PHOTOPLETHYSMOGRAPHY DEVICE

Abstract

A photoplethysmography device comprises a light source configured to direct source light towards an external object; a light sensor arranged and configured to provide a sensor signal indicative of an intensity of a first source light fraction, which has been scattered by the external object; a casing for housing the light source and the light sensor, and having a cover plate transparent for the source light and an outer face to be facing the external object; and an optical blocking arrangement in the casing between the at least one light source and the outer face of the cover plate and configured to block a second source light fraction on its propagation path extending from the light source to the outer face of the cover plate and from the outer face of the cover plate to the light sensor without leaving the casing.

Claims

1. A photoplethysmography device, hereinafter PPG device, comprising: at least one light source configured to generate a beam of source light to be directed towards an external object; at least one light sensor which is arranged and configured to provide a sensor signal indicative of an intensity of a first source light fraction, which has been scattered by the external object and detected by the light sensor; a casing housing the at least one light source and the at least one light sensor, the casing having a cover plate, which is transparent for the source light and which has an outer face to be facing the external object; and an optical blocking arrangement, which is arranged in the casing between the at least one light source and the outer face of the cover plate and which is configured to block a second source light fraction on its propagation path extending from the light source to the outer face of the cover plate and from the outer face of the cover plate to the light sensor without leaving the casing.

2. The PPG device according to claim 1, wherein the optical blocking arrangement is arranged to block the propagation path of the second source light fraction propagating from the outer face of the cover plate to the light sensor.

3. The PPG device according to claim 1, wherein the optical blocking arrangement comprises at least one absorption element on an inner face of the cover plate facing the at least one light source and the at least one light sensor.

4. The PPG device according to claim 1, wherein the optical blocking arrangement forms an integrated part of the cover plate.

5. The PPG device according to claim 3, wherein the at least one absorption element has a lateral extension in a direction pointing from the light source to the light sensor, and wherein the lateral extension equals a thickness of the cover plate.

6. The PPG device according to claim 1, wherein the light source is configured to provide the source light with a first polarization, and wherein the optical blocking arrangement comprises a polarization filter in the propagation path of the second source light fraction between the cover plate and the at least one light sensor, the polarization filter being configured to block a propagation of light of the first polarization.

7. The PPG device according to claim 6, wherein the light source comprises a light emitter configured to emit unpolarized light and a first polarization filter that is configured to provide the source light with the first polarization, and wherein the polarization filter of the optical blocking arrangement forms a second polarization filter exclusively allowing a transmission of source light having a second polarization, the first and second polarizations being mutually orthogonal.

8. The PPG device according to claim 7, wherein the second polarization filter is arranged, with respect to the propagation path of the second source light fraction, in front of the light sensor.

9. The PPG device according to claim 8, wherein a separation wall is arranged between the light source and the light sensor and blocks a third source light fraction propagating from the light source directly to the light sensor.

10. The PPG device according to claim 9, further comprising a silicone layer, which is transparent for the source light and arranged on a base plate, and which surrounds those sides of the at least one light source and of the at least one light sensor that are not attached to the base plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the following drawings:

[0043] FIG. 1 shows a schematic illustration of an embodiment of a PPG device;

[0044] FIG. 2 shows a schematic illustration of another embodiment of the PPG device, which has an optical blocking arrangement that comprises absorption stripes;

[0045] FIG. 3 shows a diagram, which illustrates a dependency between a power of unwanted source light that reaches a light sensor directly after a reflection at the cover plate, for an embodiment of a PPG device;

[0046] FIG. 4 shows a schematic illustration of an embodiment of the PPG device, which has an optical blocking arrangement that comprises a plurality of polarizing filters;

[0047] FIG. 5 shows an illustration of a first embodiment of the PPG device that is arranged in a watch.

DETAILED DESCRIPTION OF EMBODIMENTS

[0048] FIG. 1 shows a schematic illustration of an embodiment of a photoplethysmography (PPG) device 100. The drawing is not to scale and the extensions of the individual structural elements are not intended to be indicative of actual extensions and relations between extensions of different structural elements. The drawing is also schematic in that components not required for elucidating features in the context of the present embodiment are omitted. In particular, electrical interconnections are not illustrated.

[0049] The PPG device 100 comprises a casing 102. A base plate 104 such as a circuit board with mounted electronic components is arranged in the casing 102. As one of the electronic components, a light source 106 is provided on the base plate 104. are The light source 106 is a light emitting diode (LED) that in operation generates source light generally referenced under the label 108 that is at least in part directed towards an external object 110, which for instance is a finger or arm of a user. A number of arrows labelled 108a, 108b, 108c, and 108d are shown to illustrate different fractions of the source light and will be explained further below. The wavelength of the source light is suitable for entering the external object 110 and being scattered in particular by blood inside blood vessels in the external object 110.

[0050] A light sensor 112 in the form of a photodiode is arranged on the base plate 104. It is positioned with a lateral distance from the light source 106. The light sensor 112 is sensitive in the spectral range of the source light 108 and thus allows providing at its output 114 an electronic sensor signal that is indicative of an intensity of a first source light fraction 108a, which has been scattered inside the external object 110 towards the light sensor 112 and then detected by the light sensor 112. The first fraction of scattered source light is redirected towards the light sensor 112 by source light 108a and 108d subject to single (108a) or multiple (108d) scattering events inside the external object 110, as indicated by arrows 108a and 108d in a very simplified manner.

[0051] The casing 102 housing the light source 106 and the light sensor 112 has a cover plate 116, which is transparent for the source light 108 and which has an inner face 116i facing the light source 106 and the light sensor 112, and an outer face 116o facing the external object 110. The cover plate 116 gives rise to scattering of source light, in particular in the form of reflection of source light. A small fraction of the source light is reflected at the inner face 116i. A non-negligible fraction of the source light, which is herein referred to as the second source light fraction 108b, is reflected at the interface between the outer face 116o of the cover plate 116 and the ambient air outside the casing 102, and is redirected towards the light sensor 112 without having left the casing 102.

[0052] An optical blocking arrangement 118 is arranged in the casing 102 between at the inner face 116i of the cover plate 116. It blocks the propagation path of the second source light fraction 118b on its way from the light source 106 light sensor 112 after reflection at the outer face 116o of the cover plate 116. The optical blocking arrangement 118 of the present embodiment is an absorption stripe applied to the inner face 116i of the cover plate 116. At least a portion of source light that is reflected at the inner face 116i of the cover plate 116 can be blocked by the optical blocking arrangement 120 of this embodiment by proper design of the width of the optical blocking arrangement. To this end, in addition to providing a material with particularly high absorption of the source light, the absorption stripe is made of a material with particularly low reflectance.

[0053] A separation wall 120 serves to block a fraction 108c of the source light, which herein is also referred to as the third source light fraction and would in absence of the separation wall directly propagate from the light source 106 to the light sensor 112. The use of a separation wall can be avoided by providing the light source with a sufficiently small beam aperture of the emitted source light, which can for example be achieved with a lens as an optical collimation element. The lens can be incorporated as an integral part into the light source.

[0054] A protection layer 122 made of a silicone, which is transparent for the source light 108, covers the light source and the light detector.

[0055] Thus, while the casing 102, supported by the silicone layer 122, protects the light source 106 and the light sensor 112 against physical damage, a useful component of the light sensor signal corresponding to the first source light fraction 108a, 108d, which carries the vital-sign information, is provided with particularly small perturbation, despite the use of the cover plate. This eliminates or at least reduces an intensity of the unwanted second source light fraction 108b that does not reach the external object butin absence of the optical blocking arrangement 118would have reached the light sensor 112 without leaving the casing 102, in particular after reflection at the outer face 116o of the cover plate 116 of the casing 102. This elimination or at least reduction of the intensity of this second source light fraction 108b in the detected signal increases the relative amount of the useful first source light fraction 108a, 108d, which has been scattered by the external object 110 and thus contains desired vital-sign information when it is detected by the sensor 112. This allows extracting and evaluating the vital-sign information with high accuracy.

[0056] The most suitable position and shape of the optical blocking arrangement 118 can determined by simulation of the optical pathways during the design process, in consideration of the geometry and arrangement of the light source 106 and the light sensor 112 and the light propagation inside the casing 102, including light propagation inside the cover plate undergoing multiple reflections at transitions from the cover plate to an adjacent air space 124 inside the casing 102 and to ambient air outside the casing 102.

[0057] A PPG processing unit (not shown) may be provided either inside or outside the casing and receive the detection signal from the output 114 of the light sensor 112 for processing and determining the vital-sign information.

[0058] FIG. 2 shows a schematic illustration of a second embodiment of a PPG device 200.

[0059] The following description focuses on differences between the embodiments of FIGS. 1 and 2. Reference labels differing only in the first digit are used to indicate components of the PPG device 200 which are also comprised by the embodiment of the PPG device 100 of FIG. 1. Reference is thus additionally made to the description of FIG. 1 for such features, unless differences are explained in the following.

[0060] The PPG device 200 of FIG. 2 has two light sources 206.1 and 206.2, which in the present case are identical in their physical characteristics, in particular in their emission wavelength. This increases the scattered source light intensity and thus improves the accuracy of the vital-sign information to be determined. However, it is not a requirement that only light sources with identical physical characteristics are used. For instance, different wavelengths of source light may be used to obtain different types of vital-sign information, such as pulse frequency and blood oxygen saturation.

[0061] The first and second light sources 206.1 and 206.2 are arranged to the left and to the right of the light sensor 212 on the base plate 204. Separation walls 220.1 and 220.2 are provided between the first and second light sources 206.1 and 206.2, respectively, and the light sensor 212.

[0062] The optical blocking arrangement comprises absorption elements 218.1, 218.2 in the form of absorption stripes. The absorption elements 218.1 and 218.2 are printed on the inner face 216i of the cover plate 216 and of a black color and a dull surface.

[0063] The first absorption element 218.1 is printed on the inner face 216i in a lateral position that is substantially in the middle between the first light source 206.1 and the light sensor 212. The second absorption element 218.2 is printed on the inner face 216i in a lateral position that is substantially in the middle between the second light source 206.2 and the light sensor 212. The positions are selected such that the absorption elements 218.1 and 218.2 block a propagation of the described second source light fraction from the outer face 216o of the cover plate 160 to the light sensor 212. At the same time the absorption elements 218.1 and 218.2 avoid a reflection of source light at the inner face 216i of the cover plate 216.

[0064] Both absorption stripes have an extension E in a direction parallel to the inner surface of the cover plate and in the paper plane of FIG. 2 which substantially equals a thickness T of the cover plate 216. The thickness T of the cover plate 160 is typically between 0.3 mm and 1.5 mm. The air gap 224 between the silicone layer 222 and the cover plate 116 has a height H which is in some embodiments between 0.02 mm and 0.15 mm.

[0065] The extension E of the absorption elements 218.1 and 218.2 is suitably determined on the basis of simulation results discussed in the context of FIG. 3 below.

[0066] FIG. 3 shows a diagram 300, which illustrates a dependency between a relative amount I of the unwanted second source light fraction that reaches the light sensor 212 and the stripe width E for an embodiment of a PPG device substantially corresponding in construction to that shown in FIG. 2.

[0067] The relative intensity amount of the unwanted second source light fraction reaching the light sensor 130 is plotted on the ordinate in units of % of the total emitted source light intensity. The extension E of the absorption elements 218.1 and 218.2 is plotted on the abscissa in linear units of millimeter. The thickness T of the cover plate 216 underlying the simulation is 0.5 mm and the height H of the air gap 224 is 0.1 mm. A lateral distance between the light source and the light sensor is about 1.5 mm. Two different scenarios are represented by two simulation curves 302 and 304. The scenario underlying the curve 302 is a reference scenario, in which there is no space (and thus no air) between the outer face of the cover plate and skin of a user (forming the external object in the language of the claim). The scenario underlying the curve 304 has an air gap between the outer face of the cover plate and skin of the user. The simulations were calculated assuming a user having white skin.

[0068] The extension of the absorption elements in the direction perpendicular to the paper plane is assumed to be identical and sufficient for blocking any light that reaches a lateral position along the extension E in the paper plane.

[0069] As determined by the simulation and visible in FIG. 3 from the curve 304, the relative amount of unwanted light (second source light fraction) detected by the light sensor 130 can be reduced by a factor of at least about 3 by using the absorption stripes 212, 214 with a stripe width E of 0.5 mm, while the reference scenario exhibits a reduction by a factor of about 2 only.

[0070] It is noted that simulations done with dark skin using the same scenarios show that the received second source light fraction, which may also be called shortcut light, is in fact equal to the useful signal formed by the first source light fraction, when no absorption stripes 212, 214 are used. In other words, in this case the curve 304 would start at I=50% for a stripe width E=0 mm. Still, in that case, for a stripe width of 0.5 mm, the relative intensity of the shortcut light can be reduced to 10%. As a rule of thumb therefore, the strip width is preferably chosen equal to the thickness of the cover plate. In general, for a given sensor, proportionality is observed between the stripe width suitable for achieving a relative intensity I of the shortcut light of 10% and the thickness T of the cover plate. The thicker the cover plate, typically a glass plate, is, the wider the stripe width E has to be.

[0071] The suitable stripe width E for achieving I=10% also depends on a lateral distances between the light source and the light sensor. As a rule of thumb, in comparison with the simulation data given above, an increase of the lateral distance by a factor F results in a suitable stripe width E that is equally increased by the factor F in comparison with the values presented for the simulations above. As an example, if the lateral distance is increased by a factor F=3 in comparison to the example of FIG. 3, then the suitable is preferably at E=3T.

[0072] FIG. 4 shows a schematic illustration of a further embodiment of a PPG device 400. The following description focuses on differences between present embodiment and the embodiments of FIGS. 1 and 2. Reference labels differing only in the first digit from those used in FIGS. 1 and 2 are used to indicate components of the PPG device 400 which are also comprised by the embodiment of the PPG device 100 of FIG. 1 or the PPG device 200 of FIG. 2. Reference is thus additionally made to the description of FIG. 1 and FIG. 2 for such features, unless differences are explained in the following.

[0073] The PPG device 400 of FIG. 4 has two light sources 406.1 and 406.2, which in the present case are identical in their physical characteristics, in particular in their emission wavelength and in the fact that they provide unpolarized light. The first and second light sources 406.1 and 406.2 are arranged to the left and to the right of the light sensor 412 on the base plate 404. Separation walls 420.1 and 420.2 are provided between the first and second light sources 406.1 and 406.2, respectively, and the light sensor 412.

[0074] Two polarization filters 407.1 and 407.2 are provided on top of the silicone layers 422 and the light sources 406.1 and 406.2, respectively. The have also been referred to as first polarization filters and serve to provide polarized source light. In the present example, both polarization filters 407.1 and 407.2 allow the transmission of linearly polarized light with a first direction that is only schematically indicated in FIG. 4. As is well known, the polarization vector of a light wave is in a plane that is perpendicular to the direction of propagation of the light wave. The actual direction of polarization of the source light provided behind the polarization filters 407.1 and 407.2 therefore points in a direction perpendicular to the paper plane of FIG. 4. The optical blocking arrangement 418 comprises a further polarization filter, which has been referred to as the second polarization filter above. The polarization filter 418 is arranged on top of the light sensor 412 and the silicone layer 422. It allows transmission only of linearly polarized light that has a polarization vector in the paper plane of FIG. 4 and parallel to the cover plate 416. In particular, the allowed polarization direction of the second polarization filter 407.2 is orthogonal to the allowed polarization direction of the first polarization filter 407.1.

[0075] The PPG device 400 makes use of the depolarizing effects of scattering by biological tissue to block the propagation of the second source light fraction by the second polarization filter 418. Specifically, since scattering of incident polarized light by biological tissue depolarizes the light, and thus the detected backscattered light is substantially unpolarized, polarized source light that is backscattered or reflected by inorganic material used for the cover plate can be filtered out by the second polarization filter 418 because it has not lost its high degree of polarization achieved by the first polarization filters 407.1 and 407.2. By providing the optical blocking arrangement in the form of the polarization filter 418 in the propagation path of the second source light fraction between the cover plate 416 and the light sensor 412, i.e., after scattering or reflection of the source light by the cover plate 416 towards the light sensor, a blocking of the propagation of source light of the polarization induced by polarization filters 407.1 and 407.2 is achieved. On the other hand, source light depolarized by scattering at the external object includes at least a light portion of roughly 50% of its intensity which does not have the polarization induced by polarization filters 407.1 and 407.2. This portion also contains the desired signal and thus the vital-signal information.

[0076] In alternative embodiments not shown, the polarizing filters allow a passing of source light of respective first and second mutually exclusive circular or ellipsoidal polarizations.

[0077] FIG. 5 shows an illustration of a first embodiment of the PPG device 500 that is arranged in a watch 510. The only difference between the PPG device 500 and the PPG device 200 with the absorption elements 218.1, 218.2, as shown in FIG. 2, is that the PPG device 500 is arranged in the watch 510 and connected with a processing unit 520.

[0078] The processing unit 520 is configured and arranged to receive the sensor signal 530 and to process the sensor signal 530 in order to provide a PPG information signal 540 indicative of PPG characteristics, which can be displayed by the watch 510 upon a respective activation of the PPG device 500 by a user of the watch 510.

[0079] The cover plate 160 of the PPG device 500 protects the PPG device 500 against physical contact with the user as well as against moisture of the arm or wrist of the user.

[0080] In summary, the invention relates to a photoplethysmography device that comprises a light source configured to direct source light towards an external object; a light sensor arranged and configured to provide a sensor signal indicative of an intensity of a first source light fraction, which has been scattered by the external object; a casing for housing the light source and the light sensor, and having a cover plate transparent for the source light and an outer face to be facing the external object; and an optical blocking arrangement in the casing between the at least one light source and the outer face of the cover plate and configured to block a second source light fraction on its propagation path extending from the light source to the outer face of the cover plate and from the outer face of the cover plate to the light sensor without leaving the casing.

[0081] While the present invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0082] In particular the invention is not restricted to the use of a whole medical apparatus containing a PPG processing unit or to monochromatic light sources. The invention is furthermore not restricted to medical applications.

[0083] In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality.

[0084] A single step or other units may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0085] Any reference signs in the claims should not be construed as limiting the scope.