NL2034027B1

NL2034027B1 - Multi-frequency speckle sensing

Info

Publication number: NL2034027B1
Application number: NL2034027A
Authority: NL
Inventors: Bramer Cas; Venialgo Esteban; Nathan Santosa Moonen Leonard; Bhattacharya Nandini; Jan Johannes Van Brussel Jaap
Original assignee: Praxagoras B V
Priority date: 2023-01-26
Filing date: 2023-01-26
Publication date: 2024-08-13
Also published as: WO2024158286A1

Abstract

25 Methods and systems are disclosed for multi-frequency speckle sensing. The system comprises a first light source for illuminating a target region with coherent light of a first 5 wavelength, a second light source for illuminating the target region with coherent light of a second wavelength, different from the first wavelength, a first image detector arranged for detecting a first speckle pattern created by interaction of the coherent light of the first wavelength with the target region, the first image detector having pixels with a first pixel size, and a second image detector arranged for detecting a second speckle pattern created by 10 interaction of the coherent light of the second wavelength with the target region, the second image detector having pixels with a second pixel size. A ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector. 15 + Fig. 1.

Description

NL35427/Sv-TD

Multi-frequency speckle sensing

Technical field

The disclosure relates to multi-frequency speckle sensing, and in particular, though not exclusively, to methods and systems for multi-frequency speckle sensing.

Background

Wearable devices for monitoring vital functions have become increasingly popular over the last year, with applications both in medical/clinical settings and in a consumer settings. One method to monitor vital functions is photoplethysmography (PPG).

Photoplethysmography is a well-known optical, non-invasive method to detect volumetric changes in blood in peripheral circulation. A photoplethysmography system comprises one or more (non-coherent) light sources (typically LEDs) for illuminating a body part (e.g., finger, wrist, or ear lobe) and one or more light sensors {e.g., photo diodes) for detecting an intensity of the light reflected by or transmitted through the illuminated body part. As the blood volume in the body part changes, the ratio between reflected (or transmitted) and absorbed light changes, leading to a time-varying light intensity on the sensor. This allows determination of the heart rate and variations therein. These, in turn, can be used to derive additional vital signs, such as respiration rate.

When two or more suitably chosen wavelengths are used (typically one in the red part and one in the near-infrared part of the spectrum), additional information such as blood oxygenation may be determined, a technique known as pulse oximetry. Typically, the light sources are switched on and off in an alternating manner, and a single photodetector is used to detect both wavelengths.

However, photoplethysmography signals are very sensitive to perturbations, in particular to motion artifacts but also to varying ambient light conditions. This makes standard photoplethysmography devices less suitable for, e.g., outdoor use during sports events or revalidation activities and makes it difficult to make clinically validated conclusions in an ambulatory or non-clinical environment.

Speckle plethysmography (SPG) is an optical technique that is related to standard photoplethysmography, but more robust against, e.g., motion artifacts. Speckle plethysmography uses a coherent light source (e.g., a laser), and uses a two-dimensional image detector (e.g., e.g., a CMOS or CCD camera). The interaction of the coherent light with the body part creates a pseudo-random interference pattern on the image detector, known as a speckle pattern. This speckle pattern additionally provides information on blood flow (compared to only blood volume information obtained with photoplethysmography); i.e, both volume changes and velocity changes can be detected using speckle plethysmography.

Again, if multiple wavelengths are used, additional parameters such as blood oxygenation may be determined.

US 2020/0359948 A1 describes a wearable device for monitoring a person's vascular health comprising a coherent source and an image sensor which may be used for

PPG and/or SPG measurements. Multiple wavelengths may be used to calculate e.g., an oxygen saturation level.

However, all these components, i.e., laser source, image sensor, processor, et cetera, require power and space. On the other hand, it is desirable to minimise power usage in order to increase battery life and/or reduce battery dimensions. Moreover, it is desirable to minimise weight and volume of the device, both for wearable devices in general and even more so for sports applications.

Hence, from the above, it follows that there is a need in the art for systems and methods for non-invasive determination of a vital function that are robust yet light-weight.

Summary

It is an aim of embodiments in this disclosure to provide a method and system that avoids, or at least reduces the drawbacks of the prior art.

In a first aspect, this disclosure relates to a system for multi-frequency speckle sensing. The system comprises a first light source for illuminating a target region with coherent light of a first wavelength, a second light source for illuminating the target region with coherent light of a second wavelength, different from the first wavelength, a first image detector arranged for detecting a first speckle pattern created by interaction of the coherent light of the first wavelength with the target region, the first image detector having pixels with a first pixel size, and a second image detector arranged for detecting a second speckle pattern created by interaction of the coherent light of the second wavelength with the target region, the second image detector having pixels with a second pixel size. A ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.

For speckle plethysmography measurements, the signal to noise ratio (SNR) is related to the relation between the pixel size and the size, or more typically size distribution, of the individual speckles. As used herein, the speckle size is defined as the correlated area on the image detector. In a diffraction-limited system, a measure of (radial) speckle size might be the distance between the centre and the first minimum of the Airy disk, or another intra-Airy disk distance. The speckle size distribution may be represented using a representative speckle size, e.g., a minimum, mean, median, or modal speckle size. The representative speckle size may also be referred to as the typical speckle size. The size may be represented as a length (e.g., radius or diameter), or as an area. However, the speckle size is proportional to the wavelength of the coherent light, and therefore, in general, the first detector for detecting the first wavelength has a different optimal pixel size than the second detector for detecting the second wavelength— unless the speckle size is adjusted, e.g., using optical elements. In a diffraction-limited imaging system— meaning an in-focus system without aberrations—the minimum speckle size d may be approximated as d=12(1+ MAS where Mis the magnification of the imaging system (M = 1 for a system without a focussing lens}, A is the wavelength under consideration, and f is the f-number of the imaging system, i.e., the ratio between the focal length and the aperture diameter. It may be noted that in practice, many systems, e.g. so-called wearables, do not meet the diffraction limit, as optical quality may be traded off against price and/or size.

In systems as described in this disclosure, the speckle detection may be optimised for each wavelength. Which ratio between speckle size and pixel size is optimal, depends on the intended use. For example, for laser speckle contrast imaging based on spatial speckle contrast, the speckles need to be substantially larger than the pixels, whereas for speckle plethysmography, the speckles should be at least the same size, and preferably twice as large as the pixels for optimal efficiency.

As used herein, speckle sensing comprises both speckle imaging and its derivatives, such as speckle contrast imaging, and single-value sensing, where the entire speckle image is reduced to a single value (such as typical speckle plethysmography applications). The image detectors typically detect two-dimensional images, but some embodiments may use a different image detector, e.g. one-dimensional image detector (also known as a line detector).

The target region can be a two-dimensional region, e.g., an essentially opaque surface, or a three-dimensional region, e.g., an at least partially translucent volume. In some embodiments, the lateral dimensions of the sensed volume are much larger than the penetration depth of the coherent light. The first and second light sources may be configured to illuminate substantially identical parts, partly overlapping parts, or substantially disjunct parts of the target region.

The ratio between the speckle size and the pixel size can be adjusted by adjusting the pixel size and/or the speckle size. For example, different detectors with different pixel size may be used, and/or different pathways between the tissue and the detector that affect the speckle size on the detector.

In some embodiments, a plurality of light sources may be used for each of one or more of the wavelengths illuminating the target region. Similarly, in some embodiments, a plurality of image detectors may be used for detecting light of one or more of the wavelengths. Or additionally, a combination of a plurality of light sources, each of one or more of the wavelengths illuminating the target region and, a plurality of image detectors may be used.

In an embodiment, the representative speckle size is a mean speckle area, the pixel size is the pixel area, and the ratio is between 0.8 and 4, for example between 0.9 and 2.7, or between 1.9 and 2.3. In general, the optimal ratio of speckle size to pixel size depends on the application. It has been found that for certain applications the signal to noise ratio is optimal if the mean speckle area is approximately equal to or slightly larger than two times the pixel area.

Thus, the pixel count and, hence, power consumption of the image sensors may be minimised. A lower pixel count typically also results in a lower production cost than a higher pixel count.

In an embodiment, the system further comprises a third light source for illuminating the target region with coherent light of a third wavelength, and a third image detector arranged for detecting a third speckle pattern created by interaction of the coherent light of the third wavelength with the target region, the image detector having pixels with a third pixel size. A ratio of the third pixel size and a third representative speckle size of the third speckle pattern at the third image detector is substantially the same as the ratio of the first pixel size and the first representative speckle size.

The system may be similarly expanded with additional light sources and detectors, wherein the system is arranged such that the ratio between the pixel size and the representative speckle size is substantially the same for all wavelengths.

In an embodiment, the second pixel size is different from the first pixel size, preferably the ratio between the first pixel size and the second pixel size being substantially the same as the ratio between the first wavelength and the second wavelength.

By varying the pixel size of the image detectors, the image detectors may be arranged in an otherwise identical manner. This is particularly advantageous if a set-up without focussing optics (e.g., lenses) is used.

In an embodiment, wherein the second image detector is the first image detector. In other words, the first and second image detectors may be the same image detector. This reduces the complexity and cost of the device. In such an embodiment, light of the first and second wavelengths may be guided to the image detector via different paths, or via a path with adjustable optical properties.

In an embodiment, the first representative speckle size and/or the second representative speckle size is adjusted using optical elements and/or path length differences 5 and/or an illumination spot size of the coherent light of the first and/or second wavelengths on the target region.

By adjusting the size of the speckles on the detector, any suitable off-the-shelf image detector may be used. For example, identical detectors may be used for the light of the first and second wavelengths, but differently arranged or used in combination with different optics. This may reduce the complexity and/or the cost of the device.

In an embodiment, the system further comprises a first narrow-bandpass filter, preferably an interference filter, configured for transmitting light of the first wavelength in front of the first image detector; and a second narrow-bandpass filter configured for transmitting light of the second wavelength in front of the second image detector. This way, only the light emitted by the first and second light sources, respectively, is detected by the image sensors.

Thus, both light sources and image detectors may be used simultaneously, maximising the signal over time per pixel element. Thus, a smaller image sensor and/or lower read-out rate may be used. Moreover, such filters reduce noise from environmental light, which has a wide range of wavelengths. Hence, lower power light sources may be used while keeping the same signal quality.

As used herein, a filter can be any element that prevents at least some unwanted wavelengths from reaching the detector while allowing at least part of the desired wavelengths to pass; thus, in this context, filters include dispersive elements such as prisms and gratings. As used herein, a filter is considered to be placed in front of a detector as it filters the light reaching the pixels of the detector at the moment the detector is active.

In an embodiment, the system is configured to alternate between a first state and a second state. In the first state, the first light source and the first detector are activated, and the second light source and the second detector are deactivated, and in the second state, the second light source and the second detector are activated, and the first light source and the first detector are deactivated. This prevents cross talk between the light of the first and second wavelengths, without the need for an optical filter.

In an embodiment, the first wavelength is selected in a first atmospheric absorption band within the range 0.4—2 pm inclusive, for example an O2 or HO absorption band. For instance, the first wavelength may be selected between 0.75 um and 0.78 um, e.g. about 0.762 um. The second wavelength may be selected in a second atmospheric absorption band within the range 0.4—2 um inclusive, for example an O2 or HO absorption band. For instance, the second wavelength may be selected between 1.3 um and 1.4 ym.

By choosing wavelengths for which there is a high atmospheric absorption, noise due to environmental light may be reduced. The amount of the reduction may depend on, e.g., properties of the emitter, properties of the target region, and light conditions.

However, for biosensing applications, the H2O absorption band may be less suitable, as many biological specimens also have a high H:O absorption, which may lead to a reduced signal quality.

In an embodiment, the first wavelength is selected between 0.65 um and 0.78

Um, for example about 0.66-0.67 um. In an embodiment, the second wavelength is selected between 0.8 um and 1.4 um, for example about 0.81-0.82 um, about 0.89-0.90 um or about 0.94 um. At these wavelengths, the relative absorption between unoxygenated haemoglobin (Hb) and oxygenated haemoglobin (HbO;) is large while other common blood pigments feature small spectral differences at these wavelengths and tissue absorption at these wavelengths is low, leading to a strong oximetry signal.

In an embodiment, the interaction of the light of the first and/or second wavelengths with the target region comprises reflection by the target region. Additionally or alternatively, the interaction of the light of the first and/or second wavelengths with the target region may comprise transmission through the target region.

A reflective geometry can be advantageous if at least one of the used wavelengths has a small penetration depth. Moreover, it allows close positioning of the light sources and detectors in a fixed arrangement. On the other hand, a transmissive geometry can be advantageous where light transmission is good but where the flow (i.e, the signal source) is relatively weak, as the light path is typically longer than in a reflective geometry, leading to a more developed speckle pattern (i.e., a stronger signal than would be obtained in a reflective geometry). However, coherent light often has an associated coherence length, after which the light is no longer, in general, coherent. This may, in some cases, limit the depth of transmissive geometries to a depth substantially less than the coherence length of the used light.

Systems that are used for monitoring biological signals typically use a reflective geometry when they are worn on the wrist or torso, while finger or ear lobe sensors may use either geometry. Sensors for neonatal care often use a transmissive geometry in order to obtain a sufficiently strong signal.

In an embodiment, the light of the first wavelength and the light of the second wavelength interact with essentially the same region of the perfused tissue. This leads to a signal with a high fidelity, as both measurements relate to the same blood volume. In other embodiments, different, but preferably nearby regions are measured; this may simplify design of the device.

In an embodiment, the system further comprises a controller configured to determine a quantity of interest based on a first signal obtained by the first detector and a second signal obtained by the second detector. The quantity of interest may be a vital function. In such cases, the target region may comprise a perfused tissue, e.g., a finger, wrist, foot, or earlobe.

The system may further comprise an output module, e.g., a screen, to display output indicative of the determined vital sign, and/or a communication module to transmit information indicative of the determined vital sign to a further system, e.g., an external computer system or a smart phone.

The system may be a single device, e.g., a smart-watch-like device.

Alternatively, the system may comprise a plurality of devices, which may be communicatively connected, e.g., using well-known wireless communication methods.

In an embodiment, the vital function is a blood parameter, preferably at least one of SpO:, Sa0z, or an oxygenation status. The vital function can also be, e.g., a heart rate and/or heart rate variation, a respiration rate, blood pressure, et cetera. In some embodiments, a plurality of vital functions may be determined based on the signals obtained by the first and second detectors.

In an embodiment, the system further comprises a power source for powering the first and second light sources and the first and second detectors. The power source is typically a battery, preferably a rechargeable battery.

In an embodiment, the system is a wearable device, e.g. a wristband, a chest band, or a plaster.

In a second aspect, this disclosure relates to a method for multi-frequency speckle sensing. The method comprises illuminating a target region with coherent light of a first wavelength, illuminating the target region with coherent light of a second wavelength, different from the first wavelength, determining, using a first image detector, a first signal representative of a first speckle pattern created by interaction of the coherent light of the first wavelength with the target region, the first image detector having pixels with a first pixel size, determining, using a second image detector, a second signal representative of a second speckle pattern created by interaction of the coherent light of the second wavelength with the target region, the second image detector having pixels with a second pixel size; and determining a quantity of interest, e.g., a vital function, based on the first signal and the second signal. A ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.

The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the blocks may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

The embodiments will be further illustrated with reference to the attached schematic drawings. It will be understood that the disclosure is not in any way restricted to these specific embodiments. Identical reference signs refer to identical, or at least similar elements.

Brief description of the drawings

Fig. 1 schematically depicts a cross-section of a system according to an embodiment;

Fig. 2A-C schematically depicts relations between speckle size and pixel size, Fig. 2D schematically depicts a pixel size, and Fig. 2E depicts a relation between speckle contrast and number of pixels per speckle;

Fig. 3A and 3B schematically depict a speckle size;

Fig. 4A-D schematically depicts various detector configurations according to embodiments;

Fig. 5A and 5B schematically depict systems according to embodiments;

Fig. 6A and 6B depict various light spectra; and

Fig. 7 is a flow chart of a method according to an embodiment.

Detailed description

The embodiments in this disclosure aim to provide methods and systems for multi-frequency speckle sensing. Although most of the examples provided below relate to biosensing, i.e., sensing a property of a living being, such as a human, other embodiments are readily apparent to the skilled person and are similarly envisaged. For example, instead of studying properties of blood in a blood vessel, similar systems may be used to monitor {mixtures of) fluids or gases in, e.g., industrial applications in a context of quality control or safety control. In such an application, the system may be embodied as an Internet of things (IoT) device, e.g., a mesh device. A lot of the same considerations apply to these kinds of devices as to wearable biosensors, such as changing ambient lighting (especially outdoors), motion artifacts (in particular for vehicle-mounted sensors), and a need to reduce power consumption while maintaining a high accuracy.

Fig. 1 schematically depicts a cross-section of a system according to an embodiment. The system 100 comprises at least one laser source 102 and at least one image detector 106. In the depicted example, the at least one laser source is driven by an analogue front end 116. The at least one laser source generates light of a plurality of wavelengths. The plurality of wavelengths comprises a first wavelength 126 and light of a second wavelength 128, different from the first wavelength. Typically, there is one laser source for each wavelength, but in some embodiment, multi-frequency lasers generating light of two or more wavelengths may be used. This may reduce the number of components needed but may limit the choice of wavelengths. In principle, the wavelengths are not very restricted, but wavelengths in the visible, infrared, and/or ultraviolet part of the electromagnetic spectrum are mostly used, in particular wavelengths in the visible and near- infrared part of the spectrum, e.g., between about 0.4 um and 2.0 pm.

Laser speckle analysis may be used for a wide variety of applications, including both biological applications, such as agricultural, medical, and food-related applications, and non-biological applications. In both cases, laser speckle analysis may be used for non-invasive and non-destructive analysis and/or monitoring of a sample. Some examples include monitoring of fruit ripening, food degradation, or paint drying processes; detection and measurement of parasites, contaminations, or blood contents (including oxygenation); flow measurement such as blood flow measurement, et cetera. The suitable and optimal wavelengths for each application depend on the target substance and, where applicable, the medium comprising the target substance.

The at least one laser source 102 is arranged to illuminate a target region. The target region can be a two-dimensional region, e.g., an essentially opaque surface, or a three-dimensional region, e.g., an at least partially translucent volume. In some embodiments, the lateral dimensions of the volume are much larger than the depth, e.g., when the target region is a part of the (subcutaneous) microvasculature. In the depicted example, the target region comprises a conduit 120 through which two kinds of particles 122,124 flow. The conduit may be a blood vessel, and the particles can be, e.g., red blood cells and white blood cells, or oxygenated and unoxygenated haemoglobin molecules. The conduit can also be a pipe or other structure containing moving particles. In other applications, the conduit may comprise only a single kind of particle, or more than two kinds of particles (such as in the blood example).

The light is scattered one or more times by the particles 122,124 in the conduit 120. Because coherent light is used, the pseudo-random scattering results in a pseudo- random interference pattern, the speckle pattern. Thus, a speckle pattern is generated for each of the plurality of wavelengths. The plurality of wavelengths may be selected such that the different kinds of particles in the conduit interact differently with both wavelengths. For example, the different kinds of particles may have different absorption rates at the plurality of wavelengths. This way, the relative presence of each kind of particle may be determined, e.g, the ratio between Hb and HbO: during SpO: measurements. The plurality of wavelengths may also be used for other purposes, for example, to obtain information at different depths (longer wavelengths typically having larger penetration depths), or for error correction (certain artifacts may have different effects at different wavelengths).

The scattered light is detected by the at least one image detector 106. Various optical elements may be positioned in the path of the light beam between the target region and the at least one image detector. In the depicted example, these optical elements comprise a lens 112 and a filter 114. The filter can be a narrow-band filter, e.g., an interference filter. The at least one image detector and the optical elements are arranged such that the ratio between a representative speckle size and a pixel size of the pixels detecting that speckle pattern, is essentially the same for all wavelengths. This way, the speckle detection may be optimised for each wavelength, as is discussed in more detail below with reference to Fig. 2. There are various ways in which this may be achieved, some examples of which are described in more detail below with reference to Figs. 4A-D. In the depicted example, the device further comprises a protective cover 118.

Thus, for a system with two wavelengths, In a first aspect, this disclosure relates to a system for multi-frequency speckle sensing. The system comprises a first light source for illuminating a target region with coherent light of a first wavelength, a second light source for illuminating the target region with coherent light of a second wavelength, different from the first wavelength, a first image detector arranged for detecting a first speckle pattern created by interaction of the coherent light of the first wavelength with the target region, the first image detector having pixels with a first pixel size, and a second image detector arranged for detecting a second speckle pattern created by interaction of the coherent light of the second wavelength with the target region, the second image detector having pixels with a second pixel size. A ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.

In an embodiment, the system further comprises a third light source for illuminating the target region with coherent light of a third wavelength, and a third image detector arranged for detecting a third speckle patterns created by interaction of the coherent light of the third wavelength with the target region, the image detector having pixels with a third pixel size. A ratio of the third pixel size and a third representative speckle size of the third speckle pattern at the third image detector is substantially the same as the ratio of the first pixel size and the first representative speckle size.

In the depicted example, the optical axes of the laser source 102 and the image detector 106 are essentially parallel. This configuration can make efficient use of a limited space. In other embodiments, the laser source and the image detector may make an angle, e.g., such that the optical axes intersect within the target region, for instance close to a centre of the target region.

In other examples, changes in the target region may not be due to motion but due to other processes, e.g., chemical and/or physical processes affecting a composition of a material in the target region.

Fig. 2A-C schematically depict relations between speckle size and pixel size.

In particular, Fig. 2A depicts a first image sensor 202 with a square 10x10 array of pixels 204 (shown as squares). Most image sensors feature a regular array of square pixels. Pixel size may be defined as a length (e.g., edge length or diameter) of the pixel or as an area of the pixel. As used in this disclosure, pixel size may also refer to pixel pitch, i.e., the heart-to- heart distance between neighbouring pixels.

The pixel size of the pixels 204 of the first image detector 202 is optimised for the speckle size of the speckles 206 (shown as circles) detected by the image detector. In this example, there is essentially one speckle per pixel, i.e., the ratio between the pixel pitch and the speckle diameter is approximately equal to 1. This way, the speckle detection may be optimised for each wavelength. It generally depends on the use case which size definitions and which ratios are optimal. In this example the detector detects 100 speckles.

As was noted above, laser speckle analysis (or speckle sensing) is used for a wide range of applications. The optimal ratio between speckle size and pixel size may depend on the application. For example, for laser speckle contrast imaging based on spatial (or spatio-temporal) speckle contrast, it might be justified for the speckles to be substantially larger than the pixels (for example, about 3-5 pixels per speckle), whereas for single-value sensing applications such as speckle plethysmography, the speckles should have roughly twice the area as the pixels for optimal efficiency, i.e., pixel to speckle ratio close to or slightly larger than two. The optimal pixel to speckle ratio may further be affected by other factors such as motion artifacts, ambient light noise, et cetera.

Thus, for example, for speckle plethysmography, the ratio between mean speckle area and pixel area may be selected between 0.8 and 4.0, for example between 0.9 and 2.7, or between 1.9 and 2.3. Evidently, when different size measures are used, a different numerical value for the ratio may be found.

Fig. 2E shows the effect of the number of pixels per speckle on the global speckle contrast, and is adapted from Figure 2 of S.J. Kirkpatrick et al., “Detrimental effects of speckle-pixel size matching in laser speckle contrast imaging,” Optics Letters, vol. 33:24 (2008) pp. 2886-2888, which is hereby incorporated by reference. Fig. 2E depicts the results of simulations (squares), subjective speckle experiments (triangles) and objective speckle experiments (circles) which relate the pixels per speckle to the achievable global contrast of a static speckle pattern; the lines are just guides to the eye. In all cases, the global contrast tends to O for O pixels per speckle, taking a sharp uptick as pixels per speckle increase up to about 2 (corresponding to the Nyquist criterion). After this, the global contrast has reached 1 where it is asymptotically limited, leading to, at most, minor improvements despite increasing the spatial resolution up to 9 pixels per speckle. The graph thus shows the advantage of sampling the speckle pattern at least at its spatial Nyquist frequency of 2 pixels per speckle, in order to avoid unpreventable loss in contrast.

It may be noted that while in this example, the speckle pattern is shown as substantially following a square array, actual speckle patterns typically have different spatial distributions. Furthermore, in this example, all speckles are shown equally large. However, speckle sizes in a speckle pattern may also follow a different size distribution. In such cases, a typical or representative speckle size may be determined. The representative speckle size, can be, e.g., a minimum, mean, median, or modal speckle size. The representative speckle size may be represented as a length (e.qg., radius or diameter), as an area, or using some other suitable parameter.

Fig. 2B shows the same detector 202 as Fig. 2A, but now used to detect light of a different (longer) wavelength, resulting in differently dimensioned (larger) speckles 208.

For example, the speckle-generating light in Fig. 2A may have a first wavelength of about 700 nm, and the speckle-generating light in Fig. 2B may have a second wavelength of about 1000 nm. As the speckle size is proportional to the wavelength of the light generating the speckle, the pixel size is no longer optimal for the currently detected speckles. Instead of the 100 speckles detected for the first wavelength, only 49 speckles (so, less than half) are detected for the second wavelength. If the amount of information per speckle is substantially constant, this reduces the effective signal by a factor of more than 2.

A potential solution could be to increase the size of the detector, and use a detector with, e.g., a 14x14 pixel array. However, this increases the cost and power consumption of the image sensor.

Therefore, Fig. 2C schematically depicts a second image sensor 212. This image sensor again has a 10x10 array of pixels 214. However, in this case, the pixel size has been increased (compared to the first image sensor 202) proportional to the increase in size of the speckles 208 (compared to speckles 206), so that the same amount of pixels may be used to detect the same amount of speckles. Thus, the pixel count and, hence, power consumption of the image sensors may be minimised, while maintaining a good signal to noise ratio.

In this example, the ratio between (representative) speckle size and pixel size is kept substantially constant by increasing the pixel size. As will be explained in more detail below with reference to Fig. 4A-D, in other embodiments, the size of the speckles on the image detector may be reduced, in addition to or as alternative to increasing the pixel size, such that the ratio between speckle size and pixel size remains substantially constant between the first and second image detectors.

Fig. 2D schematically depicts a pixel size. In general, pixel size may be denoted using a length or an area. Examples of length are the width a, the height b, the diagonal d, or the (horizontal) heart-to-heart distance h. Examples of an area are the pixel area axb, or the heart-to-heart area gxh.

Fig. 3A and 3B schematically depict a speckle size. In particular, Fig. 3A depicts projection of two point sources A, B on the surface of a scattering volume 318 on a photodetector 302. The photodetector comprises a plurality of pixels 304 for detecting the images A’, B’ of the point sources A, B. The depicted example further comprises a lens 306 and an aperture stop 308 reducing the effective diameter of the lens, which is denoted with

Diens.

The surface of the scattering volume 318 defines an object plane 310 comprising the (possibly virtual) point sources A, B, which is imaged on the image plane 312.

The entrance of the first (and in this example, single) lens between the object plane and the image plane defines a pupil plane 314. The distance between the object plane and the pupil plane may be denoted by p, and the distance between the pupil plane and the image plane may be denoted by q.

Fig. 3B depicts the light intensity distributions 320,322 on the image plane 312 for light emitted by, respectively point sources A, B on the object plane 310. Two speckles can generally be distinguished if they satisfy the Rayleigh criterion, i.e., if the maximum of the first light intensity distribution is separated from the maximum of the second light intensity distribution by at least the distance between the maximum and the first minimum of the first light intensity distribution (here, this distance is denoted by dm). This same distance may be used as a measure of the speckle size.

The minimum (resolvable) speckle radius dmin may then be given by:

Omin = 1.22 Aq / Diens, Or Amin = 1.22 (1+ MAF where M is the magnification of the imaging system, A is the wavelength under consideration, and f is the f-number of the imaging system, i.e., the ratio between the focal length g and the aperture diameter Diens. It follows that, in general, the diameter of the speckle is (at least approximately) linearly proportional to the wavelength. For various reasons, some speckles may be larger than the minimal resolvable speckle size Amin.

In other embodiments, speckle size may be determined differently; for example empirically, after performing a measurement.

Fig. 4A-D schematically depicts various detector configurations according to embodiments. In general, the ratio between the speckle size and the pixel size can be adjusted by adjusting the pixel size and/or by adjusting the speckle size (as determined on the image detector). Fig. 4A and 4B show examples of detectors with different pixel sizes, whereas Fig. 4C and 4D show examples of different pathways between the target region and the detector that affect the speckle size on the detector.

Fig. 4A depicts a first detector configuration according to an embodiment. In this configuration, the image detector 402 comprises two types of pixels. The pixel size of the pixels 404,3 used to detect the light of the first wavelength 126 is different from the pixel size of the pixels 4061-3; used to detect the light of the second wavelength 128. In this example, the pixels for the first and second wavelength are placed in alternating order. This ensures that essentially the same (part of the) target region is imaged by both wavelengths. If the image detector is a two-dimensional detector, the pixels may be arranged, e.g., in stripes or in a checkerboard pattern.

Furthermore, pixels 4044; are provided with a filter 4084-3 that blocks at least the light of the second wavelength, and similarly, pixels 4064-3 are provided with a filter 4104- 3 that blocks at least the light of the first wavelength. That way, both light sources may be used simultaneously without undesirable crosstalk. The filters can be narrow-bandpass filter, e.g., an interference filter, that only transmits light in a small window around the first and second wavelength, respectively, so that also ambient light influences are minimised. Other embodiments may be prevent cross-talk between the first and second wavelengths by alternately switching on and off the light sources; i.e., the system may be configured to alternate between a first state in which the first light source and the first detector are activated and the second light source and the second detector are deactivated, and a second state, in which the second light source and the second detector are activated and the first light source and the first detector are deactivated. Such embodiments may not need a filter to prevent cross-talk, but may still use a filter to e.g. block ambient light.

A lens 412 focusses the light of the first and second wavelengths on the detector. Other embodiments may not use a lens. This may save space, allowing for e.g. a thinner device. On the other hand, the use of a lens may allow for a smaller detector.

Fig. 4B depicts a second detector configuration that is identical to the first detector configuration, except that the ‘small’ pixels 4144-3 for detecting the light of the first wavelength 126 and the ‘large’ pixels 4164-3 for detecting the light of the second wavelength 128 are placed in groups next to each other. This may simplify detector production.

Moreover, the distance between the pixels may be optimised individually for each wavelength. Other configurations are readily apparent to the skilled person, e.g., a centre region of ‘small’ pixels surrounded by a border of ‘large’ pixels, et cetera.

In the depicted example, each pixel is provided with a filter; however, in other embodiments, the filter may cover all adjacent pixels of the same type (size).

Fig. 4C depicts a third detector configuration according to an embodiment. In this example, the pixels 4244s of the first detector 420 and the pixels 4261-5 of the second detector 422 are identical. However, different lenses 428,430 are used which are selected such that the speckle size on the detector is substantially the same for both wavelengths. In this example, the filters 432,434 are positioned in front of the lenses, but other suitable positions could also be selected.

Fig. 4D depicts a fourth detector configuration according to an embodiment. In this example, identical detector elements 440,442 are used, similar to those 420,422 shown in Fig. 4C. However, in this example, no focussing element is used. As the pixel size may increase with the distance from the target region, a substantially identical speckle size may be achieved by proper selection of the distance between target region and image detector.

The skilled person can readily devise further alternative configurations, for example by combining elements of two or more of the embodiments described with reference to Fig. 4A-D, or by using different methods to manipulate the apparent speckle size and/or the pixel size.

Fig. 5A and 5B schematically depict systems according to embodiments. In particular, Fig. 5A depicts a top view of a system 502 comprising a first light source 504 for generating coherent light of a first wavelength and a second light source 506 for generating coherent light of a second wavelength, different from the first wavelength. The light sources may be lasers, e.g., VCSEL or LED lasers. The lasers may be controlled using an analogue front end 524. The system further comprises a first image detector 508 for detecting light of the first wavelength and a second image detector 510 for detecting light of the second wavelength.

When in use, the first and second light sources 504,506 are arranged for illuminating a target region. The light sources may illuminate the same part of the target region, or different parts of the target region. The interaction of the coherent light with the target region results in speckle patterns. The first and second image detectors 508,510 are arranged for detecting the speckle patterns created by the light of the first and second wavelengths, respectively.

Although only single light sources are shown, in some examples, a plurality of light sources may be used for illuminating the target region with light of the first wavelength, and/or a plurality of light sources may be used for illuminating the target region with light of the wavelength. The pluralities of light sources may be more or less equally distributed around the respective image sensors. Additionally or alternatively, respective pluralities of image detectors may be used to detect speckle patterns of the first and/or second wavelengths.

In the depicted example, the first and second image detectors are arranged as described above with reference to Fig. 4C. Thus, the first image detector 508 comprises pixels 5121-4 having a first size and the second image detector 510 comprises pixels 5144, having a second size, in this example identical to the first size. A first interference filter 516 configured to transmit only light of the first wavelength is positioned in front of the first image detector and a second interference filter 518 configured to transmit only light of the second wavelength is positioned in front of the second image detector. A first lens 520 is provided in front of the first image detector and a second lens 522 is provided in front of the second image detector. The first and second lenses are configured to optimise the size of the speckles on the first and second detectors. As a result, a ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.

Other embodiments may use differently arranged image detectors, for example as discussed above in more detail with reference to Figs. 4A-D.

In the depicted example, the light sources 504,506 and the image detectors 508,510 are provided in essentially the same plane. This plane may be flat or slightly curved.

In such an embodiment, the interaction of the light of the first and/or second wavelengths with the target region comprises reflection by the target region. In other embodiments, the light sources and the image detectors may be positioned on essentially opposite sides of the target region; in such an embodiment, the interaction of the light of the first and/or second wavelengths with the target region comprises transmission through the target region.

Mixtures of reflection and transmission geometries are also possible.

A reflective geometry can be advantageous if at least one of the used wavelengths has a small penetration depth. Moreover, it allows close positioning of the light sources and detectors in a fixed arrangement. On the other hand, a transmission geometry can be advantageous where light transmission is good but where the flow (signal source) is relatively weak, as the light path is typically longer than in a reflection geometry, leading to a stronger signal.

Systems that are used for monitoring biological signals typically use a reflection geometry when they are worn of the wrist or torso, while finger or ear lobe sensors may use either geometry. Sensors for neonatal care often use a transmission geometry in order to obtain a sufficiently strong signal.

In the shown example, the light of the first wavelength and the light of the second wavelength interact primarily with neighbouring parts of the target region. This leads to a relatively simple design of the device 502. In other embodiments, the light sources 504,506 and the image detectors 508,510 are arranged to interact with essentially the same part of the target region, e.g. by using a cross-like configuration. This can increase the accuracy of a quantity of interest determined based on a combination of measurements of both speckle patterns.

Fig. 5B is a block diagram of a system according to an embodiment. A system 550 comprises one or more light sources 552 for generating coherent light of a plurality of wavelengths and configured, when in use, to illuminate a target region. The one or more light sources 552 may include a laser and a laser controller. Alternatively, the laser may be controlled by controller 556. The system also comprises one or more detectors 554 configured, when in use, to separately detect a plurality of speckle patterns created by interaction of the coherent light with the target region. The one or more light source and the one or more detectors can be as described above with reference to Fig. 5A, for example.

The system 550 further comprises a controller 556, communicatively connected to the one or more image detectors 554. The controller may comprise a processing unit and a memory storing executable instructions. The controller may be configured to determine one or more quantities of interest based on a plurality signals obtained by the one or more detectors, the plurality of signals being indicative of the speckle patterns of the respective plurality of wavelengths. The one or more quantities of interest may comprise one or more vital functions. The one or more vital functions may comprise one or more blood parameters, preferably at least one of SpO:, SaO:, or an oxygenation status.

The one or more vital functions may, additionally or alternatively, comprise, e.g., a heart rate and/or heart rate variation, a respiration rate, et cetera. In such cases, the target region may comprise a perfused tissue, e.g., a finger, wrist, foot, or earlobe.

The system 550 may comprise an optional output module 558 communicatively coupled to the controller 556. The output module can comprise a screen to display output indicative of the determined quantity of interest. Additionally or alternatively, the output module may comprise a loudspeaker. For example, the controller may be configured to generate a sound (e.g., an alarm) when a quantity of interest is above or below a threshold.

The system 550 may comprise an optional communication module 560 communicatively coupled to the controller 556. The communication module may be configured to transmit information indicative of the determined quantity of interest to a further system, e.g., an external computer system or a smart phone.

The system 550 further comprises a power source 562 for powering the at least one light source 552, the at least one detector 554. The power source may further power the controller 556 and, if present, the output module 558 and the communication module 560. The power source can comprise battery, preferably a rechargeable battery. The power source may also comprise a power generator, e.g., solar cells.

The system may be a single device, e.g., a smart-watch-like device.

Alternatively, the system may comprise a plurality of devices, which may be communicatively connected, e.g., using well-known wireless communication methods. For example, a first device may comprise a controller configured to obtain a plurality of signals from the one or more image detectors and to transmit the plurality of signals to a further device, and the further device may be configured to receive the plurality of signals and to determine a quantity of interest based on the plurality of signals and to output the determined plurality of signals.

The system can be a wearable device, e.g. a wristband, chest band, or a plaster. In other embodiments, the system can be, e.g., an industrial monitoring device.

Fig. 6A and 6B depict various light spectra. In particular, Fig. 6A depicts a typical sunlight spectrum near sea level. For practical reasons, the wavelengths used by the device are typically chosen in the visible or near-infrared region, for example within the range 0.4-2 um inclusive. However, this largely coincides with the sunlight spectrum. Hence, in particular environment with natural light, such as in outdoor applications, there may be substantial noise caused by natural light.

However, it can be seen in Fig. 6A that the spectrum features several absorption valleys, where a substantial amount of light of a (relatively small) band of wavelengths is absorbed by the atmosphere. For example, at around 0.762 um, there is an absorption band caused by atmospheric O2, as indicated in the left-most figure. By using a wavelength in such an absorption band (as shown in the centre figure) and filtering out other wavelengths using a narrow-bandpass filter, the effect of environmental light on the measurement signal may be reduced. This is shown in the right-most figure, where the peak of the laser spectrum overlaps with the trough in the sunlight spectrum (at ground level).

Thus, wavelengths corresponding to one or more atmospheric absorption bands may be used, for example O2 or H2O absorption bands. More in particular, one wavelength may be selected at about 0.762 um, e.g., between 0.75 um and 0.77 um.

Additionally or alternatively, a wavelength may be selected at about 1.13 um or between 1.3 um and 1.4 ym, corresponding to an HzO absorption band. It is noted that a wavelength in an H2O absorption band may be less suitable for samples with a high water content, such as many biological samples. In some applications, other absorption bands may be selected based on the (local) atmosphere, e.g., in a CO: rich atmosphere, a CO, absorption band may be selected.

In some cases, the selectable wavelengths may be limited by or optimised for the material that is being observed. For example, SpO: is typically determined based on the relative absorption between unoxygenated haemoglobin (Hb) and oxygenated haemoglobin

(HbO:z). Fig. 6B depicts the absorption spectra oxygenated and unoxygenated haemoglobin.

It can be seen that the difference in absorption is maximal between about 600 nm and 750 nm, and above about 920 nm. For wavelengths larger than about 1050 nm, especially the unoxygenated haemoglobin signal may become very weak. Therefore, for oximetry, a first wavelength between 600 nm and 750 nm may be used and a second wavelength above 920 nm, and preferably less than 1050 nm, as this leads to a maximal oximetry signal.

Further refinement of the wavelength may be based on the presence of other absorbers in the target region and/or on the availability of suitable light sources.

Typical examples of suitable wavelength pairs for oximetry are 660/940 nm, 665/894 nm, 780/808 nm and 761/818 nm. Other combinations of these and other wavelengths may also be used. These wavelengths may reduce the scattering variability and/or may account for the presence of other blood pigments than Hb and HbO:.

Fig. 7 is a flow chart of a method according to an embodiment. A step 701 comprises illuminating a target region with coherent light of a first wavelength. The interaction of the coherent light of the first wavelength with the target region creates a first speckle pattern. In a step 703, a first signal representative of the first speckle pattern is detected using a first image detector. The first image detector has pixels with a first pixel size.

A step 705 comprises illuminating a target region with coherent light of a second wavelength, different from the first wavelength. The interaction of the coherent light of the second wavelength with the target region creates a second speckle pattern. In a step 707, a second signal representative of the second speckle pattern is detected using a second image detector. The second image detector has pixels with a second pixel size.

A ratio of the first pixel size and a first representative speckle size of the first speckle pattern (determined at the first image detector) is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern (determined at the second image detector). Thus, each detector may be optimised for the wavelength detected by the detector.

In some embodiments, steps 701 and 703 are performed simultaneously with steps 705 and 707. In such embodiments, the detectors may comprise filters to prevent detecting unwanted light, i.e., the second detector may have a filter that blocks the light of the first wavelength, and vice versa. In other embodiments, the illumination with the first and second wavelength is alternated, and the read-out of the detectors is synchronised with the illumination.

Other embodiments may comprise analogous steps for one or more additional wavelengths.

A step 709 comprises determining a quantity of interest based on the first signal and the second signal. The quantity of interest can be, e.g., a vital function such as a blood parameter, for instance an oxygenation level.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention.

The embodiments were chosen and described in order to best explain the principles and the practical application, and to enable others of ordinary skill in the art to understand the various embodiments with various modifications as are suited to the particular use contemplated.

Claims

CONCLUSIONS

1. A system for multi-frequency speckle sensing, the system comprising: a first light source for illuminating a target area with coherent light of a first wavelength; a second light source for illuminating the target area with coherent light of a second wavelength different from the first wavelength; a first image detector configured to detect a first speckle pattern resulting from interaction of the coherent light of the first wavelength with the target area, the first image detector having pixels having a first pixel size; and a second image detector configured to detect a second speckle pattern resulting from interaction of the coherent light of the second wavelength with the target area, the second image detector having pixels having a second pixel size; wherein a ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.

2. The system of claim 1, wherein the representative speckle size is an average speckle area and wherein the ratio is between 0.8 and 4, preferably between 0.9 and 2.7, more preferably between 1.9 and 2.3.

3. The system of claim 1 or 2, further comprising a third light source for illuminating the target area with coherent light of a third wavelength; a third image detector configured to detect a third speckle pattern resulting from interaction of the coherent light of the third wavelength with the target area, the image detector having pixels having a third pixel size; wherein a ratio of the third pixel size and a third representative speckle size of the third speckle pattern at the third image detector is substantially the same as the ratio of the first pixel size and the first representative speckle size.

4. The system of any preceding claim, wherein the second pixel size differs from the first pixel size, the ratio of the first pixel size to the second pixel size preferably being substantially the same as the ratio of the first wavelength to the second wavelength.

5. The system of any one of claims 1 to 3, wherein the second image detector is the first image detector.

6. The system of any preceding claim, wherein the first representative speckle size and/or the second representative speckle size is adjusted using optical elements and/or path length differences and/or an illumination spot size of the coherent light of the first and/or second wavelengths on the target area.

7. The system of any preceding claim, further comprising: a first narrow bandpass filter, preferably an interference filter, configured to pass light of the first wavelength to the first image detector; and a second narrow bandpass filter configured to pass light of the second wavelength to the second image detector.

8. The system of any preceding claim, wherein the system is configured to switch between a first state and a second state, wherein in the first state the first light source and the first detector are activated, and the second light source and second detector are deactivated, and wherein in the second state the second light source and the second detector are activated, and the first light source and first detector are deactivated.

9. The system of any preceding claim, wherein the first wavelength is selected in a first atmospheric absorption band between 0.4-2 µm including, preferably an O; or HO absorption band, more preferably the first wavelength is selected between 0.75 and 0.77 µm, most preferably about 0.762 µm, and wherein the second wavelength is selected in a second atmospheric absorption band between 0.4-2 µm including, preferably an O; or HO absorption band, more preferably the second wavelength is selected between 1.3 and 1.4 µm.

10. The system of any one of claims 1 to 9, wherein the first wavelength is selected between 0.65 µm and 0.78 µm, and wherein the second wavelength is selected between 0.8 µm and 1.4 µm.

11. The system of any preceding claim, wherein the interaction of the light of the first and/or second wavelength with the target area comprises reflection through the target area, and/or wherein the interaction of the light of the first and/or second wavelength with the target area comprises transmission through the target area.

12. The system of any preceding claim, wherein the light of the first wavelength and the light of the second wavelength interact with substantially the same region of the target area.

13. The system of any preceding claim, further comprising a control element configured to determine a quantity of interest based on a first signal obtained by the first detector and a second signal obtained by the second detector, wherein the quantity of interest is preferably a vital sign and the target area comprises a perfused tissue.

14. The system of claim 13, wherein the vital sign is a blood parameter, preferably one of SpO:, SaO:, or an oxygenation status.

15. The system of any preceding claim, further comprising a power source for powering the first and second light sources and the first and second detectors.

16. The system of any preceding claim, wherein the system is a wearable device, for example a wristband, a chest strap, or a patch.

17. A method of multi-frequency speckle sensing, the method comprising: illuminating a target area with coherent light of a first wavelength; illuminating the target area with coherent light of a second wavelength different from the first wavelength; determining, using a first imaging detector, a first signal representative of a first speckle pattern resulting from interaction of the coherent light of the first wavelength with the target area, the first imaging detector having pixels having a first pixel size; determining, using a second imaging detector, a second signal representative of a second speckle pattern resulting from interaction of the coherent light of the second wavelength with the target area, the second imaging detector having pixels having a second pixel size; and determining a quantity of interest, e.g., a vital sign, based on the first signal and the second signal; wherein a ratio of the first pixel size and a first representative speckle size of the first speckle pattern at the first image detector is substantially the same as a ratio of the second pixel size and a second representative speckle size of the second speckle pattern at the second image detector.