Patent Publication Number: US-2022225886-A1

Title: Optical vital signs sensor

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical vital signs sensor for monitoring vital signs of a user. 
     BACKGROUND OF THE INVENTION 
     Optical heart rate sensors are well known to monitor or detect vital signs like a heart rate of a user. Such a heart rate sensor can be based on a photoplethysmograph (PPG) sensor and can be used to acquire a volumetric organ measurement. By means of pulse oximeters, changes in light absorption of a human skin is detected and based on these measurements a heart rate or other vital signs of a user can be determined. The PPG sensors comprise a light source like a light emitting diode (LED) which is emitting light into the skin of a user. The emitted light is scattered in the skin and is at least partially absorbed by the blood. Part of the light exits the skin and can be captured by a photodiode. The amount of light that is captured by the photo diode can be an indication of the blood volume inside the skin of a user. A PPG sensor can monitor the perfusion of blood in the dermis and subcutaneous tissue of the skin through an absorption measurement at a specific wave length. If the blood volume is changed due to the pulsating heart, the scattered light coming back from the skin of the user is also changing. Therefore, by monitoring the detected light signal by means of the photodiode, a pulse of a user in his skin and thus the heart rate can be determined. Furthermore, compounds of the blood like oxygenated or de-oxygenated hemoglobin as well as oxygen saturation can be determined. 
       FIG. 1  shows a basic representation of an operational principle of a heart rate sensor. In  FIG. 1 , a heart rate sensor is arranged on an arm of a user. The heart rate sensor  100  comprises a light source  110  and a photo detector  120 . The light source  110  emits typically green light onto or in the skin  1000  of a user. Some of the light is reflected and the reflected light can be detected by the photo detector  120 . Some light can be transmitted through tissue of the user and be detected by the photo detector  120 . Based on the reflected or transmitted light, vital signs of a user like a heart rate can be determined. 
     WO 2006/110488 A2 shows a PPG sensor with coupling gel proximate to a light source of the PPG sensor. 
     US 2012/0078116 A1 discloses an optical vital signs sensor with a contact surface, a light source and a photo detector as well as a filter adapted to remove part of the light spectrum. 
     EP 2 139 383 B1 discloses an optical vital signs sensor with a light source, a photo detector and a filter for removing part of the lights spectrum. 
     JP 2001025462 A discloses an optical vital signs sensor with a light source, a photo detector and a filter in form of a coated acrylic board. 
     US 2014/0243648 A1 discloses an optical vital signs sensor with a light source, a photo detector and a colored converting plate. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an optical vital signs sensor which is able to more efficiently detect vital signs of a user. 
     According to an aspect of the invention, an optical vital signs sensor is provided. The optical vital signs sensor is configured to measure or determine vital signs of a user. The optical vital signs sensor comprises a contact surface and at least one light source configured to generate light. The light is directed towards a skin of a user. Furthermore, at least one photo detector unit is configured to detect light which is indicative of a reflection of the light beam from the at least one light source in or from the skin of the user. Between the light source and the contact surface, a color converting plate is provided which converts a color of the light from the light source. The color converting plate unit comprises an angle selective optical coating or film which is able to reflect or redirect light having a large angle of incidence and to transmit light having a small angle of incidence. The color converting plate unit comprises a diffusion chamber which is configured to recycle light having a large angle of incidence (i.e. light which is reflected or redirected by the angle selective optical coating) and which is arranged around the at least one light source. 
     The diffusion chamber has a recycling function, namely it is re-trying to convert the unconverted light. 
     According to a further aspect of the invention, the color converting plate comprises a low-wave pass filter coating or film which is able to transmit light having a long wavelength while reflecting light having short wavelengths. 
     According to a further aspect of the invention, the at least one light source comprises an InGaN light emitting diode. 
     According to a further aspect of the invention, a method of producing an optical vital signs sensor configured to measure or determine vital signs of a user is provided. A contact surface of the optical vital signs sensor is provided. The contact surface is configured to be placed directly against a skin of a user. At least one color converting plate unit is arranged in or at the contact surface. At least one light source configured to generate light such that the light generated by the at least one light source is directed towards a skin of a user via the at least one color converting plate unit. At least one photo detector unit is provided. The photo detector unit is configured to detect light which is indicative of a reflection of light emitted via the at least one color converting plate unit in or from the skin of a user. The color converting plate unit comprises an angle selective optical coating or film which is able to reflect or redirect light having a large angle of incidence and to transmit light having a small angle of incidence. The color converting plate unit comprises a diffusion chamber which is configured to recycle light having a large angle of incidence (i.e. light while is reflected or redirected by the angle selective optical coating) and which is arranged around the at least one light source. 
     According to an aspect of the invention, the vital signs sensor comprises a vital signs sensor which can be a LED based PPG sensor. The LED light penetrates the skin of the user and some of it can reach a photo detector. The output of the photo detector can be used to monitor a blood volume fraction and blood compounds like oxygenated and de-oxygenated hemoglobin. In particular, the amount of absorption or reflectance of the light from the LED light source can be used to determine the heart rate as well as the blood volume fraction or blood compounds. The heart rate relates to the blood volume fraction. Furthermore, the PPG sensor according to the invention is therefore an optical sensor allowing a non-invasive measurement of vital signs of a user. 
     It shall be understood that a preferred embodiment of the present invention can also be a combination of the dependent claims or above embodiments or aspects with respective independent claims. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following drawings: 
         FIG. 1  shows a basic representation of an operational principle of a vital sign monitoring system, 
         FIG. 2  shows a schematic representation of an optical vital signs sensor according to an aspect of the invention, 
         FIG. 3  shows a schematic representation of an optical vital signs sensor according to a further aspect of the invention, 
         FIG. 4  shows a schematic representation of an optical vital signs sensor according to a further aspect of the invention, 
         FIG. 5  shows a graph indicating the function of the transmittance over the wavelength of the optical vital signs sensor according to  FIG. 4 , 
         FIG. 6  shows a basic representation of part of an optical vital signs sensor according to a further aspect of the invention, 
         FIG. 7  shows a basic representation of a part of an optical vital signs sensor according to a further aspect of the invention, 
         FIG. 8  shows a graph for illustrating a relative DC power and AC/DC signal of an optical vital signs sensor according to an aspect of the invention, 
         FIG. 9  shows a basic representation of a vital signs sensor according to a further aspect of the invention, and 
         FIG. 10  shows a basic representation of a vital signs sensor according to a further aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     According to an aspect of the invention, an optical vital signs sensor is provided which is based on a photoplethysmograph PPG sensor. Such a PPG sensor is depicted in  FIG. 1 . A light source  110  emits light onto or into the skin  1000  of a user and some of the light is reflected and this reflected light can be detected by a photo detector  120 . The output of the photo detector can be analyzed to determine a heart rate or other vital signs of a user. 
     The output signal of the PPG sensor gives an indication on the blood movement in vessels of a user. The quality of the output signal of the PPG sensor can depend on the blood flow rate, skin morphology and skin temperature. In addition, optical losses in the PPG sensor may also have an influence on the quality of the output signal of the PPG sensor. The optical efficiency of the PPG sensor can depend on reflection losses when light penetrates from one media into another. Furthermore, a scattering of light at the surface of the skin of the user may also have an influence on the optical efficiency of the PPG sensor. 
     The PPG sensor or optical vital signs sensor according to an aspect of the invention can be implemented as a wearable device which can be arranged or attached to a skin of a user. The wearable device can be a wrist device (like a watch or smart watch). A device worn behind the ear of a user, e.g. like a hearing aid. 
     According to an aspect of the invention, at least one of the light sources  110  can be implemented as a phosphor converted light emitting diode LED which comprises a color converting plate unit which is arranged at a contact surface of the sensor. The contact surface is that surface of the PPG sensor which is placed against the skin of a user. In other words, the color converting plate unit can be part of the contact surface and can thus be in direct contact with the skin of the user. The color converting plate unit can thus be arranged between the skin of the user and the light source or a light emitting diode LED in the light source. The color converting plate unit according to an aspect of the invention receives light and output light with a new emission spectrum. This can e.g. be performed by wavelength conversion through photo luminescence. The color converting plate can be thus implemented as wavelength conversion unit, wherein the wavelength conversion is based on photo luminescence. 
     According to the invention, an optical interface is provided between the light delivery system, namely the light source  110  and the skin  1000  of the user. This optical surface (e.g. in form of the color converting plate) is used to reduce reflectance losses and to increase the efficiency of the vital signs sensor. 
       FIG. 2  shows a schematic representation of an optical vital signs sensor according to an aspect of the invention. The PPG sensor  100  comprises a contact surface  101  which is placed in direct contact to a skin  1000  of a user. Optionally, the converting plate can be directly mounted onto the light source area. Furthermore, the PPG sensor  100  also comprises at least one light source  110  as well as at least one photo detector unit  120 . The at least one light source  110  emits light into the direction of the contact surface  101 . Between the at least one light source  110  and the contact surface  101 , a color converting plate unit  200  is provided. The color converting plate unit  200  can for example be implemented as a ceramic phosphor color converting plate. The at least one light source  110  can be implemented as a InGaN light emitting diode which is down-converted to for example yellow (having a wavelength of 570 nm) for example by means of the color conversion plate unit  200 . As the color converting plate unit  200  is arranged at the contact surface  101  of the sensor, the color converting plate  200  will also be in direct contact to the skin  1000  of a user when the PPG sensor is placed onto the skin  1000  of the user. 
     According to an aspect of the invention, the light from the at least one light source  110  which is emitting from the contact surface  101  of the PPG sensor should preferably have a wavelength in the green/yellow range (e.g. in the area of 500 to 600 nm). This can either be achieved by a light source or a light emitting diode which is directly outputting light at this wavelength or this can be achieved by using a color converting plate unit  200  to change the color of the light from the light source to a desired color temperature. In other words, the wavelength of the light from the light source  110  is changed by the color converting plate  200 . 
       FIG. 3  shows a schematic representation of an optical vital signs sensor according to a further aspect of the invention. The PPG sensor according to  FIG. 3  substantially corresponds to the PPG sensor according to  FIG. 2  with a coating  210  on top of the color converting plate  200  and optionally with a diffusing chamber  220 . The coating or layer  210  on top of the color converting plate  200  can be implemented as an angle selective film which transmits light at small angles of incidence while reflecting light at large angles of incidence. The angle selective film  210  may comprise a multi-layer thin film interference filter like a dielectric mirror. 
     The optional diffusing chamber  220  can optionally be arranged around the light source or light emitting diode  110  and is used to recycle light as shown in  FIG. 3 . 
     The light source  110  emits light and a part of this light  103  passes through the color converting plate  200  and the angle selective film  210 . Other parts of this light with different angles of incidence  104  are reflected from the angle selective film or coating  110 . In addition, further light  105  can be recycled by the diffusion chamber  220  and can be redirected towards the color converting plate unit  200  with a different angle. 
       FIG. 4  shows a schematic representation of an optical vital signs sensor according to a further aspect of the invention. The PPG sensor according to  FIG. 4  substantially corresponds to the PPG sensor according to  FIG. 4  with a diffusion chamber  220  around the light source  110  as well as a low wave pass filter LWPF  230  on top of the color converting plate  200 . The low-wave pass filter  230  can comprise a dielectric multi-layer stack which allows long wave like green/yellow light to be transmitted while reflecting short waves like blue light. According to this aspect of the invention, the low-wave pass filter coating  230  on top of the color converting plate  200  is part of the contact surface  101  of the PPG sensor such that the coating  230  is in direct contact with the skin of a user. 
     A part  103  of the light from the light source  110  passes through the color converting plate  200  and the low-wave pass filter  230  and enters the skin  1000  of a user. A further part  104   a  is reflected by the low-wave pass filter  230  and can be recycled  105   a  by the diffusion chamber  220 . 
       FIG. 5  shows a graph indicating the function of the transmittance over the wavelength of the optical vital signs sensor according to  FIG. 4 . In  FIG. 5 , a long-wave pass filter with a blue spectrum as well as a down-converted yellow spectrum A 1 , A 2 , A 3  is depicted. 
       FIG. 6  shows a basic representation of part of an optical vital signs sensor according to a further aspect of the invention. The aspect of the invention according to  FIG. 6  is a combination of the PPG sensor of  FIGS. 3 and 4 . Accordingly, a light source  110  is optionally surrounded by a diffusing chamber  220  and a color converting plate  200 . On top of the color converting plate  200 , a low wave pass filter coating  230  is provided. On top of this low wave pass filter coating  230 , an angle selective filter coating  210  is provided. With the PPG sensor according to this aspect of the invention, only green/yellow light  103  at small angles is transmitted through the two coatings  210 ,  230  while unconverted light (i.e. short wavelength pump-light  104   c ) is reflected by the low-wave pass filter coating  230 . Furthermore, converted light  106  which still has large angles of incidence is reflected by the angle selective filter coating  210 . Once again, the diffusion chamber  220  can be used to recycle light. 
       FIG. 7  shows a basic representation of a part of an optical vital signs sensor according to a further aspect of the invention. The PPG sensor according to this aspect substantially corresponds to the PPG sensor according to  FIG. 6 , wherein the order of the angle selective for a film coating  210  and the low wave pass filter coating  230  is changed. 
     According to the invention, the PPG sensor according to  FIG. 6  appears to be more effective than the PPG sensor according to  FIG. 7 . This is due to the fact that the angle selective film coating  210  is designed for a narrow wavelength range. On the other hand, with the PPG sensor according to  FIG. 7 , the low wave pass filter coating  230  is able to reflect unconverted light at large angles of incidences. 
     To further reduce the optical losses at the interface between the PPG sensor and the skin of the user, optical coupling material such as a gel, liquid or oil can be provided at the contact surface  101  of the PPG sensor. 
       FIG. 8  shows a graph for illustrating a relative DC power and AC/DC signal of an optical vital signs sensor according to an aspect of the invention. In  FIG. 8 , one important property of the output signal, namely the modulation signal is depicted. 
     The modulation signal relates to the ratio of the AC component to the DC component. The modulation AC/DC signal is important, because it is related to intrinsic properties of the skin. It covers the peek-to-peek value of the change in blood volume fraction in one heart pulse (AC signal), but also the skin-dependent reflectance (DC-component DC) which is important to know because a low reflectance can be compensated with LED power boost, preserving the same modulation signal. 
     In  FIG. 8 , the output signal of the PPG sensor is depicted. Furthermore, the influence of different parts of the body, namely pulsating arterial blood PA, non-pulsating arterial blood NA, venous blood VB and other tissue is depicted. Moreover, incident light I 0  as well as transmitted light TL and absorbed light AL is depicted. It should be noted that according to the invention, the AC signal AC represents the component that contains the information which the sensor requires in order to determine a heart rate of a user. In other words, the AC signal represents the information regarding the pulsating arterial blood, i.e. the change in the blood volume while the DC component of the output signal represents the unwanted background signal, namely the influence of the other tissue, the venous blood VB and the non-pulsating arterial blood NA. The DC component can have 0 Hz or may also have a low frequency component which can be caused by leakage light shunted from the light source to the light detector without passing through the skin or tissue of the user (static), a dynamic variation of leakage light caused by motion (dynamic) and light detected by the detector which is reflected by the tissue or skin of the user or other matter like the venous blood VB, fat, bone, water, cell membranes, etc. 
     Typically, in a PPG sensor, the AC component of the output signal is smaller than the DC component. Hence, in order to obtain a good output signal, the DC component should be minimized while the AC component should be maximized in order to achieve a maximum modulation signal. 
     In  FIG. 8 , two measurements, namely M 1  and M 2  are depicted, wherein the first measurement M 1  is measured at the minimum value of the output signal while the second value is measured at the maximum output signal. 
     The modulation signal can be expressed by the following equation: 
     
       
         
           
             
               
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     It should further be noted that the modulation signal, i.e. the AC/DC signal is sensitive towards the beam pattern and the angle of incidence. The greater the distance between the light source and the photodiode, the lower the sensitivity regarding the angle of incidence. Furthermore, according to an aspect of the invention, an angle of incidence of greater than 45° should be avoided while small beam angles around 0° and a beam angle pointing in the opposite direction as towards the photodiode can also be used. According to an aspect of the invention, an improved PPG signal can be obtained if the beam angle of the light source is &lt;±20°. 
       FIG. 9  shows a basic representation of a vital signs sensor according to a further aspect of the invention. The vital signs sensor according to an aspect of the invention according to  FIG. 9  comprises at least one light source  110 , at least one photodiode  120  as well as at least one light guide  400 . According to this aspect of the invention, the light guide  400  is arranged between the at least one light source  110  and the at least one photodiode  120 . The light guide  400  is implemented as a light transport unit  450  which is able to transport light from the at least one light source (for example a LED which is implemented as a side emitter) towards the at least one photodiode  120 . The distal end of the light transport unit  450  has an inclination  451  such that the light  111  from the at least one light source  110  is redirected towards the skin of the user  1000 . With such a light guide unit  400 , the distance between the photodiode  120  and the output end of the light guide unit  450  can be significantly reduced and a flat design with a low building height is possible. Furthermore, a color converting plate  200  as described above can be provided between the light guide  400  and the skin  1000  of a user. 
       FIG. 10  shows a basic representation of a vital signs sensor according to a further aspect of the invention. The vital signs sensor can comprise at least one light unit  110 , a photo detector  120  and an optical angle selective foil  200 . The angle selective optical foil as color converting plate unit  200  is able to allow light to transmit within a selected angle range. Alternatively, the color converting unit can also be implemented with an optical holographic light shaping diffuser or direction turning film DTF. 
     The color converting unit  200  is used to shape, direct, redirect, control or manage the light beam from the light source such that the angular range of the beam is limited or restricted. 
     Other variations of the disclosed embodiment 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. 
     In the claims, the word “comprising” does not exclude other elements or steps and in the indefinite article “a” or “an” does not exclude a plurality. 
     A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutual different dependent claims does not indicate that a combination of these measurements cannot be used to advantage. A computer program may be stored/distributed on a suitable medium such as an optical storage medium or a solid state medium, supplied together with or as a part of other hardware, but may also be distributed in other forms such as via the internet or other wired or wireless telecommunication systems. 
     Any reference signs in the claims should not be construed as limiting the scope.