Abstract:
An optical vital signs sensor configured to measure or determine vital signs of a user comprises a light source configured to generate a light beam having an angular range of angles of incidence. The light beam is directed towards the skin of the user. A photo detector is provided and is configured to detect light which is indicative of a reflection of the light beam from the light source in or from the skin of the user. The light source and the photo detector are arranged adjacent to each other and on the same side of the skin of the user. A light shaping unit is configured to shape the light beam of the light source before the light beam enters the skin by limiting the angular range of angle of incidence to less than 20°.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of International Application No. PCT/EP2015/070911, filed on Sep. 14, 2015, which claims the benefit of European Application No. 14187495, filed Oct. 2, 2014. These applications are hereby incorporated by reference herein for all purpose. 
     
    
     TECHNICAL FIELD 
       [0002]    The embodiment relates to an optical vital signs sensor for monitoring vital signs of a user. 
       BACKGROUND 
       [0003]    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. 
         [0004]    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. 
         [0005]      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 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 . 
         [0006]    U.S. 2009/033937 shows a sensor for measuring living body information having a light guide panel with a light extracting unit for outputting a first light towards the skin of a user and a light coupling pattern for guiding light from the skin towards a photo detector. 
       SUMMARY 
       [0007]    It is an object of the embodiment to provide an optical vital signs sensor which is able to efficiently detect vital signs of a user while still having a reduced building height. 
         [0008]    According to an aspect of the embodiment, 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 photoplethysmographic sensor PPG having a housing and a contact surface which is placed directly onto a skin of a user and at least one light source configured to generate a light beam having an angular range of an angle of incidence. The light beam is directed via the contact surface 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. The at least one light source and the at least one photo detector unit are arranged adjacent to each other inside the housing. A distance is present between the at least one light source and the at least one photo detector unit. The vital signs sensor furthermore comprises at least one light shaping unit arranged between the light source and the contact surface and is configured to shape the light beam from the at least one light source. This can be performed by limiting the angular range of the angle of incidence to less than 20°. The light shaping unit is a unit that is shaping, directing, controlling or managing the light beam to limit its angular range. 
         [0009]    According to an aspect of the embodiment, the light shaping unit comprises an optical film which is able to reflect or redirect light beams having a large angle and to transmit light having a small angle of incidence. 
         [0010]    According to a further aspect of the embodiment, the light shaping unit comprises a diffusion chamber which is configured to recycle light having a large angle of incidence. 
         [0011]    According to a further aspect of the embodiment, the light shaping unit comprises a separating wall having a mirror side between the at least one light source and the at least one photo detector unit. 
         [0012]    According to a further aspect of the embodiment, the light shaping unit comprises an optical refractive unit. 
         [0013]    According to a further aspect of the embodiment, the light shaping unit comprises an optical collimation plate. 
         [0014]    According to a further aspect of the embodiment, the at least one light source comprises a side emitting light emitting diode. The at least one light shaping unit comprises a light transport unit which is coupled with its first end to the at least one light source and wherein its distal end comprises an inclination such that light travelling through the light transport unit is reflected towards the skin of the user. 
         [0015]    The embodiment also relates to a method of measuring or determining vital signs of a user. A contact surface of a housing of a PPG sensor is placed directly against a skin of a user. A light beam having an angular range of angles of incidence is generated by at least one light source in the PPG sensor. The light beam is directed via the contact surface towards the skin of the user and at least one photo detector unit in the housing 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 a user. 
         [0016]    A light shaping unit is arranged between the light source and the contact surface and is shaping the light from the light source to an angular range of less than 20°. 
         [0017]    According to an aspect of the embodiment, a computer program product comprising a computer readable memory storing computer program code means for causing the optical vital signs sensor to carry out the steps of the method for measuring or determining vital signs of a user when the computer program is run on a computer controlling the optical vital signs sensor is provided. 
         [0018]    According to an aspect of the embodiment, 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 embodiment is therefore an optical sensor allowing a non-invasive measurement of vital signs of a user. 
         [0019]    It shall be understood that a preferred embodiment of the present embodiment can also be a combination of the dependent claims or above embodiments or aspects with respective independent claims. 
         [0020]    These and other aspects of the embodiment will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In the following drawings: 
           [0022]      FIG. 1  shows a basic representation of an operational principle of a vital sign monitoring system, 
           [0023]      FIG. 2  shows a graph depicting simulated reflectance for different angles of incidences, 
           [0024]      FIG. 3  shows a graph depicting measured reflectance for different angles of incidence and different skin types, 
           [0025]      FIG. 4  shows a graph depicting light reflected from a skin of a user at two different angles of incidence, 
           [0026]      FIG. 5  shows a schematic cross section of a vital signs sensor according to an aspect of the embodiment, 
           [0027]      FIG. 6  shows a graph indicating the relation between the relative power on the photodiode and the beam angle, 
           [0028]      FIG. 7  shows a graph depicting the function of the relative DC power as a function of the beam angle at different distances of the light source, 
           [0029]      FIG. 8A  shows a schematic representation of a vital signs sensor according to an aspect of the embodiment, 
           [0030]      FIG. 8B  shows a schematic representation of a vital signs sensor according to a further aspect of the embodiment, 
           [0031]      FIG. 9  shows a graph indicating the relative DC power and the AC/DC signal has a function of the light source distance, 
           [0032]      FIG. 10  shows a graph indicating the relative DC power and AC/DC signal as a function of angle of incidence, 
           [0033]      FIG. 11  shows a graph indicating a relative DC power and AC/DC signal as a function of the angle of incidence without a separation wall in the vital signals sensor according to an aspect of the embodiment, 
           [0034]      FIG. 12  shows a graph indicating the relative DC power and AC/DC signal as a function of the light source distance, 
           [0035]      FIG. 13  shows a graph indicating a relative DC power and AC/DC signal as a function of angle of incidence with and without a separation wall in the vital signs sensor according to an aspect of the embodiment, 
           [0036]      FIG. 14A  shows a graph indicating a beam pattern in a vital signs sensor according to an aspect of the embodiment, 
           [0037]      FIG. 14B  shows a beam pattern in a vital signs sensor with angle of incidence with a negative direction, 
           [0038]      FIG. 15  shows a schematic representation of a vital signs sensor according to an aspect of the embodiment, 
           [0039]      FIG. 16  shows a basic representation of a vital signs sensor according to an aspect of the embodiment, 
           [0040]      FIG. 17  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment, 
           [0041]      FIG. 18  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment, 
           [0042]      FIG. 19  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment, 
           [0043]      FIG. 20  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment, and 
           [0044]      FIG. 21  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0045]    According to an aspect of the embodiment, an optical vital signs sensor is provided which is based on a photoplethysmograph PPG sensor. Such a PPG sensor  100  is depicted in  FIG. 1  and comprises a housing  101  with a contact surface  100   a . 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 contact surface  100   a  of the sensor  100  is placed in direct contact with the skin  1000  of the user. 
         [0046]    The PPG sensor or optical vital signs sensor according to an aspect of the embodiment can be implemented as a wearable device or a wrist device (like a smart watch). 
         [0047]      FIG. 2  shows a graph with a simulated reflectance on different skins of a user. In  FIG. 2 , the total reflectance R for different angles of incidences α are depicted for three different skin types. The simulation has been performed for a single wave length of 450 nm. In particular, three different skin types ST 1 -ST 3  are depicted. The first skin type ST 1  is a light skin with a melanin amount of 1.3%. The second skin type ST 2  refers to a fair skin with a melanin amount of 4%. The third skin type ST 3  refers to a dark skin with a melanin amount of 40%. 
         [0048]      FIG. 3  shows a graph indicating measured reflectance as a function of different angles of incidences and different skin types. In  FIG. 3 , the total reflectance R in % is shown as a function of the angle of incidence a for five different skin types ST 4 -ST 8 . The skin types include the European light skin type ST 4 , an Asian skin type STS, a south European skin type ST 6 , a European skin type (black hairy) ST 7  and an African skin type ST 8 . The measurements of  FIG. 3  have been performed at a center wave length of 450 nm. In  FIG. 3 , the measured reflectance as a function of angle of incidences is depicted for the different skin types, namely the skin type Ito VI on the Fitzpatrick scale. 
         [0049]    From  FIG. 2  and  FIG. 3 , it can be seen that the reflectance significantly increases for high angles of incidences (for example&gt;60°) as compared to a normal angle of incidence of 0°. The reason for this significant increase is due to Fresnel losses, i.e. reflectance losses on the skin surface. 
         [0050]    Based on the simulated reflectance according to  FIG. 2  as well as the measured reflectance according to  FIG. 3 , it becomes clear that about 20 to 30% of the light is reflected at a normal angle of incidence for the fair to white skin types, namely the skin types I-III. Furthermore, the specular component of the reflected light is high for larger angles of incidence like 70°. Accordingly, the diffuse reflectance is larger than the specular reflectance at all angles of incidence. 
         [0051]      FIG. 4  shows a graph depicting reflected light from a skin at two different angles of incidences. In particular, in  FIG. 1 , the angle A [°] and the intensity I of the reflected light is depicted. In  FIG. 4 , the reflected light I 0  for an angle of incidence of 0° as well as the reflected light  170  for an angle of incidence of 70° is depicted. 
         [0052]      FIG. 5  shows a schematic cross section of a vital signs sensor according to an aspect of the embodiment. The vital signs sensor comprises a light source  110 , a photo detector  120  as well as a separation wall  220 . Optionally, the separation wall can surround the light source. The separation wall  220  is used to separate the light source  110  from the photo detector  120 . This can be done in order to avoid that light from the light source directly reaches the photodiode without any interaction with the skin  1000  of a user.  FIG. 5  also shows some beam patterns to illustrate possible beam paths. 
         [0053]      FIG. 6  shows a graph indicating the relation of the relative power on the photodiode for different beam angles. In  FIG. 6 , the beam half angle BA as well as the relative power on the photodiode RP are depicted. As can be seen from  FIG. 6 , the power on the photodiode RP is decreasing with approximately 2% per 10° of the beam half angle (based on simulations for a Gaussian beam shape) if a half beam angle of 20° is considered.  FIG. 4  shows the result of the reflected intensity per angle of incidence while  FIG. 2  shows the reflectance per angle of incidence. As already mentioned above, because of Fresnel losses on the skin, the DC power of the photodiode will decrease with increasing beam angle. 
         [0054]      FIG. 7  shows a graph indicating the relative DC power as a function of the beam half angle for different distances of the light source towards the skin. In particular, three distances D 1 -D 3  are depicted, wherein the first distance D 1  corresponds to 3.2 mm, the second distance D 2  corresponds to 4.05 mm and the third distance D 3  corresponds to 5.5 mm. As can be seen, the distance between the light source and the diode has a major influence on the relative DC power RDCP. 
         [0055]      FIG. 8A  shows a basic representation of a vital signs sensor according to an aspect of the embodiment. Here, the sensor  100  comprises a housing  101 , a contact surface  100   a  as well as a light source  110  and a diode  120  inside the housing. The light from the light source  110  is directed towards the skin  1000  of the user and reflected light can be detected by the diode  120 . According to this aspect of the embodiment, the distance D between the light source  110  and diode  120  can be varied. 
         [0056]      FIG. 8B  shows a schematic representation of a vital signs sensor according to a further aspect of the embodiment. Here, the beam angle of incidence a of the light from the light source  110  is varied. 
         [0057]      FIG. 9  shows a graph indicating the relative DC power RDCP as function of the distance between the light source and the photodiode. Furthermore, the relation between the AC and DC power is also depicted as a function of the distance D between the light source and the diode. 
         [0058]      FIG. 10  shows a graph indicating the relation between the relative DC power RDCP and the relation between AC and DC as a function of angle of incidence.  FIG. 10  represents the results of the variation of the beam angle of incidence according to  FIG. 8B . 
         [0059]    In  FIG. 9  as well as in  FIG. 10 , the relative DC power is shown for two different skin types and the AC/DC is shown for two different skin types. 
         [0060]      FIG. 11  shows a graph indicating the relationship between the relative DC power and the AC/DC ratio versus the beam angle of incidence α. In particular, in  FIG. 11 , the situation is shown where the separation wall between the light source and the photodiode as shown in  FIG. 5  is removed. 
         [0061]    The difference between the graphs in  FIGS. 10 and 11  is that in the situation of  FIG. 10 , a separation wall is present between the light source and the photo detector while in  FIG. 11 , the separation wall is missing. By the comparisons of the graphs of  FIGS. 10 and 11 , the effect on the beam angle on the AC/DC signal can be seen. 
         [0062]      FIG. 12  shows a graph depicting the relative DC power and the DC/AC as a function of the distance between the light source and the diode. In particular, in  FIG. 12 , the DC signal DCW with a separation wall and the DC signal DCNW without a separation wall is depicted. Furthermore, the AC/DC signal with a wall ACDCW and the AC/DC signal ACDCNW without a wall is depicted. 
         [0063]      FIG. 13  shows a graph depicting the relative DC power and the AC/DC signal as a function of angle of incidence. In particular, in  FIG. 13 , the relative DC power is shown with and without a wall DCW, DCNW. The AC/DC signal is also shown with a wall and without a wall ACDCW, ACDCNW. For all cases in this graph, the distance between the light source and the photo diode is 4,05 mm. 
         [0064]    According to the measurements as shown in the  FIGS. 9 to 13 , there is a relationship between the relative DC power and the AC/DC signal. One important property of the output signal is the modulation signal. 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) which is important to know because a low reflectance can be compensated with LED power boost, preserving the same modulation signal. 
         [0065]    In particular, the AC/DC signal decreases when the DC signal increases. If no separation wall is present between the light source and the photodiode, this relationship is valid for beam angles of &gt;0°. Furthermore, if the distance between the light source and the diode increases, this also leads to an increase of the AC/DC signal. If the beam angle increases, the DC signal increases up to 50° and then starts to decrease for larger angles. This is probably due to the Fresnel losses on the skin surface. As can be seen from  FIG. 13 , beam angles of incidences above 45° and a small distance (e.g. less than 4 mm) between the light source and the photodiode should be avoided. 
         [0066]    Furthermore, as can be seen in the Figs. above, in order to obtain an optimal 
         [0067]    AC/DC signal, the beam angle of incidence can be adapted. 
         [0068]      FIGS. 14A and 14B  show an angle of incidence of 45° in the positive direction and 45° in the negative direction. As seen from  FIGS. 14A and 14B , two complete different modulation signals are obtained. 
         [0069]    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 embodiment, 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 embodiment, an improved PPG signal can be obtained if the magnitude of the beam angle of the light source is less than 20°. 
         [0070]      FIG. 15  shows a schematic representation of a vital signs sensor according to an aspect of the embodiment. The vital signs sensor  100  can be embodied as a PPG heart rate sensor and can comprise a housing  101 , a contact surface  100   a , at least one light source  110  and a photo detector  120 . The light source  110  and the at least one photo detector  120  can be arranged on a mutual base material  130 . The light source  110  can for example be embodied as a light emitting diode LED. Furthermore, the vital signs sensor comprises a light shaping means  200  which serves to guide, shape or direct or redirect the light  111  from the at least one light source  110 . The light shaping unit  200  can also be used to shape a beam profile of the light  111  emitted by the at least one light source  110 . In particular, the shaping unit is adapted to guide the light or light beam  111  such that it has angle of incidence of less than 20°. Advantageously, the range of the angle of incidence is between −20° and +20°. This will lead to an improved modulation signal, i.e. AC/DC signal. 
         [0071]    In  FIG. 15 , the shaping unit  200  is implemented by a diffusing chamber  210  having a top plate  211  which is adapted to transmit light  111  from the at least one light source  110  if the light has a small angle of incidence while light with larger angles of incidences are reflected or redirected by the diffusing chamber. The top plate is the angle selective film or directional turning film (transmitting small angles and reflecting large angles). Thus, the diffusing chamber  210  only allows those light beams  111  to be transmitted or to pass through it if the angle of incidence is small. In fact, the diffusing chamber  210  can act as a mixing chamber and is used to recycle part of the light  111  from the at least one light source  110 . 
         [0072]      FIG. 16  shows a basic representation of a vital signs sensor according to an aspect of the embodiment. The vital signs sensor according to  FIG. 16  substantially corresponds to the vital signs sensor according to  FIG. 15 . Thus, the vital signs sensor also comprises a light shaping means  200 . In the aspect of the embodiment according to  FIG. 16 , the light shaping means  200  is implemented as a separation wall  220  wherein the separation wall comprises a mirror side  221  which is the side towards the light unit  110 . By means of the separation wall  220  which is arranged between the at least one photodiode  120  and the at least one light source  110 , the light or light beam  111  from the light source is guided such that the light reaching the skin  1000  of the user has an angle of incidence between −60° and +20°, in particular between −20° and +20°. By means of the mirror side  221  of the separation wall  220 , the light  111  is redirected such that it points towards the photodiode  120  in negative direction, i.e. it is pointing away from the photodiode  120 . 
         [0073]      FIG. 17  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. The vital signs sensor according to the aspect of the embodiment according to  FIG. 17  substantially corresponds to the vital signs sensor according to  FIG. 15 or 16 . Thus, the vital signs sensor  100  comprises at least one light source  110 , at least one photo detector  120  and a light shaping unit  200  for guiding or shaping the light  111  from the at least one light source  110 . Optionally, the at least one light source  110  and the at least one photo detector  120  can be arranged on a mutual base material  130 . In the aspect according to  FIG. 17 , the light shaping unit  200  comprises an optical refractive unit  230 . Such an optical refractive unit  230  can for example be a Fresnel lens. Here, any light  111  from the at least one light source  110  which has a large angle is refracted and can be redirected based on the property of the optical refractive unit  230 . Thus, it can be ensured that the range of angle of incidences of the light  111  from the at least one light source which is entering the skin  1000  of the user is within a range of between −60° and +20°, in particular between −20° and +20°. 
         [0074]      FIG. 18  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. The vital signs sensor according to the aspect of the embodiment of  FIG. 18  substantially corresponds to the vital signs sensor according to  FIG. 15, 16 or 17  and thus comprises at least one light source  110  and a photodiode  120 . In addition, a light shaping unit  200  is provided for ensuring that the light which is entering the skin  1000  of a user has a specific range of angles of incidence, for example between −60° and +20°. According to the aspect of  FIG. 18 , this is achieved by a light shaping unit  200  which is implemented as an optical collimation plate. The optical collimation plate  240  is arranged in front of the at least one light source  111  and the light  111  from the at least one light source is reflected at the optical collimation plate  240  such that the angle of incidence of the light  111  which is entering the skin of the user is within a specific range of angle of incidence, namely between −60° and +20°. 
         [0075]      FIG. 19  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. The collimation plate  240  is used together with a mixing box  241 . The collimation plate  240  comprises several recesses  242  which can be of compound parabolic concentrator-shape  243 . 
         [0076]      FIG. 20  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. The vital signs sensor according to an aspect of the embodiment according to  FIG. 20  comprises at least one light source  110 , at least one photodiode  120  as well as at least one light guide  200 . According to this aspect of the embodiment, the light guide  200  is arranged between the at least one light source  110  and the at least one photodiode  120 . The light guide  200  is implemented as a light transport unit  250  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  250  has an inclination  251  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  200 , the distance between the photodiode  120  and the output end of the light guide unit  250  can be significantly reduced and a flat design with a low building height is possible. 
         [0077]      FIG. 21  shows a basic representation of a vital signs sensor according to a further aspect of the embodiment. The vital signs sensor can comprise at least one light unit  110 , a photo detector  120  as well as a light shaping unit  200  which is arranged between the light source  110  and the skin  1000  of the user. The light shaping unit  200  can be implemented as an optical angle selective foil  260 . The angle selective optical foil  260  is able to allow light to transmit within a selected angle range. Alternatively, the light guide unit can also be implemented with an optical holographic light shaping diffuser or direction turning film DTF. 
         [0078]    The light shaping 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. Thus the light shaping unit can be considered as a light directing or redirecting unit, a light control unit or a light management unit. 
         [0079]    Other variations of the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed embodiment from a study of the drawings, the disclosure and the appended claims. 
         [0080]    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. 
         [0081]    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. 
         [0082]    Any reference signs in the claims should not be construed as limiting the scope.