PATENT DOCUMENT

Publication Number: US-10687718-B2
Application Number: US-201715592016-A
Country: US
Kind Code: B2

Title: Systems and methods for non-pulsatile blood volume measurements

Abstract:
This relates to systems and methods for determining one or more of a user&#39;s physiological signals. The one or more of the user&#39;s physiological signals can be determined by measuring pulsatile blood volume changes. Motion artifacts included in the signals can be canceled or reduced by measuring non-pulsatile blood volume changes and adjusting the signal to account for the non-pulsatile blood information. Non-pulsatile blood volume changes can be measured using at least one set of light emitter-light sensor. The light emitter can be located in close proximity (e.g., less than or equal to 1 mm away) to the light sensor, thereby limiting light emitted by the light emitter to blood volume without interacting with one or more blood vessels and/or arterioles. In some examples, the systems can further include an accelerometer configured to measure the user&#39;s acceleration, and the acceleration signal can be additionally be used for compensating for motion artifacts.

Claims:
What is claimed is: 
     
       1. A method for determining a physiological signal, the method comprising:
 emitting a first light at a user; 
 detecting a first reflection of the first light; 
 generating a first signal indicative of the detected first reflection of the first light, the first signal including non-pulsatile blood information; 
 detecting an acceleration of the user; 
 generating a second signal indicative of the acceleration; 
 comparing the second signal to a threshold value; 
 in response to the second signal being greater than or equal to the threshold value, emitting a second light at the user; 
 detecting a second reflection of the second light; 
 generating a third signal indicative of the detected second reflection of the second light, the third signal including pulsatile blood information; 
 adjusting the third signal to compensate for information included in the first signal; and 
 determining the physiological signal based on the adjusted third signal. 
 
     
     
       2. The method of  claim 1 , wherein the first light is emitted at a first portion of the user and the second light is emitted at a second portion, different from the first portion, of the user. 
     
     
       3. The method of  claim 1 , further comprising:
 determining one or more peaks included in the first and third signals; and 
 determining one or more locations of the one or more peaks, wherein adjusting the third signal includes scaling the third signal at the one or more locations. 
 
     
     
       4. The method of  claim 1 ,
 wherein adjusting the third signal further includes using information included in the second signal. 
 
     
     
       5. An electronic device comprising:
 a housing at least partially defining a first cavity and a second cavity separate from the first cavity; 
 a first light emitter positioned in the first cavity and configured to generate a first light; 
 a first light sensor positioned in the first cavity and configured to:
 detect a first reflection of the first light; and 
 generate a first signal indicative of the first reflection of the first light, the first signal including non-pulsatile blood information; 
 
 a second light emitter positioned in the first cavity and configured to generate a second light; 
 a second light sensor positioned in the second cavity and configured to:
 detect a second reflection of the second light; and 
 generate a second signal indicative of the second reflection of the second light, the second signal including pulsatile blood information; and 
 
 a controller coupled to the first light sensor and the second light sensor, the controller configured to:
 receive the first signal and the second signal; and 
 determine at least a portion of a physiological signal using the first signal and the second signal. 
 
 
     
     
       6. The electronic device of  claim 5 , further comprising an isolation member positioned at least partially within the housing and configured to separate the first cavity and the second cavity. 
     
     
       7. The electronic device of  claim 5 , further comprising an isolation member positioned between the first light emitter and the first light sensor and configured to optically isolate the first light emitter and the first light sensor. 
     
     
       8. The electronic device of  claim 7 , wherein:
 the electronic device further comprises a window optically coupled to the first light emitter; and 
 an end of the isolation member extends to an inner surface of the window. 
 
     
     
       9. The electronic device of  claim 7 , wherein:
 the electronic device further comprises a window optically coupled to the first light emitter; and 
 an end of the isolation member extends to an outer surface of the window. 
 
     
     
       10. The electronic device of  claim 5 , wherein the first light emitter is spaced less than 1 mm from the first light sensor. 
     
     
       11. The electronic device of  claim 5 , wherein:
 the electronic device further comprises one or more third light emitters configured to generate a third light; 
 at least one of the first light sensor or the second light sensor is, further configured to:
 detect a third reflection of the third light; and 
 generate a third signal indicative of the third reflection of the third light; and 
 
 the controller is further configured to receive the third signal and include the third signal in the determination of the physiological signal. 
 
     
     
       12. The electronic device of  claim 11 , wherein the first light includes light with a wavelength between 570-750 nm, the second light includes light with a wavelength between 495-570 nm, and the third light includes light with a wavelength between 750-1400 nm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/334,363, filed May 10, 2016, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     This relates generally to architectures for PPG systems, and, more particularly, to PPG systems capable of generating signals including little-to-no pulsatile blood information and capable of measuring non-pulsatile blood volume changes. 
     BACKGROUND 
     A user&#39;s physiological signals (e.g., pulse rate or arterial oxygen saturation) can be determined by photoplethysmogram (PPG) systems. In a basic form, PPG systems can employ one or more light sources that illuminate a user&#39;s tissue and one or more light detectors to receive light that enters and probes a subsurface volume of tissue. The light sources and light detectors can be in contact with the tissue or can be remote (i.e., not in contact) to the tissue surface. The received light can include light with an amplitude that can be modulated in time as a result of interaction with pulsatile blood flow and parasitic, non-signal light that can indirectly sample pulsatile tissue volumes with an amplitude that can be modulated (i.e., “noise” or “artifacts”) and/or unmodulated (i.e., DC). 
     Although PPG systems measure the pulsatile blood flow to determine a user&#39;s physiological signals, these measurements may be corrupted by noise introduced by, for example, the user&#39;s motion, motions from within the user&#39;s body (e.g., tendon motion and/or muscle motions that can affect venous blood volume information), tilt and/or pull of the device, ambient light variations, or any combination thereof. While some PPG systems can utilize accelerometer measurements to correct for such noise, accelerometer measurements can be limited to the gross, periodic motion. Given that a user&#39;s motion may not be limited to gross, periodic motion, a PPG system capable of differentiating pulsatile blood volume changes from anatomical motion can be desired. In some examples, anatomical motion can be measured by measuring non-pulsatile blood volume changes. 
     SUMMARY 
     This relates to systems and methods for determining one or more of a user&#39;s physiological signals. The one or more of the user&#39;s physiological signals can be determined by measuring pulsatile blood volume changes. Motion artifacts included in the signals can be canceled or reduced by measuring non-pulsatile blood volume changes and adjusting the signal to account for the non-pulsatile blood information. Non-pulsatile blood volume changes can be measured using at least one set of light emitter-light sensor. The light emitter can be located in close proximity (e.g., less than or equal to 1 mm away) to the light sensor and/or emitting light at specific wavelengths (e.g., greater than 600 nm), thereby limiting light emitted by the light emitter to interaction to venous blood (non-pulsatile blood) volume changes. In some examples, the systems can further include an accelerometer configured to measure the user&#39;s acceleration, and the acceleration signal can be additionally be used for compensating of motion artifacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  illustrate systems in which examples of the disclosure can be implemented 
         FIG. 2A  illustrates a top view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 2B  illustrates a cross-sectional view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 3A  illustrates a top view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 3B  illustrates a cross-sectional view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 3C  illustrates exemplary circuitry coupled to the light sensors and light emitters and utilized for estimation of the user&#39;s physiological signals according to examples of the disclosure. 
         FIG. 3D  illustrates an exemplary process flow for estimating the user&#39;s physiological signals according to example of the disclosure. 
         FIG. 4A  illustrates a top view of an exemplary electronic device including a dedicated sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 4B  illustrates a cross-sectional view of an exemplary electronic device including a dedicated light sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIGS. 4C-4D  illustrate cross-sectional views of exemplary electronic devices including a light sensor optically coupled to a light emitter in the same cavity, but divided by an isolation according to examples of the disclosure. 
         FIG. 5A  illustrates a top view of an exemplary electronic device including at least one separate light sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 5B  illustrates a cross-sectional view of an exemplary electronic device including at least one separate light sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 5C  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a light emitter in the same cavity, but divided by an isolation according to examples of the disclosure. 
         FIG. 6A  illustrates a top view of an exemplary electronic device including a light sensor optically coupled to a common light emitter used for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 6B  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a common light emitter used for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 6C  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a common light emitter used for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. 
         FIG. 6D  illustrates a cross-sectional view of an exemplary electronic device including angled isolation according to examples of the disclosure. 
         FIG. 7A  illustrates a top view of an exemplary electronic device including at least two different cavities, each cavity can include at least one light sensor and a plurality of light emitters according to examples of the disclosure. 
         FIG. 7B  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity can include at least one light sensor and a plurality of light emitters according to examples of the disclosure. 
         FIG. 7C  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters divided by an isolation according to examples of the disclosure. 
         FIG. 8A  illustrates a top view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters according to examples of the disclosure. 
         FIG. 8B  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters according to examples of the disclosure. 
         FIG. 8C  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters divided by an isolation according to examples of the disclosure. 
         FIGS. 8D-8F  illustrate top views of exemplary configurations for light emitters, light sensors, and isolation for electronic devices according to examples of the disclosure. 
         FIG. 9A  illustrates exemplary oxy-hemoglobin and deoxy-hemoglobin absorption signals measured across a plurality of wavelengths according to examples of the disclosure. 
         FIG. 9B  illustrates exemplary signals measured at the plurality of light sensors included in an exemplary electronic device according to examples of the disclosure. 
         FIG. 10A  illustrates an exemplary circuit diagram for motion artifact removal according to examples of the disclosure. 
         FIG. 10B  illustrates an exemplary process for motion artifact removal according to examples of the disclosure. 
         FIG. 11A  illustrates an exemplary circuit diagram for motion artifact removal according to examples of the disclosure. 
         FIG. 11B  illustrates an exemplary process for motion artifact removal according to examples of the disclosure. 
         FIGS. 12A-12C  illustrate exemplary measurement modes according to examples of the disclosure. 
         FIG. 13A  illustrates an exemplary process illustrating time-based operation of a PPG system according to examples of the disclosure. 
         FIG. 13B  illustrates an exemplary process illustrating operation of a PPG system including a light emitter-light sensor set for motion detection according to examples of the disclosure. 
         FIG. 13C  illustrates an exemplary process illustrating operation of a PPG system including an accelerometer for motion detection according to examples of the disclosure. 
         FIG. 13D  illustrates an exemplary process illustrating operation of a PPG system according to examples of the disclosure. 
         FIG. 14  illustrates an exemplary block diagram of a computing system comprising light emitters and light sensors for measuring a signal associated with a user&#39;s physiological state according to examples of the disclosure. 
         FIG. 15  illustrates an exemplary configuration in which an electronic device is connected to a host according to examples of the disclosure. 
         FIG. 16A  illustrates a perspective view of an underside or back surface of a wearable device according to examples of the disclosure. 
         FIG. 16B  illustrates a schematic side view of the back surface of  FIG. 16A  in contact with the skin surface of an individual. 
         FIG. 17A  illustrates a perspective view of another variation of an underside or back surface of a wearable device according to examples of the disclosure. 
         FIG. 17B  illustrates a schematic side view of the back surface of  FIG. 17A  in contact with the skin surface of an individual. 
         FIGS. 18A and 18B  illustrate perspective views of other variations of an underside or back surface of a wearable device according to examples of the disclosure. 
         FIG. 18C  illustrates a schematic side view of the back surface of  FIG. 18A  in contact with the skin surface of an individual. 
         FIG. 19A  illustrates a perspective view of another variation of an underside or back surface of a wearable device according to examples of the disclosure. 
         FIG. 19B  illustrates a schematic side view of the back surface of  FIG. 19A  in contact with the skin surface of an individual. 
         FIG. 20  illustrates a cross-sectional view of one variation of a protrusion. 
         FIG. 21A  illustrates a cross-sectional view of one variation of a device comprising a protrusion and a Fresnel lens. 
         FIG. 21B  illustrates a cross-sectional view of another variation of a device comprising a protrusion and a Fresnel lens. 
         FIG. 22A  illustrates a cross-sectional view of one variation of a protrusion comprising an isolation or optical barrier. 
         FIG. 22B  illustrates a cross-sectional view of one variation of a device comprising an isolation or an optical barrier, and a Fresnel lens disposed between the protrusion and the light emitter and light sensor. 
         FIG. 22C  illustrates a cross-sectional view taken across line  22 C- 22 C of one variation of a wearable device (e.g., the device depicted in  FIG. 22D ) comprising an isolation or an optical barrier and a Fresnel lens disposed between the protrusion and a plurality of light emitters and light sensor. 
         FIG. 22D  illustrates a top view of the underside of a wearable device comprising one variation of a Fresnel lens. 
         FIG. 22E  illustrates a top view of the underside of a device comprising another variation of a Fresnel lens. 
         FIGS. 23A-23C  illustrate cross-sectional views of exemplary configurations of light emitters for co-localizing the noise reference and PPG channels according to examples of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples. 
     This relates to systems and methods for determining one or more of a user&#39;s physiological signals. The one or more of the user&#39;s physiological signals can be determined by measuring pulsatile blood volume changes. Motion artifacts included in the signals can be canceled or reduced by measuring non-pulsatile blood volume changes and adjusting the signal to account for the non-pulsatile blood information. Deeper tissue of a user can be more susceptible to motion artifacts due to, for example, muscle movement, tendon movement, non-pulsatile blood movement, or a combination thereof. The effect of the motion artifacts can be less pronounced in the superficial layers of the user due to absence of the muscles and tendons. In some examples, non-pulsatile blood volume changes can be measured using at least one set of light emitter-light sensor. The light emitter can be located in close proximity (e.g., less than or equal to 1 mm away) to the light sensor to limit the depth within the tissue that is measured. Light can be emitted at one or more wavelengths (e.g., greater than 600 nm) less sensitive to oxy-hemoglobin, which can reduce the interaction of light to venous blood volume changes. In some examples, the systems can further include an accelerometer configured to measure the user&#39;s acceleration, and the acceleration signal can be additionally be used for compensating of motion artifacts. 
     Representative applications of methods and apparatus according to examples of the present disclosure are described in this section. These examples are being provided solely to add context and aid in the understanding of the described examples. It will thus be apparent to one skilled in the art that the described examples may be practiced without some or all of the specific details. In other instances, well-known process steps have been described in detail in order to avoid unnecessarily obscuring the described examples. Other applications are possible, such that the following examples should not be taken as limiting. 
       FIGS. 1A-1C  illustrate systems in which examples of the disclosure can be implemented.  FIG. 1A  illustrates an exemplary mobile telephone  136  that can include a touch screen  124 .  FIG. 1B  illustrates an exemplary media player  140  that can include a touch screen  126 .  FIG. 1C  illustrates an exemplary wearable device  144  that can include a touch screen  128  and can be attached to a user using a strap  146 . The systems of  FIGS. 1A-1C  can utilize the reconfigurable apertures and methods for detecting a PPG signal as will be disclosed. 
       FIG. 2A  illustrates a top view and  FIG. 2B  illustrates a cross-sectional view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. The top view in  FIG. 2A  can be viewed as the underside of wearable device  144  of  FIG. 1C , for example. Device  200  can include light sensor  204 , light sensor  214 , light emitter  206 , and light emitter  216 . Light sensor  204  can be optically coupled to light emitter  206 . Light sensor  214  can be optically coupled to light emitter  216 . Device  200  can be situated such that light sensor  204 , light sensor  214 , light emitter  206 , and light emitter  208  are proximate to a skin  220  of a user. For example, device  200  can be held in a user&#39;s hand or strapped to a user&#39;s wrist, among other possibilities. 
     Light emitter  206  can be configured to emit light (e.g., light  222 ). A portion of the one or more light paths can be absorbed by one or more blood vessels  242 , and a portion of the one or more light paths can reflect back to be detected by a light sensor. For example, as illustrated in  FIG. 2B , a portion of light  222  (emitted from light emitter  206 ) can be absorbed by blood vessel  242 , and a portion of light (e.g., light  223 ) can reflect back for detection by light sensor  204 . Light emitter  206  can also be configured to emit light, and a portion of the light can reflect back for detection by light sensor  214 . Similarly, light emitter  216  can be configured to emit light towards light sensor  204  and light sensor  214 . 
     Signal  250  can include the measured total signal (i.e., sum of the measured modulated light and unmodulated light) detected by the light sensor (e.g., light sensor  204 ). In some examples, the device or system can include an accelerometer  202 . Accelerometer  202  can be any type of sensor capable of measuring acceleration. Signal  255  can include the measured acceleration signal detected by accelerometer  202 . Device  200  can include a processor or controller  209  configured to determine the user&#39;s physiological signal from signal  250  and signal  255 . The user&#39;s physiological signal can be determined using any number of algorithms or simple mathematical functions including, but not limited to, subtracting, multiplying, and/or scaling. 
     In some examples, the capabilities of the accelerometer included in the PPG system may be limited to measuring gross motion (e.g., the user waving his or her hand) and may not be capable of measuring anatomical motion (e.g., the user clenching his or her fist). In some examples, the capabilities of the accelerometer can be limited to periodic motion artifacts. As a result, signal  250  can include distortion from the anatomical motion, and the system may erroneously include the distortions in its determination of the user&#39;s physiological signal (due to the inability to distinguish anatomical motion). Examples of anatomical motion can include surface motion or motion induced by blood re-distribution (e.g., increases or decreases in venous blood caused by user motion). In some examples, the system can be capable of measuring solely non-pulsatile blood volume changes—where a system relying entirely on an accelerometer for noise correction may not be capable of measuring non-pulsatile blood volume changes. In some examples, the system can be capable of measuring non-periodic motion artifacts. By measuring the modulation of the optical signal from non-pulsatile blood volume changes, motion artifacts can be accurately determined. 
       FIG. 3A  illustrates a top view and  FIG. 3B  illustrates a cross-sectional view of an exemplary electronic device including light sensors and light emitters for measuring a user&#39;s physiological signal according to examples of the disclosure. Device  300  can include light emitter  306 , light emitter  308 , light emitter  316 , light emitter  318 , light sensor  304 , and light sensor  314 . Each light emitter can be optically coupled to each light sensor. For example, light emitter  306  can be optically coupled to both light sensor  304  and light sensor  314 . Similarly, light emitter  316  can be optically coupled to both light sensor  304  and light sensor  314 . Device  300  can be situated such that light sensor  304 , light sensor  314 , light emitter  306 , light emitter  308 , light emitter  316 , and light emitter  318  are proximate to a skin  320  of a user. For example, device  300  can be held in the user&#39;s hand or strapped to the user&#39;s wrist, among other possibilities. 
     Light emitter  306  can be configured to emit light and generate one or more light paths detected by light sensor  304  and one or more light paths detected by light sensor  314 . Light emitter  308  can also be configured to emit light and generate one or more light paths detected by light sensor  304  and one or more light paths detected by light sensor  314 . Light emitter  316  can be configured to emit light and generate one or more light paths detected by light sensor  304  and one or more light paths detected by light sensor  314 . Light emitter  318  can also be configured to emit light and generate one or more light paths detected by light sensor  304  and one or more light paths detected by light sensor  314 . 
     Device  300  can include a controller  309  configured to utilize the signal(s) from one or more lights paths to correct the signal(s) from one or more other lights paths to determine the user&#39;s physiological signal. The correction can be performed to cancel out any noise due to, for example, the user&#39;s motion, motions from within the user&#39;s body (e.g., tendon motion and/or muscle motion), tilt and/or pull of the device, ambient light variations, or any combination thereof. Device  300  can be in close proximity to skin  320  of a user and configured such that light emitter  306 , light emitter  308 , light emitter  316 , and light emitter  318  can emit light towards skin  320 . A plurality of blood vessels can be located in skin  320 . For example, as illustrated in the  FIG. 3B , one or more blood vessels  342  can be located in one or more deeper layers, such as layer  346  (e.g., the subcutaneous tissue), in skin  320 , and one or more arterioles  334  can be located in one or more shallower layers, such as layer  345  (e.g., the dermis tissue), in skin  320 . 
     In some examples, light emitter  306  and light sensor  304  can be located such that light path  322  emitted by light emitter  306  can reach layer  346  (e.g., a layer including the subcutaneous tissue), which can be located deeper within skin  320  than layer  345  (e.g., a layer including dermis tissue). A portion of light  322  can be absorbed by one or more arterioles  334  and/or one or more blood vessels  342  located in layer  345  and layer  346 , and a portion of light (i.e., light  323 ) can reflect back for detection by light sensor  304 . Light sensor  304  can generate signal  350 , which can be measured by controller  309 . Light emitter  308  and light sensor  304  can be located such that light  324  emitted by light emitter  308  can be sensitive to arterial blood volume changes. A portion of light  324  can be absorbed by one or more arterioles  334  in layer  345 , and a portion of light (i.e., light  325 ) can reflect back for detection by light sensor  304 . Light sensor  304  can generate signal  355 , which can be measured by controller  309 . 
     Signal  350  can include measured total signal (i.e., sum of the measured modulated light and unmodulated light) representative of light  323  detected by light sensor  304 . Signal  355  can be the measured signal representative of light  325  detected by light sensor  304 . In some examples, the user&#39;s motion (and/or motions from within the user&#39;s body (e.g., tendon motion and/or muscle motion)), can distort light  323  and light  325 , which can change both signal  350  and signal  355 . Since light  324  can be sensitive to arterial blood volume changes, signal  355  can include both pulsatile blood information and motion artifacts (e.g., non-pulsatile blood information from either deep or shallow tissue structures). Controller  309  can utilize an algorithm or simple mathematical functions can be applied to signal  350  and signal  355  to determine the user&#39;s physiological signal (e.g., signal  360  illustrated in  FIG. 3C ). However, given that light  324  can be absorbed by one or more arterioles  334 , a portion of signal  355  may include pulsatile blood information. Thus, signal  355  may not be entirely representative of motion artifacts. 
     In some examples, the signals from one or more sets of light emitter-light sensor can be utilized to perform other functions. For example, light emitter  308  and/or light emitter  316  can be configured for monitoring off-wrist detection. In some examples, light emitter  308  and/or light emitter  316  can be configured to measure the background user&#39;s physiological signal (e.g., heart rate) when the user may not be moving. The system can monitor the user&#39;s motion through an accelerometer to determine whether the user is moving. 
       FIG. 3C  illustrates exemplary circuitry coupled to the light sensors and light emitters and utilized for estimation of the user&#39;s physiological signals according to examples of the disclosure.  FIG. 3D  illustrates an exemplary corresponding process flow according to example of the disclosure. System  300  can include light emitter  306 , light emitter  308 , light emitter  316 , and light emitter  318  configured to emit light (e.g., light  322  and light  324 ) towards the user (step  372  of process  370 ). A portion of the emitted light can reflect back (e.g., light  323  and light  325 ) towards one or more light sensors (e.g., light sensor  304  and/or light sensor  314 ). Light sensor  304  and light sensor  314  can be configured to generate a plurality of signals  350  in response to the detected reflected light (e.g., light  323  or light  325 ) (step  374  of process  370 ). System  300  can include a plurality of filters  310 . Each filter  310  can be configured to receive a plurality of signals  350  from a light sensor and can filter the signals (step  376  of process  370 ). Plurality of filters  310  can be any type of filter capable of selection based on one or more properties, such as a bandpass filter capable of selecting a range of frequencies. In some examples, plurality of filters  310  can be adaptive filters. Each of the plurality of signals  350  generated from the light sensor can represent detected reflected light from different light emitters. For example, filter  310   a  can receive signal  350   a  and signal  350   b . Signal  350   a  and signal  350   b  can be generated from light sensor  304 , where signal  350   a  can represent detected reflected light from light emitter  306 , and signal  350   b  can represent detected reflected light from light emitter  308 . That is, signal  350   a  can represent signal information, and signal  350   b  can represent a noise reference channel. In some examples, for a given filter  310 , the signal from one light emitter can represent the user&#39;s physiological signal and noise, and the signal from the other light emitter can represent noise. For example, light emitter  306  can be configured to emit light in the wavelength range of 495-570 nm, and signal  350   a  can represent the pulsatile blood volume changes of the user. Light emitter  308  can be configured to emit light in the wavelength range of 750-1400 nm, and the reflected light (e.g., light  325 ) can represent noise. 
     Plurality of signals  352  from plurality of filters  310  can be input into controller  309 . System  300  can also include accelerometer  302 . Accelerometer  302  can be configured to generate signal  355  indicative of the user&#39;s acceleration or gross motion (step  378  of process  370 ). Controller  309  can receive plurality of signals  352  from plurality of filters  310  and signal  355  from accelerometer  302  to determine the user&#39;s physiological signal  360  using one or more algorithms or simple mathematical functions (step  380  of process  370 ). 
       FIG. 4A  illustrates a top view and  FIG. 4B  illustrates a cross-sectional view of an exemplary electronic device including a dedicated light sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. Device  400  can include light emitter  405 , light emitter  406 , light emitter  415 , and light emitter  416 . Device  400  can further include light sensor  404 , light sensor  407 , light sensor  414 , and light sensor  417 . Light emitter  406  can be configured to emit light towards light sensor  404  and light sensor  414 . Light emitter  405  can be configured to emit light towards light sensor  407 . Light emitter  416  can be configured to emit light towards light sensor  404  and light sensor  414 . Light emitter  415  can be configured to emit light towards light sensor  414 . In some examples, light emitter  406  and light emitter  416  can be located such that the path lengths to light sensor  404  are different from the path lengths to light sensor  414 . 
     Device  400  can be configured such that one or more light emitters are optically coupled to one or more light sensors, where the one or more light emitters are located in a different cavity than the one or more light sensors. For example, light emitter  406  can be optically coupled to light sensor  404 , where light emitter  406  can be located in cavity  466  and light sensor  404  can be located in cavity  464 . In some examples, each cavity can be associated with a different aperture  401  (where light exits and enters device  400 ) and/or window. Device  400  can also be configured such that one or more light emitters can be optically coupled to one or more light sensors, where the one or more light emitters can be located in the same cavity as the one or more light sensors. For example, light emitter  405  can be optically coupled to light sensor  407 , where both can be located in cavity  466 . In some examples, the cavities included in device  400  can be separated by isolation  419 . 
     In some examples, one or more sets of light emitter-light sensor located in different cavities can be configured to measure pulsatile blood volume changes. In some examples, one or more light emitter-light sensor sets located in the same cavity can be configured to measure non-pulsatile blood volume changes (from shallow tissues structures, deep tissue structures, or both) and/or serve as a noise reference channel. For example, the set comprising light emitter  406  and light sensor  404  can be configured to be sensitive to pulsatile blood volume changes. Light emitter  406  can emit light  422 . Light  422  can be incident on blood vessel  442  located in layer  446 , and a portion of the light can reflect back as light  423 . Light sensor  404  can measure light  423  and can generate signal  450 , where signal  450  can include both pulsatile blood volume changes and noise information. The set comprising light emitter  405  and light sensor  407  can be less sensitive to arterial blood volume changes (than the set comprising light emitter  406  and light sensor  404 ) and can be configured to generate a signal indicative of the non-pulsatile blood changes. Light emitter  405  can be located in close proximity (e.g., less than or equal to 1 mm away) to light sensor  407 . Light emitter  405  can emit light  426 . A portion of light  426  can penetrate through skin  420 , and a portion of the light can reflect back as light  427 . Light sensor  407  can detect light  427  and can generate signal  455 , where signal  455  can include noise information. The spacing between light emitter  405  and light sensor  407  can prevent light  426  from reaching one or more deep layers (e.g., layer  446 ). Deeper tissue of a user can be more susceptible to motion artifacts due to, for example, muscle movement, tendon movement, non-pulsatile blood movement, or a combination thereof. The effect of the motion artifacts can be less pronounced in the superficial layers of the user due to absence of the muscles and tendons. In some examples, light  427  can be emitted at one or more wavelengths (e.g., greater than 600 nm) less sensitive to oxy-hemoglobin, which can reduce the interaction of light to venous blood volume changes. Controller  409  can receive signal  450  and signal  455  and can apply one or more algorithms to determine the user&#39;s physiological signal. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
       FIG. 4C  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a light emitter in the same cavity, but divided by an isolation according to examples of the disclosure. Device  400  can include isolation  421  located between a light emitter-light sensor set included in the same cavity (e.g., cavity  466 ). Isolation  421  can be any material configured to optically isolate light emitter  405  from light sensor  407 . Exemplary materials for isolation can include, but are not limited to, carbon. In some examples, window  403  can be configured to reject one or more angles of light. In some examples, the rejected angles can include high angles (e.g., greater than 50 degrees) such that reflections from the surface of skin  420  and/or from the surface of window  403 . In some examples, window  403  can be a Fresnel lens. 
     In some examples, device  400  can be located in close proximity (e.g., less than 5 mm away) or in contact with skin  420  to help prevent light  427  from including any light that has merely reflected off the surface of skin  420  and/or the surface of device  400 . In this manner, penetration of light  426  can be better controlled. The close spacing of light emitter  405  and light sensor  407  can prevent the reflected light  427  from including non-pulsatile blood information. Isolation  421  and/or close proximity of the surface device  400  to skin  420  can prevent reflected light  427  from including reflections off the surface of skin  420  and/or surface of device  400 . In some examples, the light sensor&#39;s numerical aperture can be configured to prevent light  427  from including any light that has merely reflected off the surface of skin  420  and/or the surface of device  400 . Although  FIG. 4C  illustrates isolation  421  ending at the inner surface (i.e., surface closest to light emitter  405  and light sensor  407 ) of window  403 , examples can include isolation  421  ending at the outer surface (i.e., surface furthest from light emitter  405  and light sensor  407 ) of window  403  as illustrated in  FIG. 4D . In some examples, isolation  421  can comprise a plurality of materials, where the material(s) within the cavity can be different from the material(s) within the window. In some examples, isolation  421  can be continuous and/or the same material along the cavity and the window. 
       FIG. 5A  illustrates a top view and  FIG. 5B  illustrates a cross-sectional view of an exemplary electronic device including at least one separate light sensor and light emitter set for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. Device  500  can include light emitter  505 , light emitter  506 , light emitter  515 , and light emitter  516 . Device  500  can also include light sensor  504 , light sensor  507 , light sensor  514 , and light sensor  517 . Light emitter  505  can be configured to emit light towards light sensor  504  and light sensor  514 . Light emitter  506  can be configured to emit light towards light sensor  507 . Light emitter  515  can be configured to emit light towards light sensor  504  and light sensor  514 . Light emitter  516  can be configured to emit light towards light sensor  517 . In some examples, light emitter  505  and light emitter  515  can be located closer to the center of device  500 , whereas light emitter  506 , light sensor  507 , light emitter  516 , and light emitter  517  can be located closer to the outer edges of device  500 . In some examples, light emitter  505  and light emitter  515  can be located such that the path lengths to light sensor  504  are the same as the path lengths to light sensor  514 . 
     Device  500  can be configured such that one or more light emitters are optically coupled to one or more light sensors, where the one or more light emitters are located in a different cavity than the one or more light sensors. For example, light emitter  505  can be optically coupled to light sensor  504 , where light emitter  505  can be located in cavity  566  and light sensor  504  can be located in a cavity  564 . In some examples, each cavity can be associated with a different aperture (where light exits and enters the device) and/or window. Device  500  can also be configured such that one or more light emitters are optically coupled to one or more light sensors, where the one or more light emitters are located in the same cavity as the one or more light sensors. For example, light emitter  506  can be optically coupled to light sensor  507 , where both can be located in cavity  566 . In some examples, the cavities included in device  500  can be separated by isolation  519 . 
     Similarly, light emitter  515  can be optically coupled to light sensor  504  and light sensor  514 , where each light sensor can be located in a different cavity than light emitter  515 . Light emitter  516  can be optically coupled to light sensor  517 , where each can be located in the same cavity. 
     In some examples, one or more light emitter-light sensor sets located in different cavities can be configured to measure pulsatile blood volume changes. In some examples, one or more light emitter-light sensor sets located in the same cavity can be configured to measure non-pulsatile blood volume changes and/or serve as a noise reference channel. The set comprising light emitter  515  and light sensor  504  and/or the set comprising light emitter  515  and light sensor  514  can be configured to measure pulsatile blood changes. The signals generated from these sets can include both pulsatile blood volume changes and noise information. The set comprising light emitter  516  and light sensor  517  can be configured to measure non-pulsatile blood changes. Light emitter  516  can be located in close proximity (e.g., less than or equal to 1 mm away) from light sensor  517 . The spacing between light emitter  516  and light sensor  517  can prevent the emitted light from reaching one or more arterioles  534  and/or one or more blood vessels  542 , and hence, the associated signal can include little-to-no pulsatile blood information. Controller  509  can receive one or more signals that include pulsatile blood volume changes (e.g., signals, such as signal  550 , from light sensor  504 ) and one or more signals that includes little-to-no pulsatile blood information (e.g., signals, such as signal  555 , from light sensor  507 ) and can apply one or more algorithms to determine the user&#39;s physiological signal. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
       FIG. 5C  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a light emitter in the same cavity, but divided by an isolation according to examples of the disclosure. Device  500  can further include isolation  521  located between a light emitter-light sensor set included in the same cavity. Isolation  521  can be any material configured to optically isolate light emitter  506  from light sensor  507 . Exemplary materials for isolation can include, but are not limited to, carbon. In some examples, isolation  521  can be configured to focus and/or collimate light  526  such that light  526  can exit cavity  566  and/or aperture  501 . In some examples, device  500  can be located in close proximity (e.g., less than 5 mm away) or in contact with skin  520  to help prevent light  527  from including any light that has merely reflected off the surface of skin  520  and/or surface of device  500 . In this manner, the penetration of light  526  can be controlled. The close spacing of light emitter  506  and light sensor  507  can prevent reflected light  527  from including non-pulsatile blood information. Isolation  521  and/or close proximity of device  500  to skin  520  can prevent reflected light  527  from including reflections from the surface of skin  520  and/or surface of device  500 . Although  FIG. 5C  illustrates isolation  521  ending at the inner surface (i.e., surface closest to light emitter  505  and light sensor  507 ) of window  503 , examples can include isolation  521  ending at the outer surface (i.e., surface furthest from light emitter  505  and light sensor  507 ) of window  503  (not shown). In some examples, isolation can comprise a plurality of materials, where the material(s) within the cavity can be different from the material(s) within the window. In some examples, isolation can be continuous and/or the same material along the cavity and the window. 
     In some examples, light sensor  507  can be coupled to a passband filter and/or can be configured to detect only those wavelengths of light emitted by light emitter  506 . In this manner, light sensor  507  may not detect light emitted from light emitter  505 . In some examples, light sensor  507  can be configured to detect wavelengths of light emitted by light emitter  506  and wavelengths of light emitted by light emitter  505 . In some examples, the different wavelengths of light can provide different types of information. For example, light emitter  506  can emit red light (or light within 570-750 nm), and light emitter  505  can emit green light (or light within 495-570 nm). Light sensor  507  can be configured to detect both red light and green light, where detected red light can be used for determining motion artifacts, and detected green light can be used for off-wrist detection. Moreover, light sensor  504  can detect light emitted from light emitter  505  that can pass through the multiple layers of skin  520  and pulsatile blood flow (i.e., one or more blood vessels  542  and/or one or more arterioles  534 ). 
     Although  FIGS. 4A and 5A  illustrate four light emitters, examples of the disclosure can include any number of light emitters. In addition, examples of the disclosure can include one or more common light sensors that can be used for detecting signals including pulsatile blood information and signals including non-pulsatile blood information. 
       FIG. 6A  illustrates a top view and  FIG. 6B  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a common light emitter used for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. Device  600  can include light emitter  606 , light emitter  608 , light emitter  616 , and light emitter  618 . Device  600  can also include light sensor  604  and light sensor  614 . Light emitter  606  can be configured to emit light towards light sensor  604  and light sensor  614 . Light emitter  608  can also be configured to emit light towards light sensor  604  and light sensor  614 . Light emitter  616  can be configured to emit light towards light sensor  604  and light sensor  614 . Light emitter  618  can also be configured to emit light towards light sensor  604  and light sensor  614 . In some examples, light emitter  606  and light emitter  616  can be located such that the path lengths to light sensor  604  and to light sensor  614  are the same. In some examples, light emitter  608  and light emitter  618  can be located such that the path lengths to light sensor  604  and to light sensor  614  are the same. In some examples, light emitter  606  and light emitter  616  can be located closer to the center of device  600  than light emitter  608  and light emitter  618 . 
     Device  600  can further include light emitter  605  and light emitter  615 . Light emitter  605  can be located in close proximity to and can be configured to emit light towards light sensor  604 . Light emitter  615  can be located in close proximity to and can be configured to emit light towards light sensor  614 . Light sensor  604  can be configured as a common light sensor that can detect light reflected from one or more blood vessels  642  and/or one or more arterioles  634 , where pulsatile blood volume changes can be determined based on the detected reflect light. For example, pulsatile blood volume changes can affect light  623  and light  627 . Light  623  can include information from layer  644  and layer  645 , and light  627  can include information from layer  644 , layer  645 , and layer  646 . Light sensor  604  can also detect light reflected from light  625 , which can be sensitive to venous blood (non-pulsatile blood) volume changes due to the light emitter being located in close proximity (e.g., less than or equal to 1 mm away) to the light sensor and/or emitting light at specific wavelengths (e.g., greater than 600 nm), for example. 
     In some examples, light emitter  606  and light emitter  608  can be located in the same cavity  666 , and light emitter  616  and light emitter  618  can be located in the same cavity. In each cavity, at least one light emitter can be configured to emit light at a wavelength different from another light emitter. Different types of information can be extracted from the different wavelengths of light. For example, light emitter  606  can be configured to emit light  622 . Light  622  can travel through one or more layers of skin  620 , and a portion of the light can reflect back as light  623  to be detected by light sensor  604 . Light emitter  608  can be configured to emit light  626 . Light  626  can travel through one or more layers of skin  620 , and a portion of the light can reflect back as light  627  to be detected by light sensor  604 . The separation distance between light emitter  606  and light sensor  604  can be shorter than the separation distance between light emitter  608  and light sensor  604 . Additionally or alternatively, light  622  can have a shorter wavelength than the wavelength of light  626 . In some examples, the shorter separation distance and/or shorter wavelength can lead to light  622  and light  623  having a shorter path length than light  626  and light  627 . As a result, light  622 / 623  may not penetrate as deep in skin  620  as light  626 / 627 . Light sensor  604  can generate a plurality of signals, including signal  652  representative of light  623 , signal  650  representative of light  627 , and signal  655  representative of light  625 . Although  FIG. 6A  illustrates light emitter  605  and light emitter  615  located in a center of light sensor  604  and light sensor  614 , respectively, examples of the disclosure can include light emitter  605  and light emitter  615  located in other locations (e.g., to one side) with respect to light sensor  604  and light sensor  614 , respectively. 
       FIG. 6C  illustrates a cross-sectional view of an exemplary electronic device including a light sensor optically coupled to a common light emitter used for noise correction utilized in measuring a user&#39;s physiological signal according to examples of the disclosure. Device  600  can include isolation  617  located between light emitter  605  and light sensor  604 . Device  600  can also include isolation  621  located between light emitter  608  and light emitter  606 . In some examples, isolation  621  can be configured to prevent light mixing between light emitted by light emitter  606  and light emitted by light emitter  608 . Isolation  617  and isolation  621  can be any material configured for optical isolation. Exemplary materials for isolation can include, but are not limited to, carbon. 
     In some examples, isolation  617  can be configured to focus and/or collimate light  624  such that light  624  can exit cavity  664 . In some examples, device  600  can be located in close proximity (e.g., less than 5 mm away) or in contact with skin  420  to help prevent light  625  from including any light that has merely reflected off the surface of skin  620  and/or the surface of device  600 . In this manner, the penetration of light  624  can be controlled. The close spacing of light emitter  605  and light sensor  604  can prevent reflected light  625  from including pulsatile blood information. Isolation  617  and/or close proximity to the surface of device  600  to skin  620  can prevent reflected light  625  from including reflections from the surface of skin  620  and/or surface of device  600 . Additionally or alternatively, isolation  621  can be configured to focus and/or collimate light  622  and/or light  626  such that light  622  and/or light  626  can exit cavity  666 . Penetration of light  622  and light  626  can be controlled such that reflected light  623  and reflected light  627  can include pulsatile blood information. 
     Although  FIG. 6C  illustrates isolation  617  and isolation  621  ending at the inner surface (i.e., surface closest to light emitter  605 , light sensor  604 , light emitter  606 , and light emitter  608 ) of windows  603 , examples of the disclosure can include isolation  617  and/or isolation  621  ending at the outer surface (i.e., surface furthest from light emitter  605 , light sensor  604 , light emitter  606 , and light emitter  608 ) of windows  603  (not shown). In some examples, isolation can comprise a plurality of materials, where the material(s) within the cavity can be different from the material(s) within window. In some examples, isolation can be continuous and/or the same material along the cavity and the window. 
       FIG. 6D  illustrates a cross-sectional view of an exemplary electronic device including angled isolation according to examples of the disclosure. Device  600  can include isolation  617  located between light sensor  604  and light emitter  605 . Device  600  can also include isolation  621  located between light emitter  606  and light emitter  608 . Isolation  617  and/or isolation  621  can be angled or non-orthogonal to windows  603 , which can focus and/or collimate light  624  and light  626 . In some examples, isolation  617  and isolation  621  can steer light  624  and light  626 , respectively, more than isolation that is orthogonal to the windows (e.g., isolation  421  illustrated in  FIG. 4D ). In some examples, one or more of isolation  617  and isolation  621  can be angled towards (i.e., spacing between isolations can be located closer to the windows  603 ) isolation  619 . 
       FIG. 7A  illustrates a top view and  FIG. 7B  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity can include at least one light sensor and a plurality of light emitters according to examples of the disclosure. Device  700  can include cavity  743  and cavity  746 . Cavity  743  can include light emitter  705 , light emitter  706 , light emitter  708 , and light sensor  704 . Cavity  746  can include light emitter  715 , light emitter  716 , light emitter  718 , and light sensor  714 . Device  700  can be configured such that each light sensor can be surrounded by light emitters and/or the edge of device  700 . For example, light sensor  704  can be located between a first column of light emitters (e.g., column formed by light emitter  705 , light emitter  706 , and light emitter  708 ) and a second column of light emitters (e.g., column formed by light emitter  715 , light emitter  716 , and light emitter  718 ). 
     Device  700  can be configured such that in each cavity, at least one light emitter can be optically coupled to a light sensor in the cavity, and at least one light emitter can be optically coupled to a light sensor in another cavity. A plurality of overlapping light paths can be formed by the plurality of light emitted from light emitters that can be optically coupled to a light sensor in another cavity. In this manner, multiple light paths can “cross” over each other, which can increase the locations on skin  720  that can be sampled. 
     Light emitter  705  can be configured to emit light  722 . Light  722  can enter skin  720 , and a portion can reflect back as light  723  to be detected by light sensor  714 , which can be a light sensor optically coupled to a light emitter in a different cavity. Light emitter  705  can be located relative to light sensor  714  such that one or more areas of skin  720  located along the optical path of light  722 / 723  can be measured. The measurement can include light  722  and/or light  723  undergoing optical changes due to pulsatile blood volume changes from, for example, one or more blood vessels  742  and/or one or more arterioles  734 . 
     Light emitter  706  can be configured to emit light  724 . Light  724  can enter skin  720 , and a portion can reflect back as light  725  to be detected by light sensor  704 , which can be a light sensor optically coupled to a light emitter in the same cavity. Light emitter  706  can be located in close proximity to light sensor  704  such that the penetration of light  724  can be limited to shallower layers (e.g., layer  744  and/or layer  745 ). In some examples, light  724  can include specific wavelengths (e.g., greater than 600 nm), thereby limiting light  724 / 725  to be sensitive to venous blood (non-pulsatile blood) volume changes. 
     Light emitter  715  can be configured to emit light  726 . Light  726  can enter skin  720 , and a portion can reflect back as light  727  to be detected by light sensor  704 , which can be a light sensor optically coupled to a light emitter in a different cavity. Light emitter  715  can be located relative to light sensor  704  such that one or more areas of skin  720  located along the optical path of light  726 / 727  can be measured. The measurement can include light  726  and/or light  727  undergoing optical changes due to pulsatile blood volumes changes from, for example, one or more blood vessels  742  and/or one or more arterioles  734 . In some examples, light emitter  715 -light sensor  704  set can measure one or more areas of skin  720  different than the one or more areas of skin  720  measured by light emitter  705 -light sensor  714  set. In some examples, one or more blood vessels  742  and/or one or more arterioles  734  measured by light  726 / 727  can be different from the one or more blood vessels  742  and/or one or more arterioles  734  measured by light  722 / 723 . In some examples, the light path from light  722 / 723  can cross or intersect with the light path from light  726 /light  727 . The angle of intersection between the light paths can be adjusted based on the location of the corresponding optical components, which can then adjust the measurement profile of the one or more areas in skin  720 . 
     Light emitter  716  can be configured to emit light  728 . Light  728  can enter skin  720 , and a portion can reflect back as light  729  to be detected by light sensor  714 , which can be a light sensor optically coupled to a light emitter in the same cavity. Light emitter  716  can be located in close proximity (e.g., less than or equal to 1 mm away) to light sensor  714  such that the penetration of light  728  can be limited to shallower layers (e.g., layer  744  and/or layer  745 ). In some examples, light  728 / 729  can include specific wavelengths (e.g., greater than 600 nm), thereby limiting the sensitivity of light  728 / 729  to venous blood (non-pulsatile blood) volume changes. In some examples, the separation distance between light emitter  706  and light sensor  704  can be the same as the separation distance between light emitter  716  and light sensor  714 . In some examples, the separation distance between light emitter  706  and light sensor  704  can be different from the separation distance between light emitter  716  and light sensor  714 . In some examples, light  725  can include the same noise artifacts as light  729 . In some examples, light  725  can include different noise artifacts than light  729 . If the noise artifacts are different, then device  700  can utilize the difference in noise artifacts to determine whether the noise originates from multiple sources. For example, a difference in noise artifacts can be indicative of a tilt and/or pull experienced by one side of the device and not by the other side of the device. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
       FIG. 7C  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters divided by an isolation according to examples of the disclosure. To prevent light  725  and/or light  729  from including reflected light at the interfaces (e.g., at the surface of skin  720 , at the surface of window  703 , and/or at the surface of device  700 ), device  700  can include isolation  717  and/or isolation  721 . In this manner, light  725  and/or light  729  can include information related to non-pulsatile blood and/or other noise artifacts (e.g., noise from a tilt and/or pull of the device or ambient light variations). 
       FIG. 8A  illustrates a top view and  FIG. 8B  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters according to examples of the disclosure. Device  800  can include cavity  843  and cavity  846 . Cavity  843  can include light emitter  805 , light emitter  806 , light emitter  808 , and light sensor  804 . Cavity  846  can include light emitter  815 , light emitter  816 , light emitter  818 , and light sensor  814 . Device  800  can be configured such that light sensor  804  and light sensor  814  are adjacent optical components. In some examples, light sensor  804  and light sensor  814  can be symmetrically (horizontally) placed on device  800  with respect to its center. Light emitter  805 , light emitter  806 , and light emitter  808  can be located on one side of device  800 , and light emitter  815 , light emitter  816 , and light emitter  818  can be located on the opposite side of device  800 . 
     Device  800  can be configured such that in each cavity, at least one light emitter can be optically coupled to a light sensor in the cavity, and at least one light emitter can be optically coupled to a light sensor in another cavity. A plurality of overlapping light paths can be formed by the plurality of light emitted from light emitters that can be optically coupled to a light sensor in another cavity. In this manner, multiple light paths can “cross” over each other, which can increase the locations within skin  820  that device  800  can sample. 
     Light emitter  805  can be configured to emit light  822 . Light  822  can enter skin  820 , and a portion can reflect back as light  823  to be detected by light sensor  814 , which can be a light sensor optically coupled to a light emitter in a different cavity. Light emitter  805  can be located relative to light sensor  814  such that one or more areas of skin  820  located along the optical path of light  822 / 823  can be measured. The measurement can include light  822  and/or light  823  undergoing optical changes due to pulsatile blood volume changes from, for example, one or more blood vessels  842  and/or one or more arterioles  834 . 
     Light emitter  806  can be configured to emit light  824 . Light  824  can enter skin  820 , and a portion can reflect back as light  825  to be detected by light sensor  804 , which can be a light sensor optically coupled to a light emitter in the same cavity. Light emitter  806  can be located in close proximity (e.g., less than or equal to 1 mm away) to light sensor  804  such that the penetration of light  824  can be limited to shallower layers (e.g., layer  844  and/or layer  845 ). Light  824 / 825  can include specific wavelengths (e.g., greater than 600 nm), thereby limiting the sensitivity of light  824 / 825  to venous blood (non-pulsatile blood) volume changes. 
     Light emitter  815  can be configured to emit light  826 . Light  826  can enter skin  820 , and a portion can reflect back as light  827  to be detected by light sensor  804 , which can be a light sensor optically coupled to a light emitter in a different cavity. Light emitter  816  can be located relative to light sensor  804  such that one or more areas of skin  820  located along the optical path of light  826 / 827  can be measured. The measurement can include light  826  and/or light  827  undergoing optical changes due to pulsatile blood volumes changes from, for example, one or more blood vessels  842  and/or one or more arterioles  834 . In some examples, light emitter  815 -light sensor  804  set can measure one or more areas of skin  820  different than the one or more areas of skin  820  measured by light emitter  805 -light sensor  814  set. In some examples, one or more blood vessels  842  and/or one or more arterioles  834  measured by light  826 / 827  can be different than the one or more blood vessels  842  and/or one or more arterioles  834  measured by light  822 / 823 . In some examples, the light path from light  822 / 823  can cross or intersect with the light path from light  826 / 827 . The angle of intersection between the light paths can be adjusted based on the location of the corresponding optical components, which can then adjust the measurement profile of the one or more areas in skin  820 . By locating light emitters (e.g., light emitter  805 , light emitter  806 , and light emitter  808 ) on one side of device  800  and locating light emitters (e.g., light emitter  815 , light emitter  816 , and light emitter  818 ) on another side of device  800 , the light paths can have a greater separation distance relative to one another (compared to, for example, the light paths illustrated in  FIG. 7B ). Different characteristics (e.g., size, shape, and/or location) of the measurement areas on the skin  820  can be obtained. 
     Light emitter  816  can be configured to emit light  828 . Light  828  can enter skin  820 , and a portion can reflect back as light  829  to be detected by light sensor  814 , which can be a light sensor optically coupled to a light emitter in the same cavity. Light emitter  816  can be located in close proximity (e.g., less than or equal to 1 mm away) to light sensor  814  such that the penetration of light  828  can be limited to shallower layers (e.g., layer  844  and/or layer  845 ). Light  828 / 829  can include specific wavelengths (e.g., greater than 600 nm), thereby limiting the sensitivity of light  828 / 829  to venous blood (non-pulsatile blood) volume changes. In some examples, the separation distance between light emitter  816  and light sensor  814  can be the same as the separation distance between light emitter  806  and light sensor  804 . In some examples, the separation distance between light emitter  816  and light sensor  814  can be different from the separation distance between light emitter  806  and light sensor  804 . In some examples, light  825  can include the same noise artifacts as light  829 . In some examples, light  825  can include different noise artifacts than light  829 . If the noise artifacts are different, then device  800  can utilize the difference in noise artifacts to determine whether the noise originates from multiple sources. For example, a difference in noise artifacts can be indicative of a tilt and/or pull experienced by one side of the device and not the other. 
     In some examples, the system can be configured with a plurality of sets of light emitter-light sensor, where the light emitters and light sensors have a common optical axis. For example, light emitter  806  can be configured to emit light towards light sensor  814 . Multiple light paths can exist. For example, one light path can be between light emitter  806  and light sensor  804 , and another light path can be between light emitter  806  and light sensor  814 . The light path between light emitter  806  and light sensor  804  can be utilized for noise correction, and the light path between light emitter  806  and light sensor  814  can be utilized for pulsatile blood information. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
       FIG. 8C  illustrates a cross-sectional view of an exemplary electronic device including at least two different cavities, each cavity including at least one light sensor and a plurality of light emitters divided by an isolation according to examples of the disclosure. To prevent light  825  and/or light  829  from including reflected light at the interfaces (e.g., at the surface of skin  820 , at the surface of window  803 , at the surface of device  800 ), device  800  can include isolation  817  and/or isolation  821 . In this manner, light  825  and/or light  829  can include non-pulsatile blood information. In some examples, light  825  and/or light  829  can exclude pulsatile blood information. 
     Although  FIGS. 7A and 8A  illustrate the light emitters arranged to form columns (relative to the long axis of device  700  and device  800 ), examples of the disclosure can include the light emitters arranged to form rows.  FIGS. 8D-8E  illustrate top views of exemplary configurations for light emitters, light sensors, and isolation for electronic devices according to examples of the disclosure. As illustrated in  FIG. 8D , light sensor  804  and light sensor  814  can be located on one side of device  800 , and the rows of light emitters (e.g., row formed by light emitter  805 , light emitter  806 , and light emitter  808  and row formed by light emitter  815 , light emitter  816 , and light emitter  818 ) can be located on another side of device  800 . The light sensors and rows of light emitters can be separated by isolation  817  and isolation  821 . Additionally, cavity  843  can and cavity  846  can be separated by isolation  819 . The configuration can lead to one or more intersection light paths. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
     As illustrated in  FIG. 8E , light sensor  804  and one row of light emitters (e.g., row formed by light emitter  815 , light emitter  816 , and light emitter  818 ) can be located on one side of device  800 . Additionally, light sensor  814  and another row of light emitters (e.g., row formed by light emitter  805 , light emitter  806 , and light emitter  808 ) can be located on another side of device  800 . Each row of light emitters can be divided by isolation (e.g., isolation  817  or isolation  821 ) from a light sensor. This configuration can lead to non-overlapping light paths as shown in the figure. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
     Examples of the disclosure are not limited to rows of light emitters, but can include any configurations such as illustrated in  FIG. 8F . For example, light emitter  805  and light emitter  806  can be located in a row different from light emitter  808 . Light emitter  808  can be located in the same column as light emitter  805  or light emitter  806  or can be located in a column different from light emitter  805  and light emitter  806 . Similarly, light emitter  815  and light emitter  816  can be located in a row different from light emitter  818 . Light emitter  818  can be located in the same column as light emitter  815 , or light emitter  816  or can be located in a column different from light emitter  815  and light emitter  818 . In some examples, the light emitters and light sensors can be symmetrically located (e.g., light sensor  804  can be located the same distance away from the center of the device as light sensor  814 ) with respect to the center of device  800 . In some examples, the light emitters and light sensors can be asymmetrically located with respect to the center of device  800 . In some examples, the light sensors can be located closer to the edges of device  800  than the light emitters. In some examples, the light emitters can be located closer to the edges of device  800  than the light sensors. Additional light paths formed between light sensors and light emitters can be included in examples of the disclosure and are not shown in the figure for clarity purposes. 
       FIG. 9A  illustrates exemplary oxy-hemoglobin and deoxy-hemoglobin absorption signals measured across a plurality of wavelengths according to examples of the disclosure. The spectrum can include a plurality of wavelength ranges, such as wavelength range  963 , wavelength range  964 , wavelength range  965 , wavelength range  966 , and wavelength range  967 . Signal  950  can include the oxy-hemoglobin absorbance signal, and signal  955  can include the deoxy-hemoglobin absorbance signal. At one or more wavelengths (e.g., wavelength  961 ) in the spectrum, signal  950  and signal  955  can intersect. That is, signal  950  and signal  955  can have the same or similar absorbance values. A PPG system configured to measure the user&#39;s physiological signal at or within close proximity to these one or more wavelengths (e.g., wavelength  961  corresponding to an intersection of the signals) may not be capable of discerning whether the measured reflected light associated originates from oxy absorbance or de-oxy absorbance. 
     Examples of the disclosure can include a system capable of measuring both oxy-hemoglobin and deoxy-hemoglobin absorbance values in one or more wavelength ranges, where the oxy-hemoglobin and deoxy-hemoglobin absorbance signals are non-intersecting. In some examples, the system can be configured to measure at one or more wavelengths where the difference in absorbance values of the signals are greater than a pre-determined threshold (e.g., 10% difference). In some examples, the one or more measured wavelengths (e.g., wavelength  962 ) can correspond to a “minimum” (i.e., zero derivative) in the de-oxy absorbance signal. 
     Examples of the disclosure can include at least one light emitter (e.g., light emitter  206  illustrated in  FIG. 2A , light emitter  306  illustrated in  FIG. 3A ) configured to emit within wavelength range  764  (i.e., 495-570 nm). Examples of the disclosure can include at least one light emitter (e.g., light emitter  308  illustrated in  FIG. 3A ) configured to emit within wavelength range  765  (i.e., 570-750 nm). Examples of the disclosure can include at least two light emitters (e.g., light emitter  306  and light emitter  308  illustrated in  FIG. 3A ) configured to emit within the same wavelength range (e.g., wavelength range  764 , wavelength range  765 , and/or wavelength range  767 ). Examples of the disclosure can include at least one light emitter (e.g. light emitter  308  illustrated in  FIG. 3A ) configured to emit within wavelength ranges  966  and  967  (i.e., 750-1400 nm). 
     Examples of the disclosure can include a system capable of emitting light across a spectrum of wavelengths or a plurality of wavelength (e.g., greater than two wavelengths). For example, the system can include at least one light emitter (e.g., light emitter  606  illustrated in  FIG. 6A ) configured to emit light within wavelength range  964  (i.e., 495-570 nm), at least one light emitter (e.g., light emitter  605  illustrated in  FIG. 6A ) configured to emit light within wavelength range  966  and wavelength range  967  (i.e., 750-1400 nm), and at least one light emitter (e.g., light emitter  608  illustrated in  FIG. 6A ) configured to emit light within wavelength range  965  (i.e., 570-750 nm). In some examples, one light emitter can be configured to emit at 525 nm, one light emitter can be configured to emit at 660 nm, and one light emitter can be configured to emit at 890 nm. Measuring reflected light within wavelength range  965  can lead to signals with little-to-no pulsatile blood information. Measuring reflected light within wavelength range  966  and wavelength range  967  can lead to light that can be invisible to the user&#39;s eye. Examples of the disclosure can include a system configured with a common (i.e., shared) light emitter capable of emitting light to a plurality of light sensors, where at least one set of light emitter-light sensor can be configured for measuring pulsatile blood flow and at least one set of (the same) light emitter-light sensor can be configured for measuring non-pulsatile blood flow. 
     In some examples, at least two sets of light emitter-light sensor can be configured to measure (e.g., light passes through) the same volume of the user&#39;s skin. By measuring the same volume of skin, the non-pulsatile blood information can be accurately associated with the corresponding pulsatile blood information. In some examples, at least two light emitters and optically coupled one or more detectors can be located along the same optical axis. 
     In some examples, at least two sets of light emitter-light sensor can be configured to measure different volumes of the user&#39;s skin. The locations of optical components in such a configuration may be limited due to the size of a package or the separation distance between optical components, for example. 
       FIG. 9B  illustrates exemplary signals measured at the plurality of light sensors included in an exemplary electronic device according to examples of the disclosure. Signal  950  can include pulsatile blood information and noise artifacts. Signal  955  can include noise artifacts using any of the above disclosed examples. In some examples, signal  950  can be the signal generated by one of the sets of light emitter-light sensor. In some examples, signal  955  can be the signal generated by another one of the sets of light emitter-light sensor. The PPG system can include a controller configured to determine a user&#39;s physiological signal by removing noise artifacts from signal  950 . In some examples, at least a portion of the noise artifacts included in signal  950  can be determined using signal  955 . For example, frequency  971  can correspond to a fundamental frequency for the user&#39;s physiological signal (e.g., PPG), and frequency  972  can correspond to a harmonic frequency for the user&#39;s physiological signal. The controller can be configured to determine the fundamental and harmonic frequencies and can utilize signal  950  and signal  955  to determine the user&#39;s physiological signal. 
     In some examples, signals associated with one or more light emitters (e.g., light emitter  306  and light emitter  316 ) can be used for determining the user&#39;s physiological signals while the user is in motion. In some examples, signals associated with one or more light emitters (e.g., light emitter  306  and light emitter  316 ) can be used for determining the user&#39;s physiological signals while the user is stationary. In some examples, one or more signals can include heart rate PPG signals. In some examples, one or more signals can be used for off-wrist detection (i.e., the device is located a far distance away from the user). In some examples, the device can include an accelerometer to detect the user&#39;s acceleration, and such acceleration information can additionally be used for canceling/correcting motion artifacts in the user&#39;s physiological signal. In some examples, one or more of the light emitters and/or light sensors can be disabled, powered off, or their signals can be ignored. For example, light emitter  308  and/or light emitter  316  (illustrated in  FIG. 3A ) can be powered off when the user is moving. 
       FIG. 10A  illustrates an exemplary circuit diagram and  FIG. 10B  illustrates an exemplary process for motion artifact removal according to examples of the disclosure. The system can include light emitter  1006  optically coupled to light sensor  1004  and light emitter  1005  optically coupled to light sensor  1007 . Light emitter  1006  can emit light towards light sensor  1004  (step  1052  of process  1050 ). Light sensor  1004  can detect the reflected light from light emitted by light emitter  1006  and can generate a signal  1050  (step  1054  of process  1050 ). The Fourier transform of signal  1050  can be taken using FFT  1010  (step  1056  of process  1050 ). Light emitter  1005  can emit light towards light sensor  1007  (step  1058  of process  1050 ). Light sensor  1007  can detect the reflected light from light emitted by light emitter  1005  and can generate a signal  1055  (step  1060  of process  1050 ). In some examples, light emitter  1005  can emit light at the same time as light emitter  1006 . The Fourier transfer of signal  1055  can be taken using FFT  1010  (step  1062  of process  1050 ). The locations of the peaks (i.e., “maximum”/zero derivative) in signal  1050  and signal  1055  can be determined using component  1011  (step  1064  of process  1050 ). In some examples, the values of signal  1050  can be scaled (e.g., a Gaussian weight can be applied) at locations where a peak exists (step  1066  of process  1050 ). In some examples, the corrected (or adjusted) signal  1050  can include peaks from the fundamental and harmonic frequencies of the user&#39;s physiological signal. 
     The system can further include an accelerometer  1002 . Accelerometer  1002  can measure the user&#39;s acceleration (step  1068  of process  1050 ). In some examples, the acceleration measurement can be concurrent with the optical measurements from the light sensors. The Fourier transform of the acceleration signal can be taken using FFT  1010  (step  1070  of process  1050 ). The locations of the peaks in the corrected signal  1050  and acceleration signal can be determined using component  1011  (step  1072  of process  1050 ). Controller  1009  can apply one or more algorithms and/or simple mathematical functions to determine the user&#39;s physiological signal  1060  (step  1074  of process  1050 ). 
       FIG. 11A  illustrates an exemplary circuit diagram and  FIG. 11B  illustrates an exemplary process for motion artifact removal according to examples of the disclosure. The system can include light emitter  1106  optically coupled to light sensor  1104  and light emitter  1105  optically coupled to light sensor  1107 . Light emitter  1106  can emit light towards light sensor  1104  (step  1152  of process  1150 ). Light sensor  1104  can detect the reflected light from light emitted by light emitter  1106  and can generate a signal  1150  (step  1154  of process  1150 ). The Fourier transform of signal  1150  can be taken (step  1160  of process  1150 ). 
     Light emitter  1105  can emit light towards light sensor  1107  (step  1162  of process  1150 ). Light sensor  1107  can detect the reflected light from light emitted by light emitter  1105  and can generate a signal  1155  (step  1164  of process  1150 ). The system can further include an accelerometer  1102 . Accelerometer  1102  can measure the user&#39;s acceleration (step  1156  of process  1150 ). In some examples, the acceleration measurement can be concurrent with the optical measurements from the light sensors. Principle component analysis (PCA) can be performed on signal  1155  and the acceleration signal using motion estimator  1103  (step  1158  or process  1150 ). PCA  1102  can be configured to utilize an orthogonal transformation to convert signal  1155  and the acceleration signal into three orthogonal components. The Fourier transform of the orthogonal components can be taken using FFT  1110  (step  1166  of process  1150 ). 
     The locations of the peaks in the signals from both FFTs  1110  can be determined using component  1111  (step  1168  of process  1150 ). In some examples, one or more gait frequencies in signal  1150  can be determined. At the one or more gait frequencies, the signals can be attenuated. The controller  1009  can be configured to apply one or more algorithms and/or simple mathematical functions to determine the user&#39;s physiological signal  1160  (step  1170  of process  1150 ). 
     In the one or more of the above disclosed systems, the sets of light emitter-light sensor can be operated serially or in parallel (i.e., concurrently).  FIGS. 12A-12C  illustrate exemplary measurement modes according to examples of the disclosure. The PPG system can include light emitter  1206 , light emitter  1208 , light emitter  1216 , light emitter  1218 , light sensor  1204 , and light sensor  1214 . 
     As illustrated in  FIG. 12A , the system can be configured to cycle through the sets. Between time t 0  and t 1 , light emitter  1206  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1206  followed by light sensor  1214  measuring the reflected light from light emitter  1206 . Between time t 1  and time t 2 , light emitter  1208  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1208 , followed by light sensor  1214  measuring the reflected light from light emitter  1208 . Between time t 2  and time t 3 , light emitter  1216  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1216 , followed by light sensor  1214  measuring the reflected light from light emitter  1216 . Between time t 3  and time t 4 , light emitter  1218  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1218 , followed by light sensor  1214  measuring the reflected light from light emitter  1218 . Between time t 4  and time t 5 , the system can be configured for off-wrist detection. In some examples, the cycle can be repeated. In some examples, subsequent cycles can have a different order. 
     The system can further include light emitter  1205  and light emitter  1215 . As illustrated in  FIG. 12B , the measurements can include operation of those additional light emitters in time periods following the operation of all other light emitters. Between time t 4  and time t 5 , light emitter  1205  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1205 , followed by light sensor  1214  measuring the reflected light from light emitter  1205 . Between time t 5  and time t 6 , light emitter  1215  can be active/on. Light sensor  1204  can measure the reflected light from light emitter  1215 , followed by light sensor  1214  measuring the reflected light from light emitter  1215 . Between time t 6  and time t 7 , the system can be configured for off-wrist detection. In some examples, the cycle can be repeated. In some examples, subsequent cycles can have a different order. 
     In some examples, two or more measurements can operate concurrently. As illustrated in  FIG. 12C , a first measurement and a second measurement can operate concurrently between time t 0  and t 1 . The first measurement can include light emitter  1206  active/on, while light sensor  1204  measures the reflected light and light sensor  1214  measures the reflected light. The second measurement can include light emitter  1205  active/on, while light sensor  1207  measures the reflected light and light sensor  1217  measures the reflected light. In some examples, the measurements can operate in a staggered (i.e., the start of one measurement can be delayed from the start of another measurement). In some examples, a time period can include the first measurement having a different set of light emitter-light sensor (e.g., light emitter  1208 , light sensor  1204 , and light sensor  1214 ), while the second measurement can be the same (e.g., light emitter  1205 , light sensor  1207 , and light sensor  1217 ), as illustrated between time t 1  and t 2 . In some examples, a time period can include the first measurement having the same set of light emitter-light sensor (e.g., light emitter  1206 , light sensor  1204 , and light sensor  1214 ), as illustrated between time t 2  and t 3 . In some examples, a time period can include more than two measurements operating concurrently. For example, all the measurements can be taken at the same time (not shown). In some examples, off-wrist detection can occur at any time between measurements or concurrently with measurements. 
       FIG. 13A  illustrates an exemplary process illustrating time-based operation of a PPG system according to examples of the disclosure. The first light emitter can emit light towards the first light sensor (step  1302  of process  1300 ). The first light sensor can generate the first signal indicative of the reflection from the first light emitter (step  1304  of process  1300 ). The second light emitter can emit light towards the second light sensor (step  1306  of process  1300 ). The second light sensor can generate a second signal indicative of the reflection from the second light emitter (step  1308  of process  1300 ). In some examples, first light emitter can operate concurrently with the second light emitter. In some examples, first light emitter can operate before second light emitter. In some examples, second light emitter can operate before first light emitter. The second signal can be compared to a threshold value (step  1310  of process  1300 ), and a controller can determine whether noise correction should be performed (step  1312  of process  1300 ). If noise correction is to be performed, the first signal can be corrected or adjusted with the second signal (step  1314  of process  1300 ). One or more algorithms can be applied to determine the user&#39;s physiological signal(s) (step  1316  of process  1300 ). 
       FIG. 13B  illustrates an exemplary process illustrating operation of a PPG system including a light emitter-light sensor set for motion detection according to examples of the disclosure. The first light emitter can emit light towards the first light sensor (step  1332  of process  1330 ). The first light sensor can generate the first signal indicative of the reflection from the first light emitter (step  1334  of process  1330 ). The second light emitter can emit light towards the second light sensor (step  1336  of process  1330 ). The second light sensor can generate the second signal indicative of the reflection from the second light emitter (step  1338  of process  1330 ). An accelerometer can detect the user&#39;s acceleration and generate an acceleration signal (step  1344  of process  1330 ). The acceleration signal can be compared to a threshold value (step  1346  of process  1330 ). A controller can determine whether optical-based noise correction should be performed (step  1348  of process  1330 ), and if so, the first signal can be corrected or adjusted with the second signal (step  1340  of process  1330 ). If not, the controller can determine whether acceleration-based noise correction should be performed (step  1350  of process  1330 ). If so, the first signal can be corrected or adjusted with the acceleration signal (step  1342  of process  1330 ). One or more algorithms can be applied to determine the user&#39;s physiological signal(s) (step  1352  of process  1330 ). 
       FIG. 13C  illustrates an exemplary process illustrating operation of a PPG system including an accelerometer for motion detection according to examples of the disclosure. The first light emitter can emit light towards first light sensor (step  1362  of process  1360 ). The first light sensor can generate the first signal indicative of the reflection from the first light emitter (step  1364  of process  1360 ). An accelerator can detect the user&#39;s motion and generate an acceleration signal (step  1366  of process  1360 ). A controller can determine whether the user is moving (step  1368  of process  1360 ). If the user is moving, the second light emitter can emit light towards second light sensor (step  1370  of process  1360 ). The second light sensor can detect the reflected light and generate a second signal (step  1372  of process  1330 ). In some examples, the second light sensor can remain in an inactive state until the accelerometer detects user movement. The first signal can be corrected or adjusted using the second signal (step  1374  of process  1330 ). A controller can apply one or more algorithms to determine the user&#39;s physiological signal(s) (step  1376  of process  1330 ). 
       FIG. 13D  illustrates an exemplary process illustrating operation of a PPG system according to examples of the disclosure. An accelerometer can detect user motion (step  1382  of process  1380 ). If user motion is detected, a first light emitter can emit light towards a first light sensor (step  1384  of process  1380 ). The first light sensor can generate a first signal indicative of the reflection from the first light emitter (step  1386  of process  1380 ). A second light emitter can emit light towards a second light sensor (step  1388  of process  1380 ). The second light sensor can generate a second signal indicative of the reflection from the second light emitter (step  1390  of process  1380 ). The first signal can be corrected or adjusted using the second signal (step  1392  of process  1380 ). If user motion has not been detected, a third light emitter can emit light towards the first light sensor (step  1394  of process  1380 ). The first light sensor can generate a third signal indicative of the reflection from the third light emitter (step  1396  of process  1380 ). A controller can apply one or more algorithms to determine the user&#39;s physiological signal(s) (step  1398  of process  1380 ). 
       FIG. 14  illustrates an exemplary block diagram of a computing system comprising light emitters and light sensors for measuring a signal associated with a user&#39;s physiological state according to examples of the disclosure. Computing system  1400  can correspond to any of the computing devices illustrated in  FIGS. 1A-1C . Computing system  1400  can include a processor  1410  configured to execute instructions and to carry out operations associated with computing system  1400 . For example, using instructions retrieved from memory, processor  1410  can control the reception and manipulation of input and output data between components of computing system  1400 . Processor  1410  can be a single-chip processor or can be implemented with multiple components. 
     In some examples, processor  1410  together with an operating system can operate to execute computer code and produce and use data. The computer code and data can reside within a program storage block  1402  that can be operatively coupled to processor  1410 . Program storage block  1402  can generally provide a place to hold data that is being used by computing system  1400 . Program storage block  1402  can be any non-transitory computer-readable storage medium, and can store, for example, history and/or pattern data relating to PPG signal and perfusion index values measured by one or more light sensors such as light sensors  1404 . By way of example, program storage block  1402  can include Read-Only Memory (ROM)  1418 , Random-Access Memory (RAM)  1422 , hard disk drive  1408  and/or the like. The computer code and data could also reside on a removable storage medium and loaded or installed onto the computing system  1400  when needed. Removable storage mediums include, for example, CD-ROM, DVD-ROM, Universal Serial Bus (USB), Secure Digital (SD), Compact Flash (CF), Memory Stick, Multi-Media Card (MMC) and a network component. 
     Computing system  1400  can also include an input/output (I/O) controller  1412  that can be operatively coupled to processor  1410 , or it can be a separate component as shown. I/O controller  1412  can be configured to control interactions with one or more I/O devices. I/O controller  1412  can operate by exchanging data between processor  1410  and the I/O devices that desire to communicate with processor  1410 . The I/O devices and I/O controller  1412  can communicate through a data link. The data link can be a one-way link or a two-way link. In some cases, I/O devices can be connected to I/O controller  1412  through wireless connections. By way of example, a data link can correspond to PS/2, USB, Firewire, IR, RF, Bluetooth or the like. 
     Computing system  1400  can include a display device  1424  that can be operatively coupled to processor  1410 . Display device  1424  can be a separate component (peripheral device) or can be integrated with processor  1410  and program storage block  1402  to form a desktop computer (e.g., all-in-one machine), a laptop, handheld or tablet computing device of the like. Display device  1424  can be configured to display a graphical user interface (GUI) including perhaps a pointer or cursor as well as other information to the user. By way of example, display device  1424  can be any type of display including a liquid crystal display (LCD), an electroluminescent display (ELD), a field emission display (FED), a light emitting diode display (LED), an organic light emitting diode display (OLED) or the like. 
     Display device  1424  can be coupled to display controller  1426  that can be coupled to processor  1410 . Processor  1410  can send raw data to display controller  1426 , and display controller  1426  can send signals to display device  1424 . Data can include voltage levels for a plurality of pixels in display device  1424  to project an image. In some examples, processor  1410  can be configured to process the raw data. 
     Computing system  1400  can also include a touch screen  1430  that can be operatively coupled to processor  1410 . Touch screen  1430  can be a combination of sensing device  1432  and display device  1424 , where the sensing device  1432  can be a transparent panel that is positioned in front of display device  1424  or integrated with display device  1424 . In some cases, touch screen  1430  can recognize touches and the position and magnitude of touches on its surface. Touch screen  1430  can report the touches to processor  1410 , and processor  1410  can interpret the touches in accordance with its programming. For example, processor  1410  can perform tap and event gesture parsing and can initiate a wake of the device or powering on one or more components in accordance with a particular touch. 
     Touch screen  1430  can be coupled to a touch controller  1440  that can acquire data from touch screen  1430  and can supply the acquired data to processor  1410 . In some cases, touch controller  1440  can be configured to send raw data to processor  1410 , and processor  1410  can process the raw data. For example, processor  1410  can receive data from touch controller  1440  and can determine how to interpret the data. The data can include the coordinates of a touch as well as pressure exerted. In some examples, touch controller  1440  can be configured to process raw data itself. That is, touch controller  1440  can read signals from sensing points  1434  located on sensing device  1432  and can turn the signals into data that the processor  1410  can understand. 
     Touch controller  1440  can include one or more microcontrollers such as microcontroller  1442 , each of which can monitor one or more sensing points  1434 . Microcontroller  1442  can, for example, correspond to an application specific integrated circuit (ASIC), which works with firmware to monitor the signals from sensing device  1432 , process the monitored signals, and report this information to processor  1410 . 
     One or both display controller  1426  and touch controller  1440  can perform filtering and/or conversion processes. Filtering processes can be implemented to reduce a busy data stream to prevent processor  1410  from being overloaded with redundant or non-essential data. The conversion processes can be implemented to adjust the raw data before sending or reporting them to processor  1410 . 
     In some examples, sensing device  1432  can be based on capacitance. When two electrically conductive members come close to one another without actually touching, their electric fields can interact to form a capacitance. The first electrically conductive member can be one or more of the sensing points  1434 , and the second electrically conductive member can be an object  1490  such as a finger. As object  1490  approaches the surface of touch screen  1430 , a capacitance can form between object  1490  and one or more sensing points  1434  in close proximity to object  1490 . By detecting changes in capacitance at each of the sensing points  1434  and noting the position of sensing points  1434 , touch controller  1440  can recognize multiple objects, and determine the location, pressure, direction, speed and acceleration of object  1490  as it moves across the touch screen  1430 . For example, touch controller  1440  can determine whether the sensed touch is a finger, tap, or an object covering the surface. 
     Sensing device  1432  can be based on self-capacitance or mutual capacitance. In self-capacitance, each of the sensing points  1434  can be provided by an individually charged electrode. As object  1490  approaches the surface of the touch screen  1430 , the object can capacitively couple to those electrodes in close proximity to object  1490 , thereby stealing charge away from the electrodes. The amount of charge in each of the electrodes can be measured by the touch controller  1440  to determine the position of one or more objects when they touch or hover over the touch screen  1430 . In mutual capacitance, sensing device  1432  can include a two layer grid of spatially separated lines or wires (not shown), although other configurations are possible. The upper layer can include lines in rows, while the lower layer can include lines in columns (e.g., orthogonal). Sensing points  1434  can be provided at the intersections of the rows and columns. During operation, the rows can be charged, and the charge can capacitively couple from the rows to the columns. As object  1490  approaches the surface of the touch screen  1430 , object  1490  can capacitively couple to the rows in close proximity to object  1490 , thereby reducing the charge coupling between the rows and columns. The amount of charge in each of the columns can be measured by touch controller  1440  to determine the position of multiple objects when they touch the touch screen  1430 . 
     Computing system  1400  can also include one or more light emitters such as light emitters  1406  and one or more light sensors such as light sensors  1404  proximate to skin  1420  of a user. Light emitters  1406  can be configured to generate light, and light sensors  1404  can be configured to measure a light reflected or absorbed by skin  1420 , vasculature, and/or blood of the user. Device  1400  can include a plurality of sets of light emitter-light sensor. At least one of the sets of light emitter-light sensor can be configured to measure pulsatile blood, and at least one of the sets of light emitter-light sensor can be configured to measure non-pulsatile blood. In some examples, device  1400  can include an accelerometer (not shown). Light sensor  1404  can send measured raw data to processor  1410 , and processor  1410  can perform noise and/or artifact cancelation to determine the PPG signal and/or perfusion index. Processor  1410  can dynamically activate light emitters and/or light sensors and dynamically reconfigure the aperture properties based on an application, user skin type, and usage conditions. In some examples, some light emitters and/or light sensors can be activated, while other light emitters and/or light sensors can be deactivated to conserve power, for example. In some examples, processor  1410  can store the raw data and/or processed information in a ROM  1418  or RAM  1422  for historical tracking or for future diagnostic purposes. 
     In some examples, the light sensors can measure light information and a processor can determine a PPG signal and/or perfusion index from the reflected or absorbed light. Processing of the light information can be performed on the device as well. In some examples, processing of light information need not be performed on the device itself.  FIG. 15  illustrates an exemplary configuration in which an electronic device is connected to a host according to examples of the disclosure. Host  1510  can be any device external to device  1500  including, but not limited to, any of the systems illustrated in  FIGS. 1A-1C  or a server. Device  1500  can be connected to host  1510  through communications link  1520 . Communications link  1520  can be any connection including, but not limited to, a wireless connection and a wired connection. Exemplary wireless connections include Wi-Fi, Bluetooth, Wireless Direct and Infrared. Exemplary wired connections include Universal Serial Bus (USB), FireWire, Thunderbolt, or any connection requiring a physical cable. 
     In operation, instead of processing light information from the light sensors on the device  1500  itself, device  1500  can send raw data  1530  measured from the light sensors over communications link  1520  to host  1510 . Host  1510  can receive raw data  1530 , and host  1510  can process the light information. Processing the light information can include canceling or reducing any noise due to artifacts and determining physiological signals such as a user&#39;s heart rate. Host  1510  can include algorithms or calibration procedures to account for differences in a user&#39;s characteristics affecting PPG signal and perfusion index. Additionally, host  1510  can include storage or memory for tracking a PPG signal and perfusion index history for diagnostic purposes. Host  1510  can send the processed result  1540  or related information back to device  1500 . Based on the processed result  1540 , device  1500  can notify the user or adjust its operation accordingly. By offloading the processing and/or storage of the light information, device  1500  can conserve space and power-enabling device  1500  to remain small and portable, as space that could otherwise be required for processing logic can be freed up on the device. 
     An electronic device is disclosed. The electronic device can comprise: one or more first light emitters configured to generate a first light; one or more first light sensors configured to detect a reflection of the first light and configured to generate a first signal indicative of the reflection of the first light, the first signal including non-pulsatile blood information; and logic coupled to the one or more first light sensors, the logic configured to: receive the first signal, and determine at least a portion of a physiological signal from the first signal. Additionally or alternatively, in some examples, the device can further comprise: one or more second light sensors configured to detect a reflection of a second light and configured to generate a second signal indicative of the reflection of the second light, the second light generated by the one or more first light emitters and the second signal including pulsatile blood information, wherein the logic is further coupled to the one or more second light sensors, and further configured to receive the second signal and include the second signal in the determination of the physiological signal. Additionally or alternatively, in some examples, the one or more first light emitters, one or more first light sensors, and one or more second light sensors are located along a common optical axis. Additionally or alternatively, in some examples, the device further comprises: one or more second light emitters configured to generate a second light, wherein the one or more first light sensors are further configured to generate a second signal indicative of the reflection of the second light, the second signal including pulsatile blood information, wherein the logic is further configured to receive the second signal and include the second signal in the determination of the physiological signal. Additionally or alternatively, in some examples, the one or more first light emitters, one or more first light sensors, and one or more second light emitters are located along a common optical axis. Additionally or alternatively, in some examples, the device further comprises: one or more second light emitters configured to generate a second light; and one or more second light sensors configured to detect a reflection of the second light and configured to generate a second signal indicative of the reflection of the second light, the second signal including pulsatile blood information, wherein logic is further coupled to the one or more second light sensors and is further configured to receive the second signal and include the second signal in the determination of the physiological signal. Additionally or alternatively, in some examples, the one or more second light emitters and the one or more second light sensors are located in different cavities. Additionally or alternatively, in some examples, the second light includes light with a wavelength between 570-750 nm. Additionally or alternatively, in some examples, the second light includes light with a wavelength between 495-570 nm. Additionally or alternatively, in some examples, the first light and second light intersect. Additionally or alternatively, in some examples, the device further comprises: one or more third light emitters configured to generate a third light, wherein the one or more second light sensors are further configured to detect a reflection of the third light and configured to generate a third signal indicative of the reflection of the third light, the third signal including pulsatile blood information, wherein logic is further configured to receive the third signal and include the third signal in the determination of the physiological signal. Additionally or alternatively, in some examples, the first light includes light with a wavelength between 570-750 nm, the second light includes light with a wavelength between 495-570 nm, and the third light includes light with a wavelength between 750-1400 nm. Additionally or alternatively, in some examples, the one or more first light emitters and the one or more first light sensors are located in a same cavity. Additionally or alternatively, in some examples, the first light includes light with a wavelength between 495-570 nm. Additionally or alternatively, in some examples, at least one of the one or more first light emitters is spaced less than 1 mm from at least one of the one or more first light sensors. Additionally or alternatively, in some examples, the device further comprises: an isolation configured to optically isolate at least one of the one or more first light emitters from at least one of the one or more first light sensors. Additionally or alternatively, in some examples, the device further comprises: a window optically coupled to at least one of the one or more first light emitters, wherein an end of the isolation contacts an inner surface of the window, the inner surface located closer to the one or more first light emitters than an outer surface of the window. Additionally or alternatively, in some examples, the device further comprises: a window optically coupled to at least one of the one or more first light emitters, wherein a first end of the isolation contacts an outer surface of the window, the outer surface located further from the one or more first light emitters than an inner surface of the window. Additionally or alternatively, in some examples, the isolation includes a continuous section disposed between the first end and a second end, the second end located proximate to the at least one of the one or more first light emitters. Additionally or alternatively, in some examples, the isolation comprises: a first section disposed between the first end and a third end, the third end located at the inner surface of the window, and a second section disposed between the third end and a second end, the second end located proximate to the at least one of the one or more first light emitters. Additionally or alternatively, in some examples, the device further comprises: a second isolation, wherein a first end of the isolation is laterally spaced a first distance away from the second isolation and a second end of the isolation is laterally spaced a second distance, different from the first distance, away from the second isolation. 
     A method for determining a physiological signal is disclosed. The method can comprise: emitting a first light at a user; detecting a reflection of the first light; generating a first signal indicative of the detected reflection of the first light, the first signal including non-pulsatile blood information; emitting a second light at the user; detecting a reflection of the second light; generating a second signal indicative of the detected reflection of the second light, the second signal including pulsatile blood information; adjusting the second signal to compensate for information included in the first signal; and determining the physiological signal based on the adjusted second signal. Additionally or alternatively, in some examples, the first light is emitted at a first portion of the user and the second light is emitted at a second portion, different from the first portion, of the user. Additionally or alternatively, in some examples, the first and second light are emitted at a first portion of the user. Additionally or alternatively, in some examples, the method further comprises: determining one or more peaks included in the first and second signals; and determining one or more locations of the one or more peaks, wherein adjusting the second signal includes scaling the second signal at the one or more locations. Additionally or alternatively, in some examples, the method further comprises: detecting an acceleration of the user; and generating a third signal indicative of the acceleration, wherein adjusting the second signal further includes information included in the third signal. Additionally or alternatively, in some examples, the method further comprises: detecting an acceleration of the user; generating a third signal indicative of the acceleration; comparing the third signal to a threshold value, wherein the second light is emitted at the user when the third signal is greater than or equal to the threshold value. 
     The light emitter(s) and light sensor(s) may be located such that their illumination field(s) and field-of-view(s), respectively, extend from the back surface of the device housing. In some variations, at least a portion of the back surface of the device housing (e.g., the underside of a wearable device) may contact skin when worn by an individual. The back surface of the device may comprise one or more protrusions or raised regions that may be optionally sized and shaped to facilitate skin contact, and/or apply pressure to the skin in order to facilitate movement of non-pulsatile blood away from skin regions that are within, or in the vicinity of, the illumination field(s) and/or field-of-view(s) of the light emitter(s) and light sensor(s) when the device is worn by the individual (e.g., attached to the wrist, arm, chest, leg, etc.). Since non-pulsatile blood flow may be a significant contributor of motion artifacts in pulsatile blood measurements, reducing the flow of non-pulsatile blood in this region this may help to improve the optical measurements of pulsatile blood. 
     In some variations, the protrusions may have one or more curves or contours that apply pressure to the skin when the device is worn by the individual. For example, the one or more protrusions may comprise one or more curves or contours that may be convex, and/or concave, and/or convex in some regions and concave in other regions. In some variations, the convex regions of the one or more protrusions may be disposed over the light paths of the light emitter(s) and/or light sensor(s). In other variations, the concave regions of the one or more protrusions may be disposed over the light paths of the light emitter(s) and/or light sensor(s). The one or more protrusions may comprise transparent and/or opaque regions. The regions of the protrusion(s) that are located within the illumination field and/or field-of-view (i.e., optical path or light path) of the light emitter(s) and light sensor(s) may be transparent or translucent, while other regions of the protrusion may be opaque. For example, one or more protrusions may be disposed within the optical path of the light emitter(s) and/or light sensor(s). The back surface (i.e., underside of the device) may comprise an opening or a window in the housing that is aligned with the illumination field and/or field-of-view of the light emitter(s) and/or light sensor(s) and an optically transparent cover structure disposed over or within the opening. For example, the cavity within which the light emitter(s) and/or light sensor(s) reside may comprise an opening or window. The cover structure may be flush with respect to the housing surface, or may be concave or convex. In some variations, a protrusion may comprise a convex cover structure. Some protrusions or cover structures may comprise an optical barrier or isolation as described herein that extends through the thickness of the protrusion. The isolation may obstruct or prevent light from one side of the barrier from interfering with light from the other side of the barrier. In some variations, the isolation may extend continuously from within the cavity and through the thickness of the protrusion. The isolation may be a single component that extends through the cavity and the protrusion, or may be comprised of one or more isolation segments that connected together. The isolation may be approximately parallel to the optical path of the emitters or detectors. The size and shape of an optical opening or window of a cavity may correspond with the size and shape of the illumination field and/or field-of-view of the light emitter(s) and/or light sensor(s). Alternatively or additionally, the diameter of an optical opening or window may vary from about 1 mm to about 20 mm, for example, about 3 mm, about 4 mm, about 5 mm, about 6 mm, about 8 mm, about 10 mm, about 12 mm, about 15 mm, etc., for example, about 5.4 mm, about 6.4 mm. 
     Some variations of a wrist-worn device may have a housing that comprises a protrusion that circumscribes and/or at least partially surrounds and/or encloses the optical opening(s) or window(s) of one or more cavities within which the light emitter(s) and/or light sensor(s) are disposed. The protrusion may not be located in the optical path of the light emitter(s) and/or light sensor(s). One variation of a protrusion that at least partially surrounds the one or more optical openings of one or more cavities of a device is depicted in  FIGS. 16A-16B .  FIG. 16A  depicts a back surface  1600  of a wrist-worn device (such as any of the devices depicted in  FIGS. 1A-1C ), and a protrusion  1602  that at least partially surrounds the optical openings  1604  of the cavities of the device. One or more optical components (e.g., light emitter(s), light sensor(s), or a combination thereof) may be located within the housing of the device and aligned with an optical opening  1604  of a corresponding cavity. A transparent or translucent cover structure may be disposed over or within each of the optical openings or cavities. The protrusion  1602  may be ring-shaped, which may be an open or closed ring. In still other variations, the protrusion may be arc-shaped. The enclosed region  1606  of the underside or back surface  1600  that is surrounded by the protrusion  1602  may have a convex curvature, as depicted in  FIG. 16B , or may have a concave curvature. In still other variations, the enclosed region  1606  may not have any curves, and may be substantially flat. As depicted in  FIG. 16A , the protrusion  1602  may surround all of the optical openings  1604  and corresponding cavities, but it should be understood that the protrusion  1602  might surround a subset of the openings and corresponding cavities. For example, some devices may comprise a first protrusion that surrounds a first set of the cavities and a second protrusion that surrounds a second set of the cavities. A device may comprise two or more protrusions that may not surround or enclose any of the cavities, but may span a length or width of the housing of the device.  FIG. 16B  depicts a side view of the back surface  1600  having a ring-shaped protrusion  1602  when attached to skin  1608  of an individual. In this example, the protrusion  1602  may apply pressure that is focused in a ring around the optical windows  1604 . That is, the skin area under the protrusion  1602  may be displaced more (i.e., subject to greater levels of pressure) than the skin area under the region  1606  or optical openings  1604 . A protrusion that surrounds the optical openings and/or corresponding cavities may subject skin that surrounds the optical openings and/or corresponding cavities to greater levels of pressure as compared to skin located directly underneath the optical openings and/or corresponding cavities. As described previously, the optical components that are disposed within each of the cavities that correspond to each of the optical openings  1604  may comprise one or more light emitters, one or more light sensors, or a combination of one or more light emitters and one or more light sensors, as described above. The cover structures may each include an isolation that extends from the cavity and through the thickness of the cover structure. 
     In some variations, the underside or back surface of a device may comprise a protrusion that comprises a raised region that extends from the surface of the housing. The cavities within which the light emitter(s) and/or light sensor(s) and their corresponding optical openings may be located on the protrusion. For example, the protrusion may form a plateau that extends from the surface of the housing, and the optical openings and/or corresponding cavities may be located on the surface of the plateau. The plateau may extend over or across a substantial portion of the area of the back surface (e.g., the surface area of the plateau may be about 30%, or about 40%, or about 50% or about 60% or more, of the surface area of the entire back surface). For example, the surface area of the plateau may be approximately the same as the surface area of the underside or back surface (e.g., covers the entire back surface), or the surface area of the plateau may be about 20% less, about 30% less, about 40%, about 50% less than the surface area of the back surface. One variation of a device having a back surface that comprises a protrusion or raised region that extends from and across the back surface is depicted in  FIGS. 17A-17B . Underside or back surface  1700  may comprise a protrusion  1702  comprising a surface that is raised relative to the other regions  1706  of the back surface  1700 . The device may comprise four optical openings  1704  corresponding to four cavities that are located on the raised surface of the protrusion  1702 . The surface of the protrusion  1702  that contacts the skin may be flat (i.e., without any curves), or may have a convex curve, as depicted in  FIGS. 17A and 17B . The cover structures disposed over or within the optical openings  1704  may be flush with the surface of the protrusion  1702 , or may protrude even further from the surface of the protrusion  1702 .  FIG. 17B  depicts a side view of the back surface  1700  when attached to skin  1708  of an individual. The skin regions in contact with the protrusion  1702  may be subject to greater levels of pressure as compared to the skin in contact with the non-raised regions  1706  of the back surface  1700 , as schematically represented by the arrows in  FIG. 17B . That is, the skin region directly underneath and in the vicinity of the optical openings may be subject to increased pressure levels. While the protrusion  1702  is depicted as having a circular shape, it should be understood that the protrusion might have any shape (e.g., ellipse, oval, rectangle, etc.). In other variations, a back surface may comprise two or more raised regions or protrusions that are co-located with the optical openings. For example, a back surface may comprise a first semi-circular protrusion that extends over the portions of the back surface that include a first subset of the cavities and/or corresponding optical openings and a second semi-circular protrusion that extends over the portions of the back surface that includes a second subset of the cavities and/or corresponding optical openings. As described previously, the optical components that are disposed within each of the cavities that correspond to each of the optical openings  1704  may comprise one or more light emitters, one or more light sensors, or a combination of one or more light emitters and one or more light sensors, as described above. The cover structures may each include an isolation that extends from the cavity and through the thickness of the cover structure. 
     In some variations, a back surface of a wearable device may be similar to the protrusion(s) described above and depicted in  FIGS. 17A and 17B , however, the protrusion(s) may comprise one or more recessed regions within which the optical opening or windows of the cavities may be located. The surface of the cover structure disposed over each optical window may be set within each recess such that the cover structure surface is not flush with, nor does it extend beyond, the surface of the protrusion. One variation is depicted in  FIG. 18A . As depicted there, back surface  1800  of a wearable device may comprise a protrusion  1802  that comprises recesses (i.e., recessed regions)  1803  that are each located over an optical opening or window  1804 . That is, the cavities within which the light emitter(s) and/or light sensor(s) are located may themselves be located within a recess of a protrusion. The height of the cover structures located over each of the optical openings may not exceed the depth of each of the recesses  1803 .  FIG. 18C  depicts a side view of the back surface  1800  when contacting skin  1808  of an individual. The skin regions in contact with the regions  1805  of the protrusion  1802  between the recessed regions  1803  or optical windows  1804 , or outside of the recessed regions (e.g., around or near the outer edge or perimeter of the protrusion  1802 ) may be subject to higher levels of pressure as compared to the skin regions in contact with the recessed regions  1803  and/or cover structures of the optical openings  1804 . For example, skin regions that may be subject to increased pressure levels are represented by the arrows in  FIG. 18C . The surface area of the protrusion  1802  may be similar to the surface area of the back surface  1800 , as depicted in  FIG. 18A , or may be less than the surface area of the back surface  1810 , as depicted in  FIG. 18B . As depicted there, the protrusion  1812  may comprise recesses  1813  disposed over the optical openings or windows  1814 , as described previously. The surface area of the protrusion  1812  may be about 20% less, about 30% less, about 40%, about 50% less than the surface area of the back surface  1810 . The skin regions that contact the regions  1816  of the back surface that surround the protrusion  1812 , or that contact the recessed regions  1813  of the protrusion  1812  may be subject to reduced levels of pressure as compared to the skin regions in contact with the protrusion  1812 . As described previously, the optical components that are disposed within the cavities that correspond to each of the optical openings  1804 ,  1814  may comprise one or more light emitters, one or more light sensors, or a combination of one or more light emitters and one or more light sensors. The cover structures may each include an isolation that extends from the cavity and through the thickness of the cover structure. 
     In some variations, the underside or back surface of a wearable device may comprise protrusions disposed in the optical path of the light emitter(s) and/or light sensor(s). In such variations, the protrusions may be optically transparent or translucent. For example, the back surface of a wearable device may comprise one or more cavities each having a corresponding optical opening and a protrusion located over each of the optical openings. In some variations, the cover structure disposed over each of the optical openings may be itself a protrusion that applies focal regions of higher pressure directly on the skin regions located under the optical path of the light emitter(s) and/or detector(s). In other words, the skin region(s) that may be subject to increased levels of pressure may co-localize with the illumination field(s) of the one or more light emitters and/or the field-of-view(s) of the one or more light sensors (in contrast to, for example, a protrusion that applies focal regions of increased pressure to skin that is located between the illumination field(s) and/or field-of-view(s) of the emitters and/or detectors, such as is depicted in  FIG. 18C ). One variation of a device having an underside or back surface comprising protrusions disposed within the optical path(s) of the light emitter(s) and/or light sensor(s) is depicted in  FIGS. 19A-19B . Back surface  1900  may comprise one or more optical openings or windows  1904  and a convex cover structure or protrusion  1902  disposed over each of the optical openings  1904  of the corresponding cavities. The protrusion  1902  may comprise an optically transparent or translucent material such as acrylic, glass, and the like.  FIG. 19B  depicts a side view of the back surface  1900  when the device is worn by an individual and the back surface is located against skin  1908  of the individual. As depicted there, the skin regions located under the protrusion  1902  (which are schematically represented by the arrows in  FIG. 19B ) may be subject to increased levels of pressure as compared to the skin regions located under non-protruding portions  1906  of the back surface  1900 . The radius of curvature of the protrusions  1902  may be consistent across the surface of the protrusion (i.e., the curvature of the protrusion approximates the curvature of a sphere), or may vary (i.e., the curvature of the protrusion may be similar to the curvature of an ovoid). As described previously, the optical components that are disposed within each of the cavities that correspond to each of the optical openings  1904  may comprise one or more light emitters, one or more light sensors, or a combination of one or more light emitters and one or more light sensors, as described above. The cover structures may each include an isolation that extends from the cavity and through the thickness of the cover structure. 
     The height and/or curvature of the one or more protrusions of a back surface of a wearable device may vary, as may be desirable to attain a desired contact and/or pressure profile in the skin of the individual.  FIG. 20  depicts examples of various protrusion surface geometries of protrusions similar to the protrusions depicted in  FIGS. 19A and 19B  (though such sizes and geometries may be applicable to any of the protrusions and/or optical window cover structures described previously).  FIG. 20  depicts an underside or back surface  2000  of a wearable device comprising a protrusion  2002  that is located over an optical opening or window  2004  of a cavity  2006 . In some variations, the protrusion may comprise the cover structure disposed over the optical opening. The protrusion  2002  may have a height  2003  from about 0.3 mm to about 2 mm, for example, about 0.5 mm, or about 0.9 mm, about 1.1 mm, about 1.3 mm, etc. The radius of curvature of the protrusion  2002  may be from about 2.5 mm to about 8.5 mm, for example, about 3.23 mm, about 3.43 mm, about 4.25 mm, about 4.47 mm, about 6.5 mm, about 7.47 mm, etc. The width  2005  of base of the protrusion  2002  may span the width of the optical opening  2004 , or may be less than the width of the optical opening. In some variations, the width  2005  may be from about 3 mm to about 10 mm, for example, about 3.5 mm, about 4.6 mm, about 5.4 mm, about 6 mm, about 7.3 mm, about 8.8 mm, etc. 
     In some variations, the cover structure and/or protrusion may comprise a Fresnel lens or similar optical component. Since the wearable device may include several optical components and associated wiring, it can be desirable to obscure the components and prevent internal components from being visible to a user&#39;s eye. In addition to obscuring the internal components, it may be desirable that the light emitted from a light emitter retains its optical power, collection efficiency, beam shape, and collection area so that the intensity of light is unaffected. To obscure internal components, one or more lenses such as Fresnel lenses may be located in the protrusion, and/or between the protrusion and cover structure, and/or in the cover structure, and/or within the thickness of the housing material, and/or underneath the housing (e.g., within the volume enclosed by the housing). For example, a Fresnel lens  2102  may be located between the protrusion  2100  and a light emitter  2106  that is located within a cavity  2110 , as shown in  FIG. 21A . In this variation, the Fresnel lens  2102  may be located above the optical opening  2104  of the cavity  2110 , that is, extending from the surface  2101  of the device housing  2103 . Fresnel lens  707  can have two regions: an optical center  2109  and a cosmetic zone  2111 . Optical center  2109  can be placed in substantially a same area or location as light emitter  2106  to collimate the emitted light into a smaller beam size. Cosmetic zone  2111  can be located in areas outside of optical center  2109 . The cosmetic zone  2111  may comprise ridges that may help to obscure the underlying internal components. Optionally, a light sensor  2108  disposed within the same cavity  2110  as the light emitter  2106  may be covered by the same or different Fresnel lens, which may or may not have an optical center (i.e., a large-area light sensor may be a large-area photodiode that may not require shaping of the light field may not require a Fresnel lens with an optical center and instead may use a Fresnel lens having one or more regions comprising ridges configured for a cosmetic zone). The ridge shapes of the Fresnel lens  2102  may vary to help facilitate obscuration, especially in cosmetic zones. For example, deep and sharp saw tooth patterns can be used for high obscuration needs. Other types of ridge shapes can include rounded cylindrical ridges, asymmetric shapes, and wavy shapes (i.e., ridges that move in and out). The Fresnel lens  2102  may be used additionally or alternatively for light collimation. By collimating light, the optical signal efficiency can be improved. Without a lens or similar collimating optical element, emitter light may be directed at an angle away from the light sensor and can be lost. Additionally or alternatively, light may be directed at an angle toward the light sensor, but the angle may be shallow. The Fresnel lens  2102  may redirect light to directions that otherwise may be lost or enter into the tissue at shallow angles. Such redirected light can be collected instead of being lost and/or may militate against parasitic non-signal light, resulting in improved optical signal efficiency. In some examples, a diffusing agent may be used alternatively or additionally to a Fresnel lens. A diffusing agent may be surrounding, touching, and/or covering one or more components of a light emitter. In some examples, diffusing agent may be a resin or epoxy that encapsulates the dies or components and/or wire bonds. Diffusing agent may be used to adjust the angle of the light emitted from the light emitter. By narrowing the beam of light emitted, more light may be collected by the lens and/or window resulting in a larger amount of detected light by the light sensor. 
     In another variation depicted in  FIG. 21B , a Fresnel lens  2122  may be located between the protrusion  2120  and a light emitter  2126  that is located within a cavity  2130 . In this variation, the Fresnel lens  2122  may be located within the optical opening  2124  of the cavity  2130 , that is, within the thickness of the device housing  2123 . There may optionally be a light sensor  2128  within the cavity  2130 . The Fresnel lens  2122  may have any of the characteristics described above, and may or may not have an optical center located over either the light emitter  2126  and/or light sensor  2128  (the variation in  FIG. 21B  uses a Fresnel lens that does not have an optical center). 
     As indicated above, some variations of protrusions may comprise an isolation that extends through the entire thickness of the protrusion, where the isolation is configured to separate the light paths of the optical components on one side of the protrusion from the other side. The isolation may extend from within the cavity, through the cavity and through the thickness of the protrusion.  FIG. 22A  depicts one variation of an underside or back surface  2200  of a wearable device comprising a protrusion  2202  disposed over an optical opening  2204  of a cavity  2206 , where the protrusion  2202  comprises an isolation or optical barrier  2203  extending through the thickness of the protrusion. In this example, the isolation  2203  extends from inside the cavity  2206  to and through the protrusion  2202 . While the isolation  2203  is depicted as being substantially perpendicular to the base of the cavity, it should be understood that the isolation  2203  may be at an angle with respect to the base of the cavity. A first optical component  2208  (e.g., a light emitter or light sensor) may be located on one side of the cavity and a second optical component  2210  (e.g., a light sensor or a light emitter) may be located on the other side of the cavity such that the isolation  2203  separates the light paths of these first and second optical components within the protrusion and the cavity. Isolation  2203  may be similar to any of the isolation variations described previously.  FIG. 22B  depicts another variation of an underside or back surface  2220  of a wearable device comprising a protrusion  2222  disposed over an optical opening  2224  of a cavity  2226 , where the protrusion  2222  comprises an isolation or optical barrier  2223  extending through the thickness of the protrusion. In this variation, a Fresnel lens assembly  2225  may be located within the optical opening and/or cavity  2226  (e.g., as part of the thickness of the housing and/or located within the volume enclosed by the housing). Alternatively or additionally, a Fresnel lens assembly may be located within the protrusion  2222 , as previously described. The Fresnel lens assembly  2225  may comprise a first Fresnel lens and a second Fresnel lens that are each coupled to one side of the isolation  2223 . The first and second Fresnel lenses may be manufactured as two separate and/or independent Fresnel lenses that are attached to the isolation  2223 . Alternatively, the first and second Fresnel lenses may be manufactured as a single Fresnel lens and then cut into two components and attached to the isolation  2223 . The one or more Fresnel lenses of a Fresnel lens assembly may have any of the lens characteristics described previously. 
     In some variations of a wearable device, there may be two or more light emitters in the same cavity and one or more Fresnel lens disposed between the protrusion and the light emitters. The Fresnel lens may comprise one optical center for each light emitter within the cavity.  FIG. 22C  depicts a wearable device  2230  comprising three light emitters  2232   a,b,c  located within a cavity  2234 , and a Fresnel lens  2236  disposed between the light emitters  2232   a,b,c  and a cover structure and/or protrusion  2238  located over the opening of the cavity  2234 . In this example, there may be a light sensor  2240  disposed in the same cavity of the light emitters and an isolation  2242  that provides an optical barrier between the light sensor and the light emitters. The light emitters  2232   a,b,c  may be collinearly arranged, or may be offset with respect to each other in any pattern. In some variations, light emitters of a particular wavelength may be located closer to the light sensor than the other light emitters. For example, red and/or infrared light emitters may be in closer proximity to the light sensor than a green light emitter.  FIG. 22D  is a top view of the cavity  2234  of the device of  FIG. 22D . The Fresnel lens  2236  may comprise three optical centers  2237   a,b,c  that are each located over (e.g., aligned with) a corresponding light emitter  2232   a,b,c . The ridge pattern as viewed from above the underside of the device may appear to have three sets of concentric rings, or spirals, or a plurality of concentric and/or merged arcs. In some variations, where the light emitters are not arranged collinearly, the optical centers of the Fresnel lens may be offset with respect to each other. Alternatively, the Fresnel lens may not have any optical centers located over any of the light emitters (and/or light sensors).  FIG. 22E  depicts an example of a cavity of a wearable device similar to that of  FIG. 22D , but where the Fresnel lens does not have an optical center that is located over (e.g., aligned with) a light emitter. The ridge pattern of the Fresnel lens  2250  as viewed from above the underside of the device may appear to have arc-shaped edges. In some variations, a Fresnel lens without any optical centers may have the appearance of a plurality of concentric semicircles, partially truncated circles (e.g., having one, two, three, four or more truncated sides), concentric arcs and the like. The ridge pattern shape, size, edge density, etc. may vary from those depicted in  FIGS. 22C and 22D , as may be desirable. 
     Any number, size, shape/geometry, etc. of the protrusions described above may be applied to any of the devices described herein, as may be desirable. 
     As discussed above, noise correction can be performed by using a noise reference channel (e.g., including a red or infrared light emitter) to correct or adjust the signal measured by a PPG channel (e.g., including a green light emitter). In some instances, the optical attenuation coefficient associated with the noise reference channel can be smaller than the optical attenuation coefficient associated with the PPG channel. As a result, the light from the noise reference may traverse deeper into the user&#39;s tissue compared to the light from the PPG channel, even if the separation distance between the light emitters and light sources are the same. With light traversing deeper, the sensing volume in the user&#39;s tissue of the noise reference channel may differ from the sensing volume of the PPG channel, thereby possibly reducing the effectiveness of the noise correction. One way to enhance the effectiveness of the noise correction can be to configure the light emitter and light sensor for the noise reference channel to have a shorter separation distance than the light emitter and light sensor for the PPG channel. In some instances, the differing separation distances can result in the optical components occupying a larger area on the back of the device, which can lead to larger windows. 
     Another way to enhance the effectiveness of the noise correction can be to co-localize the noise reference and PPG channels.  FIGS. 23A-23C  illustrate cross-sectional views of exemplary configurations of light emitters for co-localizing the noise reference and PPG channels according to examples of the disclosure. In some examples, the light emitters can be configured with different light emission angles, as illustrated in  FIG. 23A . Device  2300  can include light emitter  2305 , light emitter  2306 , and light sensor  2304 . Light emitter  2305  and light emitter  2306  can be co-localized such that the separation distance between light emitter  2305  and light sensor  2304  relative to the separation distance between light emitter  2306  and light sensor  2304  can be substantially the same (e.g., within 10% difference). For example, light emitter  2305  can be located in close proximity (e.g., less than or equal to 1 mm away) to light emitter  2306 . In some examples, light sensor  2304  can be located in a cavity different from light emitter  2305  and light emitter  2306 , separated by isolation  2319 . In some examples, one or more light emitter-light sensor sets located in different cavities can be configured to measure pulsatile blood volume changes. In some examples, one or more light emitter-light sensor sets located in the same cavity can be configured to measure non-pulsatile blood volume changes (from shallow tissues structures, deep tissue structures, or both) and/or serve as a noise reference channel. For example, the set comprising light emitter  2306  and light sensor  2304  can be configured to be sensitive to pulsatile blood volume changes. The set comprising light emitter  2305  and light sensor  2304  can be less sensitive to arterial blood volume changes (than the set comprising light emitter  2306  and light sensor  2305 ) and can be configured to generate a signal indicative of the non-pulsatile blood changes (e.g., noise). 
     Light emitter  2305  can be configured to emit light towards skin  2320  and in the direction of light sensor  2304 , while light emitter  2306  can be configured to emit light towards  2320  and away from the direction of light sensor  2304 . In this manner, light emitted by light emitter  2306  can probe deeper into skin  2320 , and light emitter by light emitter  2305  can probe shallower into skin  2320 . The depth of penetration can be achieved by configuring the angles of light emission from light emitter  2305  and light emitter  2306  to be different, for example. The angles of light emission can be configured such that light from both light emitter  2305  and light emitter  2306  are incident on the same location  2301  and penetrate the same volume of tissue in skin  2320 . As a result, device  2300  can include multiple channels configured to measure a similar optical sensing volume even if the attention coefficients of the multiple channels differ and with reduced area constraints. In some variations, the light emitter (e.g., light emitter  2306 ) included in the PPG channel can be located closer to light sensor  2304  than the light emitter (e.g., light emitter  2305 ) included in the noise reference channel. Controller  2309  can receive signal  2350  and signal  2355  and can apply one or more algorithms to determine the user&#39;s physiological signal. Although the figure illustrates a single light ray emitted by the light emitters, examples of the disclosure can include multiple light rays emitted by the light emitters; a single light ray is illustrated for clarity purposes. 
     In some examples, the device can include one or more reflective walls for changing the depth of penetration of the different light emitters, as illustrated in  FIG. 23B . The device can include reflective walls  2315 . At least a portion of light emitted by light emitter  2306  can be directed towards light sensor  2304  and can reflect off a reflective wall  2315 . A portion of light emitted by light emitter  2305  can be directed away from light sensor  2304  and can reflect off a reflective wall  2315 . Both light rays can be incident on the same location  2301  and/or same volume of tissue in skin  2320 . 
     In some examples, the device can include one or more Fresnel lenses for changing the angles of light emitted by the light emitters, as illustrated in  FIG. 23C . The device can include Fresnel lens  2322 . In some examples, light emitted by light emitter  2305  and light emitter  2306  can include the same angle of incidence on the surface of Fresnel lens  2322 . The Fresnel lens can be configured to change the angle of incidences of the light beams. For example, Fresnel lens  2322  can redirect light emitted by emitter  2306  towards one direction and can redirect light emitted by light emitter  2306  towards another direction. In some examples, the angles of light exiting Fresnel lens  2322  can be between 30-60°. The angles for light originating from light emitter  2306  can be the same or different from light originating from light emitter  2305 . Both light rays can be incident on the same location  2301  and/or same volume of tissue in skin  2320 . 
     Although the disclosed examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosed examples as defined by the appended claims.

Metadata:
Filing Date: 20170510
Publication Date: 20200623
Grant Date: 20200623
Priority Date: 20160510
Inventors: ALLEC, NICHOLAS PAUL JOSEPH
PETERSON, Rui
BLOCK, UEYN L.
VENUGOPAL, VIVEK
Assignee: APPLE INC
CPC Classifications: [{"code": "A61B2562/0238", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/7264", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/7214", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/721", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6898", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6843", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/681", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/14552", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/11", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0295", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0261", "inventive": true, "first": true, "tree": "[]"}, {"code": "A61B5/02438", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/02427", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/02427", "inventive": true, "first": true, "tree": "[]"}, {"code": "A61B5/0205", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/0219", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/02427", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/7214", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/14552", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/02438", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/7264", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6898", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0261", "inventive": true, "first": true, "tree": "[]"}, {"code": "A61B5/02433", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/02433", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/7214", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/11", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/046", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/02438", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/0238", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/0205", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/681", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0295", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/721", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6843", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/681", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/185", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/6898", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/721", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/146", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/14552", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/0219", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/0205", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/7264", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6843", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/7214", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/146", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B2562/0238", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B2562/0219", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/681", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B2562/046", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/02433", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B2562/185", "inventive": false, "first": false, "tree": "[]"}, {"code": "A61B5/14552", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/6898", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/11", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/02438", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/721", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0261", "inventive": true, "first": true, "tree": "[]"}, {"code": "A61B5/02427", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0295", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 58765935