Systems and methods for calibrating speckle-based sensors

Abstract

A system and method for calibrating a speckle-based sensor. In some embodiments, the system comprises a wearable device, the wearable device comprising: a laser; an array detector; and processing circuitry configured to: obtain a calibration of the array detector using the incoherent light source; obtaining a speckle image by using a laser; and calculating a corrected speckle-based measurement, the corrected speckle-based measurement being based on the speckle image and based on the calibration.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to and the benefit of U.S. Provisional Application No. 63 / 351,316, filed on June 10, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0002] One or more aspects of embodiments according to the present disclosure relate to speckle-based measurements, and more particularly to systems and methods for calibrating speckle-based sensors. Background Art

[0003] Speckle-based measurements, such as speckle plethysmography (SPG), can use measured speckle contrast or quantities related to the speckle contrast to infer properties of a sample. Speckle contrast measurements can be affected by factors such as shot noise, dark noise, and image vignetting.

[0004] It is to this general technical environment that various aspects of the present disclosure are relevant. Summary of the invention

[0005] According to an embodiment of the present disclosure, a system is provided, the system comprising: a wearable device, the wearable device comprising: a laser; an array detector; and a processing circuit, the processing circuit being configured to: obtain calibration of the array detector using an incoherent light source; obtain a speckle image using a laser; and calculate a corrected speckle-based measurement, the corrected speckle-based measurement being based on the speckle image and on the calibration.

[0006] In some embodiments, the calculation of the corrected speckle-based measurement includes correcting the speckle image for estimated contributions from shot noise and dark noise.

[0007] In some embodiments, obtaining calibration of the array detector includes: illuminating the array detector with light from an incoherent light source; obtaining a first image through the array detector; and obtaining a second image through the array detector, wherein the intensity in the first image is different from the intensity in the second image.

[0008] In some embodiments: obtaining calibration of the array detector includes: fitting a straight line to a plurality of data points including a first data point and a second data point; the first data point includes: the intensity of a first image; and the standard deviation of a third image, the third image being based on the first image; and, the second data point includes: the intensity of the second image; and the standard deviation of a fourth image, the fourth image being based on the second image.

[0009] In some embodiments, the processing circuit is configured to form the third image by processing the first image with a high pass filter.

[0010] In some embodiments, the high pass filter is implemented by: processing the first image with a low pass filter to form a low pass filtered image; and subtracting the low pass filtered image from the first image.

[0011] In some embodiments, the low pass filter is a moving average filter.

[0012] In some embodiments, the wearable device includes an incoherent light source.

[0013] In some embodiments, the incoherent light source is a light emitting diode.

[0014] In some embodiments: the wearable device includes a photoplethysmographic sensor, the photoplethysmographic sensor includes a light emitting diode, and the incoherent light source is the light emitting diode of the photoplethysmographic sensor.

[0015] In some embodiments, the laser includes an optical amplifier and the incoherent light source includes the optical amplifier.

[0016] In some embodiments, the incoherent light source includes a laser.

[0017] In some embodiments, the incoherent light source includes a modulator configured to modulate light generated by the laser to reduce a coherence length of the light.

[0018] In some embodiments, the incoherent light source includes a laser driving circuit configured to: in a first state, drive the laser with a current that causes single-mode operation; and in a second state, drive the laser with a current that causes multi-mode operation.

[0019] In some embodiments: the processing circuit is further configured to measure a static scatter variance, and the corrected speckle-based measurement is further based on the static scatter variance.

[0020] In some embodiments, measuring the static scatter variance includes: obtaining a plurality of speckle images using a laser; averaging the speckle images in the plurality of speckle images; and calculating the variance of the average.

[0021] In some embodiments, the system further includes a motion sensor connected to the processing circuit.

[0022] In some embodiments, the processing circuit is further configured to: during the calibration measurement, detect motion exceeding a threshold; and in response to detection of motion exceeding the threshold, repeat the calibration measurement.

[0023] In some embodiments, the processing circuit is further configured to: during the speckle measurement, detect motion exceeding a threshold; and in response to the detection of motion exceeding the threshold, initiate a calibration process.

[0024] In some embodiments, the array detector is a zero degree chief ray angle detector. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] These and other features and advantages of the present disclosure will be appreciated and understood with reference to the specification, claims, and drawings, in which:

[0026] Figure 1A is a block diagram of a wearable device and samples according to an embodiment of the present disclosure;

[0027] Figure 1B is a schematic diagram of a wearable device according to an embodiment of the present disclosure;

[0028] Figure 1C is a schematic diagram of a wearable device according to an embodiment of the present disclosure;

[0029] Figure 2A is a description of calibration status and results according to an embodiment of the present disclosure;

[0030] Figure 2B is a graph of noise as a function of intensity according to an embodiment of the present disclosure;

[0031] Figure 3 is a graph of SPG measurements as a function of flow rate according to an embodiment of the present disclosure; and

[0032] Figure 4 is a flow chart according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0033] The detailed description set forth below in conjunction with the accompanying drawings is intended as a description of exemplary embodiments of systems and methods for calibrating speckle-based sensors provided according to the present disclosure, and is not intended to represent the only forms in which the present disclosure may be constructed or utilized. In conjunction with the illustrated embodiments, the description sets forth the features of the present disclosure. However, it is to be understood that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be included within the scope of the present disclosure. As indicated elsewhere herein, like element numbers are intended to indicate like elements or features.

[0034] A speckle plethysmography sensor may illuminate a sample with light from a coherent light source (e.g., a laser) and detect light scattered from or transmitted through the sample with an array detector or "imaging detector" such as a complementary metal oxide semiconductor (CMOS) image sensor (CIS). Light scattered from or transmitted through the sample may form a speckle pattern on the image sensor. The speckle pattern may change if there is motion within the sample, such as if the sample includes a blood vessel of a patient or subject and blood is flowing within the vessel. The rate at which the speckle pattern changes may be related to motion in the sample; for example, the greater the velocity of blood flow in a sample through which blood is flowing, the greater the rate at which the speckle pattern changes.

[0035] Speckle contrast may be measured based on an image acquired by an array detector sensing the speckle pattern. For example, the measured speckle contrast may be proportional to the variance in pixel values ​​calculated across the image. If the exposure time of the array detector is sufficiently large so that the speckle pattern changes significantly during the exposure, the measured speckle contrast may decrease due to motion in the sample. As such, the measured speckle contrast may be used to infer characteristics of motion in the sample, such as the flow of blood in the sample.

[0036] However, over a wide range of flow rates between 0 mm / s and 10 mm / s, raw speckle contrast measurements may not be linearly related to changes in blood flow, which may affect the ability to compare measurements between subjects and may hinder longitudinal studies. As such, in some embodiments, sensor calibration and correction methods based on the calibration may be used to calculate corrected speckle-based measurements that may exhibit more linear changes with flow velocity, and in which the effects of certain error sources (such as shot noise and dark noise) may also be reduced. A more linear relationship between measured corrected speckle contrast and blood flow velocity may improve the accuracy of a variety of physiological measurements, including blood pressure.

[0037] Figure 1A A wearable device 105 and a sample 110 are shown in some embodiments. The wearable device can be a housing containing an instrument that can be worn on a subject's wrist like a watch, or it can be a similar (e.g., larger) device that can be worn on a subject's chest or leg, or elsewhere on the subject. The wearable device can be wireless (e.g., battery powered, and connected using only wireless signals, if any). In such embodiments, the wearable device 105 can be adjacent to (e.g., in close proximity to) the subject's skin, and the sample can be a portion of the subject's skin and a portion of the tissue beneath the skin (the tissue can include blood vessels within which flow is to be measured).

[0038] The wearable device 105 may include: (i) a laser 115 and an array detector 120 for making speckle plethysmography measurements; and (ii) one or more light emitting diodes (LEDs) 125, and a photodetector (e.g., a photodiode) 130. The LED 125 and the photodiode 130 may be used to perform photoplethysmography (PPG) measurements. The LED 125 and the photodiode 130 may also (or alternatively) be used to calibrate the speckle plethysmography sensor. The wearable device 105 may also include other sensors, such as a temperature sensor 135, and one or more motion sensors (e.g., an accelerometer or a gyroscope) 140. The processing circuit 145 may receive sensing signals from other elements of the wearable device 105, or control other elements of the wearable device 105.

[0039] Figure 1B A candidate layout of the components is shown in a plan view (a top view in an orientation in which the subject's skin and the surface of the module adjacent to the skin are horizontal). A window in the enclosure of the wearable device 105 may allow light from the light emitting elements 115, 125 to reach the sample 110, and may also allow light returned from the sample 110 to return to the light detecting elements 120, 130. The light emitting elements 115, 125 may be positioned sufficiently close to the light detecting elements 120, 130 to allow light from any of the light emitting elements 115, 125 to reach both of the light detecting elements 120, 130 via a path through the sample.

[0040] Figure 2A Combinations of states of the laser 115 and states of the light emitting diode 125 that can be used for various calibration operations are shown. As discussed in more detail below, calibration of the wearable device 105 can be performed at startup (or when the wearable device 105 is first attached to a subject) and periodically thereafter (e.g., when triggered by any of a number of calibration trigger events). The calibration can include one or more of the following measurements: (i) dark noise and dark current measurements for the array detector 120, (ii) shot noise measurements, (iii) ambient light measurements, (iv) vignetting measurements, and (v) static scattering measurements. The ambient light intensity can be estimated by capturing an image without any of the light sources 115, 125 being turned on.

[0041] Turning on the LED 125 (which may be an incoherent or non-speckle generating source that may cause incoherent light to illuminate the array detector 120 via an optical path through the sample 110) at different intensity levels (e.g., at different drive currents) enables calculation of shot noise relative to intensity. If the wearable device 105 includes a PPG sensor, shot noise calibration of the array detector 120 may be performed simultaneously with an automatic gain control (AGC) sequence for the PPG sensor. For example, the light emitting diode 125 of a photoplethysmographic sensor may be operated at several different levels of drive current, thereby providing different levels of illumination to both the array detector 120 and the photodiode 130. The signal measured by the photodiode 130 during this process may be used to calibrate the automatic gain control of the photoplethysmographic sensor.

[0042] The images obtained by the array detector 120 during this process may be used to perform shot noise calibration, dark noise calibration, or vignetting calibration. Figure 2B A graph showing the standard deviation (across images) as a function of intensity. The standard deviation may be calculated by first applying a spatial high-pass filter to each image and calculating the standard deviation of the filtered image. The high-pass filter may be a suitable infinite or finite impulse response filter; for example, it may be implemented by subtracting a low-pass filtered version of the image from each image, the low-pass filter being a moving average filter (e.g., a five-pixel by five-pixel moving average filter).

[0043] Data (such as Figure 2B Data of the intensity readings of the array detector 120 can be obtained by changing the drive current of the light emitting diode 125 or changing the exposure time used to obtain different intensity values ​​(where intensity in this context is a measure of the total number of photons absorbed during the exposure time). Increasing the current of the light emitting diode 125 or the exposure time of the array detector 120 increases the intensity reading of the array detector 120. Figure 2B , the average standard deviation of the pixels increases linearly with camera intensity. A straight line can then be fit to the data, and this line can be used to calculate the contribution of shot noise and dark noise to the variance (in a subsequently acquired image, based on the average intensity in that image). In some embodiments, the gain of the analog-to-digital converter or amplifier of the array detector 120 can be adjusted to a variety of different values, and a different corresponding calibration can be performed for each value, so that if the gain of the array detector 120 is changed to a new gain setting during operation (e.g., if the sample scatters an abnormally high or low fraction of the laser light back to the array detector 120), the shot noise and dark noise calibrations obtained for the new gain setting can be used.

[0044] Static scatter variance may be a measure of the variance due to (i) speckle produced by static scattering and (ii) vignetting. To measure the static scatter variance, the sample 110 may be illuminated by a laser 115, thereby forming a speckle pattern on the array detector 120. This speckle pattern may have: (i) a static component due to scattering from stationary elements of the sample 110; and (ii) a changing component due to scattering from moving elements (e.g., flowing blood) in the sample 110. Multiple images may be acquired by the array detector 120, and these images may be averaged together, and the variance of this average may be calculated; this variance may be a measure of the static scatter variance. In performing this measurement, it may not be necessary to save the individual images; instead, each image may be discarded after having been added to the accumulated image (which may later be divided by the number of images that contributed to it to obtain an average image). In the averaged image, speckle due to dynamic scatterers may be averaged out, but speckle due to static scatterers may become sharper after averaging if the time interval over which the images are acquired is less than the decorrelation time of the speckle due to static scattering. The static scattering image also allows calculation of the variance contribution due to vignetting (and correction for that variance contribution), which scales with intensity. Vignetting and other similar low spatial frequency noise sources may occur due to miniaturization constraints, tissue inhomogeneities, sensor fixed pattern noise, or dirt or dust covering one or more of the apertures used to control speckle size. In some embodiments, as discussed in more detail below, vignetting (e.g., variance due to vignetting, or variance including contributions from vignetting (such as static scattering variance)) is measured, and corresponding corrections are then made. In some embodiments, vignetting is alternatively (or also) reduced by using a sensor (e.g., CIS) that lacks a microlens array as the array detector 120. Such a sensor may also be referred to as a zero-degree chief ray angle (CRA) detector (or, for example, as a zero-degree CRACIS).

[0045] As mentioned above, the wearable device 105 may include a temperature sensor 135 (e.g., a thermistor) and a motion sensor 140 (such as an inertial measurement unit (IMU)), one or more magnetometers, one or more gyroscopes, or one or more accelerometers. The temperature calculated from the thermistor can be used to improve calibration or to perform an estimate of Brownian motion (which can affect the measured speckle contrast). The accelerometer can be used to determine the quality of the calibration; for example, if there is too much movement, it can be determined that motion artifacts may have degraded the quality of the calibration, and the calibration steps can be repeated. If the wearable device 105 includes a photodiode 130, the photodiode 130 can also be used during calibration to confirm the intensity observed by the array detector 120 (e.g., for different light emitting elements 115, 125 or for different intensities produced by the light emitting elements 115, 125), or to help set the initial exposure time of the array detector 120. Additional sensors can also be used to determine when it is time to recalibrate for SPG measurements, such as using an IMU to determine that the subject is in a new body posture.

[0046] In some embodiments, a new calibration may be performed when the wearable device 105 is first turned on (as mentioned above), when the quality of the image obtained by the array detector 120 is unacceptable, when excessive motion of the wearable device 105 relative to the sample is detected (e.g., by a change in speckle plethysmography measurement, a change in photoplethysmography measurement, or a signal from the motion sensor 140), when a change in the subject's body posture is detected (e.g., by a change in speckle plethysmography measurement, a change in photoplethysmography measurement, or a signal from the motion sensor 140), when a change in perfusion is detected (e.g., by a change in speckle plethysmography measurement), when a change in blood volume is detected (e.g., by a change in photoplethysmography measurement), when a set time interval has passed since the most recent calibration, or when a user (e.g., a subject) initiates calibration. In some embodiments, some of the trigger conditions listed above may not result in a complete recalibration of the wearable device 105; for example, shot noise and dark noise calibration may not be repeated each time calibration is performed. Calibration may be performed discretely (e.g., when triggered by one of the conditions listed above) to reduce power consumption, or continuously to improve accuracy. Image quality assessment may include quantifying saturated pixels (potentially due to changes in ambient light), quantifying pixels at dark current, or assessing image non-uniformity outside of expected vignetting (potentially due to dust obscuring a portion of the CIS).

[0047] When the calibrations described above have been performed, they can be used to compute corrected speckle-based measurements. As used herein, "speckle-based measurements" are any measurements based on the speckles in one or more images obtained by an array detector. For example, a corrected speckle-based measurement can be a linearized corrected speckle contrast K, which can be computed using the following equation:

[0048]

[0049] where is the variance in the image, is the static scattering variance, is the variance due to shot noise and dark noise, <I_image> is the average intensity in the image, and <I_dark> is the average dark current equivalent intensity in the image. In some embodiments, additional corrections can be made in a similar manner; for example, the variance due to read noise and the variance due to any non-uniformity in the ambient light can also be subtracted from the variance in the image. As another example, the average intensity due to the ambient light can also be subtracted from the average intensity in the image.

[0050] The corrected speckle contrast can be used to compute a linearized SPG measurement (which is another example of a corrected speckle-based measurement), which is defined as follows:

[0051]

[0052] where T is the exposure time.

[0053] In some embodiments, the measurements and calibrations described above are performed on "blocks" or sub-images of the images obtained by the array detector 120. For example, the corrected speckle contrast can be computed for each block, and the results for all the blocks can be averaged together. In another embodiment, the corresponding values of each of the quantities on the right-hand side of Equation 1 above are computed for each block, the average value of each of these quantities is computed (by averaging over all the blocks), and this average value is used in Equation 1 to compute the corrected speckle contrast. The linearized SPG measurement can then be computed from the average corrected speckle contrast. The blocks can have a size of, for example, 5x5, 7x7, 16x16, 30x40, 16x16, or 480x640 pixels, or the entire set of pixels of the array detector 120 can be considered as a single block. In some embodiments, the blocks can overlap. In some embodiments, the temporal speckle contrast is computed for one pixel at a time; in such embodiments, the temporal speckle contrast can be corrected based on some of the calibrations described herein (e.g., based on the shot noise and dark noise calibrations).

[0054] Figure 3The linearized SPG measurements as a function of flow velocity are shown (in the curve labeled “Corrected Spatial SPG”); as well as similar SPG measurements derived from (i) uncorrected speckle contrast (in the curve labeled “Spatial SPG”) and similar SPG measurements derived from (ii) temporal speckle contrast (in the curve labeled “Temporal SPG”). It can be seen (e.g., from the curves labeled with the corresponding R 2 values), the straight line fit to the data for the linearized SPG measurements is significantly better than the straight line fits to the data for the other two data sets.

[0055] Figure 4 A flow chart of a method in some embodiments is shown. The method includes: initiating a calibration process at 405; acquiring an ambient light image at 410; and calculating the ambient light intensity at 415. The method also includes: repeatedly acquiring images at 420; and processing each image through a high pass filter at different intensities (e.g., different drive currents applied to the light emitting diodes 125, or different exposure times) until enough images have been acquired to span the dynamic range of the array detector 120 or to produce a reliable straight line fit to the acquired data at 425. The method also includes: creating a best fit shot noise line at 430; accumulating a plurality (e.g., N) of images at 435; and calculating the variance due to static scattering and vignetting (which may be referred to as static scatter variance) from the plurality of images at 440. The method also includes: acquiring images for speckle plethysmography measurements at 445; calculating the variance of the images at 450; and calibrating the results (e.g., calculating a corrected speckle-based measurement) at 455.

[0056] Any incoherent light source and coherent light source pairing can be used to replace LED 125 and laser 115, respectively. Another embodiment can utilize a single laser (e.g., a vertical cavity surface emitting laser (VCSEL)) as a coherent source and utilize the same laser modulated (e.g., phase modulated) as an incoherent source. Another embodiment can include a photonic integrated circuit (PIC) based device that allows direct control of the laser cavity (e.g., a tunable grating that acts as an output mirror of the laser and which can be detuned so that the device operates below threshold when incoherent light is required) that presents a laser emission output or a broadband reflective semiconductor optical amplifier (RSOA) amplified spontaneous emission (ASE) output. In another embodiment, a laser is used that can be driven (i) by sufficiently high current to cause multi-mode (short coherence length) operation or (ii) by sufficiently low current to cause single-mode (long coherence length) operation. In some embodiments, external cavity components such as deformable mirrors or optical phase arrays are used to reduce the coherence of the light as needed to reduce speckle on demand.

[0057] In some embodiments, the changing blood volume in vivo does not affect the measurement because the exposure time is much shorter than the pulse period. As such, it may not be necessary to synchronize with the pulse for acquisition, and in vivo human tissue can be used for in situ device calibration. This may be preferred to performing calibration on an external fixture or a stable tissue phantom.

[0058] As used herein, "a portion" of something means "at least some of" the thing, and as such can mean all or less than all of the thing. As such, "a portion" of a thing includes the entire thing as a special case, i.e., an example where the entire thing is a portion of a thing. As used herein, when a second quantity is "within Y" of a first quantity X, it means that the second quantity is at least XY, and the second quantity is at most X+Y. As used herein, when a second number is "within Y%" of a first number, it means that the second number is at least (1-Y / 100) times the first number, and the second number is at most (1+Y / 100) times the first number. As used herein, the word "or" is inclusive, so that, for example, "A or B" means any of the following: (i) A, (ii) B, and (iii) A and B.

[0059] Each of the terms "processing circuit" and "components for processing" is used herein to mean any combination of hardware, firmware, and software that is employed to process data or digital signals. Processing circuit hardware may include, for example, an application specific integrated circuit (ASIC), a general or dedicated central processing unit (CPU), a digital signal processor (DSP), a graphics processing unit (GPU), and a programmable logic device, such as a field programmable gate array (FPGA). In a processing circuit, as used herein, each function is performed by hardware configured (i.e., hardwired) to perform the function, or by more general hardware (such as a CPU) configured to execute instructions stored in a non-transitory storage medium. The processing circuit may be fabricated on a single printed circuit board (PCB) or distributed on several interconnected PCBs. The processing circuit may contain other processing circuits; for example, the processing circuit may include two processing circuits interconnected on a PCB, an FPGA and a CPU.

[0060] As used herein, when a method (e.g., an adjustment) or a first quantity (e.g., a first variable) is referred to as being "based on" a second quantity (e.g., a second variable), it means that the second quantity is an input to the method or affects the first quantity, for example, the second quantity may be an input to a function that calculates the first quantity (e.g., the only input, or one of several inputs), or the first quantity may be equal to the second quantity, or the first quantity may be the same as the second quantity (e.g., stored in the same location or multiple locations in memory as the second quantity).

[0061] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the concepts of the present invention. As used herein, the terms "substantially", "approximately" and similar terms are used as terms of approximation rather than as terms of degree, and are intended to account for the inherent deviations in measurements or calculations that would be recognized by those of ordinary skill in the art.

[0062] Any numerical range recorded herein is intended to include all sub-ranges of the same numerical precision that fall within the recorded range. For example, the range of "1.0 to 10.0" or "between 1.0 and 10.0" is intended to include all sub-ranges between the recorded minimum value 1.0 and the recorded maximum value 10.0 (and including both), that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Similarly, the range described as "within 35% of 10" is intended to include all sub-ranges between the recorded minimum value 6.5 (i.e., (1-35 / 100) times 10) and the recorded maximum value 13.5 (i.e., (1+35 / 100) times 10) (and including both), that is, having a minimum value equal to or greater than 6.5 and a maximum value equal to or less than 13.5, such as, for example, 7.4 to 10.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein, and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein.

[0063] Although exemplary embodiments of systems and methods for calibrating speckle-based sensors have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. It is therefore to be understood that systems and methods for calibrating speckle-based sensors constructed in accordance with the principles of the present disclosure may be implemented otherwise than as specifically described herein. The invention is also defined in the appended claims and their equivalents.

Claims

1. A system comprising: A wearable device, comprising: Lasers, array detector; and Processing circuit, The processing circuit is configured to: obtaining calibration of the array detector using an incoherent light source; obtaining a speckle image using the laser; and A corrected speckle-based measurement is calculated, the corrected speckle-based measurement being based on the speckle image and on the calibration.

2. The system of claim 1, wherein: The calculation of the corrected speckle-based measurement includes correcting the speckle image for estimated contributions from shot noise and dark noise.

3. A system as claimed in claim 1 or claim 2, wherein: The obtaining of the calibration of the array detector comprises: causing light from the incoherent light source to illuminate the array detector; Acquire a first image by means of the array detector; and A second image is obtained by the array detector, Wherein the intensity in the first image is different from the intensity in the second image.

4. The system of claim 3, wherein: said obtaining of said calibration of said array detector comprises fitting a straight line to a plurality of data points including a first data point and a second data point; The first data point includes: the intensity of the first image; and a standard deviation of a third image, the third image being based on the first image; and The second data point includes: the intensity of the second image; and A standard deviation of a fourth image, the fourth image being based on the second image.

5. The system of claim 4, wherein: The processing circuit is configured to form the third image by processing the first image with a high pass filter.

6. The system of claim 5, wherein: The high-pass filter is implemented by the following operations: processing the first image with a low pass filter to form a low pass filtered image; and The low pass filtered image is subtracted from the first image.

7. The system of claim 6, wherein: The low pass filter is a moving average filter.

8. A system as claimed in any one of the preceding claims, wherein: The wearable device includes the incoherent light source.

9. A system as claimed in any one of the preceding claims, wherein: The incoherent light source is a light emitting diode.

10. A system as claimed in any one of the preceding claims, wherein: The wearable device comprises a photoplethysmographic sensor comprising a light emitting diode, and, The incoherent light source is the light emitting diode of the photoplethysmographic sensor.

11. A system as claimed in any one of the preceding claims, wherein: The laser includes an optical amplifier and the incoherent light source includes the optical amplifier.

12. A system as claimed in any one of the preceding claims, wherein: The incoherent light source includes the laser.

13. The system of claim 12, wherein: The incoherent light source includes a modulator configured to modulate light generated by the laser to reduce a coherence length of the light.

14. The system of claim 12, wherein: The incoherent light source includes a laser drive circuit configured to: in a first state, drive the laser with a current that causes single-mode operation; And in a second state, the laser is driven with a current that causes multi-mode operation.

15. A system as claimed in any one of the preceding claims, wherein: The processing circuit is further configured to measure a static scatter variance, and, The corrected speckle-based measurement is also based on the static scatter variance.

16. The system of claim 15, wherein: The measuring of the static scatter variance comprises: obtaining a plurality of speckle images using the laser; averaging the speckle images among the plurality of speckle images; and The variance of the mean is calculated.

17. The system of any preceding claim, further comprising a motion sensor connected to the processing circuit.

18. The system of claim 17, wherein: The processing circuit is further configured to: During the calibration measurement, detecting motion exceeding a threshold; and In response to the detection of the motion exceeding the threshold, the calibration measurements are repeated.

19. The system of claim 17, wherein: The processing circuit is further configured to: During speckle measurement, detecting motion exceeding a threshold; and Responsive to the detection of the motion exceeding the threshold, a calibration process is initiated.

20. A system as claimed in any preceding claim, wherein: The array detector is a zero-degree chief ray angle detector.

Images