally of larger magnitude than PPG signals at the same wavelength. Additionally, because the SPG signal is obtained primarily from deeper lying vasculature with faster flow, it is less impacted by applied pressure. The SPG signal is comprised of very sharp peaks when collected at >20 Hz and preferably >100 Hz frame rate and so provides an excellent means of measuring HR. A cardiovascular module that combines SPG and PPG signals may provide numerous cardiovascular metrics including blood pressure, SpO2, arterial stiffness, heart rate, heart rate variability, atrial fibrillation, bradycardia, and tachycardia with just red and “IR” sources. Therefore, a green LED and/or dedicated detector for visible light is not required, which saves both battery consumption and space. Furthermore, the blood flow (SPG) measurement is very sensitive to motion and provides signals that may be interpreted as steps, gestures, or other movement-related phenomena. Thus, the module may or may not include an accelerometer.

[0061] Since the hemoglobin absorption band is quite steep in the region of 600-700 nm, SpO2 accuracy depends on the accuracy of the wavelength of the red light. Devices that utilize LEDs, with resulting broadband emission and sensitivity to temperature, may come out of calibration and give erroneous results. SPG signals require a coherent laser as light source. Lasers have much narrower emissions and less sensitivity to temperature, thereby providing a more accurate SpO2 over a wide range of conditions. Note that the “IR” light source (>800 nm) may be broader because haemoglobin absorption is relatively flat in the region 830-900 nm.

[0062] The image sensor can incorporate methods of in-pixel cancellation of DC light signals (from non-pulsatile light) or light from the ambient environment. In some embodiments, an event camera sensor may be used in place of a conventional image sensor for the detection of the SPG and PPG signals. Such a sensor offers advantages in lower power usage and processing requirements due to its capability of providing pixel change updates instead of full frame updates.

[0063] In some embodiments, the invention includes, but is not limited to a cardiovascular module that provides blood pressure, SpO2, arterial stiffness, heart rate, heart rate variability, atrial fibrillation, bradycardia, and tachycardia parameters, in a wearable, compact form factor. This module is optimized for both PPG and SPG signal collection to obtain robust measurements.

[0064] FIG. 1 shows an optical sensor module, 101 comprising a first light source 1, for illuminating the human tissue for use with SPG measurements. In this embodiment, the first light source is a laser having sufficient coherence length to acquire SPG signals (e.g. vertical-cavity surface-emitting laser (VCSEL), distributed feedback (DFB) diode, distributed Bragg reflector (DBR) diode, volume holographic (VHG) diode).

[0065] The optical sensor module also includes a second light source 2 for illuminating the human tissue for use with PPG measurements. In the embodiment shown, the second light source takes the form of an infra-red LED (e.g. having a center wavelength of 830-980 nm).

[0066] Including the second light source in the form of an LED may be advantageous as it is less susceptible to error due to wavelength drift owing to the flat hemoglobin spectrum in that region.

[0067] A plurality of optical sensor(s) is also located on the optical module for receiving light from the illuminated human tissue. In this embodiment, these sensors include a photodiode, 3 and a CMOS image sensor, 4. The photodiode may take the form of a large area photodiode with DC subtraction electronics. Electronics for the photodiode are present and may be optimized for PPG data collection to obtain SpO2 whereas the CMOS sensor may be utilized for the SPG signal collection to obtain BP and other cardiovascular parameters. This embodiment does not contain a green LED, although it would be possible to adapt it (not shown) to include a green or other wavelength LED or laser either in addition, or as a replacement to the components shown in FIG. 1.

[0068] An optical sensor module, 201 according to a second embodiment is described below in relation to FIG. 2, the second embodiment, differing from the embodiment of FIG. 2 in that the photodiode 3 is removed, and the PPG signal is instead obtained using an image sensor.

[0069] The image sensor may be a charge-coupled device (CCD), a CMOS image sensor (CIS), or an implementation of a CCD or CIS incorporating an in-pixel DC (non-pulsatile) or ambient light subtraction method such as but not limited to auto-zeroing and chopping, common mode reset, minimum charge transfer, or dual transfer gate architecture.

[0070] Alternatively, the image sensor may be an event image sensor (EIS) which produces asynchronous pixel updates according to defined pixel intensity changes as opposed to conventional synchronous frame-based CCD or CIS sensors.

[0071] A third embodiment of an optical sensor module 301 is described below with reference to FIG. 3. In this embodiment, both the first and second light source are lasers. The module comprises only the image sensor described in the second embodiment and a photonic integrated chip (PIC) 5 containing at least two lasers, one laser having a red wavelength and one laser having a NIR/IR wavelength. Red wavelength may be taken to be light with a wavelength from 620 nm to 800 nm. NIR/IR may be taken to be light with a wavelength of 800 nm to 1000 nm. The PIC may, in addition, contain multiple laser wavelengths to also perform CO-oximetry (carboxyhemoglobin, methemoglobin, etc). As with the embodiment of FIG. 2, a single image sensor is used to detect both PPG and SPG signals.

[0072] Any one or more of the embodiments described herein may utilize temporary speckle mitigation techniques for the collection of the PPG signal obtained from a photodiode, such as a deformable mirror to rapidly adjust optical pathlengths, an optical phase array, raster scanning, angle scanning, wavelength scanning or broadening, or any combination thereof. This may improve SNR as it reduces speckle noise from the intensity signal, which is more important as the size of the sensor/detector is reduced. Additionally, or alternatively, multiple photodiode acquisitions of the laser signal may be collected and averaged to improve SNR.

[0073] Any one or more of the embodiments may also include a multi-aperture array in front of the image sensor to improve SNR while maintaining the appropriate speckle to pixel ratio by virtue of the aperture diameters and distance from the sensor. Such a sensor does not require a lens to obtain the appropriate speckle to pixel ratio, although a lens or lens array may be used in conjunction with the aperture plate.

[0074] It is to be understood that the embodiments described are not limitations, e.g. green, blue, yellow, or other wavelength LEDs or lasers may be included and any LED may be replaced with a laser. The SPG signal requires, at minimum, one laser of any wavelength be present in the system along with at least one image sensor. Alternatively, in place of an image sensor that is required for speckle spatial contrast measurements, the SPG signal may be obtained by diffuse correlation spectroscopy (DCS) or interferometric diffuse correlation spectroscopy (iDCS). DCS or iDCS requires either a photodiode or balanced receiver with >500 kHz sampling rate or a single photon counting avalanche detector (SPAD) detector.

[0075] Additional modalities may be added to this module, such as ECG (electrocardiography) sensor(s), which may be utilized in addition to the SPG and/or PPG signals to calculate pulse arrival times to aid in BP estimations. ECG may also be utilized for certain arrhythmia detections as part of the suite of cardiovascular parameters.

[0076] Three further optical modules are shown in FIG. 4A and 4B. FIG. 4B differs from FIG. 4A in that it shows the link between each light source and its respective detector. Each module ((i),(ii), (iii)) includes an image sensor 403 (e.g. a CMOS image sensor (CIS)), a second sensor 404 (e.g. a photodiode), a laser (e.g. a VCSEL), and one or more LEDs (e.g. a red and an infra-red LED). A module may take the form of discrete components mounted to a printed circuit board assembly (PCBA). Example dimensions are shown and are approximate, based on discreet component sizes. However, it is important to note that these are only illustrative examples, and that other sizes would be possible.

[0077] Placement may be over an artery, 15, such as the radial or arterial artery. The arrangement shown in FIG. 4A (iii) (and corresponding FIG. 4B (iii)) is advantageous as the artery passes in the middle of the space, or “banana” (401, 402) formed between the source and detector for both the SPG and PPG modules. In the particular embodiment shown, the SPG and PPG modules are located separately on the same printed circuit board assembly (PCBA), with the PPG module corresponding to the LED(s) and photodiode, and the SPG module corresponding to the laser (e.g. VCSEL) and the image sensor (e.g. CIS).

[0078] It has been found to be advantageous for the modules to be stacked vertically with respect to an artery (i.e. along the artery when in use) in order to maximize light of similar source-detector separation. Again, such an arrangement is shown in FIG. 4A (iii) and FIG. 4B (iii).

[0079] A first example of module operation is described below in relation to FIG. 5. In this example, a first laser has a wavelength of 785 nm and separate LEDs having red (660 nm) and IR (900 or 940 nm) wavelengths respectively are also present. This gives a total of three different wavelengths used to interrogate the surface. In the example shown, the LED data acquisition is fitted within the period after camera integration (t.sub.INT ) and before max frame rate, assuming that the camera integration time is less than the period required for max frame rate operation. A time delay t.sub.SETTLE is also shown on FIG. 5, this time delay being caused by laser or light intensity stabilisation.

[0080] A further example is shown in FIG. 6, the optical module comprising a single laser and a single LED, thereby operating at two wavelengths. The laser generates light of a red (e.g. 660 nm) wavelength and the LED generates light of an IR (e.g. 900 or 940 nm) wavelength.

[0081] For convenience, operation parameters can be chosen to keep sampling rate of the IR signal the same for the PD and to use the photodiode to acquire multiple collections of the red laser in order to improve SNR due to speckle noise. Depending on the integration times used, it may be possible to carry out two or more red PD acquisitions.

[0082] A third example of an operation of an optical module is shown in FIG. 7. In this embodiment, the optical module includes a laser module or PIC configured to generate light of two different wavelengths (R and IR), and a single sensor such as a CMOS image sensor. Integration of the signal (t.sub.INT ) by the sensor starts at a time (t.sub.SETTLE) after the start of the irradiation of the sample by the laser. The PIC may contain two lasers to generate both red (660 nm) and IR (960 nm) light. It is possible that more than two wavelengths may be generated by the PIC, in which case the operation shown in FIG. 7 can be applied, and each wavelength cycled through in a given order.

[0083] A method by which the sensor can mitigate the effect of ambient background light is shown in FIG. 8.(a). The sensor can sample (SAMPLE-B) the ambient background light with the laser drive off (LASERDRV). The sensor can subsequently sample with the laser on (SAMPLE-S), the resulting reading being a combination of the signal and the ambient background light. Subtraction of the two readings results in a readout (READOUT) consisting of only the contribution of the signal. The subtraction can be accomplished by, but not limited to, in pixel charge subtraction or post-photoconversion voltage subtraction.

[0084] An example of a speckle pattern measured by an event based imaging device is shown in FIG. 8.(b). The event based image sensor functions by the detection of discrete changes in pixel intensity (events). The temporal fluctuation of the set of speckle pebbles (xi, yi) translates to the generation of an asynchronous series of pixel events, These events can then be processed by either, but not limited to, integration over a fixed time interval to create a corresponding frame of pixel intensities or in an online fashion by which a histogram of pixel update event rates relates to the temporal fluctuation of the speckle pattern.

[0085] Examples of SPG (top) and ECG (second from top) data from two subjects are shown in FIGS. 9 and 10 respectively, the data showing equivalency in performance of measuring HR (third from top) and HRV (bottom).

[0086] Examples of SPG signals (top) contain both high and low-frequency components of accelerometer data (X, Y, and Z axes in second, third, and fourth plots from the top) are shown in FIGS. 11 and 12 for two respective subjects. Motion detection and tracking may be achieved via the SPG signal and without the need for an accelerometer.

[0087] The processing of data from one or more sensor pixel arrays can be better understood with reference to the flow diagrams shown in FIGS. 13 and 14.

[0088] In the embodiment shown in FIG. 13, a sensor pixel array is first used to detect ambient background light with the light source turned off (s131). Speckle light is then detected (s132) by the sensor pixel array, when the light source is turned on. Ambient background light can then be subtracted and/or a DC set value can be subtracted from the speckle pixel array data (s133). For each pixel individually, or in a column fashion, amplification (s134) may be carried out to maximize the dynamic range of the data before digitization (s135). At digitization, pixel voltage is converted to digital unit via an analogue to digital converter. A processor may then calculate (s136) the sensor pixel array statistics, and may do so in blocks of a set size. Once these statistics have been calculated, a single data point can be obtained (s137), the single data point corresponding to SPG and PPG signals.

[0089] Calculation (s136) of the sensor pixel array statistics may include the steps of: subdividing (s138) the pixel array into N×N blocks; for each block, calculating (s139) the pixel value mean and variance and calculating speckle contrast (K) for the block; and calculating (s140) averages of speckle contrast and intensity across all blocks.

[0090] A process including event-based detection is described below in relation to FIG. 14. The process differs from that described above in relation to FIG. 13 in that the first two steps (s131, s132) are replaced by three new steps (s141, s142, s143). Firstly, ambient background light is detected (s141) through the sensor pixel array with the light source off, then speckle light is detected (s142) through the pixel array with the light source on. This may be achieved by pixel event-based detection. Next, speckle pixel events are accumulated (s. 143) over a specified time, to create a sparse speckle pixel array.

[0091] In one or more embodiments of the present invention, the optical sensor module is located on a wrist strap 1501 of a wearable device. In some of these embodiments, the optical sensor module is entirely located on a smart strap, the smart strap being a strap that includes all of the electronics and processing required by the optical sensor and can thus function completely separately from any timepiece that is to be connected to the strap. In this way, the smart strap may be used in combination with (e.g. by retrofitting onto) any pre-existing timepiece including analogue timepieces.

[0092] A first smart strap is shown in FIG. 15, the smart strap 1501 being connectable to a timepiece 1502. This embodiment is typical of non-metal (e.g. leather) straps. The optical sensor module may be located within a region 1503, 1504 on one or both sides of the strap such that the optical sensors lie over the radial and/or ulnar artery (arteries) of the user.

[0093] In an alternative smart strap shown in FIG. 16, the smart strap 1601 is made of a conductive material such as metal. Again, components of the optical sensor module may be located along the strap, and typically along the entire region 1603 of the strap. Any optical sensors of the optical sensor module may be placed so that when the conductive strap (or portionally conductive strap) is in use, being worn by the user, the one or more sensors are located above the radial and/or ulnar artery (arteries) of the user.

[0094] On a conductive (or portionally conductive), metal strap, the possibility exists for the sensing and electronic elements such as battery, Bluetooth, etc to be spread out, using the clasp (volar wrist) or other contact points along the strap as an electrical connection

[0095] It is possible to optimally design a strap using any external material (leather, silicone, plastic, metal) with sensing and electronics fully enclosed within and using the clasp for electrical attachment. The attachment area (dorsal wrist to timepiece/smartwatch) would be standard lug or compression spring or other common attachment mechanism.

[0096] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0097] All references referred to above are hereby incorporated by reference.