PHOTOPLETHYSMOGRAPHY APPARATUS FOR VITAL S IGN MONITORING

Technical Field

The disclosure relates to a photoplethysmography apparatus using photoplethysmography to examine vital signs , for example the heartrate of a patient .

Background

Vital signs such as heart rate , respiratory rate , blood pressure , body temperature , and oxygen saturation are essential for early detection of a significant illness . Many of the existing methods that are used to monitor the aforementioned vital signs are camera-based .

In the field of vital sign monitoring, a favorable and established measuring method is the use of photoplethysmography which is an uncomplicated and inexpensive optical measurement method that allows , for example , heartrate monitoring . Photoplethysmography is a non- invasive technology that uses a light source in a photon detector at the surface of skin to measure the volumetric variations of blood circulation .

For example , the arm/ skin is irradiated from light of a light source and a reflection measurement is performed by means of a photodetector . The influence of the pulsating blood flow on the ref lection/absorption behaviour of the photons emitted by the light source is used to draw conclusions about vital parameters , such as heartrate , etc . A photoplethysmography apparatus provides a photoplethysmogram ( PPG) which is an optically obtained plethysmogram that can be used to detect blood volume changes in the microvascular bed of tissue . A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption . With each cardiac cycle the heart pumps blood to the periphery . Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue . I f the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus , as a small secondary peak .

The change in volume caused by the pressure pulse is detected by illuminating the skin with the light and then measuring the amount of light either transmitted or reflected to a photodetector . Each cardiac cycle appears as a peak in the photopie thy smogram .

The main problem of a photoplethysmography apparatus based on transmitting light to a body and evaluating the reflected light is that the signal-to-noise ratio ( SNR) is insuf ficient for complex measurements , especially when the evaluation of PPT ( Pulse Propagation Time ) waveform of a photoplethysmogram is necessary . The main reason is that no "depth-resolved" measurement can be made . In this way, refractions from neighboring areas are superimposed on the signal modulated by, for example , blood flow .

There is a desire to provide a photoplethysmography apparatus which allows a signal-to-noise optimi zed evaluation of vital parameters in di f ferently deep layers of the skin . Summary

A photoplethysmography apparatus that allows the acquisition of vital parameters in selected areas of a human or animal body with a high signal-to-noise ratio is speci fied in claim 1 .

A photoplethysmography apparatus comprises a light out- coupling device for coupling light out of the apparatus and transmitting the light towards a target obj ect , and a light in-coupling device for receiving light reflected at the target obj ect and coupling the reflected light in the apparatus . The light out-coupling device is configured to transmit the light in di f ferent transmitting directions with di f ferent intensities , and/or the light in-coupling device is configured to receive the reflected light from di f ferent receiving directions with di f ferent intensities .

The light out-coupling device is configured as an emitter device having a spatial radiation characteristic, like a transmitting antenna, and/or the light in-coupling device is configured as a receiver device having a spatial radiation characteristic, like a receiving antenna . The apparatus allows vital sign monitoring with signal to noise optimi zation in that the apparatus can emit solid angle resolved light and/or detect solid angle resolved light after reflection from a body . This property of the apparatus allows signal intensities from spatial regions of no interest to be excluded . Either light is emitted by the light out-coupling device only weakly, i . e . with low intensity, into solid angle regions of low interest and/or reflected light is received by the light in-coupling device only weakly, i . e . with low signal intensity, from solid angle regions of low interest . On the other hand, light is emitted by the light out-coupling device with a preferential transmitting direction and thus with high intensity into solid angle regions of high interest and/or light is received by the light in-coupling device with a preferential receiving direction and thus with high intensity from a solid angle region of interest .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises an optical barrier being arranged between the light out-coupling device and the light in-coupling device . The optical barrier separates the light out-coupling element from the light in-coupling element . This prevents light emerging from the light-output element from entering the light-input element as scattered light , and thus ensures that the light received by the light in-coupling element is almost exclusively light that is reflected from structures of the human or animal body .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a control device for controlling the light-out coupling device to transmit the light in at least a preferred one of the di f ferent transmitting directions and/or for controlling the light-in coupling device to receive the reflected light from at least a preferred one of the di f ferent receiving directions .

By using spatially resolved emission mechanisms , it is possible , firstly, to minimise the extinction in spatial regions of no interest . By using spatially resolved detection mechanisms , it may be possible in a second step to highly suppress the residual information from these uninteresting areas by means of highly spatial angle selective detection . According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a light source that is optically connected to the light out-coupling device to generate the light to be coupled out of the apparatus by the light coupling-out device . The light source may comprise a laser or a light-emitting diode to generate the light to be coupled out of the apparatus by the light out-coupling device . The laser enables to irradiate a human or animal body with coherent light . A light-emitting diode may be used i f the generation of incoherent light is acceptable .

According to an embodiment of the photoplethysmography apparatus , the light out-coupling device comprises a plurality of light out-coupling elements . Each of the plurality of light out-coupling elements has another preferred transmitting direction . It is thus possible to emit the light generated by the light source to a human or animal body in a particular preferred transmitting direction depending from the light out-coupling element that receives the light from the light source .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a plurality of first controllable beam splitters being arranged between the light source and the plurality of the light out-coupling elements . The plurality of the first controllable beam splitters is configured to be controlled by the control device to direct the light received from the light source to one of the plurality of the light out-coupling elements .

The beam splitters enable the light generated by the light source to be guided to a particular one of the light out- coupling elements in dependence from the radiation direction of interest . It is thus possible to radiate the generated light with high intensity in a speci fic direction onto the human or animal body whose vital parameters are to be monitored by the apparatus .

According to an embodiment of the photoplethysmography apparatus , the light in-coupling device comprises a plurality of light in-coupling elements . Each of the plurality of light in-coupling elements has another preferred receiving direction . In addition to the targeted irradiation of certain areas of a body by the light out-coupling elements , the light in-coupling elements enable the targeted reading of certain preferred areas of the body that were previously irradiated and now reflect the light back . This suppresses the broad background noise from other non-interesting areas .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a plurality of second controllable beam splitters being arranged between the photodetector and the plurality of the light out-coupling elements . The plurality of the second controllable beam splitters is configured to be controlled by the control device to direct the light received from one of the plurality of the light in-coupling elements to the photodetector .

The second beam splitters enable to receive the reflected light from a particular direction of interest . It is thus possible to receive the light reflected from the area of interest of the human or animal body whose vital parameters are to be monitored by the apparatus with high intensity .

According to an embodiment of the photoplethysmography apparatus , the light out-coupling device and/or the light in- coupling device is configured as an optical phased array . In this embodiment , each light out-coupling element comprises a respective phase shi fting circuit so that the phase of the output of each light out-coupling element can be tuned . The respective phase shi fting circuits may be configured, for example , so that the respective phase delay increases linearly from the left to the right , supporting a flat phase front tiled to the left and a collimated beam in that direction .

According to an embodiment of the photoplethysmography apparatus , each of the light out-coupling elements and/or each of the light in-coupling elements is configured as a nano-antenna structure . The use of nanoantennas in particular enables the realisation of ef ficient and highly integrated light out-coupling elements .

According to another embodiment of the photoplethysmography apparatus , each of the light out-coupling elements and/or each of the light in-coupling elements is configured as an optical grating coupler . In particular, i f the complexity of an optical phased array is to be avoided and i f a reduced resolution of the system is acceptable , the depth resolution can be realised - also via photonic integration - via "directional" grating couplers .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a photodetector to receive the reflected light from the light in-coupling device . The photodetector is connected to the light in-coupling device to receive the light reflected at structures of the body . The photodetector may generate an electrical output signal as a function of the intensity of the received light . According to a possible embodiment of the photoplethysmography apparatus , the apparatus comprises a reference path that allows the reflected light and thus the reception response generated by the photodetector to be compared with the emitted light so that the reception response can be better evaluated . For this embodiment , the photoplethysmography apparatus comprises a second photodetector to at least partially receive the light from the light source .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises a third controllable beam splitter being arranged between the light source and the second photodetector . The third beam splitter makes it possible to feed the light beam generated by the light source to the light out-coupling elements or to the reference path and thus to the second photodetector .

According to an embodiment of the photoplethysmography apparatus , the apparatus comprises at least a second light source . The light source is configured to generate the light with a first wavelength . The at least one second light source is configured to generate the light with at least a second wavelength being di f ferent from the first wavelength .

Light of di f ferent wavelengths selectively interacts with di f ferent body components depending on the wavelength . For example , light of a first wavelength is particularly sensitive to haemoglobin, while light of a second wavelength is selective for other blood components , such as iron . The photoplethysmography apparatus thus enables selective measurement resolution in addition to "depth-resolved" spatial measurement of vital parameters .

Additional features and advantages of the photoplethysmography apparatus are set forth in the detailed description that follows . It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework for understanding the nature and character of the claims .

Brief Description of the Drawings

The accompanying drawings are included to provide further understanding, and are incorporated in, and constitute a part of , the speci fication . As such, the disclosure will be more fully understood from the following detailed description, taken in conj unction with the accompanying figures in which :

Figure 1A shows an embodiment of a photoplethysmography apparatus having the capability to emit solid angle resolved light and/or detect solid angle resolved light ;

Figure IB illustrates an embodiment of a photoplethysmography apparatus comprising a plurality of light sources for generating light of di f ferent wavelength;

Figure 2 shows a possible setting of light out-coupling device and a light in-coupling device of a photoplethysmography apparatus for minimising the extinction in spatial regions of no interest by using spatially resolved emission mechanisms , and for suppressing the residual information from these uninteresting areas through spatial angle selective detection;

Figure 3 illustrates a measurement method for monitoring vital parameters in various phases by a detection of the relevant areas of a target obj ect , the derivation of the relevant depths and sensitive wavelengths and the use of a continuously optimised measurement profile to determine the vital parameters of interest ;

Figure 4 shows a possible embodiment of a photoplethysmography apparatus implemented as a photonic on-chip variant using optical phased arrays for the directional reception and the directional radiation of the light ; and

Figure 5 shows a possible embodiment of a photoplethysmography apparatus implemented as a photonic on-chip variant using optical grating couplers for the directional reception and the directional radiation of the light .

Detailed Description of the Embodiments

An embodiment of a photoplethysmography apparatus 10 which enables monitoring of vital signs with signal to noise optimi zation by preferentially emitting a light signal with high intensity into areas of interest of a body to be examined, whereby the light in areas of low interest is only irradiated with low light intensity or not at all , or by receiving light signals reflected from structures of the body from a preferred direction, and thus being able to detect them with spatial angle resolution, is shown in Figure 1A.

The photoplethysmography apparatus 10 allows determing of pulse rate , breathing rate and blood constituents in a human or animal body by using light to measure varaitions in blood volume in the body . Most soft tissue in the human or animal body transmits and reflects both visible and near-infrared light . After the light incident on areas of the body interacts with tissue structures and blood constituents , time varying changes of light intensity having a relation with blood volume , known as a plethysmogram, can be detected with the photoplethysmography apparatus .

The received time varying light intensity signal will depend, inter alia, on the optical properties of the tissues and blood at the body site , and the wavelength of the light source . The received signal course results from the fact that blood absorbs light and the amount of light absorbed, and hence the intensity of remaining light detected, varies in relation with the volume of blood illuminated . Thus , variation in the plethysmogram is caused by the variation in blood volume flowing in the tissue .

The photoplethysmography apparatus 10 shown in Figure 1A comprises a light source 100 to generate light that is to be incident on a target obj ect 20 , for example an area of skin 20 of a human or animal body . The apparatus 10 comprises a light out-coupling device 110 for coupling light out of the apparatus 10 and for transmitting the light towards the target obj ect . The apparatus 10 furhter comprises a light incoupling device 120 for receiving light reflected at the target obj ect and for coupling the reflected light in the apparatus 10 .

The light out-coupling device 110 is configured to transmit the light in di f ferent transmitting directions with di f ferent intensities and/or the light in-coupling device 120 is configured to receive the reflected light from di f ferent receiving directions with di f ferent intensities . That means that the light out-coupling device 110 is configured as a transmitting antenna having a directivity, i . e . the light out-coupling device 110 irradiates the light in a preferred direction with a higher intensity than in other transmitting directions . The light in-coupling device 120 is configured as a receiving antenna having a directivity, i . e . the light incoupling device 120 receives the reflected light from a preferred receiving direction with a higher intensity than light from other directions .

Such a design of the light out-coupling device 110 and/or the light in-coupling device 120 enables the apparatus to emit solid angle resolved light and/or to detect solid angle resolved reflected light . Referring to Figure 1A, the light irradiated by the light out-coupling device/emitter 110 in di f ferently directed radiation lobes to a body area, for example an area of skin 20 , is reflected at di f ferent structures in the various layers 21 , 22 , 23 of skin 20 . The reflected light is received from di f ferent preferred directions according to the radiation characteristic of the light in-coupling device/receiver 120 .

By using spatial-angle-resolved emission and/or detection mechanisms , it is possible , on the one hand, to minimise the extinction in spatial regions of a body that are not of interest and, in a second step, to strongly suppress the residual information from these uninteresting regions of the body by means of highly spatial-angle-selective detection . In this way, signal intensities from spatial regions of a body to be examined that are of no interest can be nearly completely excluded .

The photoplethysmography apparatus 10 comprises a control device 200 for controlling the light-out coupling device 110 to transmit the light in the preferred transmitting direction with a higher intensity than in the other transmitting directions and/or for controlling the light-in coupling device 120 to receive the reflected light from the preferred receiving direction with a higher intensity than from the other directions .

The light source 100 is optically connected to the light out- coupling device 110 to generate the light to be coupled out of the apparatus 10 by the light coupling-out device 110 . The light source 100 may comprise a laser or a light-emitting diode . In particular, the light source 100 may be configured as a laser light source to emit coherent light , or the light source 100 may be embodied as a plurality of light-emitting diodes to emit incoherent light .

The light out-coupling device 110 and the light in-coupling device 120 may be separated from each other by an optical barrier 190 being arranged between the light out-coupling device 110 and the light in-coupling device 120 . The light out-coupling device 110 and the light in-coupling device 120 are located in an aperture of a housing 180 of the apparatus 10 behind a window 170 . The photoplethysmography apparatus 10 comprises a photodetector 130 to receive the reflected light from the light in-coupling device 120 . The photodetector 130 may be optically connected to the light in-coupling device 120 via an optical waveguide . The photodetector 130 is configured to generate an electrical output signal in response to the intensity of the received reflected light . The photodetector 130 may comprise a plurality of photodiodes to detect the light coupled in the light in-coupling device 120 .

The light out-coupling device 110 may comprise a plurality of light out-coupling elements 111 , wherein each of the plurality of the light out-coupling elements 111 has another preferred transmitting direction . In this embodiment , the preferred out-coupling direction of the light is determined by feeding the light generated by the light source 100 to the corresponding light out-coupling element 111 providing the preferred light out-coupling direction .

The photoplethysmography apparatus 10 comprises a plurality of controllable beam splitters 151 . The controllable beam splitters 151 are arranged between the light source 100 and the plurality of the light out-coupling elements 111 . In this embodiment of the photoplethysmography apparatus 10 , the plurality of the first controllable beam splitters 151 is configured to be controlled by the control device 200 to direct the light received from the light source 100 to at least one of the plurality of the light out-coupling elements 111 that emits the light in the direction of interest , i . e . to the area of the animal or human body of interest .

The light in-coupling device 120 may comprise a plurality of light in-coupling elements 121 , wherein each of the plurality of light in-coupling elements 121 has another preferred receiving direction . In this embodiment , the preferred incoupling direction of the reflected light is determined by feeding the light from the light-in coupling element that receives the reflected light from the direction of interest to the photodetector 130 .

The photoplethysmography apparatus 10 comprises a plurality of controllable beam splitters 152 being arranged between the photodetector 130 and the plurality of the light out-coupling elements 121 . The plurality of the controllable beam splitters 152 is configured to be controlled by the control device 200 to direct the light received from one of the plurality of the light in-coupling elements 121 that receives the light from the direction of interest , i . e . from the area of the animal or human body of interest , to the photodetector 130 .

Referring to Figure 1A, the photoplethysmography apparatus 10 may optionally comprise a reference light path 210 . The reference light path 210 is optically connected to a photodetector 140 to receive the light from the light source 100 at least partially, preferably knowing the ratio of the received power/ intensity . The reference light path 210 and the photodetector 140 allow to compare the intensity of the reflected light with the intensity of the light generated by the light source 100 . With the help of the reference light path 210 , the response received from the light in-coupling device 120 and the photodetector 130 can thus be better estimated .

In the case that photoplethysmography apparatus 10 is provided with the reference light path 210 and the photodetector 140 , the photoplethysmography apparatus 10 comprises a controllable beam splitter 160 , in order to direct the light generated by the light source 100 either to the light out-coupling device 110 or the photodetector 140 . The controllable beam splitter 160 is arranged between the light source 100 and the photodetector 140 . The controllable beam splitter 160 is controlled by the control circuit 200 to direct the light from the light source via reference path 210 to photodetector 140 or to beam splitter 151 for guiding the light to one of the light out-coupling elements 111 .

Figure IB shows an embodiment of a photoplethysmography apparatus 10 that comprises the same components as shown and explained for the embodiment of Figure 1A except that the apparatus of Figure IB comprises at least a second light source 100 ' in addition to the light source 100 . The light source 100 and the at least one second light source 100 ' are coupled to the controllable beam splitter 160 by a beam combiner 220 . The light source 100 is configured to generate light with a first wavelength . The at least one second light source 100 ' is configured to generate light with at least a second wavelength being di f ferent from the first wavelength .

According to the embodiment of a measurement method that may be performed with the photoplethysmography apparatus 10 of Figure IB, the light sources 100 , 100 ' are configured to generate the light to be emiited with di f ferent wavelengths Xl , X2 , etc . . This measurement principle makes use of the fact that di f ferent areas/ skin layers or structures in a human or animal body, particularly various tissue structures in the skin, show di f ferent ref lection/absorption behaviour for light/photons of di f ferent wavelengths . With this embodiment of the apparatus , it is therefore possible to examine certain tissue structures with the appropriate wavelength .

Figure 2 illustrates a measuring method for using the apparatus 10 to examine speci fic areas of the human body under the skin 20 by means of photoplethysmography through irradiation with solid angle resolved light and subsequent solid angle resolved detection of the reflected light . Figure 2 shows an area of the skin 20 with di f ferent layers , in particular the epidermis 21 , the dermis 22 and the hypodermis 23 .

The light out-coupling device 110 has a plurality of light out-coupling elements 111 which radiate the light generated by the light source 100 in di f ferent directions . The light output coupling device 110 thus has a spatial radiation characteristic . The light in-coupling device 120 has a plurality of light in-coupling elements 121 which can couple the light reflected from the structures of the skin into the apparatus from di f ferent preferred directions .

Figure 2 further illustrates how the di f ferent areas or layers of the skin are scanned with the photoplethysmography apparatus 10 . The signal received at the photodetector essentially represents the absorption and reflection of the emitted light from areas in the skin 20 , where high emission sensitivity E and high detection sensitivity D overlap . On the other hand, areas of low emission intensity E and/or low detection sensitivity D are hardly or only minimally considered in the signal detected at the photodetector coupled to the light in-coupling device 120 . The excitation response from "uninteresting" areas can thus be suppressed or at least signi ficantly minimi zed, allowing the vital signs of interest to be acquired with an increased signal-to-noise ratio .

Figure 3 illustrates another measurement procedure that can be performed with the apparatus . The basic idea of the illustrated method is to scan spatial elements in the body to be examined, for example in the skin area to be examined, with the light out-coupling device/emitter 110 as well as with the light in-coupling device/receiver 120 .

In a phase 1 of the measuring method, light can be directed speci fically to the individual spatial elements Al , . . . , D5 of a body/ skin area, for example , as shown in Figure 2 , by controlling the corresponding light out-coupling elements/emitters 111 . The light in-coupling device 120 detects the light reflected from each of the di f ferent spatial regions . Figure 3 shows in phase 1 an example of an increased emission on the space element Al or an increased emission on the space element D3 and the respective detection of the light reflected from the space elements Al , . . . , D5 . In the case of preferred light emission onto the space element Al , the received reflected light preferably originates from the space element Al , and in the case of preferred light emission onto the space element D3 , the received reflected light preferably originates from the space element D3 . Thus , in a first step in phase 1 , the relevant depth range in which the body structures to be examined, for example ateria, are located can be identi fied .

In phase 2 of the method, the derivation of the relevant depths and sensitive wavelengths for the examination of the di f ferent tissue areas takes place . Here , it is taken into account that a structure in the hypodermis 23 is sensitive for incident light with wavelength Xl, whereas another structure in the hypodermis 23 is sensitive for light with wavelength X2. In phase 2, the positions of ateries for blood pressure/heart rate measurement, tissue areas for tissue blood glucose measurement or, if applicable, positions for determining muscle metabolism can thus be identified.

In phase 3, a continuously optimized measurement profile can be determined. For example, certain space elements, such as space element D3 or A3 shown in Figure 3, can be irradiated with appropriately directed light. Depending on the structures to be examined, light of a suitable wavelength is used for this purpose. This results in a punctual excitation and detection. Furthermore, a specific spatial element, for example spatial element D3, can be specifically irradiated with corresponding light (punctual excitation) . This is followed by scanning over adjacent space elements, for example space elemens DI, D2, D4 and D5, examining the scatter received by the light from these space elements.

Figure 4 shows an embodiment of the photoplethysmography apparatus 10 which is realized as a photonic on-chip variant. The light source 100 may comprise light-emitting diodes or may be be realized as a laser, for example, by hybrid integration of a laser on a silicon-on-insulator (SOI) optical board. Figure 4 further shows controllable beam splitters 151 to feed the light beam generated by light source 100 to the various light out-coupling elements 111, and controllable beam splitter 160 to direct the generated light either to the beam splitters 151/the light out-coupling device 110 or to the photodetector 140. The light out- coupling device 110 and/or the light in-coupling device 120 may be realized as an optical phased array comprising phase adj ustment/of f set elements 112 , 122 and out-coupler/antenna elements 113 , 123 . In particular, each of the light out- coupling elements 111 and/or each of the light in-coupling elements 121 may be configured as a nano-antenna structure . An integrated proj ection system based on a two-dimensional optical phased array, where p-i-n phase shi fters with 200 Mhz bandwidth are used for rapid beam steering may enable real time proj ection of an image by fast vector scanning .

Figure 5 shows another embodiment of the photoplethysmography apparatus 10 which is reali zed as a photonic on-chip variant . In this embodiment , the light out-coupling elements 111 and/or the light in-coupling elements 121 are reali zed as optical grating couplers 114 , 124 . This implementation is especially recommended, when the complexity of an optical phased array is to be avoided and i f a reduced resolution of the system is acceptable . For the other components , reference is made to Figure 4 .

The technical advantages of the photoplethysmography apparatus are summari zed below .

The proposed photoplethysmography apparatus enables system response from irrelevant areas/noise areas of a body of a patient to be suppressed . Moreover, the photoplethysmography apparatus enables a depth/distance resolved provision of vital parameters so that , for example , a di f ferentiation of blood pathways/vessels in di f ferent depths of body tissue is possible . In particular, blood response in arteries/veins can be separated from blood response in "normal" vessels . Measured parameters/conditions can be recorded in di f ferent body areas , for example oxygen saturation in arteries/veins versus dermis . Moreover, the photoplethysmography apparatus 10 allows measurement of vital parameters with improved signal-to-noise ratio , increased precision and accuracy in healthcare parameters , because most of the DC component in a PPG signal is not detected . The PPG waveform comprises a pulsatile (AC ) physiological waveform attributed to cardiac synchronous changes in the blood volume with each heartbeat , and is superimposed on a slowly varying ("DC" ) baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermal regulation . An improved signal-to-noise ratio also makes a more detailed evaluation of the "PPG waveform" possible , which makes a large number of measurement parameters accessible .

The embodiments of the photoplethysmography apparatus disclosed herein have been discussed for the purpose of familiari zing the reader with novel aspects of the photoplethysmography apparatus . Although preferred embodiments have been shown and described, many changes , modi fications , equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims .

In particular, the design of the photoplethysmography apparatus is not limited to the disclosed embodiments , and gives examples of many alternatives as possible for the features included in the embodiments discussed . However, it is intended that any modi fications , equivalents and substitutions of the disclosed concepts be included within the scope of the claims which are appended hereto . Features recited in separate dependent claims may be advantageously combined . Moreover, reference signs used in the claims are not limited to be construed as limiting the scope of the claims .

Furthermore , as used herein, the term "comprising" does not exclude other elements . In addition, as used herein, the article "a" is intended to include one or more than one component or element , and is not limited to be construed as meaning only one .

This patent application claims the priority of German patent application with application No . 102022121640 . 3 , the disclosure content of which is hereby incorporated by reference .

References

10 photoplethysmography apparatus

20 skin

21 epidermis

22 dermis

23 hypodermis

100 light source

110 light out-coupling device

111 light out-coupling element

120 light in-coupling device

121 light in-coupling element

130 photodetector

140 photodetector

151 controllable beam splitter

152 controllable beam splitter

160 controllable beam splitter

170 window

180 housing

190 optical barrier

200 control circuit

210 reference light path

220 beam combiner