Method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules

Abstract

A photoplethysmography (PPG) system and method that applies derivative spectroscopy iteratively over time to increase specificity of the PPG signal to hemoglobin molecules. The system uses two or more wavelengths of light, preferably in the green spectrum near the maximum slope of the hemoglobin absorption curve. At each time point, the system calculates a derivative of absorption with respect to wavelength using a finite difference approximation. This approach differentiates hemoglobin absorption from interfering physiological materials such as skin pigments, water, and muscle tissue, thereby reducing myoelectric noise and improving signal-to-noise ratio. The innovation can be implemented with minimal hardware modifications to existing PPG platforms by using different wavelength LEDs or adding optical filters, making it suitable for various wearable devices including smartwatches, rings, and health monitoring devices.

Description

Atty. Dkt No. 10085-01-0191-PCTMETHOD FOR ENHANCING SPECIFICITY OF A PHOTOPLETHYSMOGRAPHY (PPG) SIGNAL TO HEMOGLOBIN MOLECULESCROSS REFERENCE TO RELATED APPLICATIONSThe application claims priority to US provisional patent application No. 63 / 734,656, filed on December 16, 2024, which is incorporated by reference in the present application in its entirety.BACKGROUND OF DISCLOSURE1. Field of Disclosure

[0001] The present disclosure relates to the field of physiological monitoring, and more particularly, to a method, device, and system for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules.2. Description of Related Art

[0002] Photoplethysmography (PPG) is a non-invasive monitoring technology that utilizes changes in optical impedance to measure the pulse pressure waveform. With the advancement of wearable technology, PPG sensors have been widely adopted in consumer electronics to monitor physiological parameters like heart rate and heart rate variability (HRV). Traditional PPG monitoring methods typically employ a single light source, commonly green light with a wavelength of approximately 520 nm, to track dynamic changes in the optical absorption signal.Technical Problem

[0003] However, existing PPG technology faces significant challenges in practical applications, particularly when the user is in motion. While physiological constituents such as skin pigment, muscle, and other tissues are generally considered static relative to blood flow, these non-target tissues undergo dynamic displacement during user motion. Pigment molecules in the skin, such as eumelanin and pheomelanin, exhibit high optical absorption at the preferred PPG wavelength of 520 nm. Consequently, even minor dynamic activity can introduce substantial interference into the optical absorption signal, commonly referred to as "myoelectric noise" or motion artifacts.

[0004] Current techniques for noise reduction have limitations. Filtering in the frequency domain is often ineffective because the frequency bandwidth of motion noise typically overlaps with that of the heart rate signal. Furthermore, merely altering the light source wavelength does not adequately resolve the issue: shorter wavelengths lack sufficient penetration depth to interact effectively with the capillary bed, while longer wavelengths penetrate too deeply, resulting in a signal contaminated with excessive deep-tissue noise.

[0005] Hence, a method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules is desirable.SUMMARY

[0006] An object of the present disclosure is to propose a method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules and a network node.

[0007] In a first aspect, an embodiment of the invention provides a method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules, comprising: emitting, from a light source unit, light at a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; detecting, using at least one photodetector, a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength using a finite difference approximation based on the first absorption signal and the second absorption signal; generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and determining a physiological metric based on the time-sequence PPG signal.

[0008] In a second aspect, an embodiment of the invention provides a device for photoplethysmography (PPG) signal processing, comprising: a light source unit emitting light at a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum inAtty. Dkt No. 10085-01-0191-PCT the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; a detection unit detecting a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength using a finite difference approximation based on the first absorption signal and the second absorption signal; generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and determining a physiological metric based on the time-sequence PPG signal.

[0009] In a third aspect, an embodiment of the invention provides a system for photoplethysmography (PPG) signal processing, comprising: a light source unit configured for light emitting at a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; a detection unit configured for detecting a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a derivative obtaining unit configured for obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength using a finite difference approximation based on the first absorption signal and the second absorption signal; a signal generating unit configured for generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and a determining unit configured for determining a physiological metric based on the time-sequence PPG signal.

[0010] In a fourth aspect, an embodiment of the invention provides a terminal device comprising: a processor and a memory; the memory is configured to store one or more computer programs; and the processor configured to call and run the one or more computer programs stored in the memory to cause the terminal device to perform the disclosed method.

[0011] In a fifth aspect, an embodiment of the invention provides a device for photoplethysmography (PPG) signal processing, comprising: a light source configured for emitting light through a first filter that allows light passing at a first wavelength toward a tissue site and through a second filter that allows light passing at a second wavelength toward the tissue site, wherein the light source comprises at least one of a first light source and a second light source; and signals of light through the first filter and signals of light through the second filter are pulse-modulated to be multiplexed; one or more detection units configured for being operated with the light source to demultiplex the signals of light through the first filter and the signals of light through the second filter, and detect a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

[0012] In a sixth aspect, an embodiment of the invention provides a device for photoplethysmography (PPG) signal processing, comprising: a broadband light source configured for emitting a spectrum of light covering both a first wavelength and a second wavelength; one or more detection units configured for sampling of a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength, wherein the one or more detection units comprises a first photodetector with a first optical filter configured toAtty. Dkt No. 10085-01-0191-PCT pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength; a memory storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

[0013] In a seventh aspect, an embodiment of the invention provides a chip comprising: a processor and a memory; the memory is configured to store one or more computer programs; and the processor configured to call and run the one or more computer programs stored in the memory to cause a device equipped with the chip to perform the disclosed method.

[0014] In a eighth aspect, an embodiment of the invention provides a non-transitory computer-readable storage medium, in which a computer program is stored, wherein the computer program causes a computer to perform the disclosed method.

[0015] In a ninth aspect, an embodiment of the invention provides a computer program product, comprising a computer program, wherein the computer program causes a computer to perform the disclosed method.

[0016] In an tenth aspect, an embodiment of the invention provides a computer program, wherein the computer program causes a computer to perform the disclosed method.BRIEF DESCRIPTION OF DRAWINGS

[0017] To clearly illustrate the embodiments of the present disclosure or related technical solutions, the accompanying drawings are briefly described below. These drawings represent embodiments of the present disclosure. A person of ordinary skill in the art may derive additional figures or variations based on these drawings without departing from the scope of the present disclosure.

[0018] FIG. 1 illustrates a schematic diagram showing a first embodiment of a device for photoplethysmography (PPG) signal processing.

[0019] FIG. 2 illustrates a schematic diagram showing a second embodiment of a device for photoplethysmography (PPG) signal processing.

[0020] FIG. 3 a schematic diagram showing a third embodiment of a device for photoplethysmography (PPG) signal processing.

[0021] FIG. 4 illustrates a schematic diagram showing a method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules of an embodiment of the disclosure.

[0022] FIG. 5 illustrates a schematic diagram showing a broad spectrum of hemoglobin absorption.

[0023] FIG. 6 illustrates a schematic diagram showing a narrow spectrum of hemoglobin absorption.

[0024] FIG. 7 illustrates a schematic diagram showing derivative of the absorption spectrum.

[0025] FIG. 8 illustrates a schematic diagram showing hemoglobin absorption spectrum with potentialPPG wavelengths highlighted to identify resulting absorption coefficients.

[0026] FIG. 9 illustrates a schematic view showing: the synthetically generated time-domain absorption signals for each constituent of the system; the two absorption coefficients correlating to 538 nm and 550 nm for each constituent; and the sum of all five signals at each wavelength, representing the estimated signal from the hardware platform.

[0027] FIG. 10 illustrates a schematic view showing the absorption signals zoomed to one second in time, and the derivative with respect to wavelength.

[0028] FIG. 11 illustrates a schematic view showing full ten seconds of data from two wavelengths, and the resulting derivative with respect to wavelength iterated over each point in time.

[0029] FIG. 12 illustrates a schematic view showing a chip or executing the disclosed method in a device for photoplethysmography (PPG) signal processing.

[0030] FIG. 13 illustrates a schematic view showing a system for photoplethysmography (PPG) signal processing.

[0031] FIG. 14 illustrates a schematic view showing a system for wireless communication according to an embodiment of the present disclosure.DETAILED DESCRIPTION OF EMBODIMENTS

[0032] Embodiments of the disclosure are described in detail with the technical matters, structural features, achieved objects, and effects with reference to the accompanying drawings as follows. Specifically, the terminologies in the embodiments of the present disclosure are merely for describing the purpose of the certain embodiment, but not to limit the disclosure. In one or more embodiments of the disclosure, “at leastAtty. Dkt No. 10085-01-0191-PCT one” means “one or more”.

[0033] Origin of derivative spectroscopy:

[0034] Derivative Spectroscopy is a laboratory method utilized when multiple samples exhibit overlapping optical absorption spectra, typically in the infrared (IR) or ultraviolet (UV) ranges. In this process, the wavelength of light is swept across a broad bandwidth to generate a full spectrum of light absorption within a bulk material, after which the derivative of the spectrum is calculated with respect to wavelength. Individual molecules within the bulk material are then quantified by observing the magnitude of peaks in the derivative trendline that correlate with the maximum rate of change of the molecule's absorption. The rate of change of absorption with respect to wavelength can be used to increase specificity in quantifying individual constituents within a bulk solution.

[0035] Typically, in a lab setting, this spectroscopy would be performed with a broad spectrum of wavelengths, allowing for the derivative of the full spectrum to be taken rather than using a finite difference approximation as applied herein. Using this method, a full trendline of the derivative with respect to wavelength could be plotted, and certain peaks could help to quantify ratios of the constituents.

[0036] Similar methods in health sensing:

[0037] Currently, peripheral capillary oxygen saturation (SpO2) readings utilize technology that shares similarities with derivative spectroscopy, although the application and intent differ significantly.

[0038] This method is similar in that it employs two wavelengths of light to differentiate between constituents within a bulk material. However, rather than targeting the spectral slope of a specific molecule, this method seeks to quantify the ratio of oxy-hemoglobin to deoxy-hemoglobin.

[0039] The wavelengths selected for pulse oximetry are unsuitable for derivative spectroscopy because they align with distinct, separated absorption peaks (typically red and infrared) rather than a continuous slope. Consequently, these wavelengths cannot be used to quantify the rate of change of absorption with respect to wavelength. The objective of this method is to quantify volumetric ratios of oxy-hemoglobin and deoxy-hemoglobin; therefore, the wavelengths are chosen to target specific ratios of absorption between these two molecules.

[0040] Furthermore, this method requires evaluation across points in time (e.g., across a pulse cycle), indicating that it estimates a volume change over time rather than a derivative with respect to wavelength. This technique is commonly referred to as the "ratio of ratios," as it utilizes multiple ratios of light absorption at multiple times (pulsatile versus non-pulsatile components). Thus, unlike the present disclosure, standard SpO2 methods do not, and cannot, perform derivative spectroscopy with the standard wavelengths employed.

[0041] Current methods for measuring heart rate via photoplethysmography (PPG) typically utilize a single wavelength of light to track dynamic changes in the absorption signal. PPG is a method of obtaining the pulse pressure waveform using photonic impedance. While other physiological constituents — such as skin pigment, muscle, and bone — are relatively static compared to blood flow, they are not entirely static in practice. This is particularly true during user motion, which constitutes a significant portion of wearable device use cases.

[0042] Because pigment molecules, such as eumelanin and pheomelanin, exhibit high optical absorption at the preferred PPG wavelength (approximately 520 nm), even small dynamic activities can have a disproportionately large impact on the optical absorption signal, introducing significant noise.

[0043] An innovative solution was required to increase the specificity of the absorption signal to hemoglobin molecules. While the optical impedance of these interference materials cannot be physically avoided, treating them as a baseline signal to be removed allows dynamic changes to be specifically tracked to hemoglobin.

[0044] Filtering this noise in the frequency domain is ineffective because motion noise and the heart rate signal often overlap in bandwidth. Furthermore, altering the wavelength presents significant trade-offs. Using a shorter wavelength results in insufficient penetration depth to interact effectively with the capillary bed. Conversely, longer wavelengths penetrate too deeply, significantly increasing the presence of myoelectric noise in the signal. Myoelectric noise originates from physiological sources other than the pulse pressure wave. Consequently, a wavelength near 520 nm must be maintained to optimize photon interaction with the blood.

[0045] To overcome these limitations, it was determined that the rate of change of absorption (the slope) between hemoglobin molecules and confounding materials differs sufficiently to be utilized. This difference allows for increased specificity in the measurement, distinguishing the hemoglobin signal from interference.

[0046] The integration of Photoplethysmography (PPG) sensors into consumer electronics is a relatively recent development, marked by the release of industry-standard devices around 2015. While thisAtty. Dkt No. 10085-01-0191-PCT represented a novel introduction to the consumer market, the use of PPG for heart rate sensing has been established in clinical settings for a significant period.

[0047] Miniaturizing this technology into a silicon chip for continuous user monitoring required substantial innovation. Challenges included reducing size and power consumption, scaling production for health-related devices, and managing boundary conditions and noise reduction. These initial innovations focused on adapting traditional PPG technology for a high-scale, continuously wearable platform (e.g., wearable devices). Since that time, however, advancements have primarily been iterative improvements to the existing platform rather than fundamental changes to the underlying PPG technology.

[0048] The present disclosure proposes novel methods for acquiring the PPG signal. Current PPG signals contain noise introduced by dynamic changes in optical impedance from various physiological constituents along the photon path. This disclosure seeks to isolate the photonic impedance of a single molecule — specifically oxy-hemoglobin — while ignoring dynamic impedance from other materials in the bulk tissue sample (e.g., the user's arm).

[0049] This is achieved by utilizing derivative spectroscopy, a method typically employed in laboratorybased material characterization to quantify the concentration of individual molecules within a bulk substance.

[0050] The present disclosure applies derivative spectroscopy iteratively across a time sequence. Rather than focusing on the quantitative concentration of a target molecule, this method utilizes the qualitative rate of change of the substance overtime. By employing this approach, dynamic changes in photonic impedance caused by confounding molecules are effectively removed from the signal. This increases the signal-to- noise ratio (SNR) of the PPG waveform, thereby improving the accuracy of algorithms such as heart rate monitoring, heart rate variability (HRV) analysis, arrhythmia detection, and other PPG-based assessments.

[0051] Hardware Aspects:

[0052] A significant technical advantage of the present innovation is its compatibility with existing hardware architectures. Unlike solutions requiring the development of new silicon for the analog front end (AFE), the disclosed method may be implemented using existing silicon designs. Implementation may be achieved by substituting a specific Light Emitting Diode (LED) wavelength. Alternatively, a standard broad-spectrum LED may be utilized in conjunction with an optical filter to narrow the bandwidth of photons emitted from the device (e.g., device 100, device 100a, or device 100b in FIGs. 1-3) . Optical filter can be implemented as a coating which limits the transmission of photons to a narrow wavelength spectrum. This ensures manufacturing compatibility by utilizing components similar to those already present in standard hardware platforms.

[0053] Furthermore, the scope of the present disclosure is not limited to a smartwatch form factor; it may be implemented in various devices including smartphones, telemedicine monitors, smart rings, and other physiological monitoring platforms.

[0054] The present disclosure introduces a modification to hardware platforms that utilize the Photoplethysmography (PPG) method for measuring dynamic changes in the pulse pressure wave. These measurements are utilized for estimating heart rate, heart rate variability (HRV), and detecting complex physiological features such as arrhythmias.

[0055] Conventionally, hardware platforms — including smartwatches, smart rings, telemedicine devices, smartphones, headphones, and Extended Reality (XR) devices — employ a PPG Analog Front End (AFE) to control Light Emitting Diodes (LEDs) and Photodiodes (PDs). Photodiode is a sensor used to collect photons of a broad wavelength spectrum. These AFEs typically accommodate multiple channels, often utilizing multiplexing to increase the capacity of LEDs and PDs available to the platform.

[0056] While traditional PPG waveforms are acquired using a single LED with a wavelength near 520 nm (green), the disclosed technology utilizes two distinct LED wavelengths, preferably both located within the green spectrum. Implementation may be achieved by integrating an additional LED into the hardware platform or by replacing an existing redundant LED with one emitting a narrowly shifted wavelength.

[0057] Wavelength Selection and Penetration Depth

[0058] The initial step in selecting the optimal wavelength for spectroscopy involves identifying the appropriate depth of penetration. In the present embodiment, the application is described as a consumer electronics watch worn on the arm, with the PPG module oriented toward the dorsal wrist. Furthermore, the capillary bed is identified as the target blood vessel for observation. It should be noted that these specific form factor assumptions are not strictly limiting, as discussed further in the alternatives section.

[0059] Based on these parameters, absorption peaks below 500 nm are excluded due to insufficient depth to adequately observe the capillary bed. Conversely, peaks above 650 nm are excluded because they provide unnecessary depth, which significantly increases signal noise. These exclusion zones are illustrated in FIG.Atty. Dkt No. 10085-01-0191-PCT5. Additionally, this selection process must account for user safety considerations, specifically the prevention of tissue bums.

[0060] With reference to FIG. 1, an embodiment of the disclosure provides a device 100 for photoplethysmography (PPG) signal processing, comprising: a light source 108 emitting light 401 including a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; a detection unit 112 detecting a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory 104 storing instructions; and a processor communicatively coupled to the detection unit 112 and the memory 104, wherein the processor executes the instructions to perform: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength using a finite difference approximation based on the first absorption signal and the second absorption signal; generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and determining a physiological metric based on the time-sequence PPG signal.

[0061] The detection unit 112 is configured to receive the retumed / reflected light 402. This light 402 is the light that has interacted with the tissue site (e.g., via reflection or backscattering) and represents the residual optical power remaining after the incident light has been attenuated by absorption from physiological constituents, including hemoglobin and non-hemoglobin components (i.e., hemoglobin absorption and non-hemoglobin absorption).

[0062] With reference to FIG. 2, a device 100a is an embodiment of device 100. The device 100a for photoplethysmography (PPG) signal processing comprises: a light source 108a configured for emitting light through a first filter that allows light passing at a first wavelength toward a tissue site and through a second filter that allows light passing at a second wavelength toward the tissue site, wherein the light source comprises at least one of a first light source and a second light source; and signals of light through the first filter and signals of light through the second filter are pulse-modulated to be multiplexed; one or more detection units 112a configured for being operated with the light source to demultiplex the signals of light through the first filter and the signals of light through the second filter, and detect a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory 104 storing instructions; and a processor 101 communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

[0063] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents.

[0064] In one or more embodiments of the disclosure, generating the PPG signal comprises: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; and generating a time-sequence PPG signal based on the derivatives over the plurality of time points; determining the physiological metric based on the PPG signal comprises: determining the physiological metric based on the time-sequence PPG signal.

[0065] In one or more embodiments of the disclosure, obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative ofAtty. Dkt No. 10085-01-0191-PCT absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

[0066] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

[0067] In one or more embodiments of the disclosure, the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

[0068] In one or more embodiments of the disclosure, at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

[0069] In one or more embodiments of the disclosure, the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxy-hemoglobin absorption spectrum to enhance the derivative computation.

[0070] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

[0071] In one or more embodiments of the disclosure, the light source is pulse width modulated (PWM) to decrease total energy over time while maintaining intensity.

[0072] In one or more embodiments of the disclosure, device 100a is a wearable device.

[0073] With reference to FIG. 3, a device 100b is an embodiment of device 100. The device 100b for photoplethysmography (PPG) signal processing comprises: a broadband light source 108b configured for emitting a spectrum of light covering both a first wavelength and a second wavelength; one or more detection units 112b configured for sampling of a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength, wherein the one or more detection units comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength; a memory 104 storing instructions; and a processor 101 communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of nonhemoglobin physiological constituents.

[0074] In one or more embodiments of the disclosure, generating the PPG signal comprises: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; and generating a time-sequence PPG signal based on the derivatives over the plurality of time points; determining the physiological metric based on the PPG signal comprises: determining the physiological metric based on the time-sequence PPG signal.

[0075] In one or more embodiments of the disclosure, obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

[0076] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

[0077] In one or more embodiments of the disclosure, the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

[0078] In one or more embodiments of the disclosure, at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.Atty. Dkt No. 10085-01-0191-PCT

[0079] In one or more embodiments of the disclosure, the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxy-hemoglobin absorption spectrum to enhance the derivative computation.

[0080] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

[0081] In one or more embodiments of the disclosure, the light source is pulse width modulated (PWM) to decrease total energy over time while maintaining intensity.

[0082] In one or more embodiments of the disclosure, device 100b is a wearable device.

[0083] With reference to FIG. 4, an embodiment of the disclosure provides a method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules, the method comprising:Step S 101 : emitting, from a light source unit (e.g., light source 108), light including a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents;Step S 102: detecting, using at least one photodetector (e.g., detection unit 112), a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength;Step SI 03: generating a PPG signal based on the first absorption signal and the second absorption signal; andStep S 104 determining a physiological metric based on the time-sequence PPG signal.

[0084] In one or more embodiments of the disclosure, the light source can be implemented by a light source unit which may comprise one or more light-emitting elements, such as Light Emitting Diodes (LEDs) or laser diodes, designed to cover the specified spectral range. The "light source unit" described herein, configured to emit light including a first wavelength and a second wavelength, is not limited to a specific physical configuration of light-emitting elements, and is intended to encompass various optical design solutions. In one embodiment, the light source unit may comprise a single physical light-emitting element (e.g., a single LED or a broadband source), where the emitted light spectrum comprehensively includes both the first wavelength and the second wavelength. In an alternative embodiment, the light source unit may comprise a plurality of physical light-emitting elements; for instance, a first LED may emit light including the first wavelength, and a second LED may emit light including the second wavelength. Irrespective of whether a single source or multiple sources are utilized, the first wavelength and the second wavelength are selected to be distinguishable and measurable by the subsequent photodetector(s), thereby enabling the required derivative calculation based on the corresponding absorption signals. The present invention does not impose a restriction on the specific physical composition or quantity of the light source unit.

[0085] In one or more embodiments of the disclosure, the step SI 03 comprises obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and

[0086] In one or more embodiments of the disclosure, the step S 104 comprises determining the physiological metric based on the time-sequence PPG signal.

[0087] In one or more embodiments of the disclosure, the non-hemoglobin physiological constituents comprise at least one of: eumelanin, pheomelanin, water, or muscle tissue.

[0088] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

[0089] In one or more embodiments of the disclosure, the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

[0090] In one or more embodiments of the disclosure, at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.Atty. Dkt No. 10085-01-0191-PCT

[0091] In one or more embodiments of the disclosure, emitting light at the first wavelength and the second wavelength comprises pulse-modulating at least two light emitting diodes (LEDs) to multiplex the first and second absorption signals, and wherein detecting comprises synchronizing the photodetector to demultiplex the first and second absorption signals.

[0092] In one or more embodiments of the disclosure, the light source 108 comprises a broadband light emitting diode (LED) configured to emit a spectrum of light covering both the first wavelength and the second wavelength; the detection unit 112 comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength, enabling simultaneous sampling of the first absorption signal and the second absorption signal.

[0093] In one or more embodiments of the disclosure, the light source 108 comprises: a first light emitting diode (LED) having a first optical filter configured to emit the first wavelength; and a second LED having a second optical filter configured to emit the second wavelength; and the detection unit 112 comprises a broadband photodetector; wherein a processor (e.g., processor 101) is configured to synchronize the detection unit 112 with the light source 108 to differentiate between the first wavelength and the second wavelength via time-division multiplexing.

[0094] In one or more embodiments of the disclosure, emitting light comprises modulating a first light source emitting the first wavelength and a second light source emitting the second wavelength at different time intervals.

[0095] In one or more embodiments of the disclosure, the time-sequence PPG signal is used to estimate at least one physiological parameter selected from heart rate, heart rate variability, and arrhythmia detection.

[0096] In one or more embodiments of the disclosure, the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxy-hemoglobin absorption spectrum to enhance the derivative computation.

[0097] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

[0098] In one or more embodiments of the disclosure, emitting light further comprises emitting light at a third wavelength; detecting comprises detecting a third absorption signal; and obtaining derivatives comprises computing a second derivative of absorption with respect to wavelength using finite differences among the first, second, and third absorption signals.

[0099] In one or more embodiments of the disclosure, the first and second wavelengths are selected from alternative absorption peaks of hemoglobin, including peaks between 400 nm and 500 nm for shallower tissue penetration or peaks above 550 nm for deeper penetration.

[0100] In one or more embodiments of the disclosure, the tissue site is selected from the group consisting of a wrist, finger, ear, forehead, and arm.

[0101] In one or more embodiments of the disclosure, the method is implemented in a wearable device selected from a watch, ring, phone, headphone, or telemedicine device.

[0102] In one or more embodiments of the disclosure, obtaining the derivatives reduces signal noise attributed to eumelanin or pheomelanin by utilizing a difference in absorption slope between hemoglobin and said eumelanin or pheomelanin.

[0103] FIG. 6 illustrates a narrowed spectrum, highlighting the two largest absorption peaks for oxyhemoglobin. Given that oxy-hemoglobin is more prevalent than deoxy-hemoglobin, it serves as the primary focus of this work. Notably, both hemoglobin absorption curves exhibit similar slopes, and it is presumed that deoxy-hemoglobin will be complementary in further enhancing the effectiveness of this approach.

[0104] A quick approximation is performed to identify the optimized peak for derivative spectroscopy. The steepest slope occurs between 600 nm and 620 nm, compared against the pheomelanin slope by the angle 0. The slopes between 550 nm and 600 nm are disregarded, as the oxy- and deoxy-hemoglobin spectra would interfere destructively rather than complementarily. The second-steepest slope is then from 520 nm to 550 nm, denoted by a.

[0105] Although 0 exhibits a steeper slope for the oxy-hemoglobin trendline, the skin pigments share aAtty. Dkt No. 10085-01-0191-PCT slope in the same direction. This reduces the difference in slope between hemoglobin and skin pigments. In contrast, for angle a, the hemoglobin slope is opposite to that of the skin pigments, thereby magnifying their difference. Consequently, the 520 nm to 550 nm region is considered ideal for derivative spectroscopy.

[0106] It is important to remember that derivative spectroscopy is traditionally performed using many wavelengths. This enables the technique to plot the full derivative of the absorption spectrum and identify the magnitude of peaks to estimate the concentration of the target molecule.

[0107] To leverage existing silicon while reducing size, complexity, and power consumption, this method applies a finite difference approach using only two wavelengths. These wavelengths are selected to maximize the slope between them, implying a narrow bandwidth centered around the maximum slope on the curve.

[0108] With reference to FIG. 7, by plotting the full derivative of the absorption spectrum, the maximum slope is identified near 544 nm. When selecting wavelengths evenly spaced around this point, the reduction in the mean slope between them can be estimated. Although lasers or other complex transducers can achieve a narrow emission band, these options are costly and may concentrate energy too narrowly, potentially causing bums. Therefore, the two wavelengths should account for the broad bandwidth of traditional LEDs (typically 30-40 nm), optimizing the balance between available hardware technologies and maximizing the mean slope. For a theoretical example, wavelengths of 538 nm and 550 nm have been selected.

[0109] These two wavelengths can now be applied to the original absorption curve to determine the absorption coefficients for each molecule, as illustrated in FIG. 8 for oxy-hemoglobin and pheomelanin.

[0110] The use of derivative spectroscopy was modeled using Python to evaluate the potential benefits of the method. This model did not account for deoxy-hemoglobin, as its inclusion would only strengthen the approach given its similar slope in the 520 nm to 556 nm range.

[0111] Four noise signals were generated using a Python script, each with a sample rate of 100 Hz over ten seconds and frequency content ranging from 0.1 Hz (a common 3 dB low-pass filter cutoff) to 50 Hz (the Nyquist frequency). The script intentionally maximized power near 10 Hz to simulate in-band noise that could disrupt the heart rate signal. These four signals were arbitrarily assigned to represent the absorption contributions from water, eumelanin, pheomelanin, and the baseline absorption of the hardware equipment used to empirically derive these absorption values.

[0112] Although the heart rate waveform typically exhibits higher-frequency components (e.g., up to 10 Hz) despite its fundamental one-second period (corresponding to ~1 Hz for 60 bpm), this complexity was ignored for simplicity in the model. Instead, a 10 Hz sine wave was used to represent the heart rate signal. Using the aforementioned wavelengths of 538 nm and 550 nm, the corresponding absorption coefficients were identified from the plot in FIG. 8. These coefficients were multiplied by each of the time-domain data sets (including the heart rate signal and the four noise signals), resulting in ten unique time-domain data sets (five for each wavelength). The five arrays for each wavelength were then summed at each time point, yielding two arrays representative of the total absorption signal for each wavelength. This process is illustrated in FIG. 9.

[0113] Using the equation in FIG. 10, the derivative with respect to wavelength was computed iteratively across each time point to resolve the time-domain signal focused on the absorption peak of oxy-hemoglobin. Note that the plot zooms into only one second of data to clearly show the three signals: the 538 nm absorption signal, the 550 nm absorption signal, and the derivative of these two signals with respect to wavelength.

[0114] FIG. 11 depicts the full ten-second data interval, illustrating the significant signal improvement achieved by the disclosed method within an idealized synthetic model. These simulation results serve to validate the theoretical efficacy of the approach, demonstrating the ability of the system (e.g., device 100, device 100a, or device 100b in FIGs. 1-3, system 200 in FIG. 13, or system in FIG. 14) to isolate the target signal from confounding noise.

[0115] Signal Acquisition in the Time Domain:

[0116] In the preferred embodiment, the physiological monitoring device (e.g., device 100, device 100a, or device 100b in FIGs. 1-3) is configured to acquire photonic signals in the time domain, rather than performing a spectral sweep across the wavelength domain as is typical in laboratory spectrophotometers. The detection unit (e.g., detection unit 112) continuously captures a first time-series dataset ( / ^i (t) ) corresponding to optical intensity at the first wavelength (e.g., 538 nm) and a second time-series dataset (7^2 ( ) corresponding to optical intensity at the second wavelength (e.g., 550 nm). These raw signals represent the temporal variation in optical absorption caused by pulsatile blood flow, tissue composition, and motion artifacts over a period of time. Importantly, the system (e.g., device 100, device 100a, or device 100b in FIGs. 1-3, system 200 in FIG. 13, or system in FIG. 14) does not need to construct a full absorptionAtty. Dkt No. 10085-01-0191-PCT spectrum at any single instance; instead, it relies on the continuous monitoring of these discrete, preselected spectral points.

[0117] Iterative Derivative Calculation:

[0118] The processor (e.g., processor 101) is configured to generate a derivative photoplethysmography (PPG) signal by calculating the derivative of absorption with respect to wavelength iteratively at each sampling time point (tn). Rather than analyzing the shape of a curve across a wavelength axis, the present invention computes a finite difference approximation for every synchronized time sample. Specifically, the processor (e.g., processor 101) calculates the difference in absorption between the second wavelength and the first wavelength at time t, normalized by the spectral separation (AX). This operation transforms the multi-channel raw input into a single, enhanced time-domain waveform that represents the instantaneous slope of the hemoglobin absorption coefficient. By tracking how this spectral slope changes over time, the system (e.g., device 100, device 100a, or device 100b in FIGs. 1-3, system 200 in FIG. 13, or system in FIG. 14) effectively isolates the dynamic hemoglobin signal from semi-static baselines (e.g., melanin) and broadband noise sources that exhibit different spectral slope characteristics.

[0119] Light Source and Filtering Configurations:

[0120] Standard commercial Light Emitting Diodes (LEDs) operating near the 540 nm wavelength typically exhibit a spectral bandwidth of approximately 30-40 nm. While utilizing the mean photon wavelength of such devices produces functional results, the specificity of the derivative measurement may be constrained by this broad bandwidth.

[0121] Alternative light sources, such as lasers or specialized narrow-band LEDs, may be utilized to address this bandwidth limitation. However, these components often increase system cost, size, and complexity. Furthermore, the concentration of energy in space and time associated with such coherent sources requires careful management to ensure user safety and prevent thermal injury.

[0122] Embodiment A: Emitter-Side Filtering

[0123] A practical implementation for achieving the required spectral specificity involves applying optical filters to the lens of the LED. In this embodiment, two green LEDs are utilized, each equipped with a narrow-band optical filter (e.g., a bandwidth of 10 nm or less). To allow a broadband photodiode (PD) to differentiate between the two wavelengths, the LEDs are pulse-modulated. This synchronization, or Time- Division Multiplexing (TDM), separates the signals in the time domain, allowing the PD to resolve the two distinct wavelengths. The PWM, pulse modulation, synchronization, and / or TDM can be controlled by a processor, such as processor 101.

[0124] In the present invention, pulse modulation techniques are employed to enable temporal separation of multiple wavelength signals using a single broadband photodetector. Pulse modulation encompasses a family of techniques wherein a periodic pulse train serves as a carrier wave, with information encoded by modifying pulse characteristics such as amplitude, width, or temporal position.

[0125] For the dual-wavelength PPG system disclosed herein, pulse width modulation (PWM) is particularly advantageous for controlling the LED emission timing. In this implementation, each LED wavelength (e.g., 538 nm and 550 nm) is assigned distinct time slots within a measurement cycle. The LEDs are energized sequentially rather than simultaneously, with the duty cycle and switching frequency optimized to maintain adequate signal-to-noise ratio while minimizing power consumption. The pulse repetition frequency is selected to be substantially higher than the physiological signals of interest (typically >100 Hz for heart rate measurements in the 0.5-3 Hz range), ensuring that the temporal multiplexing does not alias with the biological signals being measured.

[0126] The photodetector, being broadband in nature, receives optical signals from both wavelengths but can discriminate between them based on the synchronized timing scheme. The analog front-end (AFE) circuitry coordinates the LED driving pulses with the photodetector sampling, effectively demultiplexing the composite optical signal into separate channels corresponding to each wavelength. This temporal multiplexing approach eliminates the need for multiple narrowband photodetectors or complex optical filtering at the detector side, thereby reducing system complexity, cost, and size while maintaining the ability to perform derivative spectroscopy calculations on the separated wavelength signals.

[0127] The pulse modulation parameters, including pulse duration, interpulse spacing, and switching frequency, may be dynamically adjusted based on ambient light conditions, motion artifacts, or signal quality metrics to optimize the derivative spectroscopy performance in real-time operating conditions.

[0128] Embodiment B: Detector-Side Filtering

[0129] In a preferred embodiment, optical filters are applied over the detection unit (e.g., detection unit 112) (the Photodiodes). This configuration offers several system design advantages. A single broadband LED source, or multiple LEDs operating simultaneously, illuminates the tissue. Two separate PDs, eachAtty. Dkt No. 10085-01-0191-PCT equipped with a different narrow-band fdter, collect the return signal.

[0130] Because the signals are collected via separate filtered channels, no multiplexing is required. This allows for simultaneous sampling of both wavelengths, which significantly increases the precision of the derivative calculation by ensuring temporal alignment. Furthermore, this method improves signal integrity by preventing scattered light from the LED or coupled atmospheric light from contaminating the signal chain with extraneous wavelengths.

[0131] Technical Benefits:

[0132] The primary technical advantage of the disclosed method is the reduction of myoelectric noise within the Photoplethysmography (PPG) signal. By differentiating among the physiological constituents along the photon path of travel, the system (e.g., device 100, device 100a, or device 100b in FIGs. 1-3, system 200 in FIG. 13, or system in FIG. 14) effectively isolates the target hemoglobin signal. This resulting increase in the signal-to-noise ratio (SNR) significantly enhances downstream processing, including timeseries analysis, frequency domain analysis, machine learning models, neural networks, and other computational methods for extracting health data from time-sequence arrays.

[0133] Alternative Embodiments:I. Alternative Spectral Peaks

[0134] While the preferred embodiment targets specific slopes, additional peaks on the oxy-hemoglobin and deoxy-hemoglobin absorption curves may be utilized. For example, oxy-hemoglobin presents two peaks (four slopes) between 520 nm and 620 nm. Similarly, deoxy-hemoglobin exhibits a peak near 570 nm, with useable slopes on either side within the 500 nm to 620 nm range.II. Short Wavelengths for Shallow Tissue

[0135] For specific anatomical locations where the capillary bed is shallower — such as the ear, forehead, or finger — it is possible to utilize the shorter wavelength peak (400 nm to 500 nm), which exhibits a much steeper slope than the green spectrum.III. High-Power Arterial Sensing

[0136] To observe deeper physiological structures, such as arteries, the optical power at a given wavelength may be increased. Because the hemoglobin absorption curve lacks peaks in the Infrared (IR) range, maintaining the visible wavelength while increasing power is necessary for depth. To prevent tissue bums, the Light Emitting Diode (LED) may require Pulse Width Modulation (PWM) to decrease the total energy over time while maintaining peak intensity. This approach enables the observation of arterial flow, which provides richer physiological data than capillaries, and may be particularly applicable for blood pressure monitoring applications. The PWM, pulse modulation, synchronization, and / or TDM can be controlled by a processor, such as processor 101.IV. Second Derivative Spectroscopy

[0137] In another embodiment, a third wavelength is introduced to enable a second derivative approach. While motion noise may limit the application of second derivative spectroscopy in smartwatches during active use, this method is viable for devices with more stable contact dynamics, such as smart rings or headphones, or for smartwatch applications during rest periods.

[0138] With reference to FIG. 12, the embodiment of the disclosure also provides a chip 70 that may correspond to a device 100 in the embodiments of the disclosure. The chip 70 may implement a corresponding process realized by the device in various methods of the embodiments of the disclosure. The chip 70 includes a processor 71, and the processor 71 may call and run a computer program from memory to implement the methods in the embodiments of the present application.

[0139] Optionally, the chip 70 may also include a memory 72. In particular, the processor 71 may call and run the computer program from the memory 72 to implement the methods in the embodiments of the present application.

[0140] Moreover, the memory 72 may be a separate device from the processor 71 or may be integrated into the processor 71.

[0141] Optionally, the chip 70 may include an input interface 73. Note that the processor 71 may control the input interface 73 to communicate with other devices or chips, specifically, to obtain messages or data sent by other devices or chips.

[0142] Optionally, the chip 70 may further include an output interface 74. Note that the processor 71 may control the output interface 74 to communicate with other devices or chips, specifically, to output messages or data to other devices or chips.

[0143] With reference to FIG. 13, an embodiment of the disclosure also provides a system 200 for photoplethysmography (PPG) signal processing, comprising: a light source unit 208 configured for light emitting at a first wavelength and a second wavelength towardAtty. Dkt No. 10085-01-0191-PCT a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; a detection unit 212 configured for detecting a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a derivative obtaining unit 213 configured for obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength using a finite difference approximation based on the first absorption signal and the second absorption signal; a signal generating unit 214 configured for generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and a determining unit 215 configured for determining a physiological metric based on the time-sequence PPG signal.

[0144] In one or more embodiments of the disclosure, the non-hemoglobin physiological constituents comprise at least one of: eumelanin, pheomelanin, water, or muscle tissue.

[0145] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

[0146] In one or more embodiments of the disclosure, the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

[0147] In one or more embodiments of the disclosure, at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

[0148] In one or more embodiments of the disclosure, emitting light at the first wavelength and the second wavelength comprises pulse-modulating at least two light emitting diodes (LEDs) to multiplex the signals, and wherein detecting comprises synchronizing the photodetector to demultiplex the first and second absorption signals.

[0149] In one or more embodiments of the disclosure, the light source 108 comprises a broadband light emitting diode (LED) configured to emit a spectrum of light covering both the first wavelength and the second wavelength; the detection unit 112 comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength, enabling simultaneous sampling of the first absorption signal and the second absorption signal.

[0150] In one or more embodiments of the disclosure, the light source 108 comprises: a first light emitting diode (LED) having a first optical filter configured to emit the first wavelength; and a second LED having a second optical filter configured to emit the second wavelength; and the detection unit 112 comprises a broadband photodetector; wherein a processor (e.g., processor 101) is configured to synchronize the detection unit 112 with the light source 108 to differentiate between the first wavelength and the second wavelength via time-division multiplexing.

[0151] In one or more embodiments of the disclosure, emitting light comprises modulating a first light source emitting the first wavelength and a second light source emitting the second wavelength at different time intervals.

[0152] In one or more embodiments of the disclosure, the time-sequence PPG signal is used to estimate at least one physiological parameter selected from heart rate, heart rate variability, and arrhythmia detection.

[0153] In one or more embodiments of the disclosure, the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxy-hemoglobin absorption spectrum to enhance the derivative computation.

[0154] In one or more embodiments of the disclosure, the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

[0155] In one or more embodiments of the disclosure, emitting light further comprises emitting light at a third wavelength;Atty. Dkt No. 10085-01-0191-PCT detecting comprises detecting a third absorption signal; and obtaining derivatives comprises computing a second derivative of absorption with respect to wavelength using finite differences among the first, second, and third absorption signals.

[0156] In one or more embodiments of the disclosure, the first and second wavelengths are selected from alternative absorption peaks of hemoglobin, including peaks between 400 nm and 500 nm for shallower tissue penetration or peaks above 550 nm for deeper penetration.

[0157] In one or more embodiments of the disclosure, the tissue site is selected from the group consisting of a wrist, finger, ear, forehead, and arm.

[0158] In one or more embodiments of the disclosure, the method is implemented in a wearable device selected from a watch, ring, phone, headphone, or telemedicine device.

[0159] In one or more embodiments of the disclosure, obtaining the derivatives reduces signal noise attributed to eumelanin or pheomelanin by utilizing a difference in absorption slope between hemoglobin and said eumelanin or pheomelanin.

[0160] FIG. 14 is a block diagram of an example system 700 for wireless communication according to an embodiment of the present disclosure. Embodiments described herein may be implemented into the system using any suitably configured hardware and / or software. FIG. 14 illustrates the system 700 including a radio frequency (RF) circuitry 710, a baseband circuitry 720, a processing unit 730, a memory / storage 740, a display 750, a camera 760, a sensor 770, and an input / output (I / O) interface 780, coupled with each other as illustrated.

[0161] The processing unit 730 may include circuitry, such as, but not limited to, one or more single-core or multi-core processors. The processors may include any combinations of general-purpose processors and dedicated processors, such as graphics processors and application processors. The processors may be coupled with the memory / storage and configured to execute instructions stored in the memory / storage to enable various applications and / or operating systems running on the system.

[0162] The baseband circuitry 720 may include circuitry, such as, but not limited to, one or more singlecore or multi-core processors. The processors may include a baseband processor. The baseband circuitry may handle various radio control functions that enable communication with one or more radio networks via the RF circuitry. The radio control functions may include, but are not limited to, signal modulation, encoding, decoding, radio frequency shifting, etc. In some embodiments, the baseband circuitry may provide for communication compatible with one or more radio technologies. For example, in some embodiments, the baseband circuitry may support communication with 5G NR, LTE, an evolved universal terrestrial radio access network (EUTRAN) and / or other wireless metropolitan area networks (WMAN), a wireless local area network (WLAN), a wireless personal area network (WPAN). Embodiments in which the baseband circuitry is configured to support radio communications of more than one wireless protocol may be referred to as multi-mode baseband circuitry. In various embodiments, the baseband circuitry 720 may include circuitry to operate with signals that are not strictly considered as being in a baseband frequency. For example, in some embodiments, baseband circuitry may include circuitry to operate with signals having an intermediate frequency, which is between a baseband frequency and a radio frequency.

[0163] The RF circuitry 710 may enable communication with wireless networks using modulated electromagnetic radiation through a non-solid medium. In various embodiments, the RF circuitry may include switches, filters, amplifiers, etc. to facilitate communication with the wireless network. In various embodiments, the RF circuitry 710 may include circuitry to operate with signals that are not strictly considered as being in a radio frequency. For example, in some embodiments, RF circuitry may include circuitry to operate with signals having an intermediate frequency, which is between a baseband frequency and a radio frequency.

[0164] In various embodiments, the transmitter circuitry, control circuitry, or receiver circuitry discussed above with respect to the UE, eNB, or gNB may be embodied in whole or in part in one or more of the RF circuitries, the baseband circuitry, and / or the processing unit. As used herein, “circuitry” may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and / or memory (shared, dedicated, or group) that execute one or more software or firmware programs, a combinational logic circuit, and / or other suitable hardware components that provide the described functionality. In some embodiments, the electronic device circuitry may be implemented in, or functions associated with the circuitry may be implemented by, one or more software or firmware modules. In some embodiments, some or all of the constituent components of the baseband circuitry, the processing unit, and / or the memory / storage may be implemented together on a system on a chip (SOC).

[0165] The memory / storage 740 may be used to load and store data and / or instructions, for example, forAtty. Dkt No. 10085-01-0191-PCT the system. The memory / storage for one embodiment may include any combination of suitable volatile memory, such as dynamic random access memory (DRAM)), and / or non-volatile memory, such as flash memory. In various embodiments, the I / O interface 780 may include one or more user interfaces designed to enable user interaction with the system and / or peripheral component interfaces designed to enable peripheral component interaction with the system. User interfaces may include, but are not limited to a physical keyboard or keypad, a touchpad, a speaker, a microphone, etc. Peripheral component interfaces may include, but are not limited to, a non-volatile memory port, a universal serial bus (USB) port, an audio jack, and a power supply interface.

[0166] In various embodiments, the sensor 770 may include one or more sensing devices to determine environmental conditions and / or location information related to the system. In some embodiments, the sensors may include, but are not limited to, a gyro sensor, an accelerometer, a proximity sensor, an ambient light sensor, and a positioning unit. The positioning unit may also be part of, or interact with, the baseband circuitry and / or RF circuitry to communicate with components of a positioning network, e.g., a global positioning system (GPS) satellite. In various embodiments, the display 750 may include a display, such as a liquid crystal display and a touch screen display. In various embodiments, the system 700 may be a mobile computing device such as, but not limited to, a laptop computing device, a tablet computing device, a netbook, an Ultrabook, a smartphone, etc. In various embodiments, the system may have more or less components, and / or different architectures. Where appropriate, the methods described herein may be implemented as a computer program. The computer program may be stored on a storage medium, such as a non-transitory storage medium.

[0167] The embodiment of the present disclosure is a combination of techniques / processes that may be adopted in 3GPP specification to create an end product.

[0168] A person having ordinary skill in the art understands that each of the units, algorithm, and steps described and disclosed in the embodiments of the present disclosure are realized using electronic hardware or combinations of software for computers and electronic hardware. Whether the functions run in hardware or software depends on the condition of the application and design requirement for a technical plan. A person who has ordinary skill in the art may use different ways to realize the function for each specific application while such realizations should not go beyond the scope of the present disclosure . It is understood by a person having ordinary skill in the art that he / she may refer to the working processes of the system, device, and unit in the above-mentioned embodiment since the working processes of the above-mentioned system, device, and unit are basically the same. For easy description and simplicity, these working processes will not be detailed.

[0169] It is to be understood that the systems, devices, and methods disclosed in the embodiments of the present disclosure may be implemented in alternative configurations. The embodiments described are illustrative and not restrictive. The division of functional units is based on logical functions, and alternative divisions may be employed in practice. Multiple units or components may be combined or integrated into another system, or certain features may be omitted or not implemented. Additionally, the described couplings, whether direct, indirect, or communicative, may be achieved through various interfaces, devices, or units using electrical, mechanical, or other forms of connection.

[0170] The functional units described herein may or may not be physically separate. Displayed units may or may not constitute physical entities and may be located in a single location or distributed across multiple network entities. Some or all of the units may be selected based on the objectives of specific embodiments. Furthermore, each functional unit in the embodiments may be integrated into a single processing unit, exist as physically distinct units, or be integrated with other units into a single processing unit.

[0171] If the software function unit is realized and used and sold as a product, it may be stored in a readable storage medium in a computer. Based on this understanding, the technical plan proposed by the present disclosure may be essentially or partially realized in the form of a software product. Or, one part of the technical plan beneficial to conventional technology may be realized as the form of a software product. The software product in the computer is stored in a storage medium, including a plurality of commands for a computational device (such as a personal computer, a server, or a network device) to run all or some of the steps disclosed by the embodiments of the present disclosure. The storage medium includes a USB disk, a mobile hard disk, a read-only memory (ROM), a random-access memory (RAM), a floppy disk, or other kinds of media capable of storing program codes.

[0172] While the present disclosure has been described in connection with what is considered the most practical and preferred embodiments, it is understood that the present disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements made without departing from the scope of the broadest interpretation of the appended claims.

Claims

Atty. Dkt No. 10085-01-0191-PCTWhat is claimed is:

1. A method for enhancing specificity of a photoplethysmography (PPG) signal to hemoglobin molecules, the method comprising: emitting, from a light source unit, light including a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; detecting, using at least one photodetector, a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

2. The method of claim 1, comprising: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; generating a time-sequence PPG signal based on the derivatives over the plurality of time points; and determining the physiological metric based on the time-sequence PPG signal.

3. The method of claim 2, wherein obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

4. The method of claim 1, wherein the non-hemoglobin physiological constituents comprise at least one of: eumelanin, pheomelanin, water, or muscle tissue.

5. The method of claim 1, wherein the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

6. The method of claim 1, wherein the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

7. The method of claim 1, wherein at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

8. The method of claim 1, wherein emitting light at the first wavelength and the second wavelength comprises pulse-modulating at least two light emitting diodes (LEDs) to multiplex the first and second absorption signals, and wherein detecting comprises synchronizing the photodetector to demultiplex the first and second absorption signals.

9. The method of claim 1, wherein the light source unit comprises a broadband light emitting diode (LED) configured to emit a spectrum of light covering both the first wavelength and the second wavelength; the at least one photodetector comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength, enabling simultaneous sampling of the first absorption signal and the second absorption signal.

10. The method of claim 1, wherein the light source unit comprises: a first light emitting diode (LED) having a first optical filter configured to emit the first wavelength; and a second LED having a second optical filter configured to emit the second wavelength; and the at least one photodetector comprises a broadband photodetector; wherein a processor is configured to synchronize the at least one photodetector with the light source unit to differentiate between the first wavelength and the second wavelength via time-division multiplexing.

11. The method of claim 1, wherein emitting light comprises modulating a first light source emitting the first wavelength and a second light source emitting the second wavelength at different time intervals.

12. The method of claim 1, wherein the time-sequence PPG signal is used to estimate at least one physiological parameter selected from heart rate, heart rate variability, and arrhythmia detection.

13. The method of claim 1, wherein the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxyhemoglobin absorption spectrum to enhance the derivative computation.

14. The method of claim 1, wherein the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.Atty. Dkt No. 10085-01-0191-PCT15. The method of claim 1, wherein emitting light further comprises emitting light at a third wavelength; detecting comprises detecting a third absorption signal; and obtaining derivatives comprises computing a second derivative of absorption with respect to wavelength using finite differences among the first, second, and third absorption signals.

16. The method of claim 1, wherein the first and second wavelengths are selected from alternative absorption peaks of hemoglobin, including peaks between 400 nm and 500 nm for shallower tissue penetration or peaks above 550 nm for deeper penetration.

17. The method of claim 1, wherein the tissue site is selected from the group consisting of a wrist, finger, ear, forehead, and arm.

18. The method of claim 1 , wherein the method is implemented in a wearable device selected from a watch, ring, phone, headphone, or telemedicine device.

19. The method of claim 2, wherein obtaining the derivatives reduces signal noise attributed to eumelanin or pheomelanin by utilizing a difference in absorption slope between hemoglobin and said eumelanin or pheomelanin.

20. A terminal device comprising: a processor and a memory; wherein the memory is configured to store one or more computer programs; and the processor configured to call and run the one or more computer programs stored in the memory to cause the terminal device to perform the method of any one of claims 1 to 19.

21. A chip, comprising: a processor and a memory; wherein the memory is configured to store one or more computer programs; the processor configured to call and run the one or more computer programs stored in a memory to cause a device equipped with the chip to perform the method of any one of claims 1 to 19.

22. A non-transitory computer-readable storage medium, in which a computer program is stored, wherein the computer program causes a computer to perform the method of any one of claims 1 to 19.

23. A computer program product, comprising a computer program, wherein the computer program causes a computer to perform the method of any one of claims 1 to 19.

24. A computer program, wherein the computer program causes a computer to perform the method of any one of claims 1 to 19.

25. A device for photoplethysmography (PPG) signal processing, comprising: a light source configured for emitting light covering a first wavelength and a second wavelength toward a tissue site, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of non-hemoglobin physiological constituents; a detection unit configured for detecting a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory configured for storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

26. The device of claim 25, wherein generating the PPG signal comprises: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; and generating a time-sequence PPG signal based on the derivatives over the plurality of time points; determining the physiological metric based on the PPG signal comprises: determining the physiological metric based on the time-sequence PPG signal27. The device of claim 26, wherein obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

28. The device of claim 25, wherein obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

29. The device of claim 25, wherein the non-hemoglobin physiological constituents comprise at least oneAtty. Dkt No. 10085-01-0191-PCT of: eumelanin, pheomelanin, water, or muscle tissue.

30. The device of claim 25, wherein the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

31. The device of claim 25, wherein the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

32. The device of claim 25, wherein at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

33. The device of claim 25, wherein emitting light at the first wavelength and the second wavelength comprises pulse-modulating at least two light emitting diodes (LEDs) to multiplex the first and second absorption signals, and wherein detecting comprises synchronizing the photodetector to demultiplex the first and second absorption signals.

34. The device of claim 25, wherein the light source comprises a broadband light emitting diode (LED) configured to emit a spectrum of light covering both the first wavelength and the second wavelength; the detection unit comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength, enabling simultaneous sampling of the first absorption signal and the second absorption signal.

35. The device of claim 25, wherein the light source comprises: a first light emitting diode (LED) having a first optical filter configured to emit the first wavelength; and a second LED having a second optical filter configured to emit the second wavelength; and the detection unit comprises a broadband photodetector; wherein the detection unit is synchronized with the light source to differentiate between the first wavelength and the second wavelength via time-division multiplexing.

36. The device of claim 25, wherein emitting light comprises modulating a first light source emitting the first wavelength and a second light source emitting the second wavelength at different time intervals.

37. The device of claim 25, wherein the time-sequence PPG signal is used to estimate at least one physiological parameter selected from heart rate, heart rate variability, and arrhythmia detection.

38. The device of claim 25, wherein the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxyhemoglobin absorption spectrum to enhance the derivative computation.

39. The device of claim 25, wherein the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

40. The device of claim 25, wherein emitting light further comprises emitting light at a third wavelength; detecting comprises detecting a third absorption signal; and obtaining derivatives comprises computing a second derivative of absorption with respect to wavelength using finite differences among the first, second, and third absorption signals.

41. The device of claim 25, wherein the first and second wavelengths are selected from alternative absorption peaks of hemoglobin, including peaks between 400 nm and 500 nm for shallower tissue penetration or peaks above 550 nm for deeper penetration.

42. The device of claim 25, wherein the tissue site is selected from the group consisting of a wrist, finger, ear, forehead, and arm.

43. The device of claim 25, wherein the method is implemented in a wearable device selected from a watch, ring, phone, headphone, or telemedicine device.

44. The device of claim 26, wherein obtaining the derivatives reduces signal noise attributed to eumelanin or pheomelanin by utilizing a difference in absorption slope between hemoglobin and said eumelanin or pheomelanin.

45. The device of claim 25, wherein the light source is pulse width modulated (PWM) to decrease total energy over time while maintaining intensity.

46. A device for photoplethysmography (PPG) signal processing, comprising: a light source configured for emitting light through a first filter that allows light passing at a first wavelength toward a tissue site and through a second filter that allows light passing at a second wavelength toward the tissue site, wherein the light source comprises at least one of a first light source and a second light source; and signals of light through the first filter and signals of light through the second filter are pulse-modulated to be multiplexed;Atty. Dkt No. 10085-01-0191-PCT one or more detection units configured for being operated with the light source to demultiplex the signals of light through the first filter and the signals of light through the second filter, and detect a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength; a memory storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

47. The device of claim 46, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of nonhemoglobin physiological constituents.

48. The device of claim 46, wherein generating the PPG signal comprises: obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; and generating a time-sequence PPG signal based on the derivatives over the plurality of time points; determining the physiological metric based on the PPG signal comprises: determining the physiological metric based on the time-sequence PPG signal.

49. The device of claim 48, wherein obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

50. The device of claim 46, wherein the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

51. The device of claim 46, wherein the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

52. The device of claim 46, wherein at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

53. The device of claim 46, wherein the hemoglobin absorption spectrum comprises oxy -hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxyhemoglobin absorption spectrum to enhance the derivative computation.

54. The device of claim 46, wherein the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

55. The device of claim 46, wherein the light source is pulse width modulated (PWM) to decrease total energy over time while maintaining intensity.

56. A device for photoplethysmography (PPG) signal processing, comprising: a broadband light source configured for emitting a spectrum of light covering both a first wavelength and a second wavelength; one or more detection units configured for sampling of a first absorption signal corresponding to the first wavelength and a second absorption signal corresponding to the second wavelength, wherein the one or more detection units comprises a first photodetector with a first optical filter configured to pass the first wavelength and a second photodetector with a second optical filter configured to pass the second wavelength; a memory storing instructions; and a processor communicatively coupled to the detection unit and the memory, wherein the processor executes the instructions to perform: generating a PPG signal based on the first absorption signal and the second absorption signal; and determining a physiological metric based on the PPG signal.

57. The device of claim 56, wherein the first wavelength and the second wavelength are selected to approximate a region of maximum slope in a hemoglobin absorption spectrum, slopes in the hemoglobin absorption spectrum in the region are greater than slopes in non-hemoglobin absorption spectrum of nonhemoglobin physiological constituents.

58. The device of claim 56, wherein generating the PPG signal comprises:Atty. Dkt No. 10085-01-0191-PCT obtaining derivatives over a plurality of time points by computing, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal; and generating a time-sequence PPG signal based on the derivatives over the plurality of time points; determining the physiological metric based on the PPG signal comprises: determining the physiological metric based on the time-sequence PPG signal.

59. The device of claim 58, wherein obtaining derivatives over the plurality of time points by comprises: using finite difference approximation to compute, for each of the plurality of time points, a derivative of absorption with respect to wavelength based on the first absorption signal and the second absorption signal.

60. The device of claim 56, wherein the first wavelength and the second wavelength are within a range of 520 nanometer (nm) to 550 nm, centered around approximately 544 nm to maximize a derivative approximation corresponding to a peak slope in oxy-hemoglobin absorption spectrum.

61. The device of claim 56, wherein the first wavelength is approximately 538 nm and the second wavelength is approximately 550 nm.

62. The device of claim 56, wherein at least one optical filter applied to narrow a bandwidth of the emitted light to 10 nm or less for at least one of the first wavelength or the second wavelength.

63. The device of claim 56, wherein the hemoglobin absorption spectrum comprises oxy-hemoglobin absorption spectrum, and wherein the first wavelength and the second wavelength further account for complementary slopes in a deoxy-hemoglobin absorption spectrum with respect to slopes in the oxyhemoglobin absorption spectrum to enhance the derivative computation.

64. The device of claim 56, wherein the first wavelength and the second wavelength are selected such that a slope of an absorption coefficient of hemoglobin between the first and second wavelengths is opposite in direction to a slope of an absorption coefficient of skin pigment between the first and second wavelengths.

65. The device of claim 56, wherein the light source is pulse width modulated (PWM) to decrease total energy over time while maintaining intensity.

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